Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. Circular No. 740 COLD STORAGE for APPLES and PEARS W. V. HUKILL, Senior Agricultural Engineer, and EDWIN SMITH, Senior Horticulturist Bureau of Plant Industry, Soils, and Agricultural Engineering Agricultural Research Administration UNITED STATES DEPARTMENT OF AGRICULTURE WASHINGTON 25, D.C., February 1946 s ae ator Pan ~ PMT GS — a a a aN ca 7} ‘ Bc t — iN Y ¥ 3 ’ = | i ] x Ag A: i 5 : U. 0: ' tid ; if 4 ; oul | ~* . eae a “ | | BELTSViLi¢ RRANGH | ; Bo erie ; e = ORE et eon ore SN RN MERLE tae en February 1946 C2rcular No. 740 ¢ Washington, D.C. UNITED STATES DEPARTMENT OF AGRICULTURE Cold Storage for Apples and Pears By W. V. HUKILL, senior agricultural engineer, Division of Farm Buildings and Rural Housing, and EDWIN SMITH, senior horticulturist, Division of Fruit and Vegetable Crops and Diseases, Bureau of Plant Industry, Soils, and Agricultural Engineering, Agricultural Research Administration CONTENTS Use of cold storage for fruit____ Response of fruit to storage con- | TIONS =2iir Mee eres aS Respiration and ripening DEOCCSSCS rei weber a 7S ee Storage temperatures_______ ER UITINT CL yee eo Se Air circulation and ventila- (OTC pel atts OR a Controlled-atmosph :re, or LAS ISvOLAC ea! wee ES ee Storage sanitation__________ Storage behavior of apples and OE) Me eae es aS i ne ee Refriveratvione 22208 Cold-storage rooms_________ Required capacity of a refrig- eration= systema 2. Page 2| Cold-storage plants and equip- ment—Continued. Calculating refrigeration re- QUIGeMents = ae ee Cold-storage design _____________ Erecoohin ge) .ase alae. See uaa Capacity and height of FOOMIS) 2 sewer ae Lay-out of rooms2) 2 ss Hansiand Gucts: asa vt) AN OD OH CIR 13 athe iyaies eae kee Sh ae Cold-storage management and 15 PLAN EO PCrAGLON eS sees eee 15 Handling the truit222= "= ss 23 Control of the plant________ Operating efficiency_________ 30: | iteratureveiteds2 2232s. ss a LIST OF TABLES = . Average freezing temperature OLSVaLlOUSsLruits..— ss . Capacity and power data for typical 2-cylinder ammonia COMPECSSOES2= tose", . Relation of coil-room tempera- tures to relative humidity INS;StOraAresvoom = -2 255 fe . Data on sodium chloride (common salt) and calcium chioridejonines == o 2" 22 2" 5. Approximate refrigeration re- quired for apples if 1,000 boxes are received daily and the fruit is cooled to 82° F. bo iv) a 669297°—46——_1 Page 6. Heat-insulation values of va- 5 rious materials in dry con- GiGl One eke es ee ee ee 7. Space required for standard 22 apple packages___________ 8. Relative humidity (percent) of atmosphere by wet- and 23 dry-bulb thermometers__-__. 9. Relation of head or condens- ing and suction pressures 25 to horsepower requirements per ton for typical ammonia COMPTESSOLrS se eee 10. Temperatures of liquid ammo- nia at various gage pres- Page 2 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE USE OF COLD STORAGE FOR FRUIT Holding apples and pears in cold storage in producing areas rather than at market terminals or at points in transit has become a com- mon practice in recent years. In the Pacific Northwest this change has been more or less coincident with the decline of speculative buy- ing of the fruit by eastern interests and with the growth of cooper- ative marketing enterprises owned and controlled by the growers. As a-result the available cold-storage space in the fruit-growing dis- tricts in Washington and Oregon has been materially increased, but even yet it is inadequate for the needs of the industry. Many of the existing cold-storage plants are inadequately equipped to handle satisfactorily the tonnage stored. Year by year there is remodeling and expansion of existing plants, as well as new construction to provide additional refrigerated storage space. Some of the storages are well designed and carefully and efficiently operated; others are not. It is the purpose of this circular to present in concise language, as non- technically as possible, the essential features in the design and oper- ation of cold-storage plants and in the handling of the stored fruit in the Pacific Northwest, although the same principles will be found equally useful in other parts of the country. The principal fruits requiring refrigeration for storage are apples and pears. Grapes also are stored extensively in some places, par- ticularly in California. Refrigeration is used also for the precool- ing or short-time storage of other fruits. As rural electrification and automatic refrigeration equipment have become more available, individual fruit growers or small groups of growers have been building cold-storage plants at or near their orchards instead of relying on large plants that serve a whole com- munity or a large number of growers. This has been coincident with the development of better handling and packing methods. The time, labor, and facilities required for sorting and packing have demanded refrigeration near the orchard, so that packing and ship- ping will not be under the pressure of getting the job done in a matter of a few days after picking. Having refrigeration facilities at hand has permitted the orchardist to give his fruit optimum protection while it is awaiting packing and to employ a comparatively small crew of skilled sorters and packers instead of having to mobilize large crews, oftentimes of persons who know little or nothing about fruit handling or packing. This has been especially important under war conditions, when the utilization of efficient cold-storage facilities near the orchards has been imperative to take the fullest advantage of a short labor supply as well as to prevent wastage of fruit that is a vital part of the Nation’s food supply. Many of the cold-storage plants designed and operated along lines found satisfactory for general cold storage have been neither efficient nor economical for fruit, owing to specialized requirements for the rapid cooling of the fruit and the maintenance of its temperature within narrow limits. For best possible returns on investments, emphasis must be placed upon both the design and the efficient oper- ation of a fruit cold-storage plant. Many cold-storage operators, including foremen and plant engi- neers, will desire more detailed information on many subjects that { — COLD STORAGE FOR APPLES AND PEARS = necessarily are greatly condensed in a publication of this kind. For this reason, attention is called to other publications on refrigeration engineering and fruit storage (3, 4, 5, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 20, 21, 23, 24, 25.) RESPONSE OF FRUIT TO STORAGE CONDITIONS Before undertaking to design and operate a cold-storage plant, the nature of the product to be stored must be understood. Apples and pears are alive at the time of harvest; the length of time they may be held for consumption in the fresh state depends upon how long the end of their life can be delayed. Their storage life dates from the day they are picked, even though they may remain temporarily in the orchard or packing house. The length of storage life varies with the variety, orchard, district, and conditions of growth, the stage of maturity at which the fruit is picked, and the temperature at which it is held. RESPIRATION AND RIPENING PROCESSES When an apple or pear is harvested at a desirable stage of maturity its tissue consists largely of water and such carbohydrates as sugars, fruit acids, and, in and between the cell walls, celluloselike substances from which pectins are produced. These carbohydrates cement the cells together, and the degree of adhesion or disintegration of the cells determines whether the flesh of a fruit is firm, tough, crisp, and Juicy, or soft and mealy. The chemical changes that take place in fruit during ripening are very complex. Starch changes to sugar; sugars change form; acids decrease; and soluble pectins increase in the cell walls. These changes go on until the fruit becomes overripe and unpalatable, with subsequent collapse. As these carbohydrate constituents of the cell undergo ripening changes, oxygen is consumed from the air, water and carbon dioxide are produced, voiatile con- stituents are given off, and heat is generated. All these activities are embodied in what is spoken of as respiration. The chemical changes taking place in ripening fruit, and conse- quently the rate of respiration, are retarded as the temperature is lowered. The quicker heat is removed from fruit after picking to bring it to an optimum storage temperature, the earlier the ripening processes will be arrested and the longer the fruit can be kept. The generation of heat during the respiration and ripening proc- esses, referred to in more detail on page 31, is greater than is com- monly realized and is a factor deserving important consideration in the design and operation of fruit cold-storage houses. The faster a fruit ripens the greater the quantity of heat generated. A Bartlett pear ripens faster than an apple at a given temperature, and, there- fore, its greater heat of respiration results in larger refrigeration de- mands, even when it is taken into storage at the same temperature as the apple. Data on the generation of heat in apples and pears have been discussed in an earlier publication (20). 1 Ttalic numbers in parentheses refer to Literature Cited, p. 60. 4 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE STORAGE [TEMPERATURES Research by Magness and others (74) has shown that when apples are stored at 40° F., the rate of ripening is about double that at 32°, and at 60° the rate is about three times that at 40°. At 85° the soften- ing and respiration rates have been found to be about double those at 60°. At 30° about 25 percent longer time is required for apples to ripen than at 32°. This emphasizes the importance of having the cold storage designed to establish quickly and to maintain uniform low temperatures. UNIFORMITY OF TEMPERATURE Uniformity of temperature relates both to its range on the thermom- eter scale and to the maintenance of a hke temperature throughout a storage room. In some plants, cycles of compressor operation cause a fluctuation of 2° to 4° F. in air temperatures. Slight fluctuation does not injure fruit unless it is downward to a point resulting in freezing or in low-temperature injury. Apples or pears exposed to a temperature fluctuating from 30° to 34° will keep as long as if stored at a constant temperature of 32°. If the fruit is stored at a uniform temperature of 30°, however, its hfe may be lengthened by 25 per- cent (14). Maintaining uniformity of temperature in all parts of a storage room is more important than avoiding small fluctuations at a given point. Fruit ripens faster when stored in a part of the room where the temperature is continuously higher than in another part. This frequently results in the mixing of overripe and prime fruit in ship- ment, or 1t may result in undetected deterioration and decay of fruit in inaccessible locations. The influence of the temperature of fruit on the rate of ripening has special significance in cold-storage management. Apples at 70° F. ripen as much in 1 day as they would at 30° in 10 days; a delay of 3 days in an orchard or in a warm packing shed may shorten their storage life as much as 30 days even if they are then stored at 30°. Storage temperatures recommended for various fruits are discussed in Circular 278 (20). EFFECTS OF RAPID COOLING Apples and pears are not injured by too rapid cooling unless freez- ing takes place or the fruit is susceptible to injury by low tempera- ture occurring above the freezing point. Some low-temperature in- juries of apples are discussed on pages 9 to 12. FREEZING IN STORAGE Because of the dissolved constituents in fruits and vegetables (chiefly sugars and acids) the freezing points of these products are appreciably below that of water. The average freezing point of apples is 28.4° F. It ranges from as high as 29.7° to as low as 27.3° in some of the summer varieties, but is between 28.0° and 29.0° for the prin- cipal winter varieties that are stored. The freezing temperatures of pears are slightly below those of apples. Average freezing tempera- COLD STORAGE FOR APPLES AND PEARS 5 tures of some fruits are given in table 1; for more detailed informa- tion on this subject see Circular 447 (26). TABLE 1.—Average freezing temperature of various fruits Freezing Freezing Fruit and variety tempera- Fruit and variety tempera- ture ture Apples: OI Peaches: Sai Baldwin aoe os ee See 29. 0 I bertasts Sasi e8 So. oe Ss Dae 29.7 WY CTLCLOUS See eee eee OS Lp al 28. 4 Vee Lo) & 2) (aye Ete Ne na aa 29. 6 VON AGM ATS PARE So eee eR 28.3 || Grapes: WAINESA DE ones oe ere aE a ae 28. 2 American, or labrusca, type: Yellow Newtown_______________----- 28. 0 Moore Early_-__-----------_----- 28. 3 Pears: Catawba siete si eee 26. 7 Bartlett: Concord =a eee QTR: ar dinipe ss ea ee ye a ee 28.5 Delaware: sesae 5 pe eee ee RAS 24. 6 Softmipent sce a ae 27.8 European, or vinifera, type: Anjou: Ohanez (Almeria)______________- 25. 6 EVAR Gyr 1 ee eee es el EN eee 26.9 Alphonse Lavallee (Ribier) — __-_- 24.8 i SORG IPOs ee ae Se oe ae 27. 2 IM Peron sees eee Oe ee 24. 6 Cherries: Sultanina (Thompson Seedless) - 23. 6 Bing: Mature (black)__-_-________-_--- 24.1 Immature (bright red) -__------_- 25. 3 S\OUBI RS ghee hence ieee ae poe a et 28. 0 HuMIDITY The loss of moisture from apples and pears in storage, resulting in shriveling or wilting, is directly related to moisture in the form of water vapor in the storage atmosphere. When the humidity is main- tained at above 90 percent, the development of fruit rots is encour- aged as well as surface-mold growth on the fruit, on the walls, ceil- ings, and floors of the storage room, and on the packages. Under ideal conditions of humidity, with active air movement, apples and pears may be kept in cold-storage rooms without risk of excessive mois- ture loss, but when the relative humidity is low, shriveling is aggra- vated by moving air, particularly when the fruit is stored without wraps. A relative humidity of 85 percent is considered ideal for most fruits. Air CIRCULATION AND VENTILATION Apples and pears should be stored in an atmosphere free from pronounced odors. They acquire off-flavors when stored with pota- toes, onions, cabbage, and certain other products. If stored by them- selves, most fruits do not require a change of the air other than that occasioned by the opening of doors or ports under normal operation, provided the fruit is not overripe when received and is quickly cooled to an optimum storage temperature. In most parts of the United States it has not proved practical to substitute natural cold air for mechanical refrigeration during winter months, so that it is seldom advisable to make any special provisions in the storage designs for bringing in outside air. In the storage of apples there is an advantage in having active air movement about the packages, particularly with varieties sus- ceptible to apple scald. Less scald develops when they are stored in moving air. A heavy odor in an apple storage means that some of 6 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE the fruit is reaching an advanced stage of ripeness and is a signal to remove the apples that are approaching the end of their storage life. Ethylene, a gas given off by ripe apples, pears, and some other fruits, hastens the ripening of fruit stored at high temperatures, but has very little effect at low temperatures. Even a very small quan- tity of the gas will cause accelerated ripening at favorable temper- atures. This is an added reason for designing the cold storage for the rapid cooling of fruit in all parts of the rooms rather than attempt- ing to provide for removal of the ethylene by ventilation. CoONTROLLED-ATMOSPHERE, OR GAs, STORAGE Reducing the oxygen content and regulating the concentration of carbon dioxide in the atmosphere of a storage room generally slows up the rate at which fruits ripen. This principle has been applied as an auxiliary to refrigeration in the storage of some varieties of apples and pears. Gas storage has come into considerable use in England and to a limited extent in the northeastern part of the United States, where certain varieties of apples are susceptible to low-tem- perature disorders when stored in air at 32° F. The common practice is to seal the storage rooms until they are essentially gasproof and permit the fruit to consume oxygen until it reaches the desired level, thereafter controlling the concentration of the gas by ventilation. The concentration of carbon dioxide is built up to the desired percentage by the respiration of the fruit and there- after is controlled, when necessary, by circulating the air through an atmospheric washer containing a dilute solution of caustic soda to absorb the excess carbon dioxide. Refrigeration equipment also is necessary, since a temperature of about 40° is desired. The application of the principle of controlled-atmosphere to the storage of fruits has been limited because of the varying tolerance to carbon dioxide gas of different kinds of fruit. Likewise, different varieties of apples respond differently to a given atmosphere of the gases used. For detailed information on the use of controlled-atmos- phere storage, see references to studies made in different localities in the United States (7, 9, 22). STORAGE SANITATION A storage interior free from decayed fruit, dirt, and mold is a criterion of good management. The growth of surface molds within a storage, however, may indicate favorable conditions of relative humidity and does not particularly menace stored apples and pears packed in closed containers. The use of fungicidal paints or the annual whitewashing of walls, ceilings, posts, and air ducts and the oiling of the floors will largely prevent the growth of surface molds, which make them unsightly. Mold growth and spores may be killed by spraying the empty storage with a sodium hypochlorite solution having 0.8 percent available chlorine. The rooms should be closed for a few days following the application. Chlorine vapor from a spray of sodium hypochlorite is an irritant to the mucous membrane. Workmen should therefore be protected from injury while spraying. This may be done by use either of fans to pro- duce an air movement to carry away the fumes or of an all-service gas mask in nonventilated rooms. COLD STORAGE FOR APPLES AND PEARS 7 Fumigating the storage rooms is another method of killing molds and one that will reach areas not accessible to sprays. Sulfur dioxide is commonly used for this purpose. It is produced by burning sulfur at the rate of 5 pounds per 10,000 cubic feet of space. As soon as the sulfur has been ignited, the rooms should be closed for 24 hours. Sulfur furnaces should not be placed near motors or delicate machinery, as the fumes are corrosive and prolonged heavy concen- trations are destructive to machinery parts. Removal of such equip- ment or protection by covering is a recommended precaution. In burning sulfur, precautions against fire should be taken by placing the furnace over a 3-inch layer of sand or in a receptacle of water with a surface of 2 feet greater radius than the furnace. Sulfur dioxide is injurious to apples and pears, and no fruit should be placed in the rooms until all traces of gas have disappeared. It is likewise a strong irritant to eyes and mucous membranes, and care must be exercised to avoid contact with the fumes during and after fumi- gation. Doors should be opened to air the rooms thoroughly after fumigation and before they are entered by workmen. For this reason it is feasible to fumigate only during the season when the storage space is not in use. SIORAGE, BEHAVIOR OF-ARPPEES AND, PEARS Success in the storage of apples and pears is dependent upon giving due consideration to their inherent characteristics and to their nor- mal cold-storage life, as well as to the handling of the fruit before storage (8, 20). APPLES A temperature of 30° to 32° F. and a relative humidity of 85 to 88 percent give best results in the storage of most varieties of apples in most parts of the United States. Certain varieties, however, some- times will not tolerate continuous low-temperature storage. Yellow Newtown apples from the Pajaro Valley of California and McIntosh and Rhode Island Greening apples from New York and New England should be held at 35° to 38° to prevent the development of internal browning and brown core. Grimes Golden should be held at 34° to 36° to prevent soggy break-down. Under conditions described below, certain other varieties should be stored at temperatures higher than 32° to avoid storage disorders. The higher the storage temperature the faster the apples will ripen and the sooner the end of their storage period will be reached. Apples stored close to the place where they are to be consumed may be held until they are ripe, and if in the hands of the consumer the day fol- lowing removal from storage they will still be acceptable. Apples stored at more distant points must have sufficient life left when with- drawn from storage to withstand the higher temperatures of trans- portation and distribution. The longer apples are stored the shorter their life after removal to higher temperatures. Thus, apples that leave cold storage in apparently good condition may reach the con- sumer in an overripe and mealy condition with many decayed fruits when distribution requires 10 days to 2 weeks. Some forms of deteri- oration of apples in storage are discussed here. 8 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE AMMONIA INJURY The appearance of ammonia injury on apples is recognized by a prominence of the lenticels, which become white at the center, with some or many surrounded by bands of black on the red surfaces or of green on the yellow-green areas. Even short exposures to small con- centrations of ammonia will produce these color changes. When ammonia concentrations are 2 to 5 percent, an exposure of 5 to 8 minutes results in prominent lenticels with the surrounding discolor- ation spreading between the black or green rings. After the apples have been exposed to the fumes for a short period, they partially recover when aerated. The residual damage may be only a slight skin blemish around the lenticels or it may be more serious and affect the flesh tissue. APPLE ROTS Apple rots are either initially or finally caused by fungi commonly referred to as molds (6, 79) or are associated with them. From the standpoint of the cold-storage operator, a most important char- acteristic of rot-producing fungi is that their growth and the germi- nation of spores are either entirely stopped or greatly held in check at temperatures of 30° to 82° F. The riper the apples are before being handled the more susceptible they become to injury and rot infection. The growth of such important fungi as blue mold, gray mold, and Alternaria progresses slowly at temperatures of 30° to 32° once infection takes place. Gradual cooling over 2 to 4 weeks is a bad practice. It hastens the unseen development of rot fungi and later results in a greater percentage of decay than in fruit cooled quickly. 3 The cold-storage warehouseman needs to keep a close watch for ripening and decay in all storage lots. Certain “side rots” and the “bull’s-eye” rot from perennial canker are of slow growth until apples reach a certain stage of ripeness, whereupon the rots grow rapidly and become apparent in a few weeks, often causing severe loss before being detected. Susceptible lots should be inspected frequently and should be sold before becoming ripe, especially after the first signs of decay are noticed. The effect of cold storage upon susceptibility to decay of the fruit before it is washed and packed depends upon the character of stor- age and the degree of ripeness of the fruit when handled. The washing and packing of firm apples that are placed in good cold storage promptly after harvest may take place over a long period without increasing the danger from storage rots. When apples are to be held at temperatures conducive to ripening, it is preferable to pack them before storage unless they are to be consumed promptly after packing. BITTER PIT Bitter pit, sometimes called Baldwin spot or stippen and recog- nized by sunken areas or pits with brown spongy areas in the flesh, cannot be controlled in cold storage. Bitter pit 1s a disorder related COLD STORAGE FOR APPLES AND PEARS 6) to growing conditions and may become noticeable on the tree or after the fruit has been harvested and stored. Leaving apples on the tree until they are mature often reduces loss from bitter pit or pre- vents its subsequent development in storage. Crops of susceptible apples intended for storage should be held at 30° to 32° F. for 2 months before being packed so that affected fruits may be sorted out. INTERNAL BROWNING, OR BROWN CORE The terms “internal browning” and “brown core” are used, respec- tively, to designate the effects of low-temperature injury in Yellow Newtown and McIntosh apples. The Yellow Newtown grown in the Pajaro Valley in California is especially susceptible, and in this variety the injury commonly appears as elongated areas of brown discoloration radiating from the core. As it progresses it may spread throughout the tissue and resemble internal break-down. In MclIn- tosh, as well as in Yellow Newtown and some other varieties, it is characterized at first by a slight brown discoloration between the seed cavities that may later progress until the entire core area becomes brown, making the fruit unmarketable. Susceptible apples should not be stored at 30° to 32° F. but at 36° to 40° to prevent or minimize losses during storage. In districts where internal browning and brown core are serious storage hazards, the application of controlled-atmos- phere storage should be considered (22). INTERNAL BREAK-DOWN Internal break-down, recognized by a more or less general brownish discoloration of the flesh, usually outside the core area and at the blossom end of the apple, is essentially death from old age. It manifests itself variously in different varieties. In Jonathan an area on one side or in a zone beneath the skin may become brown and dry while the rest of the flesh is crisp and juicy. This is some- times spoken of as “Jonathan break-down.” It is associated with fruit harvested at an advanced stage of maturity and may occur early in the storage season. In other varieties internal break-down may appear as brownish streaks in ripe, mealy tissue, later becoming badly discolored, dry, and spongy. ‘This is designated as “mealy break-down” and in some varieties the skin often ruptures. Late in the storage season or after removal from storage this disorder frequently occurs beneath bad bruises, or in tissue near the core in a region affected with severe water core at the time of harvest. The risk of loss from internal break-down is negligible when apples are harvested at the proper stage of maturity and stored promptly at 30° to 32° F. for normal periods for the variety. When found in a storage lot, it should be regarded as a signal for prompt disposal of the fruit. A somewhat similar type of discoloration occurs in the fruit of some varieties in some districts before harvest. It is caused by a defi- ciency of boron. This type of break-down does not become worse while the fruit is in storage. 669297°—46—2 10 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE ? APPLE SCALD Apple scald is a browning of the skin and is distinguished from soft scald by being superficial and generally diffuse, and from a sim1- lar superficial browning following washing injury by being more. localized and more pronounced on the green or unblushed surfaces. It is associated with fruit harvested at an immature stage and may be entirely prevented in some varieties, including Delicious, by delaying picking until the fruit is sufficiently mature. It is induced by certain volatile products of respiration, and if apples are not too immature when harvested it can be largely controlled by placing paper con- taining at least 15 percent of an odorless and tasteless mineral oil in contact with the fruit as soon as possible after harvest. When shredded oiled paper is used, at least half a pound per bushel is neces- sary and it should be well distributed so as to be in contact with every apple. Where practical considerations dictate the storage of loose fruit for extended periods, scald prevention calls for (1) fruit adequately mature when picked and (2) active air movement over it. The use of slatted crates or orchard boxes and adequate spacing of containers in storage is beneficial. Apple scald ordinarily does not begin to appear earlier than 60 days after harvest, and the more mature the apples when picked the later its appearance. When it begins to appear, the fruit should be disposed of, as the scald is likely to spread and become more intense, especially after the apples are taken out into living-room temperature. Scalded fruit frequently arrives at the market in bad condition, with rots starting in the scalded tissue. SOFT SCALD AND SOGGY BREAK-DOWN Soft scald is frequently confused with apple scald but has a differ- ent appearance and is radically different with respect to cause and prevention. Soft scald seldom occurs on fruit picked at the proper stage of maturity and stored immediately at 30° to 82° F. It is usu- ally caused when susceptible varieties of apples are delayed at warm temperatures after harvesting and are then placed in low-temper- ature storage (below 36°). It cannot be prevented by the use of oiled paper or by picking at an advanced stage of maturity. In its early stages soft scald may resemble apple scald, as faint patches of brown become apparent, but soft scald develops rather rapidly into slightly depressed areas of discolored skin. The mar- gins of the affected areas are sharp, and the pattern is generally irreg- ular. The apple may have the appearance of having been rolled over a hot stove. Another distinguishing feature is the brown spongy tissue beneath affected areas. In certain varieties the disorder may be confined to the small points of contact where apples press against each other. When limited to this type of manifestation, soft scald is sometimes referred to as “contact scald” and when found in midwinter “it rarely develops to greater proportions. Jonathan and Rome Beauty are the varieties most susceptible to soft scald. At the expense of a shortened storage life these varieties should be stored at 36° to 388° F. if they cannot be given 80° to 32° within 24 hours after picking. The same applies to Golden Delicious if not COLD STORAGE FOR APPLES AND PEARS ial stored within 4 days after picking. McIntosh, Delicious, and other varieties are sometimes affected. In the Winesap, soft scald is largely confined to fruit that has been held in common storage for a period and then moved into cold-storage temperatures of 30° to 32°. Soft scald can be prevented by holding the fruit in 25-percent carbon dioxide gas for 24 hours before storage at 30° to 32°. Soggy break-down is a disease of the tissue of certain varieties of apples that has similar causes. It is largely avoided by using storage temperatures of 36° F. or above. It most commonly appears in Grimes Golden and Golden Delicious and is characterized by internal regions of brown spongy tissue, frequently with no outward signs until deteri- oration has reached advanced stages. The dead tissue appears as sharply defined islands or bands between core and skin or may extend to the skin and there coalesce with the surface manifestations of typical soft scald. SCALDLIKE DISORDERS Apples subjected unduly long to heated washing solutions, as when the washer is stopped with unrinsed fruit in the washing section, some- times get the appearance of scald without the distinguishing evidence of heat cracks. This is caused by a bleaching of the pigment and sub- sequent browning of affected areas, and becomes evident within a few days after washing. Usually the discolored areas are more intense where the wax has been removed at scratches or abrasions, although in severe cases the entire apple takes on a cooked appearance. In less severe cases a diffuse discoloration appears over the entire surface, whereas in apple scald such discoloration is first observed in patches or on the unblushed side of the fruit. Apples so injured are subject to shriveling and are unsuitable for prolonged storage. Golden Delicious and Yellow Newtown apples that hang on the tree with the cheek freely exposed to the sun may have sunburn that is not very noticeable at the time of packing, but after a period in storage _the areas take on an appearance that is difficult to distinguish from apple scald. This should be diagnosed as delayed sunburn. It does not materially shorten the storage life of the fruit and when found on occasional specimens does not require the early disposal necessary when occasional specimens are found with apple scald. The only pre- vention is a more careful sorting of sunburned apples at the time of packing. Small sunken scalded spots result from the contact of apples with Douglas-fir wood. They may result from contact with fir-tree props or from packing in fir boxes. The toxic effect of this wood will pene- trate paper wraps and box liners. FREEZING INJURY Injury from freezing ranges from no visible evidence following in- cipient ice formation in the flesh to a brown discoloration of the entire apple following “freezing to death” at prolonged low tem- peratures. Intermediate stages of injury may be only a slight soften- ing of the flesh, which, however, should be interpreted as indicating a shortened storage life; a flaky or corky character in a flesh lacking normal crispness; brown discoloration of tissue around the 10 fibrovas- 12 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE cular bundles and extending as threadlike fibers throughout the flesh ; and the appearance of sunken spots where the apples were bruised while frozen. After apples have been badly frozen, the skin becomes shriveled, the surface is discolored in irregularly shaped areas, and the tissue beneath may be translucent and water-soaked or have some shade of brown. Badly frozen tissue becomes dry and corky after pro- longed storage. When slight freezing occurs near refrigeration coils or cold-air ducts, the frost can be removed by raising the temperature at those points to 32° F., but when the apples are frozen deep in the piles, a storage-room temperature of 40° or above and an active circulation of air between the packages will be necessary to thaw them out. The fruit should not be moved while frozen, as this will result in severe injury. The thawing of frozen apples at a temperature of 70° does | not result in greater injury than thawing at 32° to 40°, but a high temperature is not recommended, because of its accelerated ripen- ing influence. To prevent shriveling, the relative humidity should be kept as high as possible during the thawing process, preferably above 80 percent. JONATHAN SPOT Jonathan spot is a skin disease giving the apple a freckled appear- ance from small black or brown spots that appear usually on the deep-colored areas. Although it sometimes develops on other varie- ties, especially Rome Beauty, from a commercial standpoint it is of importance only on the Jonathan. It may be confused with the black spots around the lenticels caused by arsenic burn or with the brown freckled appearance of Jonathans caused by other spray or washing injuries, but these are distinguished by their appearing earlier in storage, regardless of temperatures. The disease is prevented almost entirely by picking before overmature and storing promptly at 30° to 82° F. Jonathan spot is an indication of “old age,” and its appear- ance is a warning that the fruit is being kept beyond its commercial storage period. It may develop on fruit still on the tree. WATER CORE Water core occurs in the fruit before it is removed from the tree. As it is usually associated with advanced picking maturity, crops severely affected are ordinarily not considered well suited for pro- longed storage. The water-soaked areas gradually become smaller during storage and, if they are not severe, may completely disappear. Apples affected with water core never completely recover, however, because the affected tissue has been weakened and is disposed to internal break-down. In the Delicious, Rome Beauty, Stayman, and other softer varieties, internal break-down may follow slight water core at the fibrovascular bundles. Apples that have apparently made a complete recovery while in cold storage frequently become worth- less from internal break-down within 5 or 6 days after removal to living-room conditions. The disappearance of water core is not hastened by storing the apples in atmospheres of low relative humidity but rather by holding them at temperatures that produce rapid ripening. As such ripen- COLD STORAGE FOR APPLES AND PEARS 13 ing is not desirable, however, the only recommendation that can be made is to limit the storage season as much as possible and keep the fruit under refrigeration. : PEARS Pears have a slightly lower freezing point than apples and, not being subject to such low-temperature diseases as soft scald and brown core, can be stored at slightly lower temperatures, 29° to 31° being recommended. As pears are rather susceptible to shriveling, it is important to keep the relative humidity of the storage room above 85 percent, preferably about 90 percent. Pears are more responsive to high temperature than most varieties of apples, so that it is very important that heat be removed as rapidly as possible immediately after harvesting. They have a high rate of respiration, and the heat of respiration is an important consider- ation in storage, especially during the cooling period. For suc- cessful storage, therefore, the fruit at the center of packages must be cooled approximately to the storage temperature within a period of 48 hours before the packages are stacked in the permanent stor- age piles. This is usually done by circulating air at temperatures of 26° to 381° F. through widely spaced stacks of packages imme- diately after they are packed. After this initial cooling, packages should be stacked so as to provide air channels for the continuous removal of the heat of respiration and for uniform refrigeration throughout the piles. Stacking away from the walls and on strips or floor racks is necessary to prevent the conduction of heat to the fruit. Pears may be held in cold storage and subsequently washed and packed without serious injury or disfigurement, provided ripening has progressed only slightly. The prevalence of scratches and other friction marks often found on fruit thus held depends on the stage of ripeness rather than being due to the influence of refrigeration. Holding the fruit for 2 or 3 weeks prior to washing and packing is safe if the fruit is kept at 30° to 31° F. from the time it is harvested. LOSS OF RIPENING CAPACITY Following prolonged storage, certain varieties of pears may seem to be in excellent condition but when taken to high temperatures they fail to ripen. Although the color of the fruit may become yellow in the ripening temperatures, the flesh does not soften or become juicy. Bosc, Comice, and Flemish Beauty exhibit this characteristic and do so earlier in the season when stored at 36° F. rather than at 30° to 31°. It is important that these varieties be stored at optimum low temperatures and for periods not longer than the varietal storage season. Following storage, ripening must proceed promptly at opti- mum ripening temperatures. OPTIMUM RIPENING TEMPERATURES Commercial varieties of pears grown in the United States do not ripen satisfactorily for eating while held at 29° to 31° F. Some vari- eties gradually become softer at these temperatures, while others 14 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE may turn slightly more yellow but soften scarcely at all. Al unrip- ened pears need to be withdrawn from cold storage and held at higher temperatures to ripen for eating. The optimum ripening temperature for most varieties is between 65° and 70° F. Bartlett has much better quality when ripened in this range than at higher temperatures. Bosc fails to ripen normally at lower temperatures. Kieffer has optimum quality when ripened at temperatures between 60° and 65°. PEAR ROTS Blue mold rot and gray mold rot are the most important storage rots in pears. Blue mold rot usually results from skin punctures. Gray mold rot may start at ruptures of the skin or at broken stems and spreads from fruit to fruit by contact. Once established, gray mold grows slowly in cold storage, but having the capacity to enter the unbroken skin of adjacent fruits, it often produces the so-called “nest rot” when a whole group of pears is aifected. The spreading from one pear to another can be prevented by packing in wrappers impregnated with copper. Sanitary measures in harvesting and pack- ing, together with prompt cooling to temperatures of 29° to 31° F. are important factors in preventing losses from decay. Lining the orchard boxes with old newspapers is an important precaution to take to reduce mechanical injuries and resulting infection. PEAR SCALD In pear scald the skin of the fruit becomes dark brown and soft and sloughs off easily under pressure. The affected skin may become almost black and affords entrance for the decay fungi that usually follow. The disease does not appear until the fruit is aged in storage from being held too long or at too high a temperature. Pear scald, other than the type on the Anjou variety, cannot be prevented by pack- ing in oiled wrappers, but susceptibility may be lessened by picking before the fruit becomes too advanced in maturity and by storing at temperatures of 29° to 31° F. ANJOU SCALD The Anjou variety is subject to a mottled surface browning or blackening in storage. Unlike pear scald, this does not cause a skin disintegration that is deep-seated, nor does the skin slough off. Anjou scald can be largely avoided by picking fruit at the proper maturity and packing it in oiled wrappers such as are used for apple scald. CORK SPOT Cork, or cork spot, is characterized by small regions of dark-brown corky tissue appearing in the flesh of pears. When the affected tissue is near the surface a small depression frequently appears, and the skin at this spot may be shghtly dark. The sunken areas on the sur- face sometimes fail to appear until after storage. Anjou is the vari- ety frequently affected by cork spot. The disease is related to growth COLD STORAGE FOR APPLES AND PEARS 15 conditions in the orchard and is not caused by storage conditions. Affected fruit can be stored approximately as long as normal fruit, but its market value may be greatly depreciated if cork spot is very prevalent. CORE BREAK-DOWN Core break-down is characterized by an extremely soft watery con- dition about the core, followed by rapid disintegration and discolora- tion in the tissue of this region, sometimes leaving only a shell of the outer tissue unaffected. It frequently occurs during the ripening of fruit that has been left on the tree too long before harvesting and also may occur in fruit that has been held too long in storage at low temperatures. In Bartlett pears it is aggravated by ripening at too high temperatures, in which case it is not confined to the core region. COLD-STORAGE PLANTS AND EQUIPMENT REFRIGERATION The best way to become familiar with refrigeration is to work with it and use it. Each cold-storage plant has characteristics of is own, and to take advantage of its good points and to avoid difficulties that may not be common to other plants one must be familiar with that par- ticular plant. General principles of refrigeration apply to all plants, however, and knowing these principles will enable an operator to profit by his experience. They are covered in textbooks (12, 15, 16, 24), and more specific information is given in handbooks (2, 3, 23, 25) on characteristics of refrigerants, condenser, compressor, and evaporator, insulation values, fan and duct data, requirements of stored products, cooling surface, power requirements, and other matters. PUMPING HEAT The process of refrigeration might be likened to pumping air out of a tank until the pressure is lower than that of the atmosphere. Once the desired low pressure inside the tank is reached, the only additional pumping necessary is to remove any air that enters the tank by leakage, and then the pumping needed will depend entirely upon the leakage. In a refrigerated space, it is desirable to maintain a certain tempera- ture below that of the surroundings. Heat is pumped out until the de- sired low temperature is reached, whereupon further pumping is nec- essary only to remove the heat that enters the chamber by leakage through walls and open doors or heat that is generated within the space. When pumping air from a vacuum tank, if only a slight approach to vacuum is required, less power and a smaller pump are needed than - for a high vacuum. The size of the pump required and the horsepower of the motor depend upon two factors: (1) The quantity of air to be removed and (2) the pressure inside the tank. If too much air is allowed to enter the tank, the pump cannot remove it and the desired vacuum cannot be maintained. Similarly in a refrigerating system, if only a moderately low temperature is required, less power and a smaller compressor are needed than where a very low temperature is desired. Furthermore, if the refrigeration machinery does not have the capac- 16 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE ity to pump out heat as fast as it enters the chamber, the desired low temperature cannot be maintained. In extending the comparison, the factors determining the size of the pumps are, in the case of the vacuum, (1) pressures, usually expressed In pounds per square inch; and (2) quantity of air, expressed as pounds per minute. In the refrigerating system the factors are (1) tempera- ture, expressed in degrees; and (2) heat, commonly expressed as Brit- ish thermal units (B. t. u.). The term “B. t. u.” (the heat required to raise the temperature of 1 pound of water 1° F.) corresponds to the term “pound” (in pumping air), inasmuch as they both express definite quantities of the thing to be handled. QUANTITY OF HEAT In dealing with refrigeration problems it is just as necessary to con- sider the quantity of heat to be handled as to speak of pounds of air or gallons of water when computing the necessary sizes of air or water pumps for given jobs. Just as 1 pound represents a very definite and measurable quantity of air, and it is still the same regardless of the pressure under which it is placed, so 1 B. t. u. represents a definite and measurable quantity of heat, and it too remains the same regardless of existing temperatures. The refrigeration demand upon the machinery is frequently spoken of in terms of “tons.” This usage had its origin in a comparison of refrigerating capacity, or demand, with the refrigeration obtained from melting 1 ton of ice. As 144 B. t. u. of heat are required to change 1 pound of ice to water at the melting point, 288,000 B. t. u. are re- quired to melt 1 ton of ice. Where it is necessary to remove 288,000 B. t. u. of heat in 24 hours, 1 ton of refrigeration is required. If, for example, a temperature of 32° F. is to be maintained in a storage building, the refrigeration system will have to remove a quantity of heat just equal to that which enters the building. The heat entering may come from a number of sources. In the first place, if the outside temperature is above 32°, some heat will come in through the walls. This can be reduced by insulation, but not even the best of insulation will exclude all heat leakage. If there are cracks in the building, or if doors or windows are open and permit warm air to enter, an increased quantity of heat will be introduced, depending upon the outside temperature and the quantity of air. Materials having temperatures above 32° placed in the cooled space, will introduce still another quantity of heat, depending upon the temperature, weight, and nature of the material. If the materials are living, as for example, apples, they will produce heat continually; and this heat is in addition to that which they contained when first put into storage. The heat from all these sources and from other incidental sources combines into the total quantity of heat the refrigerating system is expected to remove. If the system has sufficient capacity the heat can all be pumped out. If the heat introduced into or produced within the building exceeds the capacity of the refrigeration system, some of it will remain in the fruit and cannot be taken out until the rate of heat intake drops below the rate at which it can be removed. The quantity of heat that a refrigeration system can remove may be increased or decreased by the conditions under which it operates, COLD STORAGE FOR APPLES AND PEARS 17 but no manipulation of air movement or special stacking of boxes or other adjustment can prevent the accumulation of heat if it is being introduced or produced faster than it is being removed. THREE STEPS IN THE REFRIGERATING PROCESS Heat, like air, is handled in definite quantities, but unlike air it cannot be moved bodily from one point to another. By its nature heat moves from a place of high temperature to one of low tem- perature. A refrigerating system, or heat pump, takes advantage of this tendency. Heat from the storage room moves through the walls of the evap- orator cooling coils to the ammonia or other refrigerant inside, which is at a lower temperature. The compressor then takes the vaporized ammonia with the heat it has picked up in the evaporator and, by compressing the gas, raises its temperature. The heat from the hot ammonia finally moves into the condenser water because the water is at a lower temperature. Thus the heat from the storage is now in the condenser cooling water, which may either be wasted or cooled by aeration for recirculation. These three steps in heat removal are accomplished by the three essential parts of the refrigerating sys- tem—the evaporator, the compressor, and the condenser (fig. 1)? In the evaporator, or cooling coils, the quantity of heat picked up depends upon (1) the temperature difference between the refrigerant (ammonia) in the coils and the air outside, (2) the area of coil sur- face exposed, and (3) the resistance to heat flow through the walls of the pipes. The resistance to passage of heat into the coil in turn depends not only upon the cleanness of the coil but also upon the velocity of air (or brine if a brine cooling system is used) passing the coil and the velocity of the refrigerant (whether liquid or vapor). The resistance is increased by an accumulation of frost, or if not enough piping surface is exposed a large temperature difference will be necessary between the inside and outside of the coil to permit sufh- 2 Bowen (4, pp. 2—8) describes the operation of the refrigerator shown in figure 1 as follows: To utilize its latent heat of vaporization for refrigeration and to conserve the refrigerant, application is made of the physical law that the temperature at which a fluid boils or condenses is raised or lowered, respectively, by increasing or reducing the pressure. To cause the refrigerant to boil at a low temperature in the evaporating coils and hence absorb heat on a low-temperature plane, the pressure in the coils is lowered by the suction of the compressor. ... To free the fluid of the heat absorbed in the refrigerator and return it to liquid form, the cold refrigerating gas coming from the evaporating coils is compressed until its temperature is raised above that of the water flowing through the condenser so that the contained heat can pass from the gas to the water. (In very small machines, air may be used instead of water.) The essential parts of a compression-refrigerating system are an evaporator, a compressor, and a condenser. In the evaporator (the coils in the refrigerator) the liquid boils and in the process absorbs heat from the surrounding medium. The compressor is a specially designed pump that takes the gas from the evaporator coils and compresses it into the condenser coils, reducing its volume and increasing its temperature. The condenser consists of coils of pipe over or through which water or air flows to absorb the heat from the gas, which is thereby liquefied. In some systems the cooling water passes through an inner tube, and the gas from the compressor through the annular space between the inner and the outer pipes. From the condenser the refrigerant passes first to a liquid receiver, and then through a throttling or expansion valve into the evaporator coils, to repeat the process of transferring heat from the refrigerator to the water flowing through the condenser. The temperature of the liquid ammonia is reduced from the temperature of the receiver to that of the refrigerator by vaporizing a part of the liquid. The expansion valve is of a special design and is capable of very fine adjustment. Its function is to so regulate the flow of the liquid refrigerant that suitable pressure and tem- perature conditions will be maintained. It is largely responsible for the control of tem- perature in the evaporating or cooling coils. 669297 °—_46——3, 18 CIRCULAR 740,.U. S. DEPARTMENT OF AGRICULTURE cient heat to pass into the coils. This requires a low ammonia tem- perature. If, because of resistance or insufficient surface in the cool- ing coils, it is necessary to maintain a low ammonia temperature (which means low suction pressure), the compressor is forced to boost the temperature from a low point, and it cannot handle as much heat as when the suction temperature is higher. The compressor must also discharge the ammonia at such temper- ature that heat will flow from it to the cooling water in the condenser. In general, a compressor can handle more heat if the temperature in the cooling coils is kept as high as possible and the temperature in the condenser as low as possible. The same conditions also reduce the power necessary to remove a given quantity of heat. When the ammonia enters the condenser, heat passes from it into the cooling water. As in the evaporating coils, the heat passing SN SS SSNS PR SSAA S SAAK \ SSS Q ( Voy > N LOW-PRESSURE SIDE HIGH-PRESSURE SIDE FicurE 1.—Essential parts of a compression refrigeration system. from the ammonia to the cooling water depends upon (1) the tem- - perature difference between the ammonia and the water, (2) the surface area exposed, and (3) the resistance to heat flow through the condenser pipes. Here also, the resistance to the passage of heat depends upon the water and the ammonia velocities and the cleanness of the coil. Scale, which tends to collect on the pipes from the cooling water, may increase the resistance markedly. If this scale is per- mitted to build up or if there is not sufficient cooling surface, the required quantity of heat can be transferred to the water only by hav- ing a large temperature difference between ammonia and water. As pointed out before, the high ammonia temperature in the condenser means reduced compressor capacity and high power consumption. An adequate supply of water as cold as possible will contribute toward a low ammonia temperature in the condenser and therefore to low power consumption. COLD STORAGE FOR APPLES AND PEARS 19 CONDENSER The condenser has one purpose. It must permit the passage of heat from the compressed ammonia to the cooling water (or air in an atmospheric condenser) and do so at as low an ammonia temperature as possible. It must transfer all the heat that has been taken up in the evaporator as well as that added by the work of the compressor. The passage of heat into the cooling water is facilitated by a large area of cooling surface, by a large quantity of cooling water, by a low water temperature, and by high velocity of the water and ammonia passing the surface. A high ammonia temperature also increases the quantity of heat transferred to the cooling water, but it is the function of the condenser to receive and discharge the ammonia at as low a tem- perature as possible. The design of the condenser 200 and its operation should be such as to remove the re- quired quantity of heat without excessive ammo- nia temperatures. In operation the effec- tiveness of the condenser may be judged by the head pressure indicated on the gage. If the head pressure goes too high, the effects on the system are that less heat is re- moved from the cold rooms and more power is required to operate the compressor. The effect of various high head _ pres- sures on power require- 0 I5 20 25 30 35 ments at various suction Suction pressure (pounds per sq. in.) Sec aaerasee came a 2s See FieurE 2.—Effect of condensing and suction the HAS OUI ye ae chart pressures upon power requirements of a typi- (fig. 2). For example, cal ammonia compressor. when operating at a 25- pound suction pressure, and a head pressure of 120 pounds, about 1.0 horsepower is required to remove 288,000 B. t. u. per day (1 ton of re- frigeration) ; whereas, at a head pressure of 195 pounds, about 1.5 horsepower is required for removing heat at the same rate. That is, the power cost is about 50 percent higher at a 195- than at a 120-pound pressure. At the same time, a high head pressure results in reducing the heat that the system can handle. This is illustrated in figure 3. If the head pressure is too high when the plant is running to capacity, it may be because the condenser is too small, there is not enough cool- ing water, the cooling water is too warm, noncondensable gases are present, or the condenser tubes are dirty. ‘The water used in the con- denser usually contains impurities that corrode the pipes and form Condensing pressure (pounds per sq. in.) 20 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE deposits on them. If such deposits are allowed to accumulate over long periods they interfere seriously with the exchange of heat. TYPES OF CONDENSERS While all condensers have as their purpose the cooling of the hot ammonia gas, thereby changing it to a liquid, there are several different general types. In each the hot gas is circulated through or around pipes that are exposed to a cooling fluid, usually water. In a double- pipe condenser the ammonia is passed through a bank of pipes. A smaller pipe carrying cooling water extends full length inside each section of ammonia pipe. Several banks of double-pipe condensers are usually mounted to- gether to give the re- quired capacity. In a ver- tical shell-and-tube con- denser the ammonia gas enters the top of a large vertical cylinder and the condensed liquid drains off at the lower end. Nu- merous vertical pipes in- side the cylinder are mounted so that a film of cooling water runs down the inside of each pipe. As the ammonia con- denses on the outside of the pipes it flows to the bottom of the cylinder, where it 1s drained off to 0 the receiver. WY 2 £0 2 ay 35 = The horizontal shell- Suction pressure (pounds per sq.in.) and-tube condenser is sim- - nae : ii ilar to the vertical, except IGURE 3. ect of condensing and suction that the shell is in a hori- pressures upon the capacity of a typical am- Bice monia compressor. zontal position and. the water pipes carry cooling water under pressure. The water is usually passed back and forth through several tubes in series before being discharged. In this way its velocity is increased to give more rapid “cooling without having to discharge large quantities. An evaporating condenser has the ammonia gas pass through coils that are exposed to a spray or drip of water. At the same time air is blown through the water spray past the pipes and causes some of the water to evaporate. This evaporation keeps the water cool, so that it can be recirculated, and the only waste is the water that is evaporated or carried away in the air blast. This is particularly suited to condi- tions where cooling water is limited or expensive and where the atmos- phere is relatively dry during the time large loads are expected on the refrigeration machinery. Condensing pressure (pounds per sq. in.) COLD STORAGE FOR APPLES AND PEARS Dt Where a dry climate or limited supply of cooling water makes it desirable, the effect of evaporative cooling may also be obtained with shell-and-tube or double-pipe condensers by using a cooling tower or a cooling pond. In this type, the water from the condenser, instead of being wasted, is pumped to a tower (frequently on top of the build- _ ing) or to a cooling pond adjacent to the building where it is forced through nozzles to form a spray. After falling through the atmos- phere, where it is cooled by the evaporation of a small portion, the water is recirculated through the condenser. Another type, the atmospheric condenser, as frequently used with small cooling or freezing cabinets, usually is not practical for larger installations. COMPRESSOR The compressor, by pumping ammonia from the evaporator to the condenser, takes the heat that has been absorbed in the coils and, by raising the temperature, allows the heat to be carried away by the condenser cooling water. The rate of heat removal by an ammonia compressor running at a given speed depends only upon the head pressure and the suction pressure at which it operates; the higher the suction pressure and the lower the head pressure the more heat will be removed. If the speed is increased, the rate of heat removal will increase proportionately, assuming a given set of pressure con- ditions. It is good practice, therefore, to operate a compressor at as high a speed as its design will permit, especially during the season when warm fruit is being received. In fruit storage the demand on the refrigerating equipment is at a maximum for only a short period in fall. Much of the capacity of this equipment is unnecessary during the rest of the year. To get the most out of it for this critical period, while keeping the investment in equipment at a minimum, it is sometimes economical to operate at higher speeds than would be advisable for year-round operation. Compressors, however, should be speeded up only after consulting the manufacturer regarding the particular machine. Greater capacity may be obtainable in some slow-speed compres- sors by changing the valves and lubrication system to permit con- siderably higher speeds. It is a mistake to judge the capacity of the refrigerating system by the size either of the compressor or of the motor installed. The capacity will depend upon the whole system and the conditions under which it operates. For comparative purposes the refrigerating capac- ity of a compressor is normally expressed as standard tons when operating with a head pressure of 155 pounds and a suction pressure of 20 pounds, but the actual capacity will be influenced by condi- tions in the system as a whole that cause variations in these pres- sures. The capacity of and power required for typical ammonia com- pressors of various sizes are given in table 2. DD, CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE TABLE 2.—Capacity and power data for typical 2-cylinder ammonia compressors Typical refrigerating capac- ity and power require- ments at 155-pound : : Displace- Cylinder size condenser pressure and (inches) peers Speed 20-pound suction pressure Capacity Power Cubic foot R.p.m. Tons Hp. SURw AL Shs eg cece eee BI) Nmap staat 0. 024 400 2a 3.5 At ADA LNG Sto AS STR GSM ey. eaee le eee . 058 375 4.7 Tail ST ieee: SEN ig eS Da fa IS SN ek aE aes SE PelalS 360 8.9 13. 4 Gee OS ELE et eg aS is eae ee ek . 196 360 15. 6 21.8 GS Ua eae ie ee Sete Ne . 249 360 20.0 28.0 Ta ERG aS EI Se eile! Se re. aie . 383 360 31.0 43.0 SEAS ee SI I PI Sr oan A ns ee 465 360 39.0 53. 0 Od as se SNS ee ee en ee Ee ae . 662 300 48.0 63. 0 1 Oxal Qasr es os a2 eee. SS ek ee eee eee 909 300 67.0 87.0 EV APORATOR The evaporator, or cooling coil, absorbs the heat from the room. The ammonia, having had its load of heat removed in the condenser, is expanded toa vapor. This expansion, or evaporation, under low pressure, reduces the ammonia temperature to such a point that it is ready to pick up more heat from the cold room. This is done by direct expansion coils in the room or by air circulated from the room to a bank of coils or finned surfaces. Here, as in the condenser, con- ditions should be such as to permit the heat to flow with as little tem- perature difference as possible between the ammonia and the air in the room. If there is not sufficient cooling surface, if the surface is cov- ered with frost, or if other factors retard the heat flow, the ammonia would have to be extremely cold. This would mean a low suction pressure, which reduces the capacity of the compressor, At low pres- sures ammonia gas is less dense, and the smaller quantity of gas drawn into the compressor at each ‘stroke results in lower refrigerating capacity. That the capacity of a typical compressor is increased markedly as the suction pressure is raised is shown graphically in figure 3. For example, at 140-pound head pressure and at a suction pressure of 24 pounds the compressor delivers 9 tons. An increase of 4 pounds in suction pressure changes the capacity of the same machine to 10 tons. If by increased cooling surface or careful operation the pres- sure could be increased to 36 pounds, about 12 tons of refrigeration would be obtained, a gain of 33 percent. Similar changes in suction pressure in an ammonia machine of any size would result in approx- imately the same percentage increase in capacity. Another disadvantage of operating at low suction pressures is that the coils are extremely cold and a large quantity of moisture is condensed out of the air, resulting in low storage humidity. Ample evaporator coil surface will permit the cooling to be done without excessively cooling the air that touches the coils. The results of cooling the air to low temperatures are shown in table 3. COLD STORAGE FOR APPLES AND PEARS 23 TABLE 3.—Relation of coil-room temperatures to relative humidity in storage room Maximum relative humidity when the temperature (° F.) is raised to— Degrees (F.) to which ie Rant aes LE ewes ak, ese SGD air is chilled | | 24° 26° 28° 30° 32° San SOs 38° 40° Percent | Percent | Percent | Percent | Percent | Percent | Percent | Percent | Percent eee 68 62 57 52 47 43 40 | 37 | 33 Sore cere 2 Sg Se 75 | 68 62 57 52 48 44 41 | 36 ZO Be RE rete. i se 83 76 69 63 57 53 49 45 | 39 IAP 3 Sa Re ee 91 83 75 69 63 58 54 49 44 ASS Ss ae TS att ee Sa Relies 100 91 83 76 69 64 59 54 48 746) a Se eS Se eS a a a) eer 100 91 83 76 70 64 59 53 De ae ae ee eS ee eee 100 91 83 77 71 66 58 UE se Se Se ee Se es ee ee a ee 100 91 84 78 72 64 BS ee ES Seek eee ere eee Bae ee! bone eee 100 92 85 79 70 CoLp-STORAGE Rooms Two general methods are used in the distribution of refrigeration units in cold-storage rooms for apples and pears: (1) Placing refrig- eration pipes on the ceilings and (2) circulating cold air through the rooms. The first is more commonly the direct-expansion system, though cold brine may be pumped through the pipes from a brine cooler. The gradual evolution in the use of refrigeration for fruit from the direct-expansion to the brine-spray system has included the dry-coil bunker system in the intermediate stage. The unit-cooler system is a modification of either the brine-spray or the dry-coil system and is especially convenient for small plants. Fruit keeps equally well under any of these systems, provided they are installed so that cooling will be equally fast and temperatures will be kept uniform, with atmospheric humidity at about 85 percent. The choice hinges largely upon economy in installation and oper- ation. DIRECT-EXPANSION SYSTEM In direct-expansion rooms, that is, where cold ammonia is cir- culated in exposed pipes near the ceiling, the air in contact with the coils becomes cold and, being denser than warm air, moves down- ward. As it picks up heat from the fruit it rises to the pipes to be again cooled. This gravity circulation, caused by differences in air temperatures, results in heat movement by convection. Air veloci- ties in such currents are relatively low, but take place in all parts of the room if the pipes are well distributed over the ceiling, and produce fairly fast cooling. To dispose of the accumulated frost or condensed water the pipes are usually put in groups or banks, and cutters for catching the drip are hung under them, as illustrated in fioure 4. In rooms where large areas of the ceiling are without coils, direct expansion alone cannot cool the fruit very promptly, and there may be fairly large temperature differences between various parts of the room, even after the fruit has cooled to its final temperature. In such cases, use of either portable or permanent fans operating in 24, CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE the room to stimulate air movement will tend to make the temper- atures more uniform. Fans installed to give a positive air move- ment will give even better results. Fans blowing directly over the cooling pipes are effective in reducing both condensation and the danger of localized freezing of the fruit. BRINE-PIPE SYSTEM To avoid all possibility of accidental leakage of ammonia from the cooling system into the storage rooms the cooling pipes are sometimes designed for carrying cold brine. The brine is cooled in Figure 4.—Cold-storage room piped for direct-expansion system, with gutters Suspended beneath pipes over the space to be occupied by fruit. a separate brine cooler and circulated by pumps to the various rooms. Other advantages of this method are that temperature control is simpler than in a direct-expansion system and a reserve of refriger- ation is available in the cold brine to carry over short periods of shut- down. This system however, is more costly than direct expansion and for this reason it is not commonly used in fruit districts. In comparison with an air-circulation system, brine pipes otherwise have the same advantages and disadvantages as a direct-expansion sys- tem. A brine of calcium chloride instead of common salt (sodium chloride) may be used for this type of installation. Data on the density and freezing points of sodium chloride and calcium chloride brines are given in table 4. COLD STORAGE FOR APPLES AND PEARS 25 TABLE 4.—Data on sodium chloride (common salt) and calcium chloride brines? SODIUM CHLORIDE . Salt in 100 = : Salt in 100 : Specific Freezing Densi Specific Freezing ; gravity ponds of point ensity gravity Pp eae of point Density Pounds per Pounds per Pounds ae gallon Pounds i. gallon 1A00E == sewer 32.0 3S |i} Wee 3 oe 13.5 14.9 9.17 OZR a ees 2.8 29. 1 GO) A a 16.1 10. 4 9. 34 ee ee 65 26. 0 S56 '7a||| lela ean 18.6 5.4 9. 50 1ROGHES = ant 5 8.2 22.7 SSSA 5 || ail bl Geter mile —.3 9. 67 1O82e Se See 10.9 19.0 OE OD Hy apie 23. 5 —3.6 9. 84 CALCIUM CHLORIDE L008 0 32.0 Sh 83 it TGS 17.6 7.0 9. 68 VOSA ee ee es 4.7 29.3 S567) p1e20 sees ee 21.5 —5.8 10. 01 TQSEe ese Use re 9.2 23:2 BOI i) Me peh 25, 1 —21.5 10. 35 NA ee oe ee 13.5 16.5 Sot iP Tepe. ee Be 28.7 —44.3 10. 68 1 See American Society of Refrigerating Engineers Data Book (3). DRY-COIL BUNKER SYSTEM In the dry-coil bunker system of cooling, the ammonia coils are put in a separate room or bunker and air from a large blower is passed over them, then distributed through ducts to the storage room. If large quantities of air are used, prompt cooling and even temperatures may be obtained. The problem of accumulation of frost on the pipes re- mains, although disposal of the water and frost without damage to the fruit is simpler than under direct expansion. In some installations the pipes are defrosted periodically by spraying with brine or warm un- salted water. The blower is stopped while the defrosting is taking place. In other plants defrosting is done by pumping hot ammonia into the coils. Dry-coil bunkers have largely given way to brine-spray systems in recent installations. BRINE-SPRAY SYSTEM In the brine-spray system of cooling, air from a large blower is moved over banks of ammonia coils that are continually being sprayed with a solution of salt in water. Sodium chloride (common salt) is generally used in these systems. The salt prevents accumulation of frost, and the fine spray, being in intimate contact with the air, cools it effectively. A far smaller bank of pipes can be used than in a dry bunker, and cooling can be done with a higher ammonia temperature. After cooling, the air is distributed to the storage rooms. When a continuous brine spray is used, it is necessary to use baffles, or elimina- tors, in the air stream to prevent particles of brine from being carried in the air to the storage rooms. It is also necessary to treat the brine with chemicals, as recommended by equipment manufacturers, to reduce its tendency to become unduly corrosive. Despite the necessity for eliminators, which increase the resistance to air flow, and the tendency of the brine to cause corrosion, brine-spray chambers are generally displacing both direct-expansion and dry-coil bunker sys- 669297°—46—_+4 26 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE tems for fruit refrigeration. The arrangement of blower and enclosed brine-spray compartment in a brine-spray system are illustrated in figure 5 to) e UNIT-COOLER SYSTEM A modification of the brine-spray or the dry-coil bunker is the unit cooler, which contains extended surface coils and blowers for moving the air through the coils and discharging it to the room, as shown in figure 6. Some are defrosted by a continuous brine spray, and in some FIGURE 5.—Arrangement of blower and enclosed brine-spray compartment in a brine-spray system. Motor and brine pump for forcing the brine spray over the enclosed evaporating coils are shown at left foreground. In this plant the return air ducts end in the room containing this equipment. The cold-air delivery ducts extend from the farther end of the brive-spray compartment, the coils are washed periodically with fresh water to remove the frost. In the latter case warm water from the condenser is generally used. These units usually discharge air at the top, either into ducts or through nozzles, and return it to the coils through openings near the floor. When the return air is picked up in the lower part of the room, it is difficult to get the best distribution of temperatures. A unit cooler installed with air ducts for a better distribution of refrig- eration is shown in figure 7. When defrosting is intermittent, it is important to make the cycle short enough to keep the coils fairly free from frost.