266 MODERN PAPER-MAKING
stresses set up in passing from the couch to the drying cylinders. Refining
helps to some small extent to control the wetness of the stock on the wire,
but there is no doubt that groundwood is rather unresponsive to refiner treat-
ment, and therefore it is more important that the right quality of groundwood
should be obtained from the pulp mill, rather than that any attempt should
be made to try to modify its quality by treatment in the paper mill.
The correct proportioning of sulphite and groundwood stocks is dependent
upon the maintenance of a uniform consistency, and some reference has already
been made both to the usefulness of large chests, and also to the importance of
dropping breakers regularly and diluting the stock methodically in order to
avoid excessive variation in consistency.
It may be found desirable, in addition, to control the stock automatically
using one of the many types of consistency regulator available for the purpose.
All these regulators depend, for their operation, on the stock changing in.
viscosity, as the consistency changes, and although this relationship is rather
variable, depending, among other things, on the kind of pulp and the treat-
ment it has received, a well-designed and carefully constructed regulator caa
give a very satisfactory measure of control.
The separately prepared groundwood and chemical pulp stocks are mixed
by means of a proportioner, of which a number of types and designs are
available. These depend usually on the provision of a constant head of each
kind of stock by pumping up to a receiver having an overflow for stock to
return to the storage chest. From the constant head, or constant level com-
partment, the rate of flow of the stock is controlled by a revolving variable-
speed paddle-wheel, as in the Trimbey proportioner, or by a rectangular weir
having an adjustable width, as in the Tidbury proportioner. The two pro-
portioned stocks are collected together in a receiver and conveyed by gravity
to the mixed stock chest, from which the stuff is pumped for further refining
and consistency control, finally being discharged into the machine chest ready
for the mixing pump.
In modern mills, the proportioner is the place where, in addition to adjusting
the ratio of groundwood to chemical pulp correcdy, loading, dyes, and some-
titles alum are added, in the form of a suspension or solution. As in the case
of pulp, the rate of flow may be controlled by a constant head of the liquid,.
ootipkd -with a feeding paddle rotating at the rate required to give the accessary^
<ifofifo A mote recent aad very satisfactory method of obtaining a visual control
oCalog* fk>w is to use a rotameter. This type of flow indicator consists of a very
gjcaefeafiy taperi&g glass tube* A cone rises or fells in this glass tube, according
piatiafr «l€ty$®& dbat has to esca|>e between it and the glass tube; the
*fee flow the higjber tbe cone,must rise to take advantage of tie