206 MODERN PAPER-MAKING
The improved British VJ.B. three-stage steam circulation heating system
retains these two important features in unchanged form, but uses a different
cycle of operation. The extravagant feature of the original system lay in
the fact that the same temperature was attainable on all cylinders, and most
economical working was effected only when such conditions obtained. When
the last cylinders were required very hot and the first cylinders only just warmed,
the system suffered by the recirculated steam cooling the direct steam, its
original temperature never being fully utilised in the cylinder. In the latest
British V.JB, three-stage steam circulation heating system, heating is done
in three separate stages, enabling still greater economy to be effected by the
more complete use of the heat of the steam.
The three stages are: First, the heating stage, which consists of the cylinders
using direct steam, undiluted by steam under reduced pressure, to heat the
cylinders requiring the highest temperature. Second, the mixed stage, con-
sisting of the cylinders using the exhaust steam from these hottest cylinders
mixed with direct steam. This stage is very important, because, according to
the quality of the paper, the cylinders can be heated to any required tem-
perature, high or low, thereby making the whole system very flexible. The
third or suction stage is divided into two groups: first, the cylinders receiving
a mixture of exhaust steam from the mixed and heating stages; and the
second group, which receives the steam evaporated from die hot condensate
exhausted from the cylinders, and using this 'flashed' steam to heat the low-
temperature cylinders.
In the first stage, direct steam is blown in, without the use of an ejector,
to the cylinders required to be worked at the highest temperature, so that
the full temperature of the steam is available. In the second stage, exhaust
steam from one group of the first stage, mixed with direct steam, is used for the
cylinders which are heated with a lower temperature, but which still have to
give a high evaporation of water. The steam exhausted from the first and
second stages after separation from water has sufficient pressure to he used as
the supply for heating the following cylinders with medium or low tempera-
tore- Its pressure is also sufficient to introduce exhaust steam of reduced
pressure into these cylinders through an ejector. The steam exhausted from
one group of cylinders is not put back into the same group of cylinders, but into
one ttsmg a lower temperature. The third stage is provided with a small
suction pump, which creates a vacuum sufficient to allow the use of evaporated
steam fiota tfee hot condensate of the steam traps. Suction is provided by
means eŁ a pump-operated aspirator and assisted by a cooling coil. The
coolest; wotting cylinders are virtually under suction sufficient to enable the
use of evaporated steam from the condensate, and utilise this steam as the