Page 437 - Advanced thermodynamics for engineers

18.1 LIQUEFACTION BY COOLING – METHOD (I) 427

relevant T–s diagrams are shown in Fig. 18.5(a) and (b) respectively. This is like a ternary cascade
system, but in this case, the working fluid passes through all the stages of the plant. The carbon dioxide
gas enters the plant at state ‘a’, and passes through the final stage, referred to as the snow chamber,
because by this stage the gas has been converted to dry ice. Heat is transferred from the carbon dioxide
to the final product causing some evaporation: this evaporated product is passed back to the first
compressor. The fluid then passes through the first compressor, where it is compressed to state ‘c’. It is
then passed into a flash tank, where it is cooled by transferring heat with liquid already in there: this
again causes some evaporation of the product, which is passed back into the second compressor, along
with the working fluid. In the example shown the process has three stages, and the final stage must be at

Cooling water
Q
vapour
g
h Work
liquid
f
i vapour
Flash
tank Temperature, T g Vapour
temperature
vapour Critical point e p tp
e
j h c
liquid Work Liquid a p = 1atm
j i f t = 25 C
o
d
k vapour Triple point l k d
temperature
Flash Solid
tank n m b
vapour
l
liquid c
Work Entropy, S
vapour b
m
1 kg CO gas vapour
2
a

solid carbon dioxide n
at 1 atm, -43 C
FIGURE 18.5
Plant for manufacture of dry ice (solid carbon dioxide).