Page 392 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:50 PM Page 349

Refrigeration Systems 349

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Using / 4 in. tubes, 16 ft long, the volume is the same as the The average composition in the condenser (vapor)
evaporator. 95.7% propane, 4.3% butane,
Neglect liquid volume, because it is small in the condenser. average temp. 100°F
Therefore, total volume 10.5 16 168.0 ft 3.
(1) (2) (3) (4)
Volume of tubes 63.0 ft 3 Component Mol% Mol Wt (1) (2) P c (1) (3) T c (1) (4)

Volume of vapor difference: 105.0 ft 3 Propane 95.7 44 42.0 617 590 666 638
Butane 4.3 58 2.5 551 24 766 33
Step 4c. Compressor suction and discharge volume: 44.5 614 671

Average latent heat of mixture 170 Btu/lb
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and approximate average specific vol. at suction 3.50 ft /lb Apparent mol wt 44.5
P c (mix) 614, T c (mix) 671
12,000,000 3.50
Therefore, cfm 4,100
60 170 180 560
P R 0.293 , T R 0.833
614 671
This indicates a 350 size compressor with 12 in. diam. suction Z 0.792,
piping and 8 in. discharge.
The volume of the compressor is assumed to be included in the 1,544 1,544
suction and discharge pipes. Z T 0.792 560
Suction Pipe: v ZRT mol wt 44.5
Assumed length 50 ft P P 144 180
Volume 50 0.78 39.0 ft 3
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Discharge Pipe: 0.595 ft >lb
Assumed length 75 ft
Volume 75 0.35 26.25 ft 3 Step 5c. Find the specific volume of liquid in the evapo-
rator:

Step 5. For this step, it is necessary to calculate the
(1) (2) (3) (4)
specific volume of the vapor and liquid in the system.
Component Mol% Mol Wt (1) (2) Wt % Sp Vol at 0 F (3) (4)
Step 5a. Find the specific volume of the vapor in the
evaporator and the suction pipe. Propane 75 44 33.0 69.5 0.0289 0.0201
Butane 25 58 14.5 30.5 0.0260 0.0079
47.5 100.0 0.0280 ft /lb
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(1) (2) (3) (4)
Component Mol% Mol Wt (1) (2) P c (1) (3) T c (1) (4)
Propane 93.8 44 41.3 617 580 666 625
Butane 6.2 58 3.6 551 34 766 48 Average mol wt 47.5
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44.9 614 673 Sp vol 0.0280 ft /lb
Step 6. Here the final check is made of the original
Apparent mol wt 44.9 equilibrium assumption.
P c (mix) 614, T c (mix) 673 Step 6a. First calculate the mol of propane and butane
vapor in the low pressure side:
32 460
P R 0.052 , T R 0.682
614 673
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Vapor vol in evap 071.7 ft (Step 4a).
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Therefore Z 0.920, Chart 2, Chapter 5. Vapor vol in suct pipe 039.0 ft (Step 4c).
Total 110.7 ft 3
1,544 1,544
Z T 0.920 460
ZRT mol wt 44.9 volume
v No. mol vapor
P P 144 32 sp vol average mol wt
110.7
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3.16 ft >lb 0.780
3.6 44.9
Step 5b. Find the specific volume of vapor in the con- Mol of propane 0.938 0.780 0.732
denser and the discharge line. Mol of butane 0.062 0.780 0.048