Page 393 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:50 PM Page 350

350 Applied Process Design for Chemical and Petrochemical Plants

Step 6b. Next calculate the mols of propane and butane Wt of butane = 6.511 58 = 378 lb
vapor in the high pressure side: Wt of total charge = 1,445 lb

The preceding example becomes increasingly compli-
3
Vapor vol in cond 105.00 ft (Step 4b) cated when more than two components are involved.
Another item to keep in mind is that if there is any leak-
Vapor vol in disch pipe 26.25 ft 3
(Step 4c) age in a system such as this, the leakage will be preferential.
Total 131.25 ft 3
For example, a vapor leak in the condenser would leak pro-
portionally more propane than butane. This would change
the performance of the cycle considerably.
131.25
No. of mols 4.96
0.595 44.5
Example 11-6. Other Factors in Refrigerant Selection
Mols of propane 0.957 4.96 4.747 Costs
Mols of butane 0.043 4.96 0.213
Refrigerant costs are important when considering the
Step 6c. Finally, calculate the mols of propane and investment in filling and maintaining a full charge in a par-
butane in the evaporator liquid. ticular system.
At the time of this chapter’s development for this edition,
Liquid volume in evaporator 33.3 ft 3 the phase-out of certain refrigerants (discussed earlier in
this chapter) has required careful redesign of some existing
33.3 equipment and/or replacement in order to adapt a suitable
No. of mixture mols 25.00
0.0280 47.5 fluorocarbon type refrigerant. This has required some re-
engineering including instrumentation in order to establish
Mols of propane 0.75 25.00 18.75 a reliable and workable new or upgraded system. Careful
Mols of butane 0.25 25.00 6.25 attention should be given to the system performance and
even redesign when replacement or upgrading is being con-
Mol of Mol of Mol of sidered. See Reference 32.
Propane Butane Mixture *
Refrigerant 22 is seldom used in centrifugal compressors
Evap & Suct Pipe (Vapor) 0.732 0.048 0.780 due to the high cost, and 113 and 114 are usually used only
**
Cond & Disch Pipe (Vapor) 4.747 0.213 4.960 in water-chilling applications. Refrigerant 11 is frequently
Evap (Liquid) 18.750 6.250 25.000 used in the higher temperature ranges. Refrigerant 12 *** is
Total 24.23 6.51 30.74 popular for centrifugal application due to its low cost and
favorable suction conditions. It is limited to relatively large
Thus the calculated mol% of the initial charge is as fol- tonnages due to the low cfm per ton. For reciprocating
*
lows: applications, refrigerant 22 is preferred to 12 *** due to the
low cfm per ton. The cost is not a great factor because in rec-
24.23 iprocating applications, the charge of refrigerant is relatively
Propane: 79.0%
30.74 small. Refrigerant 12 *** has the advantage of a considerably
6.51 lower heat of compression, with resulting easier duty on the
Butane: 21.0%
30.74 compressor. Ammonia, propylene, and propane require
more stages of compression in a centrifugal machine than
the chloro-fluoro-refrigerants, and this increases the com-
This checks the given composition of initial charge.
pressor costs. The cfm/ton and weight flow rates are low
If it did not, the problem must be reworked.
and, thus, give lower piping costs. *
Finally the weight of the total charge is found:
The performance characteristics must be reexamined for
Wt of propane 24.229 44 1,067 lb the replacement refrigerants, and it cannot be assumed that
they will perform as direct replacements. In fact, some hard-

***Soon to be replaced by R-507 (125/143a), R-404A (125/143a/134a), R-407A (32/125/134a), R-407B (32/125/134a), R-402A (22/125/290), R-402B
(22/125/290), R-403A (22/218/290), R-408A (125/143a/22), or R-134a, R-410A (32/125), R-410B (32/125) or R-407C (32/125/134a); see Figure 14-24.
***Soon to be replaced by R-123.
***Soon to be replaced by R-134A, R-401B, R-405A, R-406A, R-409A.