Page 345 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:49 PM Page 303

Refrigeration Systems 303

Table 11-2* Using the chart, Figure 11-17, the 103°F and 18 psig deter-
Data for Determining Utilities Required mine the 35.8% ammonia and 0 Btu/lb. Then the 25.8% as
for Absorption Plants determined previously, together with the column pressure
of 214.2 psig, determine the 267°F and 190 Btu/lb.
Single-Stage When the weak aqua enters the absorber, it flashes or is
expanded through a control valve from about a column
Btu/min Water Rate
Steam Sat. Req. in through pressure of 214.2 psig to the absorber pressure of 18 psig. At
Temp., °F Generator per Steam Rate Cond. (7.5°F equilibrium for this 25.8% ammonia solution and at 18 psig,
Evap. Req. in Ton Refrig. lb/hr/ton Temp. Rise),
Temp., °F Generators (200 Btu/Min) Refrig. gpm/ton the temperature is 138°F (Figure 11-17), and the liquid
enthalpy is 49 Btu/lb.
50 210 325 20.1 3.9
40 225 353 22.0 4.0 The weak aqua leaving the heat exchanger ahead of the
30 240 377 23.7 4.1 absorbers is assumed to be cooled to about 6—11°F (some-
20 255 405 25.7 4.3 times more) less than this equilibrium temperature of
10 270 435 28.0 4.6
0 285 467 30.6 4.9 138°F. Then the weak liquid entering the absorbers is taken
10 300 507 33.6 5.4 as 138°F 10°F 128°F. This may have to be corrected
20 315 555 37.3 5.9 later if proper balance is not reached. Figure 11-17A is also
30 330 621 42.5 6.6
40 350 701 48.5 7.7 a useful reference.
50 370 820 57.8 9.5
Two-Stage
Strong Aqua Leaving Absorber
Btu/min Water Rate
Steam Sat. Req. in Steam Rate, through Cond. lb/min (1 a/(b a)(lb NH 3 /min)
Temp., °F, Req. Generator per lb/hr/ton (7.5°F Temp. where: a weight fraction concentration of weak aqua
in Generators Ton Refrig. Refrig. Rise), gpm/ton b weight fraction concentration of strong aqua
175 595 35.9 4.3
180 625 37.8 4.5 1 0.258
190 655 40.0 4.6 lb>min 0.358 0.258 11772 1,312
195 690 42.3 4.9
205 725 44.7 5.3
210 770 47.5 5.7 gpm 1,312/7.18 lb/gal 183
220 815 50.6 6.3
230 865 54.0 6.9 Weak Aqua Entering Absorber
240 920 58.0 7.8
250 980 62.3 9.0 lb/min 1,312 177 1,135
265 1050 67.5 11.0
gpm 1,135/7.42 lb/gal (approx.) 153
Note: Water to condenser at 85°F, with 100°F condensing temperature. Water from
condenser used in absorbers.
Heat Exchanger Before Absorbers
Used by permission: Rescorla, C. L. Refrigerating Engineering, March 1953; now
merged and used by permission: ©American Society of Heating, Refrigerating, and Weak aqua temperature range: 267°F S 128°F
Air-Conditioning Engineers, Inc. All rights reserved.
Strong aqua temperature range: 103°F S ?
The temperature of the strong aqua must be evaluated.
Absorber Design
1. Heat in exchanger (weak aqua) (1,135)(189)
The range of temperature difference in the absorber
215,000 Btu/min
between strong aqua from the absorber and cooling water
2. Heat in (strong aqua) (1,312)(0) 0
entering is 9—15°F. Select 13°F for this case. Then strong
3. Heat out (weak aqua) (1,135)(49) 55,700 Btu/min
aqua temperature from absorber is 90°F 13°F 103°F. 4. Enthalpy of strong aqua liquid out of exchanger:
Assume an absorber pressure of 18.0 psig. This is 1.6 psig
lower than the evaporator pressure of 19.6 psig. 1215,000 55,7002 0
121.3 Btu>lb
Absorber Aqua Conditions (Using Chart, Figure 11-17) 1,312
Liquid Temp., °F Psig Wt% NH 3 Liquid Enthalpy, Btu/lb Strong aqua temperature from Figure 11-17 at 35.8%
ammonia and 121.3 Btu/lb 211°F. (temp. out of
Strong 103 18 35.8 0
Weak 267 00214.2** *25.8* 190 exchanger). The equilibrium value at 35.8% and column
pressure of 214.2 psig is 226°F, and the ammonia vapor con-
*Assumed difference in aqua concentration of 10%; this may range from
centration is 93.3 wt%.
6—15%, depending upon conditions, and is often varied to suit a
particular capacity situation. For exchangers, the strong aqua temperature range is
103°F S 211°F.
**Neglects effect of distillation column pressure drop.