Page 340 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:49 PM Page 299

Refrigeration Systems 299

6. Space limitation, if any. Open Circuit. Refer to Figure 11-9.
7. Utility costs for steam and cooling water if manufac- For 40°F chilled water, 100 psig high-cost steam, 90°F
turer is to make economical compromise of require- cooling water to the barometric:
ments for these services.
8. Type and nature of cooling water for condenser. Selecting cooling water at 11 gpm/ton:
Total (11)(465) 5,110 gpm
Then, the corresponding steam rate is 29.9 lb/hr/ton:
Example 11-1. Barometric Steam Jet Refrigeration Total (29.9)(465) 13,900 lb/hr
Make-up water: 100% (once through)
A process has the following requirements for chilled water
refrigeration:
Because this is a fairly large unit, consider two, 250-ton
units. This configuration would be justified only if the ton-
1. One condenser at 55°F water, 110 gpm
nage load was not steady at the 465-ton rate, but fluctuated
2. One gas cooler at 55°F water, 65 gpm
for reasonably long periods down to 250 tons. Then one
3. One direct gas absorber-cooler at 40°F water, 223 gpm
unit could be shut down and perhaps some utilities. The
500-ton unit is a good economical size, and with multiboost-
Make up water for these refrigeration units is at 80°F, and
1
1
ers can operate at / 2 (two boosters) or / 4 (four boosters)
feed water for the gas cooler unit is available at 90°F. Baro-
load. The economics should be investigated.
metric water is 90°F. Items 1 and 2 are for a closed-circuit
operation with return water at 68°F and total 175 gpm. Note
Hot Well
that in order to consolidate the temperature levels of water,
it is economical to establish a temperature, such as 55°F,
The hot well is the sump where the barometric leg is sealed.
which satisfies the bulk of the requirement, and then design
It must be designed to give adequate cross-section below the
the other phases of the plant process to also use this water
seal leg and for upward and horizontal flow over a seal dam or
temperature. Item 3 is an open-circuit operation because
weir. At sea level the hot well must be a minimum of 34.0 ft
the water is sent to waste after absorbing certain corrosive
below the base of the barometric condenser. For safety to
vapors and cooling the bulk of the gas.
avoid air in-leakage, a value of 35—36 ft is used. For an altitude
corresponding to a 26-in. Hg. barometer, the theoretical seal
Closed Circuit
height is 29.5 ft; actual practice still uses about 34 ft.
11752168 552
Tons refrigeration 94.8 tons
24 Absorption Refrigeration
The two most common industrial absorption-type refrigera-
Open Circuit
tion systems are (1) aqua ammonia and (2) lithium bromide-
12232190 402 water, with ammonia and water respectively the refrigerant for
Tons refrigeration 465 tons
24 each system.
Recommended Tonnage for Purchase Ammonia System
Closed circuit: 100 tons
In general the principle of operation depends upon the
Open circuit: 500 tons
capability of water to absorb large quantities of ammonia
vapor, which can be released from the solution by the direct
Approximate Utility Requirements
application of heat. 18
100 psig steam is high in cost, and cooling water very A typical pictorial flow diagram for the ammonia system is
cheap. given in Figure 11-14 for a single-stage system and in Figure
11-15 for a two-stage ammonia system.
Closed Circuit. Approximate values between those in Fig- As far as refrigeration is concerned, this system produces
ures 11-11 and 11-12 for 55°F chilled water temperature and ammonia liquid as the refrigerant that is evaporated in the
90°F cooling water. process unit requiring the refrigeration. In principle this is
For high-cost steam, reduce its requirements at the sacri- the same operation at the evaporator as if the system were
fice of water: lb/hr steam/ton (13.5 18.8)/2 16.1 one of compression. The method of removal of the heat of
approx. Steam required (16.1)(94.8) 1,530 lb/hr at a the ammonia vapor and then the reliquefaction are the
barometric cooling water rate of 10.1 gpm/ton; then this points of difference between the systems.
water required (94.8)(10.1) 958 gpm. Make-up process In the absorption system, as the ammonia is vaporized by
side water (0.01)(175 gpm) 1.75 gpm. the process heat load being cooled or condensed in the