Page 451 - Mechanical Engineers' Handbook (Volume 4)

440 Refrigeration

additional generator, condenser, and heat exchanger are used. Energy from an external heat
source is used to boil the dilute lithium bromide (absorbent) solution. The vapor from the
primary generator flows in tubes to the second-effect generator. It is hot enough to boil and
concentrate absorbent, which creates more refrigerant vapor without any extra energy input.
Dual-effect machines typically use steam or hot liquids as input. Coefficients of performance
above 1.0 can be obtained with these machines.

5.2 Ammonia–Water Absorption Systems
Ammonia–water absorption technology is used primarily in smaller chillers and small re-
frigerators found in recreational vehicles. Refrigerators use a variation of the ammonia
1
absorption cycle with ammonia, water, and hydrogen as the working fluids. They can be
fired with both gas and electric heat. The units are hermetically sealed. A complete descrip-
tion of this technology can be found in Ref. 1.
Ammonia–water chillers have three major differences from water–lithium bromide sys-
tems. First, because the water is volatile, the regeneration of the weak absorbent to strong
absorbent requires a distillation process. In a water–lithium bromide system, the generator
is able to provide adequate distillation because the absorbent material (lithium bromide) is
nonvolatile. In ammonia absorption systems, the absorbent (water) is volatile and tends to
carry over into the evaporator where it interferes with vaporization. This problem is overcome
by adding a rectifier to purify the ammonia vapor flowing from the generator to the con-
denser.
A second difference between ammonia–water and water–lithium bromide systems is the
operating pressures. In a water–lithium bromide system, evaporating pressures as low as 4–
8 kPa are not unusual for the production of chilled water at 5–7 C. In contrast, an ammonia
absorption system would run evaporator pressures of between 400 and 500 kPa.
A third difference focuses on the type of heat-transfer medium used in the condenser
and absorber. Most lithium bromide systems utilize water cooling in the condenser and
absorber, while commercial ammonia systems use air cooling.

6 INDIRECT REFRIGERATION
For indirect refrigeration, the process or refrigeration load is cooled by an intermediate
(secondary) liquid, which is itself cooled by refrigerant typically in a conventional vapor-
compression cycle (Fig. 8). The secondary liquid can be water, brine, alcohol, or refrigerant.
The heat exchanger used to cool the process load may need to be capable of handling
corrosive products, high pressures, or high viscosities, and is usually not well suited as a
refrigerant evaporator. Other problems preventing direct use of a vapor-compression refrig-
eration cycle may be remote location, lack of sufficient pressures for the refrigerant liquid
feed, difficulties with oil return, or inability to provide traps in the suction line to hold liquid
refrigerant. Use of indirect refrigeration simplifies the piping system because it becomes a
conventional single- phase liquid-system design.
The indirect or secondary coolant (brine) is cooled in the refrigeration evaporator and
then is pumped to the process load. The brine system may include a tank maintained at
atmospheric pressure, or may be a closed system pressurized by an inert, dry gas.
Secondary coolants can be separated into four categories:
1. Coolants with a salt base. These are water solutions of various concentrations and
include the most common brines, that is, calcium chloride and sodium chloride.