Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 116

Ammonia/Water Properties

The advantages of ammonia/water as a working pair are their high affinity, their high
stability and their property of being an environmentally friendly refrigerant. There is no
vacuum required for evaporator temperatures above 30 ◦ C. The refrigerant ammonia

has a reasonably high specific evaporation enthalpy of 1258 kJ kg − 1 (at

−

5 ◦ C), it is
lighter than air and a compact construction is possible, for instance through plate heat
exchangers.
The disadvantages are the high refrigerant operating pressure (up to 25

+

10 5 Pa)
at typical ambient air condensation temperatures, causing the production costs of the
machines to be higher than those of water/lithium bromide units. Because of the
volatility of the absorbent during desorption, a rectification through a dephlegma-
tor/rectification column is necessary. The boiling point distance between ammonia
and water of 133 K at 10 5 Pa pressure is rather small. The refrigerant ammonia is
corrosive, for example, in connection with copper and copper-based alloys, has a very
unpleasant odour and is toxic.
The thermodynamic properties of the refrigerant ammonia determine the operating
temperature range of the ammonia/water ACM. The critical temperature of the
refrigerant ammonia is 132.4 ◦ C. Ammonia has a low freezing point at

×

−

77.7 ◦ C

33.3 ◦ Cat10 5 Pa pressure. Ammonia/water
ACMs can be air cooled for air-conditioning applications but usually they are also
water cooled (Chinnappa, 1992). By using ammonia as a refrigerant, the evaporator
temperature can go down even to

−

and the boiling point temperature is

60 ◦ C. Thus, the temperature range of the
machines is suitable for air-conditioning and for industrial refrigeration processes,
for example in the chemical industry.

−

Performance

Typical performance characteristics for the closed ACM cycles described above are
given in Table 5.1 for water/lithium bromide and in Table 5.2 for ammonia/water.
In contrast to the ideal Carnot process, the COP stays nearly constant as soon as
a certain minimum driving temperature level is reached. The COP increases from
approximately 0.7 for single effect (SE) processes to 1.3 for double effect (DE)
absorption chillers and 1.7 for the triple effect (TE) as shown in Figure 5.5 (Grossmann,
2002).
For ammonia/water absorption chillers, the COP is approximately 0.6 for single
effect (SE) cycles and reduces to 0.4 for double lift (DL) cycles because of the reduced

Table 5.1 Water/lithium bromide absorption chiller characteristics

Cycle type SE DE SE/DL

Cold temperature / ◦ C 6-20 6-20 6-20

Heating temperature / ◦ C

70-110 130-160 80-100

Cooling water temperature / ◦ C 30-35 30-35 30-35

COP 0.5-0.7 1.1-1.3 0.4-0.7