Handbook of air conditioning and refrigeration / Shan K

Evaporating Temperature
Latent heat of vaporization is the primary factor that affects the heat input in an absorption chiller,
and temperature difference T con � T ev has only a minor effect on h fg. This is the primary difference
between an absorption chiller and a centrifugal chiller.
As in the centrifugal chiller, the evaporating temperature T ev and pressure p ev in an absorption
chiller depend mainly on the chilled water temperature leaving the evaporator T el. The difference
T el � T ev in current absorption chiller design is between 2 and 5°F (1.1 and 2.8°C). A smaller
T el � T ev means a higher COP and a large heat-transfer surface area in the evaporator. It is actually
a compromise between energy cost and investment. For T el � 44°F (6.7°C) and T el � T ev � 2°F
(1.1°C), the evaporating pressure is around 6.8 mm Hg abs. (0.268 in. Hg abs. or 906 Pa abs.).
The vapor pressure of the concentrated solution during absorption should be slightly less than
the evaporating pressure p ev, so that water vapor vaporized in the evaporator can be extracted to the
absorber. Both T ev and T el affect the cooling capacity of the absorption chiller as well as the heat
input to the high-temperature generator.
Cooling Water Entering Temperature
The temperature of cooling water entering the absorber T ca, in°F (°C), has the following effects on
the performance of an absorption chiller:
● Lower T ca means a higher cooling capacity.
● Lower T ca results in a lower T con.
● Lower T ca means a lower heat input to the high-temperature generator per ton of refrigeration produced
and, therefore, a high COP c.
● At high load ratios, too low of a T ca may cause crystallization.
Manufacturer recommendations should be followed. For example, one manufacturer recommends
that a minimum of T ca � 72°F (22.2°C) be maintained at all load conditions.
Heat Removed from Absorber and Condenser
Condensing Temperature
REFRIGERATION SYSTEMS: ABSORPTION 14.19
Heat input Q hg 8271.4 Btu/h�ton ref 100.0 percent
Latent heat of vaporization h fg 6985.6 Btu/h�ton ref 84.5 percent
Heat of diluted solution 1285.8 Btu/h�ton ref 15.5 percent
The total amount of heat to be removed from the absorber and condenser in a typical double-effect,
direct-fired, reverse parallel flow absorption chiller is about 1.5 times the heat rejected from the
condenser in a centrifugal chiller. The heat to be removed in the absorber is about 60 percent of the
total heat removal. Heat removed from the condenser is about 40 percent of total heat removal.
Usually, a cooling water temperature rise of 10 to 15°F (5.6 to 8.3°C) is used. For a cooling water
temperature rise of 10°F (5.6°C) when the temperature entering the absorber is 85°F (29.4°C),
the temperature of cooling water is about 91°F (32.8°C) entering the condenser and 95°F (35°C)
leaving the condenser.
Condensing temperature T con depends mainly on the temperature of cooling water available and the
heat-transfer surface area. If cooling water is at 85°F (29.4°C), allowing for a temperature increase