Handbook of air conditioning and refrigeration / Shan K

14.10 CHAPTER FOURTEEN
10
5
20
40
1
50 100
Vapor pressure, mm Hg abs
60
100
1�
5�
5
200
2�
300
500
400
3� 4�
8�
2
1000
Partload
3 4 8
Crystallization line
of adequate size, the pressure difference between the evaporator and absorber as well as between
the low-temperature generator and condenser is small.
In a typical double-effect, direct-fired, reverse parallel flow absorption chiller operating at design
load, as shown in Fig. 14.3, water usually evaporates at about 42°F (5.6°C) and at a saturated
pressure of 6.8 mm Hg abs. (906 Pa abs.) in the evaporator, point 1. Chilled water enters the evaporator
inside the tube bundle at 54°F (12.2°C) and leaves at 44°F (6.7°C). Heat is transferred from
the chilled water to the vaporized refrigerant at an amount equal to the latent heat of vaporization.
Vaporized water in the evaporator is then extracted to the absorber because of its lower vapor
pressure and is absorbed by the concentrated LiBr solution. In the absorber, concentrated solution is
supplied and sprayed at about 120°F (48.9°C) with a concentration of 63 percent, point 5. It is
cooled by the cooling water flowing inside the tube bundle at an entering temperature of 85°F
(29.4°C). As the water vapor from the evaporator is absorbed, the solution is diluted to a concentration
of about 58.5% and its temperature drops to 95°F (35°C). The vapor pressure of the solution is
then about 5 mm Hg abs. (666 Pa abs.), which is lower than the evaporating pressure of 6.8 mm Hg
abs. (906 Pa abs.). The heat of absorption is removed by the cooling water. Diluted solution is then
pumped to the low-temperature heat exchanger and the low-temperature generator by a lowtemperature
solution pump.
In the low-temperature heat exchanger, diluted solution enters at 95°F (35°C ) and leaves at
about 150°F (65.6°C). After the diluted solution enters the low-temperature generator, because of
the release of the latent heat of condensation from the condensed water vapor that flows inside the
tube bundle of the low-temperature generator, the diluted solution is first heated to a temperature of
180°F (82.2°C), and the water vapor then boils off at a pressure of about 48.5 mm Hg abs. (6.5 kPa
abs.), point 3. As the water vapor is boiled off, the LiBr solution becomes partly concentrated with
a concentration of 60.2%.
At the outlet of the low-temperature generator, partly concentrated LiBr solution is divided
into two streams: One flows through the high-temperature heat exchanger and then enters the high-
6�
40%
6
7�
50%
55%
7
Full load
70%
150 200
Solution temperature, �F
250 300 350
FIGURE 14.5 Absorption cycle of a double-effect, direct-fired, reverse parallel flow absorption chiller.
60%
65%
200
150
100
50
Refrigerant temperature, �F