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34 TRG-TRC011-EN
notes
period three
Capacity Control
The primary objective of the chiller capacity control system is to reliably
maintain the temperature of the chilled water leaving the evaporator. The
control system monitors the temperature of the leaving chilled water, compares
it to the setpoint, and adjusts the amount of solution supplied to the generator
and the heat input to the generator.
In Period One, the Equilibrium Chart for Aqueous Lithium Bromide Solutions
was introduced to explain how the combination of solution temperature and
concentration determines the pressure, and temperature, at which the
refrigerant will boil (vaporize) in the evaporator. Recall that an increase in
solution concentration ($ to %), at a constant temperature, results in a decrease
in vapor pressure. Conversely, a decrease in solution temperature ($ to &), at a
constant concentration, results in a decrease in vapor pressure. Assuming that
no air or other noncondensables are inside the chiller, the vapor pressure of the
solution determines the temperature at which the refrigerant will vaporize. In
period three
Figure 39
6
5
6
5
6
0
6
0
5
5
5
5
5
0
5
0
4
0
4
0
$
$
&
&
%
%
50°F
50°F
[10°C]
[10°C]
100°F
100°F
[37.8°C]
[37.8°C]
150°F
150°F
[65.6°C]
[65.6°C]
200°F
200°F
[93.3°C]
[93.3°C]
15
15
psia
psia
[103.4
[103.4
kPa
kPa
]
]
5
5
psia
psia
[34.5
[34.5
kPa
kPa
]
]
1
1
psia
psia
[6.9
[6.9
kPa
kPa
]
]
0.1
0.1
psia
psia
[0.69
[0.69
kPa
kPa
]
]
LiBr
LiBr
solution
solution
v
a
p
o
r
p
r
e
s
s
u
r
e
v
a
p
o
r
p
r
e
s
s
u
r
e
concentration
concentration
concentration
solution temperature
solution temperature
Figure 40