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You
will remember that we have already touched on this subject (see: various refrigeration problems page 388).
There
is usually little difficulty in controlling the temperature in
a cold room. All that is generally required is adequate refrigerating
capacity and adequate temperature control. However,
the selection of the evaporator/compressor combination will decide
what relative humidity will be finally obtained in the cold room:
so it is better not to make any mistakes in selecting the equipment!
EXERCISE
: When
the temperature control is not easily performed with a thermostat
(chilled table displays, for example), an LP pressure switch can
be used. In this case, we know that Dq total for the evaporator
is constant and that the compressor suction side pressure is equal
to the evaporation less the pressure drops (DP) along
the suction line.
A client
wants a cold-room with a temperature between 4°C and 6°C and a
humidity between 85 and 90%. Given that with a forced air evaporator,
Dq total for the evaporator
is constant, and that the pressure drop at the compressor suction
can be calculated at 0.3 bar, at what values would you
set the LP control pressure switch to cut in and out?
The refrigerant
used is R22, and a selection of pressure-temperature (bar / °C)
relationship values as shown on the LP gauge is given below:
|
To
|
8°C
|
6°C
|
4°C
|
2°C
|
0°C
|
-2°C
|
-4°C
|
-6°C
|
-8°C
|
-10°C
|
|
Po
|
5.4 bar
|
5 bar
|
4.6 bar
|
4.3 bar
|
4 bar
|
3.6 bar
|
3.3 bar
|
3 bar
|
2.8 bar
|
2.5 bar
|
(Solution
: Refrepair Manual pages 419 to 428)
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