Section 6: Water Storage Tank and Expansion
Tank
It has been mentioned in the Introduction that there is a water buffer storage tank inside the
mini chiller unit [for series A and series B versions]. The capacity of the tank is:
Model
Capacity/litres
AC 40A
AC 50A
AC 58A
32
AC 75B
AC100B
AC125B
150
The function of having this tank is to create a volume of water large enough to prevent short
cycling of the compressor, i.e. to prevent the compressor from switching ON/OFF too frequently.
This will usually happen during low load conditions, e.g. during night time, when the indoor fan
coil units may switch off by the action of the room thermostat. As a result, only one or two fan coil
units will be running while the larger capacity chiller unit continues to operate.
What is the effect of having the compressor switching ON/OFF too frequently? The main problem
is overheating of the compressor motor winding. The winding will not get sufficient cooling from
the refrigerant flow, and also there may be insufficient oil return which causes insufficient
lubrication.
Therefore, it is vital that a proper calculation of the total system water volume to be made to
determine if the chiller can operate safely.
The formula to calculate the minimum water volume is as follows:
V
min
= 860 * kW * Compressor OFF time
∆T * 60 * No. of load steps (or No. of compressors)
Units:
Conversion:
V
min
litre 1 kW = 3412 Btu/hr
OFF time minutes
∆T deg. C
Example:
A mini chiller (series A) has a cooling capacity of 40,000 Btu/hr (11.72 kW).
The mini chiller controller has been programmed to operate with a minimum
compressor run time of 3 minutes and an off time of 4 minutes (i.e. a minimum
cyle of 7 minutes.)
Section 6 Page 87
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