Dimensioning mass flow in hot gas defrost lines
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2.1 Dimensioning quality
The dimensioning quality is used to determine position of point D at the inlet to the defrost
drain line.
The term “quality” is a measure of the mass of gas compared to the total mass of refrigerant.
The dimensioning quality will be quite different based on the drain control method you select:
For liquid drain the dimensioning quality should always be 0.0 – i.e. the refrigerant in
point D is saturated liquid. The function – or the purpose – of a float valve in the defrost
drain line is exactly to avoid (as far as possible) gas to pass through the float valve, but
only let liquid pass through.
For pressure controlled drain, the defrost process will be quite different. Initially, all hot
gas supplied to the evaporator will condense and the valve will only see liquid at the
inlet. Later in the process, some gas will not condense in the evaporator and the valve
will see a mixture of liquid and gas. This process is illustrated below (from D* to D):
Selecting the right dimensioning quality for pressure controlled drain is very important for
selecting the right valve size:
If a dimensioning quality of 0.0 is selected (saturated liquid) then the resulting valve will
be relatively small, which could mean that defrost will be prolonged at the end of the
defrost cycle (gas cannot efficiently be passed through the valve)
If a dimensioning quality of 1.0 is selected (saturated gas) then the resulting valve will be
relatively large, meaning that a lot of gas will be bypassed (equals larger energy
consumption), plus the valve can become unstable when pure liquid enters the valve in
beginning of the defrost cycle.
In Coolselector®2 we recommend using a relatively low value of dimensioning quality equal to
0.05. This ensures that the valve is stable when liquid enters it and that the amount of bypassed
gas is minimized.