How to Design Refrigeration Systems

[US Iceman]

I see posts every so often that ask this question. As a result I thought it might be worthwhile to post some basic information on the subject.

First, let's assume we want to keep something cold. That's the reason for refrigeration in the first place.

Now let's say we want to maintain the temperature in the enclosed space at 0°F (-17.7°C). To produce this temperature the evaporator needs to be colder. So, for this example we select the evaporator temperature to be -10°F (-23.3°C). -10°-0°F = 10° of temperature difference to be used for selecting the evaporator capacity (or -23.3°C--17.7°C = 5.5° temperature difference in SI units).

Since we know there will be some pressure loss in the suction line we have to select the suction line diameter so that the pressure loss does not exceed what was used in designing the system. Let's say the pressure loss is 2 psi (0.14 bar). Since refrigerant flows from higher pressure to lower pressure the pressure at the evaporator is higher than the suction pressure at the compressor. What the compressor will do is produce the cooling capacity at the suction pressure.

Then the compressor increases the refrigerant pressure from the suction pressure up to the discharge pressure. Remember, the refrigerant flows from higher pressure to lower pressure. This means the discharge pressure will be greater than the condensing pressure.

Therefore if the ambient air temperature is 95°F (35°C) the condensing temperature has to be greater than the ambient temperature so that the refrigerant can actually condense. Using an example for the condenser; if the ambient air temperature is 95°F (or 35°C) the condenser capacity could be selected for 115°F (46.1°C). This indicates the condenser heat rejection capacity is based on temperature difference (condensing temperature minus ambient air temperature) of 115°F-95°F = 20° (or 46.1°C-35°C = 11.1°).

If the condensing temperature is 115° (or 46.1°C) then the condensing pressure is equal to the saturated refrigerant temperature (which is the condensing temperature). The discharge pressure of the compressor is greater than the condensing pressure as mentioned before.

What this means is; when you select the compressor capacity the operating conditions you use are; the suction pressure and discharge pressure.

Why????

Because the evaporator pressure occurs at the evaporator and the condensing pressure occurs at the condenser. The pressures at the compressor suction and discharge valves are different than the pressures at the evaporator and condenser.

I may have missed some small point in this explanation, but let's see where the discussion goes...

I'm sure someone will come here and point out any flaws.