metering device sizing

[dantoubiana]

what determines the size of metering device needed for a refrigeration system?

a. the horsepower of the compressor

b. the Btu capacity of the evaporator

c. the capcity of the compressor in Btu at the evaporator operating temperature

d. the capacity of the condenser at the operating ambient temperature

(please pick one and comment if you'd like)

[bersaga]

The evaporator capacity at the evaporating temperature and the calculated pressure difference between low side and high side , static losses and the friction losses in the pipes and fittings.

[frank]

I've got to go with (b)

Frank

Wouldn't this be better off in a "Poll" format?

[subzero*psia]

Well hopefully you have determined the btu/h requirements for the space and product you are trying to control.... which would give you a specific evaporator rating based on the type of refrigerant you are planning on using... and then the required metering device would be based off that.

[subzero*psia]

I believe what you are saying here Marc is.... its a "system dependant" question..... hehehehehheh!

[Prof Sporlan]

Amusingly, either b) or c) can be correct, depending upon the situation.

Given a single evap, single compressor system, sizing based on compressor capacity at the design operating conditions makes sense, and we assume the evaporator is properly sized so that the compressor can operate as design conditions.

If we have a multiple evaporator system connected to a compressor rack, and perhaps some of the evaporators being controlled with eprs, then one must rely on the evaporator rating at the desired operating condition to size the expansion device.

[Prof Sporlan]

The compressor(s) shall create the refrigerant flow. And condenser, evaporator, and line sizing shall determine the operating conditions. For metering device sizing, there's the rub....

[superheat]

The rub is: the valve can't pass more ***** than the compressor can pump. If the valve passes less ***** than the compressor wants to pump, operating conditions change to make the compressor pump as much as the valve passes.