Understand Evaporator Terms & Choose Right Compressor

[eaboujaoudeh]

1.superheat
Psig Saturated Temp Suction line temp superheat temp.
58 32 44 12
64 37 47 10
70 41 50 9

can some1 help me understand the ratings above..
Saturated temperature is easy ok.
Suction line temperature is the temperature before or after the evaporator? my guess is after the evaporator..
As for the superheat temperature: they say it defines the liquid content of R-22 in the system at the exit of the evaporator, but i don't understand the principle (like is it a set value? or given by the compressor in use...?)..please help with this.

2. Choosing a compressor: I'm finding that their is a conception that if u need to transfer 10 tons of cooling to a room, u choose a compressor that is rated at 10 tons. but shouldn't a smaller one do?

thnx

 

[russ_watters]

Suction temp is before the compressor, so after the evaporator. I'm not sure how the superheat temp relates to the "liquid content" (I assume that's the ratio of liquid to gas). I'll have to think about that...

10 tons is a rate, so if you need 10 tons, you need 10 tons.

 

[eaboujaoudeh]

thnx russ..
about the 10 tons, the performance of the heat exchanger lies in tubing, and gases flowing, and so and so...i felt its weird that they rates the compressor as if it was the only defining principle in the heat exchanger. but i guess ur right.

 

[russ_watters]

Air conditioning systems typically have constraints that end up dictating what the capacity is going to be. Manufacturers don't typically screw too much with the refrigerant charge for efficiency reasons, so the main variable is airflows, humidity, and temperatures. If you look at the numbers for a mass-produced unit, they give performance based on outdoor temp and indoor temp, rh, and airflow, and the variation in performance is surprisingly small.

 

[quark]

The fluid power consumed by a compressor can be calculated by mass flowrate of refrigerant times the enthalpy difference. The enthalpy of super heated gas is more than saturated gas so power consumption will be less. However, there are limitations to the extent of superheat and you can get those details from any fundamental thermodynamics or refrigeration books. (for ex. Principles of Refrigeration and Air Conditioning by RJ Dossat, Thermodynamics by Zemnsky etc.)

When the saturated vapor leaves the evaporator, it picks up heat from atmosphere during the transportation in the suction pipe. The superheat in the first case is more because you have a higher deltaT (considering constant ambient temperature). If this data is provided by the compressor manufacturer, then he is trying to give you a ballpark figure of the compressor tonnage at various evaporator saturation temperatures (i.e chilled water temperature or air temperature, indirectly)

I recommend you to possesses and refer RJ Dossat, if you seriously consider HVAC&R as a profession.

 

[eaboujaoudeh]

thank you, i will check the book out..

 

[eaboujaoudeh]

does dossat's book guide u through the whole evaporator and condenser designs?

 

[eaboujaoudeh]

well i got the book before u answer...its magnificent, what i was looking for all the way..thnx