Evap temp based on Return air temp

[njg33]

We typically size our loads based on 75Deg F 50%RH return air. We have a project requiring 20,000 BTUH sensible cooling for 86 DegF 50%RH air. Most AC system design is based on approx. 45 DegF evaporating temp. How does this now equate with the 86 deg return air?

My understanding is the evap temp will be in the area of 55 degF. However, this is the outer operating limit of our compressor performance.

Can this load be "translated" to a different coil/compressor selection to reduce the evap temperature and run the compressor in a more common operating point? I have always thought the evap temp is a function of the return air temp?

Do I simply run the coil data software at 86 entering temp and 45 deg evaporating and select a compressor to match the cooling load at this evaporating temperature?



Regards,

NJG

 

[quark]

I have always thought the evap temp is a function of the return air temp?

No, for the simple reason that you will never be able to cooldown the air temperature if it depends upon the return air temperature, in that case.

Why can't you go for chilled water application?

 

[njg33]

We usually provide chilled water for high temp return air applications. However, in this particular case our customer has specifically asked for a DX cool system.

I may have been somewhat unclear with my question. When I say evap temperature, I am speaking of the saturated suction temp. Does this not "float" with the change in return air temperature?

 

[imok2]

"Does this not "float" with the change in return air temperature?" ans: yes

The SST is dependent on the heat load on the coil, so as return temp across the coil increases then so does the SST. In other words the air off the coil may be say 60*F instead of 55*F

 

[Yorkman]

I know this is a little late to respond but, in order to determine the leaving air temperature would'nt you need to know both the sensible and latent load being presented to the DX. coil. That will allow you to find the sensible heat ratio so you can determine your leaving air temperature and CFM requirements to maintain the desired space conditions. With this information a coil selection can be made, typically the approach of a DX coil is about 10 degrees.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI