Compressor Suction Temperature

[ruccolo]

Hello All

So I had a scenario where I was required to select a roof top packaged air conditioning unit, 100% recirculation with the entering air at 86F (mining operation). Needed a capacity of 177 MBH and leaving air of 55F. I could get close, but not quite. The suction temperature was at 50F by default, I lowered it down to 46 and I hit my desired conditions right on. I understand that for each suction temp., there is only one corresponding delta_T you can have.

My question would be how exactly is the correct suction temp. achieved and/or maintained? Or, is it simply a factor dictated by the operational conditions of the condenser-evaporator system? Is this something you can have adjusted to match desired performance?

Just hoping for some insight into this modification I made, if it is correct and what are the effects. I am using a selection software that shows the equipment ratings.

 

[ruccolo]

P.S. It was a non-compressorized unit, condensing unit separate.

 

[Shophound]

How did you lower the suction temperature?

 

[ruccolo]

Like I had mentioned, I am using a selection software. It is a matter of putting in certain values in the required fields. I noted that a lower suction temp. meant a higher total capacity and lower leaving air temp. I am not too sure about the practical effects of this mechanical-wise, which is why I'd be looking for any insight/advice/source material anyone would be able to provide. If you can lower the suction temperature, what are the methods to do so?

 

[MintJulep]

roof top packaged air conditioning unit

It was a non-compressorized unit, condensing unit separate.

These two statements contradict each other. By definition, a packaged air conditioning unit has a condenser and compressor included.

 

[ruccolo]

It is a rooftop unit, only thing is that the condensing unit is elsewhere. I guess using the word "packaged" isn't appropriate in this case.

 

[Shophound]

Suction temperature can be lowered at least two ways: a) increase pumping capacity of compressor without changing size of evaporator; b) reduce airflow across the evaporator (which in turn lowers the sensible capacity of the system).

Improving liquid subcooling to the TXV may or may not net a reduction in suction temp, but it will improve net refrigeration effect (NRE).

 

[ruccolo]

Pertaining to point (a), would that mean pumping more refrigerant? Or are there various ways to increase compressor pumping capacity?

 

[MintJulep]

Suction pressure is determined by the balance between evaporator load adding energy to the refrigerant leaving the evaporator - thus trying to increase the pressure - and the compressor suction trying to decrease the pressure.

So, when you decreased the suction temperature in the selection program what you did was to tell the program that you were planning to choose a compressor/condenser unit accordingly.

It's not clear what selection program you are using, but we'll guess that it dealt with the evaporator only.

So next you need to pick a compressor/condenser unit with 177 MBH capacity at 46F suction temperature.

 

[ruccolo]

Okay, makes sense, and yes it would essentially be just the evaporator within the unit as the condenser is selected separately. The only thing I am hoping to grasp a little better is the relationship between the suction temperature and the delta_T (leaving air temperature). I have to make sure that both my condensing unit and evaporator have a suction temp.of 46 with 177 MBH capacity, but if I am able to have a leaving air temp. of 55F with these values is the question.

 

[MintJulep]

thread403-189865 from a few years ago might be interesting to you.

The selection program for the evaporator most certainly has air-side factors too.

The suction temperature is only one of several factors that determine the coil's air-side capacity.

 

[ruccolo]

okay great, thanks

 

[RJonTilt]

Just one thing. It is very unlikely that the suction temperature leaving the evaporator and entering the condenser will be the same. You must take the piping losses into account.