How do Packaged Units control air temperature

[Dymalica]

How is it possible to maintain constant supply air temperature when different heat loads are in the return air and you have a constant speed compressor? Many units only have 4 compressors for systems around 30 Tons. Is the trick to have multiple compressors so the supply air temp does not waiver too much? In chiller systems they have control valves to control the chilled water through the evap coil, while I am not aware of any control valves in a refrig loop. Is there something I am missing here, or does constant temperature mean sort of constant?

 

[ChrisConley]

DX refrigerant systems use TX valves (Thermal eXpansion) to control capacity. Spend some time on the Sporlan site to learn more about DX systems.

http://www.sporlan.com/10-9.htm

 

[NCDesign]

The short answer is you can't and constant is kinda constant. The goal for us as engineers is to match the unit as closely to the loads as possible. If you have a 30 ton max load but a base load that is about 7-1/2 tons then you would want to make sure that your selected unit has a stage that operates fairly closely to your base load. There are some things that can be done to help. For example, you can select a unit with different size compressors that can alternate the compressor staging to better match your load. Take your 30 ton unit with different compressors. If you have a 5, 10, and 15 ton compressor, you essentially have 6 stages of cooling available. Another method is hot gas bypass (control valve in your refrigerant stream). You would falsely load the system with hot gas from the compressor to keep the system running even if their wasn't enough load. Using your 30 ton example if your base load was only 5 tons but your smallest compressor was 15 tons you could install hot gas bypass to help even out the leaving air temp. If installed and tuned properly it is very effective but it is very wasteful energy wise and has fallen out of favor.
You just need to know your load and load profile and try and match your equipment as best you can.