Subcooling

[desol]

Hey guys. 3rd year commercial mechanic here(wannabe engineer ). I'm getting conflicting info from different sources regarding subcooling, so I thought I'd ask here to see what you guys thought. TXV system...As my head pressure falls (getting colder outside) my condenser is going to run cooler.

As the temperature/pressure falls in the condenser...more liquid will start to back up in it. As more liquid backs up, subcooling increases... True or false.

Cheers.

 

[EnergyProfessional]

If you properly charge the system (use manufacturer data and the OAT, etc.) and operate it in the manufacturer recommended conditions (i.e. don't use AC in winter if it is not low-ambient rated etc.) you should be fine, especially with TXV.
Also need to not extend line lengths...

 

[MintJulep]

Mostly true.

But consider that the refrigerant mass flow will change in response to the pressure change.

The compressor power changes too.

 

[desol]

Hey guys. Thanks for the replies.

Energy pro. I'm not so much interested in charging, as much as what happens after to the subcooling as the outdoor ambient drops, during normal operation.

Mint julep. That's what I thought, however, other mechanics seem to think that as outdoor ambient drops, you get less subcooling?


I understand that adding gas (during the initial charging process) increases both head pressure and subcooling....but that's charging...not operating. Mostly interested in what happens to subcooling (post charge) during operation, as outdoor ambient drops. Cheers.

 

[MintJulep]

This is one of those "all other things staying the same" type questions.

Except that all other things don't stay the same.

For building air conditioning, as the outdoor temperature falls the load on the evaporator falls. The condenser pressure falls and the compressor power goes down. If you have unloaders you will unload.

If the load is small enough, and the compressor power small enough and the condenser coil big enough there will be a point where it is possible that there is essentially no air temperature increase through the condenser. At that point the refrigerant in the condenser is basically at saturation at the outdoor temperature, so zero sub-cooling.

For process cooling the evaporator load could be independent from the outdoor temperature. In that case as outdoor temperature goes down the compressor power will still go down, but probably less. The total load on the condenser goes down only a little. Depending on the physical size of the condenser sub-cooling may initially increase and then go down, or just increase.

Start with a heat balance and work from there.

 

[desol]

I'm speaking strictly about a simple, split residential air conditioner operating in lower ambient conditions...without any fan cycling, etc.

Barebones system.

As outdoor ambient drops, liquid backs up in the condenser resulting in a flooded condenser. This should increase subcooling because there's more liquid in the condenser, however, as this lower ambient continues we loose pressure and the liquid starts to flash off before the expansion valve, because of the loss of pressure, not the loss of subcooling...I think?

Is it possible to have normal to high subcooling, while at the same time flashing off before the expansion device, solely due to the lower condensing pressure?

 

[EnergyProfessional]

what you describe is exactly why the manufacturer gives you a minimum ambient temperature. typically 45°F or so for residential. Below that OAT, you get the problems you describe.
if you want to operate below that temp, you need to get a more professional unit that is rated for that. you can get Mitsu split systems rated to -20°F.

It really depends on how the unit is built, and if it is properly charges. If charge is wrong, what you describe may happen sooner, or you get high OAT problems.

 

[berkshire]

Depending on how barebones that system is, a lot of residential units have a receiver to absorb excess refrigerant to mitigate that scenario .
Now I know that a lot of the cheaper systems do not, have, receivers, but they are not designed to run under the circumstances you describe.
B.E.

You are judged not by what you know, but by what you can do.

 

[DrRTU]

Desol,
MintJ is correct. Even the simple residential ACC, you have to look at all 4 parts (evap, exp device, compress and cond.)at any given point. This why the charging tables have a small window of indoor and OA conditions. As OA goes down your load decreases, the condenser looks bigger, power goes down, evap is adding less heat and subcool will be down.

 

[DrRTU]

FYI - Emerson has an excellent simulator training class to help with the basics of the cycle in class room setting.

https://education.emersonclimate.com/en-us/events-calendar.html