walk in freezer

[icemeister]

...I have not seen many systems running 25...

This thread isn't about an A/C system. It's a low temp R404A walk-in.

The only walk-in LT systems I ever see running at around 25ºF SH are the ones I've worked on. Others often run at 50-60ºF SH or more.

Here's an old thread that might shed some light on what we're really talking about here. (Check out Basser's answer in post #35):

http://hvac-talk.com/vbb/showthread....ssor-superheat

[ryan1088]

Whats wrong with adjusting superheat to 25 degrees at the compressor?

What if the outdoor conditions change? The heat given off by your accumulator and such wont be the same if it's 100 out as it will at 0.

[ryan1088]

This thread isn't about an A/C system. It's a low temp R404A walk-in.

The only walk-in LT systems I ever see running at around 25ºF SH are the ones I've worked on. Others often run at 50-60ºF SH or more.

Here's an old thread that might shed some light on what we're really talking about here. (Check out Basser's answer in post #35):

http://hvac-talk.com/vbb/showthread....ssor-superheat

True statement. Also that 25 degree superheat is accompanied by a huge ball of ice that is the accumulator. I don't know how many "froze up" calls I've answered.....

[cmclifton]

Do we know if this is a Copeland?

Now, it the manufacturer of THIS system says it should be 25, then fine. However, I don't think anyone in this thread, myself included, knows what the manufacturer wants to see for a sh value.

Don't know if its a Copeland, I was simply trying to point out that 25 superheat isn't high. I probably should have looked up the FAQ on the emerson website instead of throwing a number out there that has been rattling around in my head since my first Copeland class, because clearly I was incorrect. Copeland's minimum is actually 20. There is no maximum superheat, only maximum discharge temp. http://www.emersonclimate.com/en-US/...ompressor.aspx

As Icemeister mentioned, there aren't many low temp systems out there running 25 or even 35° superheat at the compressor...

[cmclifton]

What if the outdoor conditions change? The heat given off by your accumulator and such wont be the same if it's 100 out as it will at 0.

That's the exact reason that they have a minimum superheat number.

[timebuilder]

This thread isn't about an A/C system. It's a low temp R404A walk-in.

That's right. I'm not talking about AC systems. I'm talking about restaurant 404A refrig systems I service.

[dangpgt97]

You set superheat at the evap. If the lineset is short maintaining compressor superheat at 20* minimum is more important than evap superheat. I doubt that ambient would affect 2 ft of suction line that much. Learned it at copeland school actually. Regardless if I set it at 20 on a day that its 100 out I wont have any problems when its -20 out. Maybe that's why copeland says minimum 20 and not 5. Common sense.

[jpsmith1cm]

That's the exact reason that they have a minimum superheat number.

Meh. Not sure I buy this entirely.


A minimum superheat, from a manufacturer's point of view, is insurance for that compressor. With the proper superheat, a compressor can run for a VERY long time without giving the operator or the owner any problems at all.

It also ensures that the compressor will not see liquid under ANY circumstances. I don't see the heat added by ambient as a big factor in most equipment that I've ever seen. What I DO see as a factor, through, are system operating conditions.

Suction and head pressure fluctuations, liquid temperature fluctuations, frost loading on the evaporator, dirt loading on the evaporator, fan failure or possible multiple fan failures.

All of these things would affect superheat much more than the ambient temperature.


A TEV is designed and sized to be able to maintain a reasonably consistent superheat over the entire range of expected operating conditions, but we must remember that it is a mechanical device and is subject to limitations as such.

[kklobas]

hey guys, it is copeland, hermetic. I set to 25 sh at compressor because thats generally what is always recommended and is by koolpack who made this. still holding -10 fine, but checked today and superheat does fluctuate quite a bit. averaged 20 psi low side at -10 to -14 box temp. checked suction line at compressor periodically over a couple hours and it varied from -10 to 7 ?
drier only showed 2 degree temp rise inlet to outlet, sight glass clear except for a few seconds as fan cycles to it's lowest head pressure. cycle switch set to 200-275 psi

[jpsmith1cm]

Does this unit have a headmaster or an ORI/ORD valve setup?

[kklobas]

Just fan cycle. Head goes from 200-275 with cycle
Switch/fan

[cmclifton]

Meh. Not sure I buy this entirely.


A minimum superheat, from a manufacturer's point of view, is insurance for that compressor. With the proper superheat, a compressor can run for a VERY long time without giving the operator or the owner any problems at all.

It also ensures that the compressor will not see liquid under ANY circumstances. I don't see the heat added by ambient as a big factor in most equipment that I've ever seen. What I DO see as a factor, through, are system operating conditions.

Suction and head pressure fluctuations, liquid temperature fluctuations, frost loading on the evaporator, dirt loading on the evaporator, fan failure or possible multiple fan failures.

All of these things would affect superheat much more than the ambient temperature.


A TEV is designed and sized to be able to maintain a reasonably consistent superheat over the entire range of expected operating conditions, but we must remember that it is a mechanical device and is subject to limitations as such.

Yeah, I probably should have worded my response there a bit better. I didn't mean to imply that ambient temp changes are THE reason for the minimum superheat number, just that the system is dynamic and conditions aren't going to be exactly the same from day to day, so the 20° superheat setting helps keep the pump from flooding when conditions worsen.

[jpsmith1cm]

Just fan cycle. Head goes from 200-275 with cycle
Switch/fan

Probably the single WORST means of controlling head pressure.

That said, it sounds to me as if you've got it running about right.