superheat on r-404a

[Mrak]

The walkin box I'm working on, trying to achieve -10 F (which it never had problems achieveing in the past).
All of a sudden decided to creep up 5 degrees.
Here are the readings.

Head Pressure 250 psia (air temp. into condenser coil 77-78 F.)
250 head pressure = 104 deg.F

subcooling somewhere around 12-15 degrees F.

Suction Pressure around 21-22 psia = -15 to -14 deg.f.
Suction line temp. leaving evap. = -12 to -2 deg. F. (The temperature swing is -12 to -2) It stays quite stable. Isn't radical with the temp. swing.

box temp. -5

It's an empty walk in box about 7 x 10 foot square.

At the moment its gone into a defrost cycle for maybe 20-25 minutes. (got a break thought I'd post this)

My conclusion: When I first checked the superheat it was rediculously high. perhaps 25 degree S.H. So I've been slowly adjusting it the last few hours and I'm thinking the superheat setting is o.k. where it's at. I'm thinking since it's a low temp. the superheat can be a bit lower, thus the minimum 2 to 3 degree S.H.
The suction has been slowly creeping down which gives me hope that it "may" eventually (perhaps overnight) reach the
minus 10 setpoint. We'll see in the morning.

I just wanted to run this by you guys to see if there is something I'm not seeing or thinking about.

Thanks
Mark

[2sac]

You probably have a pressure limiting powerhead on the txv. Best thing to do is leave it over night and check in the morning

[Mrak]

Thanks!
I'm doing just that.....
Going to check it in the morning.

[bunny]

2-3 is pretty low considering there is some margin of error in the gauges and temperature meter. Plus, the SH you're reading on the outside of the pipe is not the true SH of the refrigerant inside the pipe.

If you're reading 2, it might really be 1/2 or 1, and the compressor says that's just too close for comfort.

[Mrak]

yeah,
That's why I'm throwing this out there.
I was thinking maybe with the low temp. that a swing to the minimum 3 deg. S.H. might be o.k.
On the other end it gets up to 10 or 11 deg. S.H.
I was concerned enough that I did my best to zero in "two" seperate gauges to hopefully get an accurate low pressure reading, but your right, it's hard to trust an absolute accurate reading with a guage.
In your opinion, what do you think might be a safe minimum S.H. ?
I appreciate your input.
Mark

[2sac]

yeah,
That's why I'm throwing this out there.
I was thinking maybe with the low temp. that a swing to the minimum 3 deg. S.H. might be o.k.
On the other end it gets up to 10 or 11 deg. S.H.
I was concerned enough that I did my best to zero in "two" seperate gauges to hopefully get an accurate low pressure reading, but your right, it's hard to trust an absolute accurate reading with a guage.
In your opinion, what do you think might be a safe minimum S.H. ?
I appreciate your input.
Mark

6-8. If the powerhead has a "P" on it it's pressure limiting. They take forever to pull down. Don't worry about it, just check it in the morning

[BBeerme]

yeah,
That's why I'm throwing this out there. In your opinion, what do you think might be a safe minimum S.H. ?

Much of that really depends on how close coupled the condensing unit is to the evaporator. And even then, if the condensing unit is on top of the evaporator, it's pretty hard to get liquid back to the compressor.

But ultimately, once you get down to that -10*F range and lower, you often have to run higher superheats. You see, sometimes it has more to do with evaporator temp than how efficient the evaporator is working. So when you screw in the TEV adjustment, you lower the suction pressure, thereby running a colder evaporator. Just keep it in mind; sometimes these things are counter intuitive.