Older 502 Bev air freezer

[freonjohn]

Replaced compressor with 404A rated compressor. Weighed in charge and have some questions. I am at about 40-60 degrees superheat and still having frost on the suction line at the compressor. I am assuming it is because the vapor temperature is so cold and not that any liquid is coming back to the compressor. With a superheat I don't think I am getting any liquid back. Is this normal? Don't have any idea how this operated before as the compressor was junk when it got here. What am I looking for in charging this system?

[tranger03]

The simple answer is that with 40-50 deg superheat you won't have any liquid coming back. It is common to see frost on low temp suction up to the compressor, under normal operating conditions due to low sst. Now the more pressing questions in my mind are: Why is the superheat so high? Could this have been the reason the existing compressor failed? What are your system pressures? Does this have a txv or cap tube?

[freonjohn]

Box= o
Suction 12psi
Discharge 205 psi
Suction line temp. 21
Cap tube system

[EEAllen]

yea, the super heat is very high for a freezer, you have the doors open the whole time or something? did you measure it at a steady state? And yes for a freezer, you should see little frost build up on the suction line.

[tranger03]

If your box is pulling down you could have high superheat. With r-404, to maintain a 0 degf box temp, you want a -10 suction temp. If you look at your pt chart you will see that for -10 degf your suction should be somewhere near 24 psig. So there are three possibilities here: the box is pulling down (high load causing high superheat), unit is under charged, or there is a liquid line restriction (restricted cap tube).

[tranger03]

It seems that you are under charged. Charge to 10 degf superheat. Given 0 box temp and 75 degf ambient your pressures should be:

Sp - 24
Dp - 250 (30 degf over ambient for air cooled condenser)

Just go slow and keep an eye on your discharge pressure.

[freonjohn]

Box is empty right now. Unit cycling on and off at o to -5 degrees. I was trying to get it around -5 to -10 degrees. I will add refrigerant to get a superheat of 10 to 20 degrees. It just through me off when I was getting frost back to the compressor with such a high superheat. I understand that the vapor is just that cold causing the freezeup.

[tranger03]

I know that feeling well. Also, with some practice you can tell if the line is cold vapor or liquid by feel. Put your hand on the pipe and then remove it (grab the pipe for a second). If the frost melts its vapor, if you still see a film of frost where your hand was it is flooded with liquid. You can also apply the same logic using a torch, just have to be more careful. Just a quick little test you can do if you suspect flooding. Have a great day.

[MicahWes]

I know that feeling well. Also, with some practice you can tell if the line is cold vapor or liquid by feel. Put your hand on the pipe and then remove it (grab the pipe for a second). If the frost melts its vapor, if you still see a film of frost where your hand was it is flooded with liquid. You can also apply the same logic using a torch, just have to be more careful. Just a quick little test you can do if you suspect flooding. Have a great day.

Hmmmm, that sounds like a fairly dubious claim.

You are running R404A in a system designed for R502 (except for the compressor, of course). The refrigerants have different densities, so you shouldn't have charged to the nameplate charge without adjusting. R505 is about 75lbs/cu.ft, and R404 is about 66lbs/cu.ft. So you would need to adjust by a factor of about 0.88 in order to have the same amount of liquid present in the system. This means that you should have overcharged the system if you put 404A in to the 502 stated charge. This doesn't jive with your high superheat, as the system should be overcharged if you didn't adjust for density.

I would get box to design temp and charge to superheat at this point.

[tranger03]

That is interesting about the difference in densities between the two refrigerants. Which part did you think was a dubious claim? I have used this dubious claim many times to check to see if a line was flooded or not. In the same way that you can find a restriction by heating the line. A line that is full of liquid will absorb the heat into the refrigerant and the pipe will remain cool, whereas a pipe that has vapor only will stay hot. Same way that a receiver level can be checked without a sight glass. Anyway, it's not meant to replace pressure gauges and temp sensors. When you have a suction manifold with multiple zones coming off of it and you suspect something is flooding, it sometimes helps to narrow down the field with sensory and then verify with instruments.