Deli Cases, Regular Headaches...

[Rusty199]

I'm an HVAC tech that has been dealing with some deli cases and the like for a regular customer. The most recent is a McCray 8 ft double duty unit. It's history includes being converted from R12 to 134 sometime in the past. Also, the compressor has been replaced at least twice in the past 3-4 years. Unknown before that.
I found it with.....a bad compressor. I got a new replacement compressor and did a heave R11 flush to the system, including the TXV separately.
I followed it up with triple evacuation and evac to 1000 microns and made sure it held for 30 min.
Charged the system and started it up. Seems to run well now, but trying to get it to cycle correctly. I was surprised to find that the only method to control the case temperature is by the low pressure switch.
I'm wondering if I can't get better temperature control by using an adjustable thermostat and set the pressure switch to shut it down as a low pressure control should.
Also I'm having trouble getting the range of pressures I should be operating in. Going by the specs for newer units, I'm finding a suction pressure of 14-18 and a ll pressure of about 115 is what I should expect. I may be a bit overcharged because my pressure runs 20-22 at times and my suction line freezes at times.
I also read to look for a superheat of 10 degrees, but this case is controlled by a TXV so I'm assuming a subcooling reading would make more sense,but don't know where to find that. I'm also assuming any readings should be when the case is down to temp.
I can find no manuals for the unit, only a spec/tearsheet on the mfr website.

[Phase Loss]

This is just a "WAG" but I think you should adjust superheat.

here is why.

This unit has had multiple compressor replacements.

You are seeing the suction line freeze at times.

Your compressor doesn't seem to be able to pull the SST down. and I feel it is because the TX Valve is slightly overfeeding.

So I think if you pinch the TX Valve down a bit, you will see your frozen suction line go away, and you will reduce the load on the compressor which will lower your SST and allow the case to achieve a lower temperature.

But this is all just a guess, I don't really know.

[jpsmith1cm]

This is just a "WAG" but I think you should adjust superheat.

here is why.

This unit has had multiple compressor replacements.

You are seeing the suction line freeze at times.

Your compressor doesn't seem to be able to pull the SST down. and I feel it is because the TX Valve is slightly overfeeding.

So I think if you pinch the TX Valve down a bit, you will see your frozen suction line go away, and you will reduce the load on the compressor which will lower your SST and allow the case to achieve a lower temperature.

But this is all just a guess, I don't really know.

This is good advice.

Also, I'm making a WAG myself, but if this system has a receiver, don't mess with subcooling as a charging metric.

Charge to clear the sightglass for starters. Does this unit have a headmaster or other form of head pressure control? If so, then you need to compute the required flooding charge.

[icemeister]

Is this something like the case you have?



McCray doesn't have service manual available online...to my knowledge.

These cases typically have a top coil, a middle coil (just under the base racks) and a bottom coil (located across the front of the bottom storage section).

The expansion valve feeds these coils in series with the bottom coil first, then the middle coil and finally the top one. The TEV bulb is located on the suction line after the top coil.

I've found during normal operation the TEV will generally run wide open during pulldown, after an off-cycle or after a defrost. It's not until the full refrigerant flow makes it all the way to the sensing bulb that the TEV actually responds.

Then you'll see the suction start to drop off more rapidly...and that's what trips the LPC.

On a blind job where I have no history, I usually start off by setting the TEV at midpoint on the adjustment stem (in case anyone has been messing with it) and then set the LPC at the manufacturer's recommended CI/CO settings. If the condensing unit has a sightglass and receiver, make sure you have sufficient charge. If in doubt, pull the charge and weigh it in per the dataplate. Charge is important, as these things have a lot of evaporator volume to fill.

Honestly, I don't see how one could effectively use either superheat or subcooling on these types of cases. They were designed many years ago by guys who probably didn't know what they were...they simply knew what it took to make these things run right.

[Phase Loss]

Ice, that youtube link is some kind of crummy movie or TV show.

[pecmsg]

Honestly, I don't see how one could effectively use either superheat or subcooling on these types of cases. They were designed many years ago by guys who probably didn't know what they were...they simply knew what it took to make these things run right.

Far be it from me to question ICE on anything concerning commercial refrigeration but I’d have to disagree with this statement.

Once the case is at temperature and stabilized (several cycles) SH plays a big part in setting these up and keeping them running for a long long time.

[2sac]

Here is what I've found out pulling teeth from McCray tech support. I had an 8ft case that at some point in time was switched to 409a. 2 gravity coils fed by 1 TXV. Went thru 2 compressors in 3 years. Come to find out that the change of freon also changed the capacity. If memory serves me correct, I removed a 1/3hp condensing unit and installed a 1/2hp med temp condensing unit with 134a. Secondly because the coils have no fans, product temp is the key, not air temp. Put a cup of water in the case and leave it there. Take water temp, not air temp. As far as SH 6-8 at the coil outlet. Continue to use the LPC to control temp. I don't remember the exact settings but a cut in of 33psi(38degrees) and a cut out around 17 or 18psi(20 degrees) should be a place to start.

[Rusty199]

The original data plate is long gone, and it's a conversion from R12 to 134. Someone in the past wrote "134a 32 oz (2 Lb) " in magic marker, which is nearly worn off as well. I weighed in the 32 oz and added another 8 oz because I had read to look for 20-22 psi suction pressure, but from checking further I think I need more like the 14-18 psi I mentioned earlier.
The unit has and evaporator in the lower cabinet that has the TXV and a second evaporator on top above the product area.
I didnt get a chance to check it today, but will stop tomorrow to see how its doing now that it's at temp.
Any thoughts about installing a temp control for better.....temp control, I guess?

Thanks to everyone for the input.

[Rusty199]

Here is what I've found out pulling teeth from McCray tech support. I had an 8ft case that at some point in time was switched to 409a. 2 gravity coils fed by 1 TXV. Went thru 2 compressors in 3 years. Come to find out that the change of freon also changed the capacity. If memory serves me correct, I removed a 1/3hp condensing unit and installed a 1/2hp med temp condensing unit with 134a. Secondly because the coils have no fans, product temp is the key, not air temp. Put a cup of water in the case and leave it there. Take water temp, not air temp. As far as SH 6-8 at the coil outlet. Continue to use the LPC to control temp. I don't remember the exact settings but a cut in of 33psi(38degrees) and a cut out around 17 or 18psi(20 degrees) should be a place to start.

That makes sense. I was amazed that the capacity on the units is so low. But thinking about it, theyre not designed to drop the temps quick, just a refrigerated display case. The product is supposed to be chilled before putting it in just for that reason.
I also failed to mention that there are two of these in tandem. I reset the pressure control to match the other one, at 25 with a diff of 20. If I'm reading it right, that will drop it out at 5 psi and restart at 25. That stopped the short cycling.
I think I'll check the TXV position and maybe go back to the 2 LB charge since my SH seems to be low or non-existent. It does have a small receiver, but I don't want to lose another compressor due to overcharge.

[2sac]

The original data plate is long gone, and it's a conversion from R12 to 134. Someone in the past wrote "134a 32 oz (2 Lb) " in magic marker, which is nearly worn off as well. I weighed in the 32 oz and added another 8 oz because I had read to look for 20-22 psi suction pressure, but from checking further I think I need more like the 14-18 psi I mentioned earlier.
The unit has and evaporator in the lower cabinet that has the TXV and a second evaporator on top above the product area.
I didnt get a chance to check it today, but will stop tomorrow to see how its doing now that it's at temp.
Any thoughts about installing a temp control for better.....temp control, I guess?

Thanks to everyone for the input.

If you have no fans, a temp control will not work in the air. It may be possibl to clamp it to the suction line, but I believe that will cause more headaches. Once you get the pressure control dialed in you should be good.

[Capz]

Hi Rusty, Make sure the bulb for your TXV is properly secured & wrapped. Secured at 4 or 8 o'clock. AC units do not wrap the bulbs but for a single wrap if any at all. Refrigeration TXV's require a good wrap of the bulb so it always senses properly. This is critical to prevent the TXV from swings possibly causing your suction line to freeze.

If this unit is being shut down using a low pressure switch & liquid line solenoid, what is commanding the solenoid to close? I ask this cause your question regarding the T-stat.

[icemeister]

Far be it from me to question ICE on anything concerning commercial refrigeration but I’d have to disagree with this statement.

Once the case is at temperature and stabilized (several cycles) SH plays a big part in setting these up and keeping them running for a long long time.

The problem I have (typically) attempting to check SH on these beasts is the system is rarely stabilized.

The original data plate is long gone, and it's a conversion from R12 to 134. Someone in the past wrote "134a 32 oz (2 Lb) " in magic marker, which is nearly worn off as well. I weighed in the 32 oz and added another 8 oz because I had read to look for 20-22 psi suction pressure, but from checking further I think I need more like the 14-18 psi I mentioned earlier.
The unit has and evaporator in the lower cabinet that has the TXV and a second evaporator on top above the product area.
I didnt get a chance to check it today, but will stop tomorrow to see how its doing now that it's at temp.
Any thoughts about installing a temp control for better.....temp control, I guess?

Thanks to everyone for the input.

Using an air-sensing thermostat can work, but often leads to excessive coil icing requiring an additional defrost period or two.

I've used a Penn A419 sensing coil temp and it works quite well.

[kim]

First this box is made to run. it should have long run cycles. You can't change anything and hope to get immedeate response. you will have to wait for it to stabalize before checking your changes. Might need to wait until the next day.

You have to let the coil freeze and thaw. Start the pressure switch at around 22 to 25F CO and 38F CI. adjust the CO as needed. I have had CO as low as 17F before. Do not trust the scale on your pressure switch. Watch your gage while testing your settings.

Give it plenty of defrost. I like Grasslin defrost clocks on these, so you can give it 1 to 2 hours every night. plus have shorter defreosts during the day. If freezing is a big problem raise the CI a little.

The easiest way to get an idea of the SH is checking compressor temp. The compressor should not get too hot. The compresser can tolerate a little cooler running temp since it is a pump down system. Heat is a big compressor killer. These boxes start with a hot compressor.
Charge is not very critical on these systems. You do have to watch out for high pressure during pump-down though. There is a lot of freon in those evaps. All of it has to be able to fit into the receiver in the pump-down. If the condenser is a remote, charge can get more critical.
I have had to rip out the temp controls somebody thought would be a good idea on a couple of large remote deli boxes