Display cabinet problem

[diljan]

Hi Folks,

This is my first post here.

By way of introduction, I am a fridgy by trade but have been out of the industry for about 10 years. My focus has always been Air Conditioning and I have very little experience in the refrigeration side.

I was looking at a small display cabinet containing slices and cakes. Located in a cafe. The evaporator was located at the top and was icing up. The drain tray beneath it was warped in the middle due to ice build up on the tray itself which meant that condensate was dripping into the cabinet rather than the drain. This is easily fixed so no big deal there.

After fitting guages I recorded the following details.

Refrigerant: R22
Head Pressure: 1900kpa (amb. temp. 21degC)
Suction Pressure: 200kpa (cabinet temp 10degC)

I assume the desired cabinet temperature would be 3degC with a 5K TD giving evap. temp of -2degC or 370kpa. I am not sure about the 5K TD. I don't have data for cakes. Are they best kept at a lower humidity than say vegetables.

In either case it seems the suction pressure is too low and the discharge pressure way too high for the ambient. I cleaned the condenser fins and checked the fan blades but could not reduce the high discharge pressure. At 21degC I would expect a condensing temp around 35degC or 1250kpa not 1900kpa.

My thought were either air in the system or a blockage in the capillary or somewhere else on the high side. Being a bit rusty in my technical skills Iam not really sure what the problem could be and how to diagnose it.

I really would appreciate some help from those with more experience and knowledge than I have.

Thanks, Dale.

[R. skiffington]

I would doubt there is a blockage on the high side, has anyone worked on the cabinet before you? If you have a partial blockage in the capillary, often this will confuse guys and they'll start charging more refrigerant thinking it's short, this will cause the high HP. I would recover the refrigerant, change the drier, re-commision system (check if a weighed charge is stated) if not you need to charge slowly untill the cabinet gets close to set point, then charge untill you get a little ice on the suction line to the comp then bleed the gas off slowly untill the ice melts, this is a good way to get the charge right. If when you are adding refrigerant the HP keeps rising and the suction presure stays low it would indicate most probably a blocked capillary, you can sometimes clear them with nitrogen, otherwise replace it. Is the evaporator passive or does it have fans, if it's passive remember it will evaporate much colder. I would expect it to have about 300 - 400 grms. Also make sure your not getting any air re circulation on the condenser. Hope this helps

[nike123]

What is expansion device type? If it is capillary, it looks like restriction and overcharged.
To check:
The strainer on a restricted cap tube should be about room temperature because of the sub cooled liquid that has backed up, and running your hand down slowly from the top of the condenser the temperature should be warm then suddenly cool to room temperature to the level where the liquid refrigerant has backed up.

[diljan]

Thanks for the replies.

The compressor was replaced by another company several months back and I suspect they overcharged the unit to try to raise the suction pressure.

I did not check the temperature of the condenser u bends but did check the drier and it was room temp (not too sure though)

Evap is forced draft.

What cabinet temp. humidity should I be aiming for?

Dale.

[amitsaxena]

Dear Diljan,

There are following possibilities :
1. Comp is undersize
2. Comp is fan cooled and fan is not working
3. Condenser air flow is getting chocked
4. System defrost cycle faulty
5. Eva fan blade not working
6.For over charging, check the temp. of the suction compared to ambient temp.
7. Also check for the cycling percentage as i feel unit is not healthy with the operating conditions mentioned.

Check for the above points and return back with some temp, as comp dome, suction etc.
Thanks
Amit Saxena

[tonyelian]

hi
u mention that other company has change the compressor
i assume that is not the same and i am leaning to a long capillary tube causing these symptom
can u provide the cap size so we can help
long cap act like a restriction