Drinks Chiller Design Issues

[Tayters]

I site we work at has had 2 custom built-in chillers fitted about a year ago and have always had problems with not maintaining the setpoint. The manufacturer has returned a couple of times regarding this but states that this system is no different to any others they make and they aren't designed to run constantly - just over luchtime to display the food.

Went back to look at the smaller drinks chiller, repaired a leak on the evaporator, replaced drier, pressure test, vac, charge and here are the results.

SCT 50*C (320psig - R404A)
Temp at drier 44*C (about the only place I could get to measure)
Subcooling 6K

SST -12*C
Temp at compressor suction 6*C
Superheat to compressor 14K

Evap air on 12*C
Evap air off -2*C
Evap dT 14K

Condenser air on 22*C
Condenser air off 46*C
Condenser dT 24K

Setpoint 2*C, capillary system, integral with condensing unit underneath. Didn't want to add more refrigerant as head pressure going through the roof and subcooling suggests liquid at drier.
Before, the capillary came out the top of the drier, now it comes out the bottom but the only way to route the pipe was across the airflow off the condensor.
Capillary doesn't run along the suction line - completley separate.
No air curtain in the display side. Evap fan motors have tape over the lower half - maybe to reduce air flow so the air can get drawn back easier? These readings were taken with the blind down but the other chiller struggles to reach temp when room temp high unless blind pulled down and you can feel the cold air being blown out the case.
New drier is smaller than the original. Had to use what I had left as my new one managed to get the inlet blocked with braze during the repair works:o. Can't see that altering the refrigeration cycle though although it confirmed my amature status.

Although it reached setpoint (blind down) the readings suggest something wrong. It appears the condensor isn't rejecting enough heat and the possibly the SST a tad low but the manufacturer says everything is fine.

Obviously could have done with filter/cap issues, air curtain sorted at the design stage but before I let the site know just wanted your thoughts.

Thanks for reading,

Andy.

[Magoo]

Try changing condenser fan motor to get more air flow.

[Tayters]

Thanks for the reply Magoo.

The fan is a bog standard multifit and spinning as you'd expect. Condensor looked clean, face of the coil partially covered by the wooden panel on the front with slots cut out of it to let air in, but the coil was an inch or 2 back from this so not a total covering of the face of the coil.
Not the best place to get the hot air out from around the condensing unit but altogether I've seen worse still running at lower pressures.

Could these conditions be caused by an oversized compressor?
The high SCT and high superheat being a pointer that the compresser is pumping the charge out quicker than it can pull it back?

Cheers,

Andy.

[Gary]

Both coils have extreme airflow problems.

[Tayters]

Hi Gary, thanks for the prompt reply.

Just wanted to be sure I wasn't missing anything before my job sheet gets quoted to the manufacturer and I get questioned on it.

The condenser readings stood out like a sore thumb. Not sure if the evaporater was still under a bit of a load to account for the high readings as well. The case was half full of drink cans but readings were taken when the case was well on it's way down. It reached setpoint and the pressures and temps hadn't changed.
As soon as the blind gets raised then a wad of air will get blown out the front causing more grief.

If airflow over the evaporator was correct then wouldn't this result in a lower SST - probably too low for a drinks chiller?

Thanks,

Andy.

[Gary]

If airflow over the evaporator was correct then wouldn't this result in a lower SST - probably too low for a drinks chiller?


No... increasing the evap airflow would increase the SST. It would also increase the SCT, which is already way too high, because there is a severe condenser airflow problem.

The cond dT should be no more than about 17K, and preferably lower than that. This is for sure a condenser airflow problem.

[Magoo]

I agree with Gary, definite condenser air flow problem, remove the wooden slotted cover and allow more air flow.
Drop the discharge pressure will drop the SST and system starts to perform.

[Tayters]

Thanks chaps for confirming my original suspicions.


No... increasing the evap airflow would increase the SST.

Obvious now I've got my thinking head on. I have a tendancy to think too hard sometimes - and still goof up.:o

Thanks for your time and advice,

Andy.

[Gary]

Could these conditions be caused by an oversized compressor?


Is there reason to believe the compressor might be oversized?

[Tayters]

Is there reason to believe the compressor might be oversized?

Just trying to sound out other reasons for the readings I got.

In my head I pictured an oversized compressor filling up the high side quicker than the low side can feed it. Not sure if that is how it would work out in reality though .
I think I'd end up with higher subcooling (if my condenser had good airflow) lower SST and higher superheat as the evaporator would starved of refrigerant. Evap dT wouldn't be high as this would be an airflow not refrigeration issue. So not fitting my case entirely but the airflow issues were causing a bit of a grey area for me.

Cheers,

Andy.

[Gary]

Just trying to sound out other reasons for the readings I got.

In my head I pictured an oversized compressor filling up the high side quicker than the low side can feed it. Not sure if that is how it would work out in reality though .
I think I'd end up with higher subcooling (if my condenser had good airflow) lower SST and higher superheat as the evaporator would starved of refrigerant. Evap dT wouldn't be high as this would be an airflow not refrigeration issue. So not fitting my case entirely but the airflow issues were causing a bit of a grey area for me.


Like so many refrigeration problems, we can't tell until we have good airflow. Once the airflow is right, the rest of the troubleshooting job becomes much clearer and easier. Airflow problems are the first thing I look for on any system... and high dT is a dead giveaway.

[Magoo]

Is the condensing unit a generic system from a supplier, if so the compressor condenser should be OK.
You mentioned the supplier did not have a problem with installation. Could be he has lots of problems similar to yours, and not wanting/ or knowing how address to problem.

[Tayters]

Is the condensing unit a generic system from a supplier, if so the compressor condenser should be OK.
You mentioned the supplier did not have a problem with installation. Could be he has lots of problems similar to yours, and not wanting/ or knowing how address to problem.

Good point. I'll have to check but think it's a generic condensing unit.

Agreed on the second point.