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bill1983
07-11-2004, 10:52 PM
i wonder if anyone knows the recommended no of cycling times/hr for a 3/4 hp hermetic compressor.
i have had a large number of cabinets with the same problem
ie compressor siezing.
details: cab temp 0/4 c run temp
refrigerant r404a
compressor run time approx 1min, comp off time 3 mins
cabinet is controlled by dixell controller, set for 0 with 3 degree diff and 3 min anticycle. control probe is sited in air off and it is uneconomical to resite the probe for more effective control due to the high number of cabinets involved
cabinet is surfrigo dairy stelvio 3000
i am working along the lines of the refrigeration system being too large resulting in too fast temp drop and compressor cycling too fast leading to overheating. the compressors fail within the first 12 months and are replaced like for like under warranty i have not had a satisfactory answer from the manufacturor as to the reason for so many failures.
what other lines should i be following?

Brian_UK
07-11-2004, 11:51 PM
control probe is sited in air off and it is uneconomical to resite the probe for more effective control due to the high number of cabinets involved
........are replaced like for like under warranty
Hi, welcome to the forum Bill.

I would have thought that the cost of moving a probe would outwiegh the cost of replacing the compressor although I understand that this may be at your cost.

If the manufacturers are funding the compressor swaps then try and talk them into moving the probes as they should not be in the 'air off' location. Mention the fact that as it is a design fault the cabinets are 'not of merchantable quality' and as such will be returned with a full refund under the Sale of Goods Act. :cool: That may grab their attention.

It is difficult to see how you can condemn the compressor size as too big when the controls are simply shutting it down as soon as it starts cooling; a probe in the air off location will always give control problems.

How about putting some temperature recorders to the cabients to monitor the air on and air off temperatures along a time base. You could then put those forward to your customer and let them chase the manufacturer.

Good luck and keep us posted.

Gary
08-11-2004, 12:10 AM
As a rule of thumb, a compressor should cycle no more than 6 times per hour.

Apparently these cabinets are cycling on the anti-cycle timer. Air off temp is going to rise very quickly when the compressor shuts off. At the very least you need to widen the differential.

These systems may have other problems as well, but without a full set of temperature readings all we can do is guess. For a full evaluation, we would need the following:

Low Side:

Evap air in temp
Evap air out temp
SST (low side pressure converted to temperature)
Suction line temp near evap outlet
Suction line temp near compressor inlet

High Side:

Cond air in temp
Cond air out temp
SCT (high side pressure converted to temperature)
Liquid line temp near cond outlet
Liquid line temp near metering device

bill1983
08-11-2004, 09:59 PM
thanks for the welcome and advice guys, the manufacturor funded option has already been investigated and ruled out by "da managament", the customer isn't interested as it is our problem under the comprehensive contract .
in the past, other cabinets i have worked on have worked ok with air off probes, eg craig nicol dairy (ette), linde etc. i would have preferred the probe in the air on or in a central position under one of the shelves, so that the temp being controlled is that which needs to be. is there a down side to any of these.
i will endevour to post the readings requested but it may take a day or two please check back if you can later in the week.

Gary
09-11-2004, 05:07 AM
Your nominal cabinet temp is 0-4C.

During the off cycle, the air off temp becomes equal (more or less) to the air on temp.

Let's imagine that during the run cycle the air off temp is 5 degrees colder than the air on temp (5K delta-T aka dT). If the air on temp is 0C, then the air off temp is -5C.

We would want to set the control to start the compressor at 4C air off, then shut off when the air off temp is -5C (with air on at 0C). The differential between 4C air off and -5C air off is 9K. IOW, assuming 5K dT, we need a 9 degree differential. As a bare minimum, for an air off control the differential must be more than the dT.

But we should not assume dT. We should measure it.

Latte
09-11-2004, 09:03 PM
Hi Bill,

Would it be posible to move the controller or fit another one and then fit the control probe on the air on.

You say you have spoken to the manufacturer. I assume in this case it is Blighline, if not give their technical dept a call on 01227 762700 they are normally very helpfull

Regards

Raymond

RogGoetsch
10-11-2004, 04:28 PM
compressor run time approx 1min, comp off time 3 mins
cabinet is controlled by dixell controller, set for 0 with 3 degree diff and 3 min anticycle. control probe is sited in air off and it is uneconomical to resite the probe for more effective control

Why not just widen the differential? Most commercial coolers use coil-sensing probes which cut out at 15F-25F and cut in at 37F-41F. Why not drop the cut-out to whatever achieves correct box temp and set cut-in to max desired box temp?

Rog

Gary
10-11-2004, 10:59 PM
Why not just widen the differential? Most commercial coolers use coil-sensing probes which cut out at 15F-25F and cut in at 37F-41F. Why not drop the cut-out to whatever achieves correct box temp and set cut-in to max desired box temp?

Rog

I totally agree.

Max desired box temp is 4C/39.2F. This is where the cut in should be set.

Cut out at 15F to 25F roughly translates to -9.5C to -4C, for a differential of 24.2F/13.5K to 14.2F/8K. IOW, he would typically need a differential of 8-13.5 degrees Kelvin (Centigrade). I offered 9K differential just as an example.

We need to find out what air out temp gives us the correct box temp (air in temp) in order to know what the correct differential should be.

bill1983
23-11-2004, 09:38 PM
thanks for all your advice,i am sorry to have taken so long to come back to the forum.
firstly moving the probes on 700 plus cabinets is out of the question due to cost ; i have put the argument forward that moving the existing, or even fitting new probes(to save rerouting the cable) is vastly cheaper than replacing compressors but unfortunately my manager is not a refrigeration engineer and quotes the standard reply these days which is always to do with price and manpower.
secondly,widening the diff leads to poor product temp unless the setpoint is set so low that the milk on the base of the cabinet freezes, or excessive condensation forms on the shelves, due to shop floor conditions,ie air conditioned to a price not to store load.
blighline too have advised new parameter settings but i can't seem to find a happy medium between controlling the cabinet at a reasonable temp and freezing the stock.
i am going to fit new probes to the cabinets in one store in my own time to prove the point and record the cabinets operation over a month to see if that will strengthen my argument.

bill1983
23-11-2004, 09:52 PM
forgot to post readings for these systems. i took measurements from 6 cabinets that were all working to assure myself that the readings i took were representative of an average cabinet
evap air in: 9c
air out: -1c
sst@2.9bar: -13c
suction temp evap: 5c
at comp : 12c
condenser air in : 23c
air out: 29c
sct@14bar: 34c
liquid temp condenser: 23c
liquid temp capillary: 22

parameter settings:
setpoint 1c
diff 3c

Gary
24-11-2004, 06:22 PM
I am assuming these readings are all taken from a single system, as opposed to averages from 6 systems. Is that correct?

Either there is a restriction or it needs a shorter cap tube, and it could also use increased evaporator airflow, but I realize these are design changes that you are not going to be able to make. Perhaps you could provide the readings you took from some of the other systems in order to judge whether these problems are only on the one system or are typical for all.

You have an evaporator delta-T of 10K. The differential must be at least 10K.

bill1983
25-11-2004, 08:42 PM
ok gary i'm all eyes can you please explain how you come to your conclusions
bill :confused:

Gary
27-11-2004, 02:18 PM
You have high subcooling, which indicates excessive refrigerant in the high side, and high superheat, which indicates either insufficient refrigerant in the low side or heavy load. The evap TD indicates heavy load, but the cond TD does not. The evap is not picking up the load, and the refrigerant is not getting from the high side to the low side (underfeeding) because the path between them (cap tube) is not allowing sufficient flow. You have a 3/4 ton condensing unit and a 1/2 ton cap tube.

Had you given us a set of temperatures when the evap air in temp was within the design range (0-4C), we could more accurately judge the relative cap tube size, but interpolating with the numbers you presented it appears the cap tube is too restrictive. Temperature measurements are most accurate under design conditions.

The evap air in temp is at 9C. When the compressor shuts down, the evap air out temp will quickly rise from -1C to near air in temp. Since you are controlling by air out temp, the 3 degree differential will very quickly close the thermostat, and the unit will cycle on the timer.

Given more airflow, the evap air out temp would be closer to the evap air in temp (lower delta-T), requiring less differential. With 10K differential, the refrigerated space temp will swing too much. Increasing the airflow would tighten the swing.

Gary
27-11-2004, 03:24 PM
Also, given the fact that the evap is not fully active it is very like oil logging. I would very carefully evaluate the piping to make sure it maximizes oil return. Did you check any of the siezed compressors for oil level?

Was the system charged according to superheat, or weighed in?

wesmax
23-12-2004, 05:47 AM
As a rule of thumb, a compressor should cycle no more than 6 times per hour.

Apparently these cabinets are cycling on the anti-cycle timer. Air off temp is going to rise very quickly when the compressor shuts off. At the very least you need to widen the differential.

These systems may have other problems as well, but without a full set of temperature readings all we can do is guess. For a full evaluation, we would need the following:

Low Side:

Evap air in temp
Evap air out temp
SST (low side pressure converted to temperature)
Suction line temp near evap outlet
Suction line temp near compressor inlet

High Side:

Cond air in temp
Cond air out temp
SCT (high side pressure converted to temperature)
Liquid line temp near cond outlet
Liquid line temp near metering device

it is fine to quote a book but a service tech
takes into consideration the operation of the system and the time of day the cooler is being used.
wesmax