Supermarket Refrigeration

[bersaga]

Hello everyone!

Need some advice/views on the following :

A supermarket had a Copeland P6-LR6-0500 semi hermetic condensing unit ( 11500 W at -10 C evap and 35 C ambient ) connected to 3 display cabinets ( medium temp application)

Cabinets C1 (3000W) , C2 (1500 W) and C3 (2000 W) were all rated at -10 C evap and 35 C ambient with a catalogue recommended operating range of 2-8 deg C but set at 0 C at the supermarket. ( the rating of the cabinets are questionable as the actual evaporating temp of the rating is actually not printed but implied verbally)

This condensing unit (CU) was replaced by a Copeland scroll 6 hp CU with a 9000 W capacity at the same rating as above.

Measurements at 1 pm gave the following :

Ambient : 29 C
Suction ( at compressor ) : 4 C
Discharge : 91 C
Liquid : 38 C
T evap : -5.5 C
T cond : 46 C
Superheat : 9K
Subcooling : 9K

The system is defrosting 4 times per day and deemed adequte.

The case temperature meets the 0 deg C setpoint at a certain period : 2 - 3pm

Problem : According to the owner, the previous CU regularly cuts-off but the new CU is running continously and he is concerned about the power consumption. ( Why he replaced the previous unit is not known to us but he claimed there were no problems with the old unit !!!)

Need advise on what areas to look at that might possibly give some answers. I have not been to the installation but the data was collected by our technician at site and I will be meeting him in the next day or two.

Regards

[Prof Sporlan]

The numbers don't seem out of line. The condenser coil TD, 17K, could be a bit high. The 9K subcooling could also be a bit high, depending where it was taken, and if the liquid line isn't passing thru a heat exchanger or refrigerated area. Which leads us to a possible slight refrigerant overcharge.

The 9K superheat at the compressor is ok, though it would be useful to know the superheats leaving the evaporator coils. The apparent loss in cooling capacity may be due to occasional floodback.

[bersaga]

Thanks Prof !

The subcool was a deduction of the actual measured temperature of the condensor discharge to the receiver from the saturated liquid temperature at the condensing pressure.

I suspect overcharge - this installation is in Southern China and they use some unorthodox method of charging refrigerant - only with viewing the sight glass and checking the running amps - no subcool measurement.

I will check on the temperature of the suction line leaving the evaporator for useful superheat.

[Gary]

In order to accurately evaluate the system, we need the following:

Low side:

evap air in temp
evap air out temp
SST (saturated suction temp)
suction line temp near evap coil
suction line temp near compressor

High side:

cond air in temp
cond air out temp
SCT (saturated condensing temp)
liquid line temp at outdoor unit
liquid line temp at metering device inlet

[Dan]

Hi, Bersaga.

Problem : According to the owner, the previous CU regularly cuts-off but the new CU is running continously and he is concerned about the power consumption. ( Why he replaced the previous unit is not known to us but he claimed there were no problems with the old unit !!!)

Well, first of all, if you reduce the size of your compressor to 80% of the capacity of the previous compressor, you should expect more run-time. The compressor that runs longer does not necessarily cost more in electricity. Often it is the opposite.

But I think the key question to answer is why he replaced the unit. And why it was replaced by a smaller compressor. Perhaps it failed because it was oversized and cycled too much.

On to other considerations: You have a 3000 W (10,230 btu/hr), a 1500 W (5100 btu/hr) and a 2000 W (6800 btu/hr) series of loads.

6500 Watts on a 9000 W compressor.... only 70% loaded. The previous compressor was a mere 50% loaded. Normal load-to-capacity ratios are 80 to 90%. By the numbers you provide, you still have an oversized compressor, and you shouldn't be experiencing constant run-times.

I would dearly love to guess the design of the cases you have connected to this compressor unit. Maybe an 8-foot multideck open meat case, and 12 foot open single deck meat case and a curved glass service deli case? I am certain I am wrong, but it is important information for you to share.

If we know this, and you have measurements such as Gary asked for, the correct question.... if not the solution... will be readily apparent.

[bersaga]

Hi Gary/Dan ,

Thanks for the reply - I will be going over to the site tommorrow - it's in Shenzen, China.

The cabinets are as follows :

C1 : 3000 W Length :3.657m Display Area : 5.946 m2

C2 : 1500 W : 1.829m : 2.974 m2

C3 : 2000 W : 2.438m : 3.964 m2

I used the CoolPack software and these are 'reference cooling demand for selected cabinet that resulted :


C1 :6054 W
C2 :3028 W
C3 :4036 W

a total of 13118 W ( 100 % compressor undercapacity !!)

Has anyone used CoolPack before ? I know for the coldstore heatloads, the results are grossly high!

But if CoolPack is high , then the solution is simple - a gross mis-match !!

Any views guys ??

[Mark]

hi bersaga
Is there a crank case pressure regulator fitted?
regards mark

[bersaga]

Hi Mark,
There is no crankcase pressure regulator fitted in this system.

By the way, after further investigation , we found that Sanyo underrates its case capacities in the brochures i.e. you would select the equipment differently from how a Sanyo sales engineer would select the units. One of the local contractors told us that you should add 40% to the rated capacities.

Why they do that - I'm not sure - my theory is although they call their capacities - total capacity , I think it only caters for Product Load , Conduction Load Defrost Loan & Fan Motor loads but NOT Infiltration ( which actually from ASHRAE is 80% for Medium Temp and 24 % for Low Temp - of total cooling load - this is for the island type open display) - Any views ?

[Peter_1]

TAsselli, Chief, Bonnet... the brands we sometimes install need appr 800 W/m, but... we have to select the compressors at an evaporating temperature of -12°C to -15°C. Otherwise, we don't have enough capacity in the cabinets.
We've seen sometimes -15°C at the compressor side but -9°C measured at the evaporator.
Have you done a recalculation of the installed lines? Are they not to litlle? Are all the suction tubes going downwaards to the compressors (somewhere oil trap, oil restrictions)?
What kind of regulation is there installed if only one cabinet is cooling? Is there some kind of capacity regulation preventing it to go very low in pressure?
The underrating is a commercial selling technique (my cabinets don't need mutch refr. capacity compared to other brands..)
Just the same as the overrating of evaporators (The not Eurovent Certified ones) Those are mainly cheap but they don't give the rated capacity (sometimes 30 % less)

[Mark]

hello bersaga
It is not uncommon to size equipment in the uk,border line ie just about correctly sized,.By fitting a crank case pressure regulator you would maintain the pressure at the compressor inlet below a high limit set point ,this would help to maintain the vaporization temperature ,this is common practice to install one where there is fluctuations in temperature due to cases defrosting and maintaining set points.In a similar application as your working on.
in my opinion.
regards mark

[Peter_1]

To Bersaga,
You were talking about Refr. installations in China. For most of us this is on the other side of the world.
What surprised me was that these Chinese peoples uses Copeland (US) compressors. Don't they have Chinese compressors? For sutch a big country.
The same for the cabinets wich are Japanese I presume.
Just because I'm interested: what brand of regulations use they?
Which common gasses are used for the moment in China?
Peter

[bersaga]

Peter,

China still uses R22 - they will move to alternate refrigerants in 2005-2006. You can find Copeland and Bitzer in China - and also Sanyo , made in Dalian China. Copeland also makes Semi hermetics in Shenyang China and scroll compressors in Suzhou China. There are no local Chinese compressors.

China adheres to its own GB and CCC standards .

Hope that answered some of those questions.