Insanity

mtoto · 08-03-2002, 05:31 AM

Can anyone out there give me some feedback. Im ashamed to admit it but im having hassle along with several of my colleagues with a walk in freezer. The design conditions are -20c but i cant get the cabinet down to temperature. Ive changed the tx valve, drier, sight glass, evacuated down to 2 milibars and still no luck. It seems to hit a barrier at around -4 and then slowly gets down to -10 overnight. Never any lower though???
Ive selected the correct valve and orifice size left the superheat setting to 6k which the manufacturer assures me is correct and im running at a back pressure of around 12psi -14c.
Im pretty sure the system has the correct charge and theres nothing wrong with the cabinet seal or compressor valves.
Is there anything left?? Or is all this insanity so logical???

Any help appreciated

**herefishy** · 08-03-2002, 02:05 PM

Could you provide more information?

How long have you (personally) or your organization been servicing this unit (do you have a history with it)?

Did it work before (according to your knowledge or the customer's)?

Is it new install? Or existing original equipment? Or has some fitting been done in the past, by others perhaps?

What is the capacity rating of the C.U. vs Evap, and have you determined what the freezer load is?

For -10c room @ -14c S.S.T. 4c (7f) T.D. seems kinda low. I'd say you've lost some capacity somewhere (assuming a typical 10f T.D. application).

Even though compressor valves are good, how are the wrist pins?

Is it a semi-hermetic? Is it noisy, but as head pressure increases, does it seem to quiet down?

I recall a freezer that drove me crazy IN THE EXACT SAME MANNER, and the problem ended up being that the crank races were worn and affected the throw of the stroke. All numbers looked good, but it wouldn't do the thingy. If the compressor seems noisy or "tapping", block your condenser coil airflow and get the head pressure to start rising. If you notice that the compressor quiets down as the head rises, you may be looking at compressor replacement. When you do this, you may notice that with the condenser completely blocked off and when you think that the discharge line should be bursting due to high head, the pressure won't even rise high enough to trip high pressure safety! I've been there!

I have actually pulled heads to check the piston position at top dead center. I will roll the compressor over till one of the pistons gets to TDC and just starts on the down stroke, then push down with my finger to see if the piston drops in some slack due to wear in the attachments. Also observe how high the piston rises in the cylinder. But the audio/head pressure check seems to be pretty dependable.

Does it "Rock", or sway harmonically on it's suspension? Curiously enough, one broken tie rod can manifest itself in a subtle manner. Wait long enough, and you'll find it sticking out of the crankcase!

A more accurate method would be to check the compressor performance chart (curve). Take exact voltage while running (exclude fans and accessories), take exact amp draw, and the suction and head pressures at the time that the power readings are taken. Compare ratings on mfgr performance chart for the refrigerant being used. If you don't have the literature, call the plant with all data in your hand. They can do it over the phone. Believe me, that will really tell you what's going on (usally).

**Andy** · 08-03-2002, 07:24 PM

Hi, your evaporation seems high temperature wise, although the pressure would seem correct for say 404a. What refrigerant are you using? Compressor wired correctly and running at the correct speed. Condenser too small, high head reducing compressor capacity? Compressor too small for the job or the wrong temperature range? Charge way too high with the liquid backed back into the condenser? On going to a job like that blind I would be inclined to recover the charge and re-charge with fresh refrigerant of the correct grade and quanity.
Regards. Andy.

**Gary** · 09-03-2002, 08:18 AM

What are the following?:

type of refrigerant
type of metering device

saturated suction temp
evap air in temp
evap air out temp
suction line temp at coil outlet
suction line temp at compressor

saturated condensing temp
cond air in temp
cond air out temp
liquid line temp at receiver outlet
discharge line temp at compressor

My inner voice says, "Inefficient compressor", but if I were on the jobsite I would gather all of the above info, so that I would know exactly what is wrong and everything that is wrong.

**Abe** · 09-03-2002, 04:24 PM

Hi Gary,

Your "book" which Marc was going to market in the UK. Is it available yet ? Im intrested in purchasing a copy, the metric version

Please advise

Thanks

Abe

**Gary** · 09-03-2002, 06:20 PM

That's a very good question.

Hey, Marc. Is it ready, yet?

**Jasper** · 09-03-2002, 10:04 PM

mtoto

I would go along with everybody else and say that the compressor is U/S

**Abe** · 10-03-2002, 11:51 PM

Not so fast Mtoto
You havent posted the operational details as listed in Gary's post, see above
In order to make a valued prognosis, you need to take into all the parameters
Ok, looks like a dodgy compressor, maybe not
If I was the "paying customer" I would expect a thorough diagnostic

**Gary** · 11-03-2002, 08:38 AM

I agree, Abe. Identifying a borderline compressor is difficult at best, and mistakes are very costly. It would be best to fully examine the system.

On the other hand, Mtoto, if you find you must immediately make the repairs, the most likely problem is an inefficient compressor.

I would strongly suggest, if this is the case, that you take a full set of temperature readings both before and after. Compressor problems almost always have an underlying cause elsewhere in the system, and you need to know where the primary problem lies.

**herefishy** · 11-03-2002, 02:00 PM

Originally posted by Gary
Compressor problems almost always have an underlying cause

I agree with that. Compressor failure is the SYMPTOM of a problem!

**frank** · 11-03-2002, 08:34 PM

I agree in principal - but you could have a compressor problem caused entirely by wear and tear! Not always evident but you have to consider the age of the system and the run hours of the comp.

**JONAH** · 26-03-2002, 10:33 PM

"im pretty sure charge is right"
Hi mtoto,hope im not stating obvious here but have u got a full column of liquid reaching tev with a decent head pressure!
I dont know how long your pipe runs are etc look at fitting sight glass as close as poss to tev at least this will eliminate poss of flash gas which ultimatelywill in turn lower tev capacity, altho should be picked up by s/heat readings ,which ideally need to be taken/altered nearer room design temp -20.

**terrygoodrich** · 26-04-2002, 02:06 AM

Loss of capacity due to worn wrist pins is usually attributed to high compression ratio. Low suction pressure can be caused by running an extra low box temperature, an under charge of refrigerant, frosting of the evaporator, failure of evaporator fan motor/s, a dirty evaporator, maladjusted TEV, maladjusted Crankcase pressure regulator, etc.. High discharge pressure can be caused by a dirty condenser, high ambient, non-condensibles in system, condenser fan failure, etc.. Any one or any combination of these things may have contributed to the loss of capacity over the years. There may not necessarily be anything wrong with the system now. I agree that the best way to diagnose this condition is to compare the pressures and amperage with the performance curve provided by the manufacturer.

**terrygoodrich** · 26-04-2002, 02:12 AM

I have a question.

Do any of the manufacturers provide compressor performance data for any of the new replacement refrigerants on the market?

What if we are using HP80 (R-402a) in place of R-502 in a freezer?
How will that affect the charts. How then, do we accurately diagnose an inefficient compressor?

**Frosty** · 28-04-2002, 07:11 PM

Herefishy

Although I will agree with you somewhat...compressor failures aren't always due to system malfunction! Maybe it's just where and tear?

Possibly a broken suction valve...or if running hot...maybe a broken disharge valve!

Has a compressor efficiency test been undertaken?

Frosty

**herefishy** · 29-04-2002, 11:23 PM

Originally posted by terrygoodrich
Do any of the manufacturers provide compressor performance data for any of the new replacement refrigerants on the market?

Call 'em on the phone. Generally the Engineer can extrapolate, and at least get you in the ballpark.

**herefishy** · 29-04-2002, 11:28 PM

Originally posted by terrygoodrich
Do any of the manufacturers provide compressor performance data for any of the new replacement refrigerants on the market?

Call 'em on the phone. Generally the Engineer can extrapolate, and at least get you in the ballpark.

Did you try the decreasing noise as the head pressure rose experiment?

**Dan** · 30-04-2002, 12:47 AM

Good comments, all. In addition to Gary's requested information, I would love to see volts, amps, and compressor model. This could be compared to the compressor curve and help indicate an inefficient compressor.

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