Compressor back pressure rating

[Cary]

I've just had to do an emergency compressor change on an 8 door display freezer replacing a Manurope ltz88 witch had short circuted after an oil shortage problem.

Being in the sticks there is a delay in obtaining a replacement so I put a Mt81 that I had handy into service after changing the oil over to ROC. It is running on 404a and has a suction temp of -28.

This is only an interim measure but got me to thinking what is the actual differance in the compressor and in a pinch how far can you push the parmeters. Can a high back pressure be used on low temp or can a low back pressure be used on air con?

While thinking about air con, can an air conditioning unit be used for refrigeration if the capacity is derated to allow for the different suction pressure?

Thank you for such a great site, I normally just sit back as a sponge an soak up the experience of a diverse group of Fridgies.

[chemi-cool]

Hi Cary,

If you have a compressor burnout because of oil shortage, you have to investigate and find out why. And repair it.

The biggest difference between AC and Refrigeration is air quantity.

Chemi

[Cary]

Forgot to mention an oil trap was located and rectified now acheving good oil return to compressor.
Also I am only wondering about the condensing side as I assume fin spacings and air volume come into consideration on the low side

[rbartlett]

i can remember seeing two hi temp hi backpressure unite hermatiques condensing units chugging away side by side for several years -
one did the coldroom the other did the freezer (and yes it was on 502...)

the only problem they ever had was in high summer the cond was too small..


chers

richard

[Gary]

High back pressure compressor are designed to pump a higher volume of refrigerant and have larger valve ports. This makes them less efficient for low temp applications. It's not a matter of what temp it can run at. It's a matter of what temp it is optimized for.

[chemi-cool]

Hi Gary,

I assume that techs who know the job, will know how to choose the right compressor for the right temp.

The small trick to avoid high head pressure and excessive wear for the compressor is using condensers with more then one fan and always, have about 15% extra in size.

But I'm sure everyone knows it.

Chemi

[Peter_1]

The difference for me is that for the same motor they fit just other pistons in it. They adapt only the pumped volume to the driven motor.

What is a compressor: one of the simples pieces of mechanic, far less complicated then for example a diesel engine.

It has an electrical motor, a crankshaft, some pistons and some valve plates and two service valves. Sometimes an oil pump. But that's all.

They don't make special plates for all the gasses, they all stay the same material, size, way of fixing it,....

We had once a small meat cold room which runned for more then 3 years with a too high humidty (95 to 100% RH)
Client called us and asked us if we could save the problem.
The previous tech had installed a speed controller on the evaporator fans.

The installed capacity was double of what there was needed. Compressor running time was +/- 6 to 8 hours/day, far too lessto remove the moisture in the room.
There were 2 evaporators installed on 1 compressor.

We removed an evaporator and changed the gas from R404a to R134a , same compressor.

Running time became 12 to 14 hours a day and humidty decreased to 80 - 85%.
We afterwards installed aslo a heater in front of the evaporator, controlled by a humidistat so that our client can set the desired humidty.
Compressor is running now for years without any problem.

The only thing I did was re-calculating the needed cooling capacity and selected afterwards a R134a compressor who gaves the calculated capacity. I looked then to the pumped volume of that compressor. Lucky, the pumped volume was +/-the same of the original R404a compressor, so this R404a compressor should give the desired capacity when we change it to R134a.

[RogGoetsch]

what is the actual differance in the compressor and in a pinch how far can you push the parmeters. Can a high back pressure be used on low temp or can a low back pressure be used on air con?

While thinking about air con, can an air conditioning unit be used for refrigeration if the capacity is derated to allow for the different suction pressure?

In a pinch, you do whatever it takes. For long service life, dig deeper. In hermetics, the suction gas is also cooling the motor windings, so at lower back pressure, the gas is cooler, but also there's less of it. Of course you're also generating somewhat less motor heat, but I try to stay within the mfg recommendations.

High back pressure on a low would make me look at motor amps and discharge temp as well. If you are operating outside of mfg recommendations for pressure and refrigerant, only time will tell, and will really be meaningful only on a statistically adequate sample (many units.)

Rog

[Cary]

Thanks for the guidance this is a bit out of lthe norm for me it is always much easier to match things with spefications on what they are designed to do than what you can make something do.
As the job is fixed and running with the correct compressor it is now only for interest and further edification .
Manurope MT copmressors are rated for R22 and LTZ compressors are rated for R404 so assuming MT is designed to run at 0 c (R22)you have a back pressuree around 375 kpa and if a LT(R404)is running about -20 c you have a back pressure around 175 kpa .
Are the design tolerances that close that it is determined by valve size and pumping volumes or is it more to do with adequate cooling in the suction gas.
One more quick question whilst talking about Manurope units is there any cheap way of reducing the noise of the condensor fan on a single fan aluminum bladed three phase 450mm.
All on the same job customer bought all the components two condensing units 8 door display frezer and a new shop .I was given the job of making it cold.
Now that I have made it cold he say's make it quiet.

[Peter_1]

As long your superheat is low enough, no matter on what suction pressure you work, the motor will cool enough to avoid problems.

The only difference between a R22 and a R404 Maneurop is the oil. In the beginning of the ester-oils, the wholesalers had only one machine in stock: one for mineral and also for ester-oil.
They asked you then for what sort of gas the compressor should be used and filled it then with the proper oil. Or they gave you a cannister with the proper oil.

What is true is when you run a HP compressor on a LP application, the motor is underloaded and will run with a smaller power factor.

Nighter, sorry I have to disagree again but it's easier to change a cooler to freezer than vice-versa. You said it just the other way.
A compressor designed for a LP application has a larger pumped volume versus the driven electrical motor. So if you use a LP compressor in a HP application, then there will be a change that you will run with an overloaded motor.

Look at the Discus range from Copeland: let's take a 15 HP. There are 3 models and all have different cooling capacities. The model with the highest capacity is limited to a suction of -20°C (sorry, SI system here)

Another problem that could arise is that your condenser will and shall be too small. Again, a 15 HP LP compressor (D4SL-150X) like you said: capacity 16, kW at -30°C (design conditions) and power consumption 10,5 kW--> needed condenser capacity +/- 27 kW.
No you use this compressor in a HP application for a cooling room, same compressor at -10°C will give 40 kW and 17 kW power consumption -->condenser of 57 kW (more then double) If condenser was designed originally like you said for a freezer, then you will be in trouble mate.

Nighter, I didn't blamed you nor someone else in this thread for asking a stupid question. But your quote "better to ask a stupid question than, to be too stupid to ask and learn about something you don't know nothing about" is a good one that I will remember.

but thats how we all learn, by making mistakes or by others making them for us, to learn from, the ones we never forget are the ones we make ourselves!

Those who are longer around here knows that I also share my mistakes here but I prefer that others make the mistakes so that I can learn from them, it's much cheaper and less embarrassing.