parallel operation of screw compressor and piston type

[cheikh ahmed]

hello everyone,
as i know it's advised to not install a screw compressor with a piston compressor in parallel on the same refrigerant circuit.
is that true?
if not, what are the conditions to have to ensure a safe running?

many thanks

[US Iceman]

...it's advised to not install a screw compressor with a piston compressor in parallel on the same refrigerant circuit.

Where did this advice come from, a screw compressor salesman?

There is no truth to that statement at all. On the other hand, there might be some problems with piping vibration if the piping is not installed properly. But in general, there is nothing wrong with having recips and screws in parallel.

[keepitcool]

There are many systems piped this way. This is not at all a problem. I fact I like to see it. On large systems it makes start-up much easier.

[Josip]

Hi,

Where did this advice come from, a screw compressor salesman?

or maybe a piston compressor salesman

as i know it's advised to not install a screw compressor with a piston compressor in parallel on the same refrigerant circuit.

.... very, very wrong advice. just opposite if you want to save some energy....

Best regards, Josip

[icecube51]

in my opinion its even better if you put the screw first and than the piston comp. reason; better performance an safer if liquid flow happens.

Ice

[cheikh ahmed]

hi every one,

I get the message , but could any one give some logical reasons,
for your information I'm an electromechanical systems engineer and i'm interested in refrigeration systems.
i still remember that the piston compressor delivers the gaz at temperature higher.
the screw type has a very specific oil circuit and might have an economiser
and if both compressors are running at the same time you may have oil blend in the condenser/evaporater if the compressor oils are different
i can't expose all here this why I asked your help regarding the conditions to ensure the safe running
someone says that putting both compressors in parallel will save energy. how?
many thanks to all

[MMMMMike]

Hi, I've been lurking for a while - but thought I might contribute a little on this:
Be careful about starting the screw machine on-load, if you can't isolate the screw and offload and/or recycle you will have to make sure your motor is man enough.
Incidentally, not sure why you assumed it would be a screw salesman that made the claim - might have been a recip salesman for the same reasons! ;-)

[US Iceman]

someone says that putting both compressors in parallel will save energy. how?

This is a function of the part-load performance of the compressors. A screw compressor has non-linear performance when a slide valve is used for capacity control. What this means is; as the slide valve unloads the compressor to reduce the capacity, the power input is higher than the percentage of capacity the compressor is running at.

On a piston compressor the part-load performance is essentially linear, so the percentage of capacity is approximately equal to the percenatge of input power.

Using both types of compressors allows you to load the screw compressor fully to meet the cooling demands at greater loads. As the loads reduce you use the piston compresor to operate for the minor load changes.

This is about using the appropriate compressor to meet the cooling demands with the lowest total power input.

Incidentally, not sure why you assumed it would be a screw salesman that made the claim - might have been a recip salesman for the same reasons! ;-)

Someone else mentioned this also. Usually when I hear comments similar to this I attribute them to a salesman trying to sell his toys. It could have very well been a recip salesman also. BTW, you have too many "M's" in your screen name. You are using 4 more than I do.

[wilko123]

Is this not parallel operation? If series then the liquid slugging protection recommendation would be true!

[RUPESH DESAI]

Dear Mr. Ahmed
There is no harm in running reci and screw chiller in parallel . The company in which i m working is having both reci and screw chiller running in parallel for -5 c temp. I thing you should operate chiller in such a manner that screw chiller will operate at full load and reci chiller will load/unload as per chilled water outlet temp setting. Also specific power consumption for screw chiller is lower that reci-chiller.