Screw Compressor Eco & Liquid Injection ?

[lana]

Dear All,

Please have a look at the attached picture. This is from Bitzer instruction manual.

Can anybody explain when and why we use both Eco and liquid injection in a screw compressor?

If liquid injection is done from the eco port then why both of them?

Generally when eco is used and when liquid injection?

If compressor capacity is decreased, do we have to stop the eco operation?

Many thanks for any reply.

Cheers
LANA

[RANGER1]

lana, liquid injection is for oil cooling & controlling discharge temperature. Economizer is for subcooling liquid line & in turn increasing refrigeration effect. This port is slightly above suction pressure.

[lana]

Thanks for the reply.

My question is this : Both methods inject cooled liquid into the Eco port. If liquid injection is for oil cooling and discharge temperature control, then economizer line does the same thing. Doesn't it?

I actually don't understand the difference but I know what their purposes are.

Thanks
LANA

[RANGER1]

lana, Economizer line into compressor is gas only, not liquid. Your picture shows a PHE economizer subcooling liquid line. Economizer is basically another evaporator, so PHE suction into compressor in your case has 5-10 deg C superheated gas (if working correctly). The economizer itself does cool machine a little, but not near enough.

[lana]

Ranger1 thanks for the explanation.


The Eco line and the sub-cooler are exactly the same as in the double-stage crecip. compressors for intermediate gas cooling. BUT in screws the main purpose of ECO is sub cooling the liquid going into the main evaporator NOT cooling the oil,... right?

BUT the main purpose of liquid injection is cooling oil and to keep the discharge gas temperature down.

Liquid injection is done by special injection valve whose bellow is connected to the discharge line. Is there any danger of liquid going into the screws?!!

Can you please explain in what applications these two systems are used (and when both together)?

And my main wonder !!! When capacity is decreased, do we have to stop the eco operation? Because ECO increases the capacity while capacity control decreases it......?

Thanks a lot.

Cheers
LANA

[RANGER1]

Ranger1 thanks for the explanation.


The Eco line and the sub-cooler are exactly the same as in the double-stage crecip. compressors for intermediate gas cooling. BUT in screws the main purpose of ECO is sub cooling the liquid going into the main evaporator NOT cooling the oil,... right?

BUT the main purpose of liquid injection is cooling oil and to keep the discharge gas temperature down.

Liquid injection is done by special injection valve whose bellow is connected to the discharge line. Is there any danger of liquid going into the screws?!!

Can you please explain in what applications these two systems are used (and when both together)?

And my main wonder !!! When capacity is decreased, do we have to stop the eco operation? Because ECO increases the capacity while capacity control decreases it......?

Thanks a lot.

Cheers
LANA

lana, you can use this scenario in nearly all systems. Liquid injection is cheaper to install than water or thermosiphon oil coolers. There is a slight capacity decrease using liquid injection in general. Best to turn economizer off at lower load as you suggest, maybe 50-66% load as it will not achieve anything & may be harder to control suction superheat.

[lana]

Ranger1, many thanks for your inputs.

Cheers

LANA

[Josip]

Hi, lana

Ranger1 thanks for the explanation.
The Eco line and the sub-cooler are exactly the same as in the double-stage crecip. compressors for intermediate gas cooling. BUT in screws the main purpose of ECO is sub cooling the liquid going into the main evaporator NOT cooling the oil,... right?

Yes, but here we have only one compressor. Economizer system with HP screw compressors (-10/+35) we can use also for some other cooling i.e. air for air-condition system or some other process fluid needed cooling.

BUT the main purpose of liquid injection is cooling oil and to keep the discharge gas temperature down.

Yes, exactly.

Liquid injection is done by special injection valve whose bellow is connected to the discharge line. Is there any danger of liquid going into the screws?!!

No danger when compressor is running, but during standstill with malfunction valve on liquid injection line we can flood compressor. Liquid injection is more dangerous when is used with piston compressor on suction pipe in case of malfunction of liquid injection valve.

Can you please explain in what applications these two systems are used (and when both together)?

as explained by RANGER1, and of course on available money and designer engineer

And my main wonder !!! When capacity is decreased, do we have to stop the eco operation? Because ECO increases the capacity while capacity control decreases it......?
Thanks a lot.
Cheers
LANA

Generally, when compressor capacity slide is below 65-70% ECO port is connected to suction port i.e. at the same pressure as main suction pressure. Refrigeration system is not affected, but economizer vessel must be designed to obtain refrigerant flow to the evaporator. To stop or not ECO operation at small compressor capacity depends on duty of economizer vessel. The point is ... at small capacity we do have not economizer system benefits i.e. we do not save any energy like when economizer is on duty.

With VFD compressor ECO is always in function.


Best regards, Josip

[lana]

Dear Josip,
Many thanks for your explanation.
I do not understand what you mean by "Economizer vessel"? Can you elaborate?
What I understand from your explanation is : when capacity is decreased there is no economizer benefit but we do not close the solenoid valve on the eco line, right?
Also, Why liquid injection is not dangerous during operation? Liquid is directly injected inside the screw.

Why is Eco operating with VFD?


Again thank you.
Cheers
LANA

[Josip]

Hi, Lana

Dear Josip,
Many thanks for your explanation.
I do not understand what you mean by "Economizer vessel"? Can you elaborate?
What I understand from your explanation is : when capacity is decreased there is no economizer benefit but we do not close the solenoid valve on the eco line, right?
Also, Why liquid injection is not dangerous during operation? Liquid is directly injected inside the screw.

Why is Eco operating with VFD?


Again thank you.
Cheers
LANA

Economizer vessel = intermediate pressure vessel = can be designed like open flash type, shell&coil, shell&tube or plate type as you can see here http://www.swep.net/en/products_solu...onomizers.aspx

Yes, when we reduce compressor capacity .... capacity slide valve is traveling towards discharge port of the compressor ... due to screw compressor mechanical construction ECO port and suction port in that moment are physically connected and are at the same pressure - i.e. suction pressure ... it is possible, but not necessary, to close solenoid to reduce power consumption ... depending on duty and control of ECO vessel

Liquid injection is always "dangerous", but when compressor is running we assume injected liquid will evaporate to do the duty... in case of compressor standstill with leaking liquid injection valve part of injected liquid will evaporate and the rest will flood the compressor and oil separator ....

VFD compressor are usually without movable capacity slide i.e. slide valve is always in 100% mechanical capacity position, because we change real compressor capacity with VFD ... thus ECO port and suction port are physically separate i.e. ECO is always in function...

This SWEP site is good to download some free handbooks too... for heating, industrial and refrigeration ... enjoy ...

http://www.swep.net/en/products_solu...handbooks.aspx

Best regards, Josip

[lana]

thanks a lot Josip
very nice site bytheway

Cheers
LANA

[sarans0]

lana

I think its not liquid injection(screw compressor). The vapour from the eco is close to half staged refrigerant in a compressor. Hence its been sent back to half stage of the compressor to reduce the work done by the compressor.