SCREW Compressor Problem

[lana]

Hi everybody,

I got a report from a spiral blast freezer with Bitzer Screw compressor. After starting the compressor both suction and discharge pressures go into the vaccume?!! I have not visited the site myself and only got this report on the phone.
Any ideas?

[afeef]

well lana,,,you didnt mention any details:
1-is it working day before?
2-what kind of gas it use?
3-how many ton capacity?
but i can tell with 90% confidance the problem is scape of gas
best wishes

[lana]

Hi afeef,

The system was off for 5 months but had been working before. The refrigerant is R22 and the capacity at the design condition (-40/+40°C) is 52kW. And the day before the receiver was frosted up? According to the documents the refrigerant charge is 170kg.
Thanks.

[afeef]

lana
now it is obvous
during the 5 month idle , all the gas scape out
when you run it its reseiver freezed cause of low gas pressure(or we say no gas pressure)
now ,when running it ,,it gives vacuum
so ,,all system gas is run out , there must be leak some where , try to charged with dry nitrogine (250psi), and find the leak by soap ,
test all soldered pipes, flanges,flares,
see where there is oil on pipes ,it indicates there is leak at that region
all regards
afeef

[US Iceman]

lana,

This looks like a good place to start the investigation.

And the day before the receiver was frosted up?

If the receiver was frosted up then the receiver shell would have to have been below freezing to do this. The most plausible explanation is the safety relief valve lifted (or a fusible plug melted).

The release of refrigerant to atmospheric pressure reduced the internal pressure of the receiver. When the refrigerant pressure was reduced (equalized with atmospheric pressure), the refrigerant was boiling at about -40C. Therefore the receiver would have frosted up very nicely.

You should pressure test the system, however I would recommend changing the relief valve before to ensure you do not have to pressure test twice.

If the receiver had a relief valve, the valve may have re-set to the closed position. Based on the information you provided, I would change the relief valve first and then pressure test.

One other item to check... Is there a possibility of other installed lines from the receiver leaking to atmosphere?

It certainly sounds like the relief valve to me.

[US Iceman]

lana,

If indeed the relief valve released the refrigerant, you might also check the relief valve setting for the correct relief pressure.

Did someone try to start the system the day before the problem, or was the ambient temperature much higher that day?

If the weather can be ruled out and the relief pressure is acceptable, something or someone caused the problem.

[lana]

USIceman, Marc and afeef thanks a lot for your help.

I will check the system and keep you posted.

Thanks again.
Cheers

[lana]

Hi everybody,

I have visited the site.

There was no leakage and the system is full of refrigerant and there is no pressure relief valve on the receiver.

There is a two stage thermostat. First stage is connected to the capacity control and the second stage to the pump-down system (solenoid valve).

The low pressure setting was brought down by the operator and the thermostat did not connect the solenoid valve, so the system was stuck on pump-down situation.

Now everything is fine and the system works well but I could not understand why the discharge pressure came to vacuum? Also, there is a check valve on the discharge line but I could not see where the pressure gauge was connected, before or after the CV (it is a compact unit).
Any idea?
Cheers

[US Iceman]

lana,

...there is no pressure relief valve on the receiver.

Does the receiver have any over-pressure protection?

I would also like to know how the receiver frosted up???

Is it possible the receiver is being used as the hot gas source for defrosting the evaporators (kool-gas defrost)?

[lana]

Does the receiver have any over-pressure protection?

No.

I would also like to know how the receiver frosted up???

I did not see this. The operator claims that the day before the receiver was frosted up from the bottom.

Is it possible the receiver is being used as the hot gas source for defrosting the evaporators (kool-gas defrost)?

No.

Thanks a lot.
Cheers.

[Andy]

Hi
blast freezers and coldstores will do this if the expansion device stays open with the compressor off. As Marc says thermosyphon, the vapour boils of the liquid and migrates to the low temperature side if the system.
The discharge at low pressure would possible with this senario. Also the discharge check valve will do this, allowing the compressor to reach the suction pressure.

Kind Regards. Andy

[US Iceman]

I could not understand why the discharge pressure came to vacuum? Also, there is a check valve on the discharge line but I could not see where the pressure gauge was connected, before or after the CV

lana, if the liquid line solenoid was de-energized then there would not be any refrigerant to create a suction pressure, hence the vacuum. At the same time, if the compressor is running and there is no refrigerant, the discharge pressure will not increase. In other words, the compressor would just be running in a vacuum.

As you pointed out the discharge pressure gauge could be upstream of the check valve, so the discharge pressure could have indeed read a vacuum also.

I cannot go for the thermosyphon theory. If the liquid line solenoid was open, the liquid could flow into the evaporator IF the evaporator was colder to allow migration (another term that could be used incorrectly for a thermosyphon). A thermosyphon effect/flow is created by a difference in densities of the volatile fluid.
The fluid density difference is created by a temperature differential between two points. Then the fluid will then actually circulate.

However, you said the system had been off several months so I cannot see the refrigerant migrating to the evaporator. The evaporator was probably just as warm as the receiver.

In all probability, the problem was caused by the defective? thermostat not allowing the liquid line solenoid to open.

[Andy]

Hi Marc
I have been keep my head down Changed jobs, took a while settling in. But I am back fighting fit
Migration or thermosyphon, still a frozen receiver
Nice to converse again Marc
Kind Regards. Andy

[lana]

Hi everybody,

I was out of town for a week, anyway thanks for your good explanations and helps.
Cheers.

[SURESH YADAV]

Please tell what are pressure during system shut down condtion i.e suction, discharge & oil.
what is ambient temp now.

[sharan]

I want Bitzer Semi Hermetic Screw compressor Installation and Working manual.

[charlie n]

I want Bitzer Semi Hermetic Screw compressor Installation and Working manual.

You can download the manual from the Bitzer web site

[moideen]

Hi everybody,

I have visited the site.

There was no leakage and the system is full of refrigerant and there is no pressure relief valve on the receiver.

There is a two stage thermostat. First stage is connected to the capacity control and the second stage to the pump-down system (solenoid valve).

The low pressure setting was brought down by the operator and the thermostat did not connect the solenoid valve, so the system was stuck on pump-down situation.

Now everything is fine and the system works well but I could not understand why the discharge pressure came to vacuum? Also, there is a check valve on the discharge line but I could not see where the pressure gauge was connected, before or after the CV (it is a compact unit).
Any idea?
Cheers

hi lana,i think it will have pressure transducer befor check valve.

moideen