2 stage compressor & pump downs

[bnhz]

Hello,
i am working with a walk-in freeze with 3 LH124/S4G12.2-35P condensing units.
When these units pumpdown sometimes the receiver (bottom) sight glass becomes

empty.
When running, both sightglasses are ok. Receiver and liquid line glass.
what would cause this to happen?

Unit was installed with 2 low pressure switches.

1 Danfoss KP1; sensor goes to the oil fill hole, according to Bitzer

diagrams. so it senses the interstage/ crankcase pressure.
cut-in = 20 psig
differential = 15 psig

1 Danfoss KP15 (hi & lo pressure switch)
Low pressure sensor attached to low side (on the suction head), i assume

sensing suction pressure.
cut-in = 2-3 psig
differential = 18 psig

can someone why there is 2 lo pressure switches?
and what should the correct temperature/ pressure setting be?
why does the receiver empty sometimes on pumpdown?

gas= R22
set temp -23 C/ -10 F

thanks for any help,
bnhz

[NoNickName]

Sometimes one LP switch is used for pump down stop and another LP switch of low pressure trip. The latter is usually having a lower setting than the former.

I'm honest when I say that a LP switch at the interstage is new to me , but it can well be the pump down LP switch, or the setpoint LP switch for a blast freezer regulated on crankcase pressure.

[Grizzly]

Hello,
i am working with a walk-in freeze with 3 LH124/S4G12.2-35P condensing units.
When these units pumpdown sometimes the receiver (bottom) sight glass becomes

empty.
When running, both sightglasses are ok. Receiver and liquid line glass.
what would cause this to happen?

Unit was installed with 2 low pressure switches.

1 Danfoss KP1; sensor goes to the oil fill hole, according to Bitzer

diagrams. so it senses the interstage/ crankcase pressure.
cut-in = 20 psig
differential = 15 psig

1 Danfoss KP15 (hi & lo pressure switch)
Low pressure sensor attached to low side (on the suction head), i assume

sensing suction pressure.
cut-in = 2-3 psig
differential = 18 psig

can someone why there is 2 lo pressure switches?
and what should the correct temperature/ pressure setting be?
why does the receiver empty sometimes on pumpdown?

gas= R22
set temp -23 C/ -10 F

thanks for any help,
bnhz

Hi,
Could you tell me where the liquid line solenoid valve is located, in relation to the other components in the liquid line?
Cheers Grizzly

[bnhz]

the liquid line solenoid is located within 5 ft (of liquid line) of the condensing unit, after the drier.

[nando]

a pressostat at the interstage, i never see this before. two pressostats, indeed, one for the pumpdown and one for safety. is the condensor outside, the receiver inside, and there is no return valva: when the condensor gets to cold cause the fans ar working, maybe no hp pressostats or something, the condensorpressure is lower than the receiverpressure, the ***** goes back from the receiver to the condensor

[sterl]

Is there an unload solenoid or even a bypass solenoid for the low stage cylinders?

I'm thinking the interstage pressure switch prevents the low stage cylinders from pumping to the high stage until the IS pressure has DROPPED to set point; so the thing doesn't overload its motor during re-start and pulldown.

Don't know about the Bitzers but other machines run that way.

[bnhz]

I am also waiting to hear from Bitzer about this situation. The only other thing that might help about the double low pressure switches is saying that these units are in Iraq and we have found some terrible mistakes on many pieces of equipment shipped here from nearby countries or installed by a locals.
this whole setup may be just another bad install. it would not surprise me.
things are just not done like they are the USA here.

[bnhz]

nando,
the receiver is outside with the condenser.
the only other solenoid in this system besides the pumpdown slnd is the subcooler and compressor cooling solenoid

[Sandro Baptista]

Hi bnhz,

This 2 stage BITZER compressors only accept a maximum corresponding suction pressure of -20ºC (for R22) and -25ºC (for R404A/R507).

Maybe the person who have installed the equipment thought that the crankcase is at the same LP pressure and try to protect against high suction pressure with a KP1 that would switch off the compressor by electrical commands.

Or maybe the person wants to protect against high intermediate pressure. But I think this makes sense if the cranckase is at the LP pressure and so the high pressure piston and other parts of them suffer high mechanical stress.

" Danfoss KP1; sensor goes to the oil fill hole, according to Bitzer diagrams" Can you show me the diagram? I think I've never see that.

About sometimes the receiver liquid is empty, since the condenser and receiver are ate same ambient temperature, an explanation could be the liquid goes almost all to the evaporators...and maybe the compressors are not be hammered with liquid because there is a good suction liquid separator...just an idea...however is strange because you would note the lack of liquid in the receiver in the first 5...10...15 minutes.