Booster system - the low temperature compressors aren't compressing a 100%

[marcelo.braga8]

Dear friends, I need a help to solve a problem in a Booster system. First of all, sorry about my english.

We have a booster system with R-507A started up about a mouth, and the medium temperature system is working good, but the low temperature system is not good, the compressor apparently is not compressing 100%, the normal eletric corrent a 100% for this compressor is about 75 A, and it works about 50 A.
This is a Bitzer compressor model HSKB85 and it has 4 solenoid valve, for capacity control. We are using 3 valves, start and 25%, 50% and 100%.
We have 3 compressors in the medium and 3 compressor in the low system.
The oil pressure arrive in the compressor at 50 psi, its enough to drive the slider valve.

The condenser temperature is about 37 C (238 psi)
Low temperature operating - 40 psi - 50 A (but I need to arrive at 15 psi)
Medium temperature operating - 65 psi - 130 A

The oil temperature is about 15 C.

Obs: to put the low temperature system in a load capacity, I need to turn of 1 ou 2 medium temp compressors to raise the electric current on the low compressor until 80 A, and when I turn on those medium compressors again, the system maintain just for 30 minutes to 1 hour than the capacity (low temp) go down again.

I dont know what to do to put the system in a normal operating load.

If someone help me, I will be very grateful.

Thanks

Marcelo

[PaulZ]

Hi Marcello
Sounds like the Low Temp compressors are not loading to 100%.
You say you are using 3 out of 4 solenoid valves, I have attached a link for the operation of a HSK85, not sure if you hav ethe same instructions.
https://www.bitzer.de/documentation/sb-110-2.pdf
Regards
Paul

[Magoo]

Agree with Paulz., plus oil temp is too low at 15 ' C. Bitzer usually insist on high oil temperature oil entering the screw. Do you have an intercooler vessel, or direct injection de super heating to low stage discharge gas.

[marcelo.braga8]

Hi Paul, thanks for your answer.
We are using 3 solenoid valves, the CAP 75% (CR1) solenoid valve we aren't using. But the solenoid valves CR2 and CR3 are de-energized, just CR4 is 10 sec on/10 sec off...I think its ok, but even though the compressors aren't operation to 100%. Maybe because the oil temperature? about 15 to 25 C

Regards
Marcelo

[marcelo.braga8]

Hi Magoo, thanks for your answer
The oil temperature is about 15 to 25 C, do you think need to be higher?
We have a coil on the oil low temperature line, it means, between the oil separator and the compressor we have the pump and the coil (heat exchanger).

Thanks
Marcelo

[PaulZ]

Hi Marcelo
I agree with Magoo about the oil temp being too low. In the operating instructions it states the max oil temp is 90oC and the oil temp should be 15 - 20 K above ambient. I would guess your oil temp should be 45oC plus.
Has the internal oil filter been changed?
What is you discharge temp?
What type of oil cooler do you have?
In the instructions is says if the internal relief valve is leaking capacity will be affected, has the internal relief valve been checked?
Regards
Paul

[Magoo]

Hi
The Bitzer screws use blocked port and not slides for capacity regulation so you need to get loading sequence correct.

[RANGER1]

Hi
The Bitzer screws use blocked port and not slides for capacity regulation so you need to get loading sequence correct.

Magoo I think with some Bitzer you can use full slide valve operation or 25,50,75,100.
Some models just ports as you say.

[marcelo.braga8]

[QUOTE=PaulZ;304472]Hi Marcelo
I agree with Magoo about the oil temp being too low. In the operating instructions it states the max oil temp is 90oC and the oil temp should be 15 - 20 K above ambient. I would guess your oil temp should be 45oC plus.
Has the internal oil filter been changed? Yes, its ok
What is you discharge temp? 30 C
What type of oil cooler do you have? Air heat exchanger
In the instructions is says if the internal relief valve is leaking capacity will be affected, has the internal relief valve been checked? Yes, checked
Regards
Paul

Hi Paul,
I answered your questions above.
I think Bitzer put this recomendation 15 - 20C above ambient just on the start up.
This is a booster system and the low discharge temperature project is 21,5C.
I was thinking that the problem could be the oil pressure arraving on the compressor is not enought to drive the slider volve (50 psi) and the superheater suction line is too low and could be arriving liquid on the compressor. I need to check.

Thanks a lot
Marcelo

[PaulZ]

Hi Marcelo
Are you having the problem with all 3 LT compressors or just 1 of them?
You said you are running 40 psi LT suction when it should be 15 psi, how many LT compressors are running when it is at 40 psi?
Can you post a drawing of the plant?
From your original post.
Obs: to put the low temperature system in a load capacity, I need to turn of 1 ou 2 medium temp compressors to raise the electric current on the low compressor until 80 A, and when I turn on those medium compressors again, the system maintain just for 30 minutes to 1 hour than the capacity (low temp) go down again.
Am I right in assuming that when you turn off 2 of the medium temp compressors the LT compressor starts to work properly and the suction drops to 15 psi or there about and then when you turn them back on the LT suction goes up?
Can you give all the pressures of both systems when all 3 MT compressors are running and how many LT compressors are running.
Regards
Paul