Help to solve problem on York chiller

hello all - i am looking for advise / help to solve a naging problem i have on the york chiller model YCAS0373SC50XFAS with two system each with DXS24LASB46/50 compressors.
Couple of days back i noticed that system 1 compressors was not perfoming as required with suction pressure isolating between 70 - 100psi.
A comprehansive service was carried out on the system - changed the oil, oil filters and core dryers. flushed the complete system with nitrogen and vaccumed the system as required.
fresh refrigerant R22 was added - approximately 60kg and the unit was started. the unit came On but the suction pressure reading was high - 90psi where as the discharge pressure looks good 200psi. The gas requirment according to name plate is 70 kg - i.e we haven't overfilled the unit and the sight glass does not show its full.
Suspecting maybe the expansion valve is bad, i repalced it with new one but still the suction pressure remains high - 80/90 psi causing the unit not to perform at all - the chilled water supply and return difference is only one degree. The discharge pressure looks ok - 200/250psi.
The other thing i noticed is that when the compressor is pumped down for shut down, the suction pressure goes down to 30psi due closure of the liquid line solenoid and compressor shuts down. Once compressor is shut down normally the suction pressure risres up and discharge pressure goes down - to equilise but in our case the suction pressure remains around 30/40 psi and only after couple of hours the pressure equilise.
I don't understand what is causing this problem of high suction pressure - i am ready to listen to your instructions to solve the problem. Thank you.

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Originally Posted by nassersh

hello all - i am looking for advise / help to solve a naging problem i have on the york chiller model YCAS0373SC50XFAS with two system each with DXS24LASB46/50 compressors.
Couple of days back i noticed that system 1 compressors was not perfoming as required with suction pressure isolating between 70 - 100psi.
A comprehansive service was carried out on the system - changed the oil, oil filters and core dryers. flushed the complete system with nitrogen and vaccumed the system as required.
fresh refrigerant R22 was added - approximately 60kg and the unit was started. the unit came On but the suction pressure reading was high - 90psi where as the discharge pressure looks good 200psi. The gas requirment according to name plate is 70 kg - i.e we haven't overfilled the unit and the sight glass does not show its full.
Suspecting maybe the expansion valve is bad, i repalced it with new one but still the suction pressure remains high - 80/90 psi causing the unit not to perform at all - the chilled water supply and return difference is only one degree. The discharge pressure looks ok - 200/250psi.
The other thing i noticed is that when the compressor is pumped down for shut down, the suction pressure goes down to 30psi due closure of the liquid line solenoid and compressor shuts down. Once compressor is shut down normally the suction pressure risres up and discharge pressure goes down - to equilise but in our case the suction pressure remains around 30/40 psi and only after couple of hours the pressure equilise.
I don't understand what is causing this problem of high suction pressure - i am ready to listen to your instructions to solve the problem. Thank you.

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Welcome.
When you mention the Suction Pressure. What is the corresponding Chilled Water Temperature.
80-90PSI is relevant to a evap (Chilled Water Temp )of say 10 - 12c.
Secondly the sight Glass flashing is irrelevant unless the system is fully loaded.
There are lots of options as to why you are seeing these symptoms.
Firstly is the Compressor loading up.
Whats the superheat, Subcooling etc etc.
All this info is available from the Millennium panel.
Also if the stated charge is 70 kg why charge to 60kg???

If you could give some more info then we may be able to help.
Cheers Grizzly

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Originally Posted by Grizzly

Welcome.
When you mention the Suction Pressure. What is the corresponding Chilled Water Temperature.
80-90PSI is relevant to a evap (Chilled Water Temp )of say 10 - 12c.
Secondly the sight Glass flashing is irrelevant unless the system is fully loaded.
There are lots of options as to why you are seeing these symptoms.
Firstly is the Compressor loading up.
Whats the superheat, Subcooling etc etc.
All this info is available from the Millennium panel.
Also if the stated charge is 70 kg why charge to 60kg???

If you could give some more info then we may be able to help.
Cheers Grizzly

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Hello Grizzly,

hereare some of the data to guide you. The suction pressure at 80psi with corresponding chilled water temperature supply 17.6deg C and return water temperature 18.3deg C.
The millennium panel indicates the compressor is fully loaded with all 75 steps IN. motor current 62Amps 50% loaded.
The superheat calculation is - suction pressure is 80psi converted to 10 deg C on the chart for R22. the suction line temperature measured is 12deg C. Therefore the superheat is 10 - 12 comes to -2 which does not look good:mad:. Subcooling - HP reading 230psi converted to 44.4 deg C using R22 chart. The lquid temperature measured is 38degC. therefore the subcooling is 44.4-38 = 6.4DegC which is not bad:(

note that we have only charged the system to about 50%. i am afraid to further charge the system because the suction pressure goes on increasing and may cause damage.

What next i can do - please advise.

thanks Grizzly for the support:)

Hi, nassersh!
I had a similar problem. The compressor has 4 stages of loadings. It looks like your compressor works on the 1st stage and cant go up to the 2,3 and 4 stages. So the suction pressure remains high and the discharge pressure looks good because of the condenser fans control system. The capacity steps are available from the Millennium panel. But they are steps the controller WONTS the compressor to work at. No feedback! You must check compressor working current. If your compressor REALLY goes up to the high step of loading, the working current will increase from its value on the first step to its value on the 4th step.
When compressor works on the first step of loading for the long time, the oil temperature and the discharge temperature may rise up to 90-100C because of the discharge gas bypassing.

check soleniod coils for loading valves these burn out quite regular which will stop compressor loading

Suction Pressure is way to low.
Try charging the system to it's correct charge.
Then let us know what is happening.
Grizzly

I would suspect faulty suction valves , you may need to pull the heads

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Originally Posted by wambat

I would suspect faulty suction valves , you may need to pull the heads

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Sorry Wombat.
I think you are correctly diagnosing what would be the likley cause if these were Recip's.
DXS Compressors are Screws, as far as I can tell basically there may be loading problems ect.
But untill the system is charged correctly everyone is wasting their time!
Cheers GRIZZLY;)