cooling system.

hi.
This isn't a plc related question,hop u will allow me to post it anyway. im using a chiller to cool juice in a pasteuriser.however,it has an evaporator,condenser and a compressor. it uses R407c as a refrigirant.my problem is everytime there is blackout,the system looses the refrigirant that it is now a routine to add more oftenly. i can't find any leakage in the system, what could be causing this problem?

Sounds like there has to be a leak somewhere if it is losing the refrigerant charge.

Does it only lose charge during a power outage, or is that only when they notice it?

I only noticed that is losses the refrigirant when there is power outage.The chiller has got 4 compressors which when all are on, there is effective cooling.there is a big cooling tower (fan at the top).when power goes off during operations, the refrigirant excess pressure relieve valve opens,and i it looses the refrigirant.this is what worries me! the equipment is from REFTECH REFRIGIRATION Denmark.

cheruiyot said:

....when power goes off during operations, the refrigirant excess pressure relieve valve opens,and i it looses the refrigirant.this is what worries me!

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This sounds like it is possibly the issue.

If there was a leak, then you would see loss of refrigerant during normal operation, and decreased ability to cool.

When you lose power, the refrigerant is likely heating up and expanding, causing excess pressure in your system. I would check your pressure relief valves to ensure they are working properly. If they are working correctly, then it is likely the pressure is exceeding the rated pressure on the relief valves. If this is undesirable, then perhaps you need to re-evaluate the pressure ratings. However if you are truly losing refrigerant through a safety relief valve, then they are working as intended! The whole point of a relief valve is to prevent excess pressure.

Perhaps consider venting your relief valves to a refrigerant recovery system?

Another thing to check is to ensure any non-safety valves are functioning properly on a power loss. Make sure that the fail safe positions are correct. If you have valves that are opening when they shouldn't be on power-loss, you could be potentially losing refrigerant.

...the equipment is from REFTECH REFRIGIRATION Denmark.

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This is most likely why the problem happened. In Denmark, the pressure relief valve could be set much lower, because the ambient temperature will be low enough to keep the pressure below that level when a power failure occurs.

In Nairobi, you are in a different ball game, and the ambient temperature is much higher, meaning that the pressure relief must be set higher (if possible), or some other solution must be used.

Definitely check that your relief valve is installed and set correctly, but to me it sounds like the system is operating as designed.

When you lose power, presumably the fan that move air over the refrigerant coils stops. Without the airflow you can't get the heat out of the refrigerant, so the pressure in the refrigerant loop increases to the point where the relief valve opens.

Is there a backup generator that comes on when the power goes out? If so, is the fan included in whatever circuits are powered by the generator? Does the fan need to be manually restarted following a power outage even if the generator starts and can power the fan?

There is a backup generator that comes on immediately after power outage.however, the fan at the coolling tower has to be switched on manually! the compressors comes on authomatically.

The first thing I would do is install a time delay that has the condensor fans run for at least a minute before starting the compressor. This will reduce the pressure in the condesor and reduce startig current on compressor. NEVER run compressor without the condensor fans. I would also run condensor fans for a couple minutes after compressor shuts down.

NEXT check with factory and see if reliefs can be set higher.

THEN check to see how refrigerant is shut off to thermal expansion valve. If a solenoid type ensure they are closing on loss of power. If not closing or leaking then thermal expansion will open to allow refrigerant to evaporator coil which then goes to the condensor where heated by sun ot create high pressure and lifting the reliefs.

cheruiyot said:

There is a backup generator that comes on immediately after power outage.however, the fan at the coolling tower has to be switched on manually! the compressors comes on authomatically.

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You say that the pressure relief only vents the refrigerant gas after the power failure, then restarts when the backup generator turns the compressor back on automatically, but fails to restart the cooling tower fan.

Now I see 2 things here that need remedied
1 The High Discharge Pressure Switch on the compressor, is either set to high, or has been bypassed, so that it fails to work at all, and stop the compressor before the safeties vent the gas, to lower the pressure to a safe level.
2 The fact the cooling tower fan is not running, I feel this should probably prevent the compressor from starting till the fan has started, especially at the air temperatures you will be seeing in Nairobi, that does the cooling of the gas hence lowers the gas pressures.

...the fan at the coolling tower has to be switched on manually! the compressors comes on authomatically.

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Big no-no! If the compressors are automatic, then the cooling tower fan should be, too! Do a little control-wiring change and you can fix this particular problem, although it may not completely solve the main problem.

There MUST be an pressostat on the discharge line of the compressor, when the pressure is too high the compressors should stop. Way before the safety is open. Do NOT fiddle the safeties.
The waterpump, and the pressostat, together with high temp discharge in a safety line.
Reset must be done manually at all times. Same as reset of compressor after blackout, must be manuual.
The start of the compressor by the suction pressure is alowwed to be automatic. it means when the suction pressure goes up th compressor starts.
some questions:
what is max operating pressure of the condenser, compressor.
what is the labeled pressure on the safety?
what is setting of high pressostat (often combined with the low one), it should be manually resettable.

when compressor stops the liquid valve to the evaporator should close.
how is system controlled, by temp?
does it have an expansionvalve

http://www.refrigeration-engineer.com/ is the better place for this question.

Way before the safety is open. Do NOT fiddle the safeties.

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Yes, if they worked correctly at one time. If not, then it may be time to contact the designer of this system and get some help. It may be that the safety relief pressure was set too low to begin with.