maintain chilling temperature at 10 deg C

Dear All,
How can we maintain a room temperature at 10 deg C.The Chiller system having 5 nos of 22.5kw compressors and 2 nos of 18.5 kw compressors.Present system is very old and maintained by only timers. 5 hours run and 30 min off.

Is there any logical procedure that by installing RTD and some controller like PLC we can maintain the temperature very acurately at 10 deg C.

Thanks ,

You have a couple of choices. If you use a temperature sensor, RTD or other, you can provide simple thermostat control logic in the PLC that switches the compressor on at a set high temperature and off at a set low temperature. You can make the deadband between the two temperatures as tight as possible, but you want to avoid short cycling the compressor. You probably want to add a time delay for on operation so the temperature has to stay low for several seconds before starting the compressor.

If you have a way to vary compressor output, say recycle or variable speed, then you could use a PID loop to continuously modulate the compressor to maintain temperature.

At the risk of seeming pedantic, I'm going to expound more on the compressor short cycling that Tom mentioned.

When a refrigeration compressor is stopped any refrigerant in the compressor case boils. This turns some of the oil in the compressor into foam. When the compressor is turned back on this foam is blown down line along with refrigerant. Most compressor systems are designed to tolerate this and have extra oil. But the compressor must be maintained on for a long enough period of time for the refrigerant flow to entrain that oil and drag it back to the compressor, usually several minutes for a compressor that is close to the evaporator, longer if the compressor is some distance away from the evaporator (condenser location also matters but usually the condenser is with the compressor). What happens if the compressor is short cycled is that the oil does not have time to return to the compressor and after a couple of short cycles all of the oil ends up in the refrigeration lines instead of in the compressor where it is needed. Typical short cycle protection schemes involve both a minimum compressor off time and a minimum compressor run time. Unless you are a refrigeration engineer or have access to one, then I wouldn't change the times. I'm not saying to not improve your controls, but to not change any time values that are used for short cycle protection.

I don't know of any variable speed compressors. For some reason that I don't understand the refrigeration industry has not adopted VFDs on compressors (maybe someone else could explain why). Copeland makes a scroll compressor that has a proportional valve that controls scroll engagement providing some compressor capacity control. Semi-hermitic compressors have a cylinder unlaoding valve (on/off) to cut compressor output in half. On hermitic compressors hot gas bypass valves can be used to cut cooling capacity by 50% but you don't get a corresponding efficiency gain. Most capacity control schemes involve a combination of cylinder unloader valves or hot gas bypass valves and switching on/off compressor groups in a bank of compressors.

Apel said:

Dear All,
How can we maintain a room temperature at 10 deg C.The Chiller system having 5 nos of 22.5kw compressors and 2 nos of 18.5 kw compressors.Present system is very old and maintained by only timers. 5 hours run and 30 min off.

Is there any logical procedure that by installing RTD and some controller like PLC we can maintain the temperature very acurately at 10 deg C.

Thanks ,

Click to expand...

When using an RTD in a room, ensure there is adequate movement of air to prevent thermal layering. Cold air sinks, hot air rises.

If the RTD is placed in an area without proper mixture of air, the RTD will sense the temperature of that particular loaction, and not the actual temperature of the rest of the room.

Alaric is right you must cycle min times, there are many types of systems out there you need to ensure the min on/off times as long as this is shorter than the required on times of the compressors then you could use temperature control via an rtd.

Variable speed control of screw compressors is becoming popular because these machines are horribly inefficient when the slide valves are part closed. It is far more efficient to run the machine slower with the valve fully open. I have never seen a VFD used on a reciprocating compressor so I cannot comment on these. If you consider using VFD's then you will need to consider the effect of running the machine at reduced speed on lubrication. As Alaric has inferred above managing lube oil is a major part of controlling these machines.
Andybr

Dear All,

Thank you all.I understand the application better from you all.We planned to use 6 RTD for one Room and one DO module make:Advantech which support modbus.We want it to control from Wander Ware SCADA script.

Thank you all.
Bye

Andybr said:

Variable speed control of screw compressors is becoming popular because these machines are horribly inefficient when the slide valves are part closed. It is far more efficient to run the machine slower with the valve fully open. I have never seen a VFD used on a reciprocating compressor so I cannot comment on these. If you consider using VFD's then you will need to consider the effect of running the machine at reduced speed on lubrication. As Alaric has inferred above managing lube oil is a major part of controlling these machines.
Andybr

Click to expand...

I googled around a bit and found some posts on a refrigeration forum about running scroll compressors on VFDs. Apparently you can. Speed ranges from 35hz through about 65hz. I assume the lower bound is for lubrication reasons. You need to de-rate the drive as well, one poster recommended sizing the drive at 160%.

Also I found an article that mentioned using a drive on a semi-hermetic compressor with a secondary lube pump. The article gave no details, just mentioned it.

I've done several chiller controls working in conjunction with a refrigeration expert which means I don't know squat about the refrigeration itself, just enough to be dangerous - so I recommend checking with a genuine rigeration engineer and the compressor manufacturer before attempting to put a drive on a compressor.

However, considering that the article mentioned above claimed that 30% of total US eneergy consumption is in refrigeration, it seems that this is one industry that could benefit from the application of drive technology and designing compressors specifically for drives.