Control Requirements for a Refrigeration Unit with Screw Compressor

It depends on how big and complex the system is and what temerature it is running at. I have run ammonia systems that were driving blast freezers at -38 C. We controlled the slide valve on the screw to maintain a suction pressure equivalent to about -40 C. The liquid was evaporated into a 6000 litre vertical vessel . Level control was maintained by a couple of Danfoss magnetic level switches and a Danfoss solenoid valve. Liquid ammonia was pumped from the evaporation vessle to the coils in the blast freezers. Again a liquid level control maintained the level in the coils, and a constant back pressure valve maintained a reasonably constant -38. (Until they iced up) Defrost was by hot gas recirculation. On the other hand, you could just load and unload the slide valve on the screw compressor using the discharge temperature from the heat exchanger. Use the usual high and low temp & pressure switches for safety . I' ve never used a variable speed fan on the condensor ,just switched fans on and off to control head pressure. Most of the time in summer (here in Melbourne) they just run flat out all the time as its mostly hot. You can use a pressure transducer in the discharge from the compressor to controll the fan speed if you want.

There's bit in it, so you might want to get a consultant if its a big (expensive) system. Don't worry about flow rate measurement as it won't do much good to know the flow rates in the system and the equipment is fairly expensive for the amount of value you get from it.

Tom,

Thanks - that took me a lot closer then I have been before. I can see having in inlet pressure controller adjusting the loading and have the setpoint of that controller set by the required cooler process outlet temperature.

For the condenser, you said you turned on/off fans to control the head pressure. What pressure did you try to achieve? I noticed that for the compressor suction, you used the pressure equivalent to the target temperture or at least something close to it so you could get some differential to move heat. What was the basis for the condenser pressure target?

Is there any value in using the condenser outlet temperature in any of these control schemes? I can see that it could be useful in determining if there are noncondensibles in the refrigerant stream but I don't see any immediate uses in control.

Thanks again, Richard

There are accasions in cold weather,where the condensing pressure can fal to a point where there is insuffient pressure to get adequet liquid flow to the low pressure side of the system. By shutting off fans or lowering their speed ,you can increase the preasure to achieve adequete flow. The lower the head pressure that you can run, the better as it reduces electricity costs. In summer here, its always better to have a BIG condenser. If you have a hot humid day, with large refrigeration loads, you need all the condensing area you can get!

I can see that it could be useful in determining if there