Chiller Control

Hi All,

Any body have experience of Chiller Control systems. I have to put an S7 plc on an old Trane unit. No documentation so any help would be appreciated.

Regards

Pouch

You will need to post more information...

What have you got exactly?

What do you want to do more precisely?

Hi there, neighbour.

What are you looking to do?

I've worked on a few Trane units... typically HVAC split systems running scrolls.

Will you be looking to program in some analogue... or let temp. controllers do the work and give you switched outputs?

Are you using speed control on evap./condenser fans?

Can you post schematics? Although you say you have no docs, can you offer any 'fag packet' sketches or photos?

Are you using S7-200?

Before attempting any chiller control, make sure you have a firm understanding of how everything in the refrigeration system works - not just what it is, but how it works.

For basic chiller control, your compressor run permissives are NOT_LOW_ PRESSURE and NOT_HIGH_PRESSURE and EVAPORATOR_FLOW_OK switches.

The compressor must be protected against short cycling. When a compressor is stopped the compressed refrigerant within the compressor crankcase boils back into gas. This creates quite a bit of foam in the compressor oil. When you restart the compressor, this oil foam blows out into the refrigeration lines. Therefore the compressor needs to run long enough to return the oil back to the compressor crankcase. For a chiller this typically takes only a couple of minutes. A typical compressor can short cycle only a couple of times before its out of oil. Shorty cycle protection usually incorporates both a minimum run time and a minimum off time between cycles. However, if the NOT_HIGH_PRESSURE permissive does not exist, the compressor should not run, regardless of a short cycle condition. High Pressure limit switches typically require a manual reset, but not always. If there isn't a manual reset in the pressure switch, then you may want to consider putting one in the PLC.

The best way to shut down a compressor when the chilled fluid is cold enough is to use a liquid line solenoid valve and low pressure shut off. Whenever the compressor is running the liquid line solenoid valve wll normally be open. But when you want to shut off the compressor, first close the liquid line solenoid and then run the compressor until the low pressure limit switch trips. This will remove the refrigerant from the evaporator and suction line and compress it all into the condenser. This will lower the load on the compressor when it restarts and also mitigate, but not eliminate, oil foam formation in the crankcase.

Smaller chillers usually have just a single compressor with on/off capacity control. On larger chillers capacity is typically regulated thru the use of multiple comrpessors, hot gas bypass valves, and when semi-hermitic compressors are used, with cylinder unloadoing valves. A hot gas bypass valve drops compressor capacity from 100% to 50%. An unloader valve also drops compressor capacity from 100% to 50%. The two in combination give you compressor capacities of 100%, 50%, and 25%. Copeland is now offering a scroll compressor with a proportional unloader valve, but I think they are the only one. I know of no compressor manufacturer who currently approves their compressors for VFD operation - but its been about two years since I last checked.

Do not forget to incorporate freezestat and low flow protection on the evaporators or you'll find yourself in for some expensive chiller repairs. Flow rate thru the evaporator must be at least 2.4 gallons/ton of capacity (I'll leave the UK conversions to you). If you don't have at least 2.4gpm/ton, you will freeze the evaporator (read damage the evaporator). Don't be tempted to put a drive on the evaporator pump. The 2.4gpm/ton minimum is dictated by the laws of physics - assuming you are using water. If you are using anything else, including water/glycol, then you have to account for the change in the specific heat of the fluid and raise the flow accordingly. For this reason I suggest you have two seperate pumps for evaporator feed and process feed, because the process flow may be less than the required evaporator flow.

If you are using a water cooled consenser instead of an air cooled consenser then I suggest putting flow swithes on those as well - though its not required. If you don't have flow on the condensers then the compressor pressure will increase until the high pressure limit switch shuts down the compressor.

Thanks for you replies,

I should be able to go to site today to trace out the existing wiring. Once I knock up a drawing I'll report back to see what we have.

Regards

Pouch.