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chemi-cool
08-11-2016, 07:34 PM
In an air separation plant I have a large air drier with two compressors, one is bigger than the other.

The unit is controlled by Carel Pco5 and superheat controller. Each compressor has its own speed controller.

When the first compressor kicks in, it accelerates up to maximum speed and then the second compressor kicks in. At that point the pressure starts to decrease and the units trips on low pressure.

I am wondering about the sequence of operation regarding the speed of the compressors, should the first one slows when the second one kicks in and maintain the pressure or keep the first one at top speed?

The sequence of starting changes by 3 min resting time.
Refrigerant is 410A.
HGBP is present but I think it might be faulty.[Sporlan HGBE-8]

Any thoughts? Different ideas?

Brian_UK
08-11-2016, 08:41 PM
After the first has reached full speed I would expect the second one to start at low speed and then ramp up as required to take the load.

Grizzly
08-11-2016, 08:50 PM
Hi Chemi.
Most control systems I work on have the scenario where the lead comp ramps up to 100% or Above 80% on others.
Once the 2nd comp is called for the 1st drops off load and they both load together.
Normally there is also some form of Suction Ramp / Pressure limiting to stop the LP situation you describe.

So your asking the right questions and you are correct the load drops off the lead as the lag is started and they then load up slowly.
I assume there will be relevant settings within the Carel controller to help achieve this?
(Proportional / Integral control settings ect?)

Let us know how you get on?
I am sure there will be others more knowledgeable of the Carel PCo controller that can help.

Good Luck Grizzly

al
09-11-2016, 12:59 AM
If HGBP is faulty, you will get low pressure trip at start up, also look at LP delay time, may need to adjust upwards by 30 seconds. Bigger compressor starting and ramping up to 100% would make sense, unless load has dramatically fallen off since purchasing the machine i would leave as is. I would possibly look at keeping a compressor running full time, even if air is dry the HGBP would allow machine to keep ticking over until load comes on, the Atlas copco ones do this, biggest problem here is keeping head pressure up, not an issue with you i'd say!

Magoo
09-11-2016, 03:50 AM
Hi Chemi.
with no HGBP you cannot control the dew point condition and water separation, so system bombs out on low LP. definitely a controller issue or S/v problem.

chemi-cool
09-11-2016, 06:07 AM
Thanks everyone, I will try to get more info on the HGBP today.
Might have to replace it.
Will keep you informed of how we get on.