View Full Version : Unloader pressure setting?

Fridgy Dave
20-04-2012, 08:21 AM
I was just wondering about cylinder unloaders for recip compressors for capacity control, setting them up if a Bitzer 4 cylinder compressor has one unloader and you have a certain cut in and cut out at which point do you unload half the compressor, is it when it almost gets to its cut out pressure or half way or is it all about running the system and getting it running smoothly with a constant back pressure graph?

20-04-2012, 10:31 AM
It depends on application and refrigerant,
Unloaders can operate on temp change, not necessarily on pressure drop.

The system requires head pressure control.

20-04-2012, 11:55 AM
Hi there. Think about the target suction pressure - usually it's a particular saturated temperature, say -10C for a medium temperature application. No point unloading the compressor before it gets to -10, but the idea is to unload the head/s slightly lower than target & load again slightly above target pressure. A good starting point is probably load compressor 2K above target & unload 2K below target. You touch on getting the system running smoothly - tweak settings to suit.

20-04-2012, 06:01 PM
Fridgy Dave.
I thought the unloaders were there to enable a part load start-up scenario.
Usually on a recip they load the cylinder banks up in a sequence.
With the comp starting on part load and once running other heads are brought on line.
There are systems which will then unload the individual heads as the control point is approached.
But they tend to have pretty clever controls.

Your point could well be correct? As to that on a Bitzer I will read up on what I have.

You are indeed correct Dave. I have a Bitzer bulletin on it and as soon as I can retrieve my mediafire password I will post it for you.

How you control is optional however there are a number of considerations required. One is a maximum of 8 compressor stops in an hour.

Also the minimum runtime allowed based upon the motor size.

20-04-2012, 07:00 PM
Hi Fridgy
In my opinion you can never have a constant suction pressure and linear with a choking in steps of 50%.
so if the power adjustment is performed with the suction pressure must calibrate the differential above the band of oscillation.

21-04-2012, 12:30 AM
On a small system like Dave is describing, I think its for balancing compressor to load.
It should be applied when yo have say 2 cooling spaces or evaporators & 1 cycles off on t/stat or turned off for defrost etc .
The suction pressure then does'nt go to low causing icing of evaporator,vacuum on suction side, keeps compressor in its range etc.
As others have mentioned keep suction pressure close to target suction pressure.
On larger systems it can be used for staging unloaders on the way up & down to keep system stable.

750 Valve
23-04-2012, 02:33 AM
Dave it would be setup like the following example

dairy cases rated at -7SST

LP cut in would be equal to the warmest discharge air temp you want to see, in this case +1 degrees C
LP cutout would be equal to the design SST minus a small margin, in this case -9SST
Unloader would be set to unload to keep design SST, in this case unload at -8SST with minimum differential if its mechanical with timer set at 1 or 2 minutes, if its controlled by the E2 or Presscon then back in at -6SST

The idea is to keep the unit running and not cycle on and off, each system will need small tweaks as some have had cases removed over the years and will be oversized anyway. You also want to keep the loads running constantly, in a single system scenario most won't have solenoids on single systems anyway, just EPRs where needed. Cycle off for defrost obviously.

You will never get totally smooth operation when you have design for +40 ambients during summer and run down to 0 degree ambients during winter, then have only 1 stage of unloading. You don't want the unloaders banging in and out, this is where the unloader timer comes into play, it should unload when needed then load back up only when unloader pressure control is made and has timed out. More importantly you don't want the unit short cycling, if you had to increase the differential between LP cut in and cut out to stop an oversized unit short cycling then go for it.