Bitzer 4 comp rack, corel controls, blast freezer, HELP

[paul_h]

Hi, I've been sent to this unit which is a freezer for a seafood place.
First life is just a -20C freezer. It has a 4 compressor bitzer rack, carel master control of the compresors, carel electronic TX valve, carel master celler main thermostat. With the added bonus that if the users press a button it overides the thermostat and goes down to -30C.
The system design works, the reason I was called in was because it didn't actually get to -30C.
Anyway, there was a sizable leak found and repaired, and about 13kg of r507 added.
Now the sightglass is full, the pressure is about 70-80KPa, but it still short cycles on pressure rather than temp setpoint.
I'm a novice at these controllers, but the SP cutout for the corel controller is 50kpa, but it cuts out at 70kpa (not just by my gauges, but by it's own reckoning)
Is there any reason why it's going off when it hasn't reached it's own LP cutout when it's fault is LP too low?
I'm looking for people who have used a similar system (hopefully)
To me a freezer on R507 trying to get to -30 shouldn't trip out on 70kpa LP (-35C for that refrigerant)
Best fix?
Is it to set the LP cutout lower?
let more refrigerant through the electronic TX valve?
Either choice you have, please tell me how
edit: no other controls or valves on the evap or suction line, just liquid line solenoid, electronic TX, headers and compressors

[Grizzly]

Paul.
How is the suction pressure monitored. Is there a pressure transducer linked to the controller and could it be faulty. Also does the control stratagy rely on any temperature probes. 1 of these may be faulty.
Just a thought, it may help cheers Grizzly

[Peter_1]

It must cut off just before atmospheric pressure an on again at an evaporate pressure of – 32°C.
If you decide to let it cut-off at -35°C, then this will be probably to low because you then suppose that the evaporator works with a DT of 5K.
This 5K seems for me to low, better assume 7K to 10K.
You have to assume that you have to evaporate at -40°C for a freezer at -30°C. don’t forget lso that the compressor efficiency drops serious when you go that low.
You will finally reach a balancing point where the reduced compressor capacity will match the heat infiltration in the room.

What’s you suction temperature at the outlet of the evaporator? You can easily adapt the SH with the Carel controller. Try to maintain reach 5K.

[PaulZ]

Hi Paul_H
Contact Ian Skelton at Carel in Vic 03 9550 0898, he is very good and knows the Carel controllers backwards, most of our guys ring him when they have a problem and he can usually sort them out.
Paul

[Pooh]

Try removing the full charge and replacing it with new refrigerant, you will probably find that it will work. I have had the same problem with systems on R404A after a major leak, change the refrigerant and it works OK, speak to the fridge GODS and they say a leak wo'nt make a difference.

Ian

[paul_h]

The main reason why we use r507 is because it's not so bad after a leak. But maybe that's worth a try if it continues to play up.
I personally think the LP is just too low, the controller uses a transducer on the suction header to measure LP.
Like mentioned before, To get the room to -30, the refrigerant should be -40, which is 40Kpa. Then there's the pressure drop.
So to me, the freezer would never get to -30C if the LP cut out is set for 50Kpa ( -34C).
Anyway, I had a play with it and set the LP to 10 kpa, before I left it was running at -26C and 50kps suction pressure measured at the controller and my gauges.

[Toolman]

Hi Paul_H
Contact Ian Skelton at Carel in Vic 03 9550 0898, he is very good and knows the Carel controllers backwards,
Paul

PaulZ is right about Ian Skelton

You said you adjusted the LP to 10kpa , is that through rack control , is there a manual mechanical safety LP on this rack ?
. Electronic TX valves should be tied into the rack controller,
you said ( carel master cella main thermostat )
The Carel Rack controller would normally run the whole show and the carel thermostat would control the room temps by buttoning off the electronic TX valves ( like a solenoid ) .
A rack like that should not cycle on an LP but rather be staged to cycle each compressor off through the Carel LP transducer telling the rack control what the pressure is at and staging comps off or on .
And to me a Bitzer 4 comp rack wouldnt even notice 13kg it would have a reciever that would be 40 to 60kg ?
Bit like pissing in a pond 13kg into a rack

[paul_h]

PaulZ is right about Ian Skelton

You said you adjusted the LP to 10kpa , is that through rack control , is there a manual mechanical safety LP on this rack ?
. Electronic TX valves should be tied into the rack controller,
you said ( carel master cella main thermostat )
The Carel Rack controller would normally run the whole show and the carel thermostat would control the room temps by buttoning off the electronic TX valves ( like a solenoid ) .
A rack like that should not cycle on an LP but rather be staged to cycle each compressor off through the Carel LP transducer telling the rack control what the pressure is at and staging comps off or on .
And to me a Bitzer 4 comp rack wouldnt even notice 13kg it would have a reciever that would be 40 to 60kg ?
Bit like pissing in a pond 13kg into a rack

All the compressors have manual HP/LP as well, which aren't tripping.
The liquid line has it's own solenoid, which the master cellar switches on/off. As far as I know the master cellar doesn't do anything to the TX, the TX has it's own module which runs off the main corel controller.
As far as the refrigerant added, that was not the total charge, I omitted by mistake that someone, (that installed the damn thing), went there the day before and added refrigerant but couldn't find the leak.
I got sent there to leak test, even though I know nothing about the controller system, they know I'm pretty good at finding leaks
Anyway, I'm sure the charge is OK, because the pressures and temps are good, sightglass at the TX is full under full load.
Problem was it was cycling off at LP cutout from the controller. Then it was cycling off at HP cutout (1900kpa!) when the controller wasn't even running all the condenser fans.
I'm pretty sure that no one has even set this controller up properly. I'm pretty sure the guy in the shop said it's never got to -30C ever!
Once I learnt how to adjust the LP/HP cut outs to something reasonable for this system I haven't had any problems.
edit: I set the LP to 10kpa on the rack controller program

[Toolman]

The liquid line has it's own solenoid, which the master cellar switches on/off. As far as I know the master cellar doesn't do anything to the TX, the TX has it's own module which runs off the main corel controller.

Its sounds a bit odd they way its being controlled and setup , it might be OK as we usually use Danfoss on larger systems , call that guy from Carel we mentioned earlier - Ian Skelton in Vic 03 9550 0898 and tell him how its setup. I'm not sure how they control the Carel TX valves but the Danfoss equivalent can be shut off like a soleniod . I hope its out of warranty if not get away from it NOW !