aaaaaaaaaaaaaaaah

[ciarnoh]

copeland condensing unit,-20 freezer room,gas 404.
problem is the unit wont change over onto defrost.
Controller lights up defrost light,de-energises solinoid
to pump down and cut out pressure switch which kicks out
comp contactor pulling in defrost heaters.
problem is when the unit pumps down its high side pressure
drops instead of building and kicking out on high pressure.
any ideas???thanks

[sedgy]

hi ciarnoh,
you could try adjusting the h-p swich settings, to test if the unit goes onto de-frost after unit trips out on its L-p switch,? sedgy,

[monkey spanners]

I think what you are saying is when it pumps down to defrost it trips the high pressure switch before its finished pumping down?

If so, it sounds like it is either overcharged or its very near being overcharged and the condenser is dirty.

The system either needs some charge removing, condenser cleaning or a bigger reciever.

What pressure does the hp switch trip at?

[ciarnoh]

hi ciarnoh,
you could try adjusting the h-p swich settings, to test if the unit goes onto de-frost after unit trips out on its L-p switch,? sedgy,


hi and thanks.
just to make sure i understand
do you mean swap it so defrost happens when switch trips on
low pressure rather than high pressure??

[ciarnoh]

hi monkey spanners
sorry i didnt explain very well.
when solinoid closes suction goes in to a vacuum and highside
drops aswell to about 130psi instead of building and tripping h.p.

[Tayters]

If the senario is as Monkey suggests then maybe you had a faulty LP switch causing system to run in a vacuum, causing air to be sucked in where there was a small leak. Now there are non condensibles in the system causing a HP trip before the LP switch does the business.

There again, I could be wrong...

Cheers,

Andy.

Edit: The last post came up as I was being creative. At least I got the vacuum bit right. If the HP switch is cutting out at 130psi then I'd check it's setting/calibration. Same goes for the LP switch.

[monkey spanners]

Aha! Now it sounds like the lp switch doesn't switch, if so check the scale to see what its set to (it could have been wrongly set, as they should turn the unit off before it reaches a vacuum)
If it looks like the settings are ok but its still not switching, check the wiring is correct (it may be switching but is linked out or connected wrong) also check the capilary is ok and not been brazed up to stop a leak for example.

If its set ok but still don't work then try adjusting it to see if it works now, if it does it would still be worth changing it as from experience, once you need to adjust them when they have been working ok in the past, they only get worse with time.

Jon :)

[mikeref]

Little confusing here!! Compressor should not go out on H.P. ever. Pump down systems will switch off on L.P. Compressor and all fans switch off when defrost heaters are switched on. Now i've lost the plot:rolleyes:... going to re-read.

[ciarnoh]

also ice build up on bottom of comp (scroll) only

[monkey spanners]

also ice build up on bottom of comp (scroll) only

If it's not defrosting, once the evaporator is frozen the load and suction temperature will drop and the risk of liquid reaching the compressor or just excessive frosting increases. Sounds more like a symptom than a cause of the problem.

[Toolman]

Rewire so it doesnt pump down at all , when it goes into defrost it just drops the compressor out and starts the heaters , then the refrigerant will sit in the coil and help transfer the heat through the evaporator and clear the ice quicker that's what refrigerant does it transfers heat ( I assume your have a suction accumulator as you need one for start up and normal low temp flood back ) , I NEVER use solenoids and pump down on any single evap system , use the HP/LP for safety only , yes check that it trips at the correct point for HP & LP

[Tayters]

So the plot thickens then.

If the evaporator is iced up then compressor should be icing up around the suction port (which it might do under normal conditions anyway) and around that part of the compressor onwards.
If there is just ice around the compressor base then my guess is it's down to a faulty crankcase heater.

Looks like the old girl has a few snags to iron out!

Cheers,

Andy.

[Tayters]

Rewire so it doesnt pump down at all , when it goes into defrost it just drops the compressor out and starts the heaters , then the refrigerant will sit in the coil and help transfer the heat through the evaporator and clear the ice quicker that's what refrigerant does it transfers heat ( I assume your have a suction accumulator as you need one for start up and normal low temp flood back ) , I NEVER use solenoids and pump down on any single evap system , use the HP/LP for safety only , yes check that it trips at the correct point for HP & LP


Interesting point but if I could enter into some technical debate here I see it another way.
Under defrost (electric) the heat is transfered by conduction throught the evap coil and some convection but not much in the immediate areas. If you introduce someting eles to heat up - i.e the refrigerant, then this wont help conduct heat around the coil as the metal does this very well. All you do is use energy to heat up the refrigerant. Then you need to get rid of this excess heat/load in the refrigerant when the system is restarted.

Agreed that for small system then it can make the setup simpler in some respect.
And doubley agreed it is vital for a hot gas defrost system.:D

Cheers,

Andy.

[Toolman]

Interesting point but if I could enter into some technical debate here I see it another way.
Under defrost (electric) the heat is transfered by conduction throught the evap coil and some convection but not much in the immediate areas. If you introduce someting eles to heat up - i.e the refrigerant, then this wont help conduct heat around the coil as the metal does this very well. All you do is use energy to heat up the refrigerant. Then you need to get rid of this excess heat/load in the refrigerant when the system is restarted.

Agreed that for small system then it can make the setup simpler in some respect.
And doubley agreed it is vital for a hot gas defrost system.:D

Cheers,

Andy.

OK Good end of story . I've seen these threads before on pump down or not and everyone has there opinion but not pumping down during defrost is the right one , you seem to understand that so get your head around the rewire and do it .
I have stumbled across "pump down systems" over the years and apart from the no gas in the coil to assist with the defrost I have found the system being held off due to the LP and solenoid not working together ,
GET RID OF THE SOLENOID its just one less thing to have fail at 2am when your sleeping :p

[Tradewinds]

The solenoid is used to pump the system down and remove liquid in the evaporator, but it also reduces the load on start up of the compressor. By having minimal refrigerant in the suction line, will reduce the start up amps and protect the life of the compressor. If you check the amp draw when you pump down compared to just switching the compressor off, you will find it has much less current.

Is it possible that your solenoid valve is leaking internally and letting liquid refrigerant into the evaporator during the off cycle?

Try pumping the unit down and see what happens to your suction pressure. Does it stay at the cutout point or rise up? If it goes up then you either have a leaking solenoid valve or faulty discharge reed valves.

[paul_h]

The solenoid is used to pump the system down and remove liquid in the evaporator, but it also reduces the load on start up of the compressor. By having minimal refrigerant in the suction line, will reduce the start up amps and protect the life of the compressor. If you check the amp draw when you pump down compared to just switching the compressor off, you will find it has much less current.
That's what CPR valve is for.

OP hasn't explained the system, or maybe I missed a bit.
Electric heaters or hot gas bypass? If electric heaters why is the compressor still running to trip out on HP?
The sentence "unit pumps down its high side pressure
drops instead of building and kicking out on high pressure." isn't making any sense to me so I need to know what the system has as far as controls, and what method it uses to defrost and for the thread starter to clarify that statement.

[mikeref]

copeland condensing unit,-20 freezer room,gas 404.
problem is the unit wont change over onto defrost.
Controller lights up defrost light,de-energises solinoid
to pump down and cut out pressure switch which kicks out
comp contactor pulling in defrost heaters.
problem is when the unit pumps down its high side pressure
drops instead of building and kicking out on high pressure.
any ideas???thanksThis is post #1 again, so i don't have to get RSI. Electric defrost yes, and why do you want high pressure to build up? The way i see it.. High side pressure drops because refrigerant load falls as solenoid is closed to allow LP to switch off control circuit for compressor and condenser fan.. Mike.

[mad fridgie]

This is confusing, unless there is a loss of charge pressure switch in the discharge line.
Toolman is partially correct defrosting is improved with a charged evaporator (non pumpdown) due to a more even distribution of energy, but you do need to have liquid solenoid valve to stop excess refrigerant migration, unless you suction acc, can hold 100% of the system charge.
Minimum starting current on the comp occurs when you have no pressure difference between suction and discharge pressure.

[monkey spanners]

Last Searle evap i fitted recommended pumpdown on defrosting to prevent uneven melting.

http://www.searle.co.uk/SiteCollectionDocuments/Searle%20NS%20Manual.pdf

[mad fridgie]

Last Searle evap i fitted recommended pumpdown on defrosting to prevent uneven melting.

http://www.searle.co.uk/SiteCollectionDocuments/Searle%20NS%20Manual.pdf

I may not agree with them, but you should do what they recommend or may loose your warranty.
Normally leaving refrigerant in the evap helps keep the return bends clear and reduces hot spots within the coil block itself. ( slightly less steam coming of the coil)