Pump down or not - Dual compressor system

[Electrocoolman]

Hi all,
Any suggestions welcome.

I'm in process of remoting a large twin compressor multideck, following burnout of one compressor.
Shop only has single phase power, so in order to get required refrigeration duty the unit (ARNEG) uses two separate refrigeration circuits with a delay start on second compressor.
Each circuit uses capillary control.
Normally when I remote units, I would opt for a pump-down system with solenoid valve etc., but my concerns are that with two remote condensing units (Silensys) on pump-down, they are possibly going to want to try to start at the same time at some stage, (Sods law!) and will trip the supply.

Fortunately the pipe run (10m) will be level for oil return, but how to avoid liquid floodback on startup without a solenoid valve?
I feel a S.V to be a necessity and different LP switch settings might work but I do not think it would be 100% foolproof in preventing dual startup.

The only way I can see is to devise two cross-linked delay timers and contactors. Has any one tried this idea before?

Adrian

[taz24]

Have you thought of just using one large remote compressor and conect the evaps together. Then you would be able to use a larger unit and pump it down.
taz

[Electrocoolman]

Hi Taz,
Cannot get a single phase unit with enough capacity to do that...ones into the realms of 3 phase only....hence x2 smaller units

Cheers Adrian.

[US Iceman]

You might try using the time delay relays with some
auxiliary interlocks and a pump down cycle.

On start-up, let only one compressor start and begin to pull down the suction pressure. After some time, the time delay would make and allow the second compressor to start IF the suction pressure was high enough.

In effect, you are locking out the second compressor until the suction pressure rises, or has been greater than the second LP switch cut-in setting.

It's sort of like a low oil pressure safety switch in reverse. The LP switches would have to have a staggered cut-in pressure to reduce overlap, or nuisance stop/starts.

[LRAC]

the easiest way we do it is the contactor on the first unit has an auxillary fiited which feeds a timer to pull the secound unit in, this must be in line with the pressure switches so both units pump down. Only 1 problem if first unit contactor fails then secound unit does not run but this should be picked up on a regular maintenance visit. very cheap and easy to make the above modification.

[bill1983]

are you going to convert to tev? if so solenoid valve fitting and lp control on both systems simple.Control circuit as on post from LRAC, but feed for 2nd unit through lp switch. contactor coil for 2nd unit fed from aux timer on 1st unit contactor. This way when 1st unit starts then timer begins countdown to pull in 2nd contactor. feed for 2nd unit through lp then if 2nd unit sog then compressor will not run even if contactor pulls in.

[monkey spanners]

Hi Adrian
SRW on 01273 455530 do a 12 way power sequencer to prevent simultanious start up of up to 12 systems
part n. 81306
I made one out of four IMO timers and two 8 pin relays (as latches) for a DX milk tank. Each of the two compressors had to start 6 minutes apart and not restart for 6 minutes, due to poor supply capacity. When the first ones timer switched on it reset the second ones timer. If the first one went off on lp or hp the second one would complete its countdown before the first one restarted , and on starting would reset the firsts timer to start its 6 minute countdown again. Can't remember how i wired it now:rolleyes: Each timer had four programing options to choose from a,b,c,d and i can't remember what i set them to:rolleyes:
Worked out just as expensive if not more with the *arting about to get it to work than the timer from SRW, but someone had said it wouldn't work and i had to prove them wrong:D

Cheers Jon

[winfred.dela]

Shop only has single phase power, so in order to get required refrigeration duty the unit (ARNEG) uses two separate refrigeration circuits with a delay start on second compressor.


What is the size of your compressor?
I have used a 5HP single phase in some of my projects.



but how to avoid liquid floodback on startup without a solenoid valve?


You may start the evaporator & condenser a little earlier (maybe 3-5 minutes) than the compressor to be able to assure that the liquid will all be gas.

Hope am right, +/- comments is just fine. :)

[Electrocoolman]

are you going to convert to tev? if so solenoid valve fitting and lp control on both systems simple.

Hi Bill,
No I was planning to leave the circuits on capillary control for now, and limit the charge, but fit solenoid valves.

I was thinking of using pressure switches as they are already fitted to the Silensys units, but I will combine it with delay timers. My concerns are during the defrost phase rather than cooling.

[taz24]

Hi Taz,
Cannot get a single phase unit with enough capacity to do that...ones into the realms of 3 phase only....hence x2 smaller units

Cheers Adrian.

single phase comps come in size's that would eat your fridge;)

If you go down the road of 2 comps just put a timer in the feed of the first comp and when No1 is running the No 2 gets power to timer and say 1 to 2 mins later the No2 comes on. They can then both be pump down. The number 2 comp will not get power until the timer tells it.

Cheers taz.

[old gas bottle]

loads of good advise as normal,just a couple of thoughts to consider,are we assuming its a off cycle defrost ? beacause if it has heaters no way you can use pumpdown with out a interlock in the compressor /heater circuit,the other thing is your starting load has been sorted in all the previuos comments but what about on a hot day when both units are flat out on max run amps,do you have the power capacity to cope?if your running high temp cooling on R407C or similar how about a inverter air con type unit, i have fitted a few of theese now on selected jobs with no probs whatsoever,your on cappilaries,fit a HP/LP combi for protection and away you go, just a thought,but if your on R134A forget it.

[Electrocoolman]

loads of good advise as normal,just a couple of thoughts to consider,are we assuming its a off cycle defrost ? beacause if it has heaters no way you can use pumpdown with out a interlock in the compressor /heater circuit,the other thing is your starting load has been sorted in all the previuos comments but what about on a hot day when both units are flat out on max run amps,do you have the power capacity to cope?if your running high temp cooling on R407C or similar how about a inverter air con type unit, i have fitted a few of theese now on selected jobs with no probs whatsoever,your on cappilaries,fit a HP/LP combi for protection and away you go, just a thought,but if your on R134A forget it.

Thanks OGB, I hadn't considered an inverter unit. Its currently on R404a (but I prefer to use R507). It is high temp (+3 dairy - so probably -5 evap) ; not sure about defrost at present - need to check. The unit copes power wise as is (with a 32A supply) running x2 ASPERA J9238GK's (duty around 5kw each), but as you say they are not on pumpdown when on defrost.

Out of intrest, what make inverter condensing unit do you use? and what control system?

Regards Adrian.

[old gas bottle]

hi matey, i started using ascon outdoor units because not all customers had a big budget and i could fit theese for the price of s/hand commecial condensing unit, there are no electronics on the out door units ,just a signal wire,so dead easy, the manurfacturer spec theese to match up with coolers for beer cellers so i thought i would give other aplications a bash,and all good upto date,even cold rooms, if using a big system i would recomend poping a bit of extra oil in first,after all there all starting to cross over now anyway.

[Electrocoolman]

Thanks for the info OGB.

Adrian

[bill1983]

re defrost cycle, arnegs in my experience do not require heater defrost (even if heaters fitted) on dairy application unless store is very well air conditioned and aisle temp is usually below 18c. i presume this is not the case due to reason for relocating unit.
def termination time needs to be max 40 minutes/min 15 term temp on coil 10c to be safe. fit cpr in case of high ambient summers or site unit on north facing aspect if possible, (but be prepared to clean off moss and algae regularly) also recommend you can lift unit off floor by 6 inches to prevent condensers from collecting rubbish.use oversize housings but ensure condenser baffled for max efficiency. some kind of stat/mag valve on cabinet any electronic controller with probe in air on or under middle shelf will do ok.:cool:

[Electrocoolman]

Bill1983,
many thanks for your input....very useful info

Cheers Adrian.