Icing Evaporator

[brad72]

Hi,

We have a cold room approx 3mx3mx4.5m high running @ a set temp of 1-4 degrees Centigrade. The Evap unit is a twin fan Muller MLV100, running on R404a gas. The compressor is a copeland running 1 other room and a a freezer. The Room cycles in/out throught the control of a bulb thermostat, opening and closing a solenoid in the supply line.

The problem we are having is the evaporator coil continuously icing, which will cause flood back to the compressor, icing it up as well. I then have to isolate the solenoid for around 6 hours to allow the unit to defrost then switch it back on again. I have cleaned the evap coil, checked both fans for air flow, which seems low but not bad. the tx valve bulb is in place. Drier was replace about 6 months ago (the drier runs all 3 rooms). The evap may take a week to ice or it might take 4 hours after a manual defrost.

The evap coil is old and was desighned for the older gasses, not the R404a. There is no defrost fitted but the room has run fine like this for years. The door seals are in sound condition and no other room leaks can be sighted but being an old room there may well be.

My question is, Would a new evap coil fix the problem ($1500 fitted), or are there some other steps that can be taken to solve the problem eg. new tx valve?

[LRAC]

Hi brad72

before going any further you really do need to check the superheat settings, this may reveal a TXV fault but you wont know without doing more tests.

Kind regards
Lrac

[S.M.Gokhale]

Hi brad72
With a set room temperature of 1 to 4 Deg. C, the compressor evaporating temperature would be -4.5 Deg.C just before cut-off, if the coil was selected for 10 Deg. F T.D. This is likely to cause frosting. However if there was no frosting problem earlier, it could mean that the evap. was sized for lower than 10 Deg. F TD - which would mean a bigger evap.
This seems to be a case of system imbalance. Either the compressor has a larger capacity or the coil efficiency has reduced due to ageing and change of refrigerant.
In case you are having an open type belt driven compressor you could try reducing the compressor speed to reduce the compressor pumping which will improve the suction pressure and hence temperature.
The compressor suction temp. should be no lower than that equivalent to 26 Deg. F evaporating temp. I.e. about 64 PSIG.
All the best
S.M.Gokhale

[old gas bottle]

hi matey, how about mounting a small simple time clock on the back wall on the electrics side of the cooler inside a plastic adatable box,run a couple of cables into the cooler or wherever the solinoid is fed from and switch the solinoid with the defrost clock,try 4 x 30 mins for a start, its worth a try,should be o/k,

as theese systems get older they do get new problems,bit like me realy, go for that before making a meal of it, cheers.

[Pawel_Wisnik]

If following are met:
-superheating about 2....5 K => at evaporating temp. ~-7oC .... -4oC at steady run for
stabilized room temperature
-at such temperature case only 2 or 3 electric defrost cycles/24 h are resonable, defreost ended by evoprator temperature, defrost sensor placed as far as possible from defrost heater in air's flow direction
-TEV mounted properly, nozzle is selected for evaporator capacity, TEV bulb mounted at rising suction pipe just after evaporator discharge outlet
-each section of evaporator is equally fed with refrigerant
-capacity of compressor and evaporator are more or less equal
no icing should appear.

If not a hughe quantity of moister is driven into coldroom.

Brgpavel

[brad72]

Hi Guys,

Thanks for the responses. Will give them a try.

- We have thought of a defrost timer but because the cold room is used to hold pallets of egg pulp, we are a little concerned of excess temperature rise. I also assume that i would have to switch the fans off at the same time to stop water bing sprayed around the room?

-I will have to find a fridgy willing to check the superheat etc. Most just want to install a new evap.

-But, if we install a new evap unit and the problem is excess humidity in the room, will this new unit also suffer from the same problem. Is it worth me caulking all the exterior joins on the room?

Thanks
Brad

[Andy W]

Has it always been used for egg pulp storage, not used or designed originally for a different product, have repairs been carried out in the last few months, as it always been troublesome or only recently, so my understanding is that there are 2 high temp cold rooms and 1 freezer coldroom run off 1 copeland comprerssor, is the other high temp cold room running ok? Now finally, has the evaporator been pressure washed to clean and is the thermostat accurate, I am a big fan of replacing old stats and timers etc with an elliwell, at least you can see exactly what it is doing from outside ofthe room. Have you thought about fitting a temporary chart recorder in the room to confirm temperatures and when ice ups actually start. Is there any capacity control on the compressor.

[old gas bottle]

i still go by what i say, ALL cold rooms with a standard spec should have a defrost system,no you DONT stop the fans, just shut the solinoid down and run the fans continiously,theres a thought,they dont stop on the thermostat do they ? if they do alter that before doing anything else and try it, you wont blow water out,it should just drain away as normal unless the evap is blocked or theres a problem, try 20 min defrosts to start then,if your product is down to temp the defrost cycle will only raise the air temp a bit and not enough to bother the product . cheers.

[Renato RR]

Install electrical heater for defrost and some Caerel for defrost regulation (controls fan,el.heater,time of defrost and goes in defrost by temperature on coil).

Total cost for example:
Heaters 100$ Australian
Carel 150$ Australian

Best regards,
Renato

[brad72]

hi
Thanks again for more advice. A cheap defrost timer would certainly help the problem.


In reply to Andy, the room has been used for this purpose for 3 years now. I pressure cleaned the evap unit for the 1st time a couple of days ago. Had noticed alot of muck on the coil. All the fins are intact and straight. The other cold room goes fine, and never ices, even if the door is left open for 5 minutes. The frrezer @ -26 centigrade is also fine. The thermostat was replaced 2 months ago and the entire sysyem was regased 1 month ago as there was a big leak in 1 of the old driers, which was redundent and no has now been removed.

The fans however don't seem to put much air through the coil. Also, the fans are mounted at an angle, about 60 degrees. I assume poor air flow through the coil can also cause problems.

Thanks

[chillin out]

hi matey, how about mounting a small simple time clock on the back wall on the electrics side of the cooler inside a plastic adatable box,run a couple of cables into the cooler or wherever the solinoid is fed from and switch the solinoid with the defrost clock,try 4 x 30 mins for a start, its worth a try,should be o/k,

What O.G.B. is saying would fix the job.

Defrosting a coil for 30 mins with the fans running will not cause any temperture concerns at all.
So long as you are keeping the temperture above 1 deg c then there is no need for heater assisted defrost.

It would take about 10 minutes to fit a timer and put an end to your troubles.

Chillin

[brad72]

Well gentlemen , i think i have fixed the problem.

Fired the room up today after pressure cleaning the coil and the air flow is alot better. Also, after reading a another thread about snow in a blast freezer, i sat in the room for 15 minutes in the dark, waited for my eyes to adjust, and found....a 4 meter x 1/4 inch gap down 1 wall. sometime in the last week our forklift drivers must have hit the wall with pallets and pushed it out. Sealed it now. It would also appear that when the room was built, they skimped on mastic as the join that split had none!

A long story short, room is cycling great and practically no ice on the coil. I will still however fit a defrost timer to the unit as the room has planty of traffic throughout the day.

Thanks again for all your help. Better do the same test for the other room and freezer...-26 centigrade....bbbrrrrrrr

[Bones]

hey brad72,

good to see you sorted the fault with the gap

good idea to add timeclock to your control circuit should be able to pick one up for around $30 cant go wrong

[raviperumal]

Hi Matey
Why don't you try out the hot gas bypassing method to avoid the liquid slugging to the compressor.This will help you to maintain the superheat also.

[Gary]

Increase the differential on the new thermostat.

[Takeshi]

Glad its sorted - but make sure you fit a timeclock, I have never come across a cold room that has no defrosts, silly if you ask me.
I would have the fans switch off on defrost - so if when on defrost and the door is opened it does not draw any warm air in.

[Gary]

I would have the fans switch off on defrost - so if when on defrost and the door is opened it does not draw any warm air in.

The air blowing through the coil is what defrosts it.

[kkboy]

What about installing a evap pressure regulator to the cool rooms this will stop the icing up.

[Deniver45]

I would go with the easiest route first and do what Gary says = increase the differential time.

Increasing the time to when the system cuts back in will give the fan a longer time to defrost the evap coil.

[chillin out]

If this system is still icing up after 3 years then the original poster should be sacked...

19-01-2007, 04:25 AM

Chillin

[markie_022]

hi guys,,

i just want to know what is the cause of icing forming in accumalator? what are things to do? thanks.....