Evaporator and spider/distributor freezing

[Talylawdi]

Hi All

Was hoping some of you old wise engineers will help me with this one.

Carrying out a service in a large holding chiller today which has 3 off identical systems. Only two systems are running at present and it look's like it's been this way for some time as the motor is missing from the compressor and the condenser coil block is also missing from the redundant system. The required room temperature is 0c. Staff have told me the room used to be used as a low temp holding freezer, running at -18c.

The two running systems consist of

Evaporator - LU-VE CONTARDO - Model no. H1534 E4
Condensing unit - FRASCOLD - Model no. SA20-71
Compressor - FRASCOLD - Model no. V20-71
Running on R404a
T.E.V - DANFOSS TE12 with No.5 orifice
1'' 5/8 suction line
7/8 liquid line
Each system has it's own control panel

I Carried out a thorough inspection and clean of all equipment.

The equipment was struggling to achieve set point.

After defrosting both systems and carrying out a test run. To begin with, at the inlet of the distributor/spider there was frosting with a even frost traveling up each of the 18 1/4 lines which entered the face of the evaporator on the fan's side of the coil. Over a period of a couple of hours the Fan's side face of the coil frosted over, frost then worked it's way back through the coil eventually causing the suction line to frost back to the compressor. There was a accumulator in the suction line before the compressor inlet which was at one point frosted over also. The motor end of the compressor where the suction rotolock is situated on top of and the accumulator were the only rusty components on the condensing unit. The defrost interval was 4 hours, endurance of 60 minutes, stop temp of 50c, so i assume someone has been treating the symptoms and not the cause and the frosting issue has been happening for some time.

Evaporator coil and condensing coil were clean with all fans running correctly.

Both systems were behaving similarly

Tried to take all readings from system at the start of cycle when evap coil was clear of frost.

Suction pressure at evaporator - 3.4 Bar
Suction line temp at evaporator remained steady at 1c before any frosting started
Expansion valve outlet pressure - 5 Bar
Suction pressure at compressor - 2.5 Bar
Suction line temp at compressor remained stedy at 1c before any frosting started
Liquid receiver outlet pressure - 13 Bar
Condenser outlet temp - 23c
Discharge temp 60c
Liquid line sight glass clear
Evaporator Air On - 2.3c
Air Off - -1.5c
Condenser Air On - 10c
Air Off - 24c

As time went on as mentioned the evaporator frosted over, restricting air flow, preventing the liquid in the coil boiling off, the suction pressure dropped and liquid was returning to compressor.

Didn't make any adjustment to T.E.V as wanted to gather some more information on equipment first.

Both systems did eventually achieve set point. At that point before any defrost cycle the evaporator fans cleared the coil of frost and the condensate drain outlet was like a open water tap.

My question is when the evaporator was clear of frost at start, the super heat was around 10c which isn't low at evap outlet.

Why was there frosting through the distributor?

This seems to be the cause of the start of the problem. I pumped system over and disconnected pipe at inlet of distributor. Visual inspection showed no blockage, Blew through with nitrogen, also confirming no restriction. Re-connected everything and re-commissioned system.

I am now thinking there is some sort of balancing issue between cond unit and evap or possible t.e.v not sized correctly. Can't find any information on this condensing unit. As mentioned, staff have told me this was a holding freezer and it looks like this equipment was pulling temp down to -18c at the time.

Is it a low back pressure compressor?

Is it operating at a to high evaporating temp, possible outside it's envelope?

Is a 1.6 Bar pressure drop through the distributor and evap coil to high?

Sorry to be so long winded, but wanted to add as much detail as possible.

Have also attached a pic of T.E.V and side of evap at start of cycle.

Regards

[Kenneth199]

You can try to raise the condensing temperature to +40c.
Is 10c the superheat or is it what you measures after evap on suction line?

Have you checked if the fan stops during defrost?

[Talylawdi]

You can try to raise the condensing temperature to +40c.
Is 10c the superheat or is it what you measures after evap on suction line?

Have you checked if the fan stops during defrost?

Hi Kennth

Thanks for reply. Suction line temp was 1c. Gauge pressure/temp was around 3.5bar/-10c. Super heat was around 10/11c.

Yes evap fans stopped during defrost.

Condenser fan control was a ranco hp switch which was faulty. Will be fitting a new one shortly.

Regards

[Kenneth199]

How do you know that the compressor gets liquid back?
When the room reaches its temperature does the fans stop?
Did you start the room with high humidity/temperature?

What's in the room since it has to be +0c? they cant go +2c - +4c?

The defrost elements will generate a lot of heat if the stop temperature is +50c.
Is it possible to run the fans continously and disconnect the defrost elements?

[al]

Does the expansion valve have an MOP element since it was originally a freezer, if everything was sized for -18 then valve is going to be well over duty, as Kenneth says as a short term measure raise head pressure and see does valve react better, you may need to get the 3rd evap going if only to evenly circulate air in the room, you may not need the condensing unit. check system is not over charged.

[Talylawdi]

Kenneth,

At the start of cycle when evap was clear of frost, no liquid was returning to compressor as super heat was sufficient enough to prevent it returning. All readings taking were taking within 1/2 hour from cycle start. As time went on and evap had heavier frosting, air flow became restricted and super heat fell away to nothing, back pressure dropped and sight glass started to flash, suction line frosting back onto compressor body. How the last 2 compressors have lasted so long, I do not know.

When room reaches set point, condensing unit pumps over and evaporator fan's continue running. Stopping during defrost.

I haven't seen room temp above +5c

Haven't taken a humidity reading as yet.

Room is used to store around 30,000kg's of packed meat. Customer requires 1c.

I am not going to leave the defrost set up like this, it's dangerous the way it's been left. This is how i found systems set up. As mentioned, i think someone has been trying to keep evap's clear of frost.

I don't think it would be possible to disconnect heaters and run fans considering the way it's running at the moment as the fans would be spitting water everywhere.

Once i get to the bottom of the frosting issue i will be return any defrost operation back to the settings you'd usually expect to see.

Al,

Thank's for input also. T.E.V is not a MOP, but there is a crankcase pressure protection valve in the suction line.

As mentioned i will be returning to site shortly to fit a new condenser fan switch to raise head pressure and i will see how it react's.

The redundant system has been disconnected electrically also. Will see if i can get onsite sparks to run a new supply to 3rd evap, to improve air flow through room.

I still have had no joy finding any info about these condensing unit. As the room was previously low temp, i have a suspicion the compressor is operating outside it's envelope at the current T.E.V setting. If these compressors have a highest operating evaporating temperature of say -20c and someone has been opening these valves right up, it might explain the problem.

What do you think, Kenneth + Al?

[monkey spanners]

Had a ex lorry body freezer on R404A that a customer wanted to run as a 0/-2C room, switched it to R134a.

[Kenneth199]

The model number of the compressor doesnt tell you much.
v = 4 cylinder
20 = hp or kw
71 is displacement.

I would send the model number and serial number to the manufactor and ask them if the compressor has motor version 1 or 2.
I think motor version 1 is med high temp and motor version 2 is low temp.


The issue with the coil freezing up is most likely too high superheat or reduced airflow through evaporator.
If the superheat is too high it would start to freeze at evaporator inlet and build up snow/ice and you would get reduced airflow.

Im not familiar with the evaporator, but does the fans have 2 speed?

It's also possible to mount a capacity control for 50% and 100% run.

[Magoo]

reselect orifice sizes for new room temperature, and keep SDT at +35'C / 15 bar. You may get away with same distributor sizes. As per Kenneth, frosted tails indicates super heat issues, with low discharge pressure the valve cannot feed correctly, and TXV will hunt, so you get saturation back to compressor. Check also you have TES 12 valve and power assembly, not a TEX.
With chill load from fresh meat packs your loads could be higher requiring bigger TXVs and bigger compressors. The whole room load has to be re engineered, and probably needs third system to be repaired and back in operation.
magoo

[chemi-cool]

but there is a crankcase pressure protection valve in the suction line.

I think you you might have a problem there. Try to open it more and increase defrost pressure.

[mbc]

be sure about TES capacity with 5 orifice match to compressor capacity and put bulb sensor of TES to right position on suction line .