Evaporator Freeze-up

[koosjr]

Good morning All!

I hope anyone could help me with ideas as I have a problem that by now have me completely dumbstruck.

About three years ago, I designed a Biltong Drier for commercial biltong producers the South Africa. (Biltong is similar to jerky - dried meat.)

It is based on a R22 refrigerant system. The working of the system is pretty simple. The room temperature is kept at a constant 18⁰C and the compressor is running 24/7. The inside unit have two coils, an evaporator coil that permanently stays cold and a heat reclaim coil for reheating.

I have absolutely no problem with the units in the field and my clients are so happy with the product that some of them already have four units on their premises.

The system is designed such that at 18⁰C and 45% RH, the suction pressure runs at around 420 kPa. (At higher humidity the suction pressure rises a bit) I did had problems with my single phase units that ran as low as 360 kPa suction pressure (because of the fan that did not give the specified airflow) and even with them I never ever had problems with freezing up of coils. I do get problems with anything lower than 360 kPa.

However, I have a client with two units, both three phase.

The one unit have a freeze-up problem and I just cannot solve the problem. The suction pressure is running nicely at 420 kPa. Sometimes the unit is running for weeks with no problem and then suddenly the coil start freezing up and the unit cuts out on it's defrost cycle. Is does however seem to be a more regular occurrence lately and it easily happens two or three times day.

I tried absolutely everything. I replaced the controller in case it was responsible for a drop in room temperature for long periods. I replaced the expansion valve in case it was sticky. The coil was cleaned. I replaced the drier and the unit was once again under vacuum if moisture was a problem.

The freezing seems to start from the bottom rows of the evaporator coil and sometimes the u-tubes of the entire front row have little ice sports on it.

Is it perhaps possible that the fins did not properly bond on the pipes during manufacturing and that the problem is becoming worse after two years of vibrations?

[NoNickName]

It may be an imperfect fin bonding.
It may also be that some oil is not draining from the suction riser, so I will look if that particular coil is any different from the others

[The Viking]

Checked the superheat ?
Vapour free liquid to the TEV?

[koosjr]

Jip, no vapour going to the TEV and superheat is fine. I checked the system for three hours and there was not a trace of funny behaviour.

The coils are all exactly the same. That same coils was manufactured in a batch of six and none of the others and all the rest in the field (about 30 units) ever had the problem.

[Andy]

Jip, no vapour going to the TEV and superheat is fine. I checked the system for three hours and there was not a trace of funny behaviour.

The coils are all exactly the same. That same coils was manufactured in a batch of six and none of the others and all the rest in the field (about 30 units) ever had the problem.

Hi Koosjr

you said you had some airflow problems in the past. This sounds like an airflow problem, perhaps a dirty coil, you could steam clean it. It could also be the way this customer is packing the room.

Kind Regards Andy

[Gary]

Does the room have a door/fan switch? If so, possibly the door is left open for extended periods and/or the switch is faulty. Or possibly employees are manually shutting off the fans while working in the room?

[DeB]

I would check the coil circuiting, the distributor tubes and the distributor postion (ie facing dowwards) and distributor size. There may be insufficient pressure drop to allow homogenising of the vapour/liquid mixture.This can manifest itself with ice-ups to the lower part of the evap coil.

[Andy]

Hi Koosjr

you said you had some airflow problems in the past. This sounds like an airflow problem, perhaps a dirty coil, you could steam clean it. It could also be the way this customer is packing the room.

Kind Regards Andy

Or they may be leaving the door open, like 90% of cold room freeze ups

Kind Regards Andy