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?