centralina evaporator ice accumulation issue

I just been on a claim survey to find out what is causing defrost heaters to fails from evaporators,thus accumulating ice. I found that besides the normal defrost timer,there is also defrost thermistor to stop short the defrost cycle once evaporator is clear. I found that both evaporator fan motors switch Off when set temperature is reached. I discussed with on site tech and se set fans to run continuously,except during defrost. what may be the cause of repetitive failure of defrost heaters ? I still have to inspect some of the replaced when they find them !! Heaters may fail open or shorted through insulation. what causes what ?
Any suggestions please ?

Hi
if your room temp is more that 4
you can use fans for defrosting ice from evaporators

Poor Quality non Genuine Heaters or maybe replacement Heaters were damaged before installation. Bending a Heater can cause it's insulation to weaken and Fail.

Hi MBC , Thanks for your reply. The fridge is 5 degrees. Yes I found the fan motors to stop when set temp is reached. We set for continuos operation. Defrost period with fans running is a bit risky to the product.

Interested to know why evaporators are freezing up with a +5°C room temperature.

Hi
What type of evap are they DX? or Flooded?
How is the expansion valve set up or are these part of a tandem system with back pressure regulators.
What's the relative suction pressure temp out of interest?

Poor air flow can easily cause iced up coolers and Heater rods will burn out if they are not doing enough work.
Over heating the rods will cause hot spots which fail if then dripped on.

I would spend some time looking at how your defrost cycle is set up!
And how many defrosts + what the duration is?

Undoubtedly you have done the right thing having the fans run on after setpoint.
Does a liquid line solenoid shut at setpoint.
At + 5.0c MBC has a point you could almost off cycle defrost.
In reality why do you need heater rods at all?

Last but not least what is being stored and how often is there product movement?

Lots of questions Sorry.
Grizzly

Hi Guys ,
sorry but misled.
I mixed two faulty systems. Fridge is air defrost but has a shorted compressor insulation.Considering compressor is part of three in a centralina with full protection,including pressure , current ( clamp) ,and all soft start type , still puzzled what caused it to fail !!

On the other hand the walkin freezer , is getting repetitive faulty condensate heaters. Today visited and checked recent replaced heaters. These are the solid 10 feet type connected two in series . Insulation test proved one heater to be open,with one end shorted to body ( spot ) , the other proved to be fine.Onsite tech explained that evaporator was found everytime blocked to half of it with ice !!!