gravity fed walk in cooler

[bunny]

First post but I’ve followed this site for a long time. One of my customers has a gravity fed 8x8x8 walk in cooler using MP-39 with remote condenser. The liquid line solenoid is controlled by an air sensing thermostat mounted on the wall about 6 feet up. The remote condenser is shut off by a low pressure control – I’m in Colorado so it gets pretty cold.
The problem has been, as long as they can remember, is that the evaporator coil would freeze up fairly often and keeping an even product temperature has been very difficult to obtain. I installed a defrost control and at present I think it defrosts 3 times a day for 45 minuets. After installing the timer the coil no longer freezes up but a constant temperature is still a problem. Apparently very slight changes in temperature result in too warm to freezing of the product.
I’m looking for suggestions on how to correct this problem. I’ve thought about using a constant cut or a Johnson Controls A419ABC thermostat with the sensor placed in the evaporator coil [best location for bulb placement] or using another low pressure control . When the 419 first came out if I placed the sensor in the evaporator coil it seemed to have a short life span and I quit placing them in the coil. Any suggestions would be appreciated.
Thanks Mike

Welcome MK. How did you become a professional member if this is your first post?

The nature of gravity coils....no air movement in the box....can easily result in what you are describing. The moment someone enters the box, the temperature will increase. With no air movement it takes a long time for the heat transfer process between the air and refrigerant, and depending on gravity to circulate the air and get even air temperature takes a long time too.

Unless the thermostat is mounted substantially above the product level it's hard to understand why the product is freezing...unless the stat isn't accurate.

It would be interesting to check the temperature at different heights....say, after a defrost period....to see how much variation there is between where the product is located and where the stat is located.

What is the product they are refrigerating and how often are they in/out of the box?

Good idea to add the head pressure control valve in a climate like Colorado. I'm with the suggestion to replace the gravity coil with a tradiional fan/coil unit. There's a reason why you rarely see a gravity coil in a walk-in box.

[Fridge Repairer]

I think your problem might be with the freon choice. Off time is very important for controlling your icing. Running MP39 means less off cycles which means more ice. If the customer would let you, you may want to suggest converting back to R-12.

My personal R-12 repalcement gas preference is hot-shot but not sure if its more efficient than MP39 though.

[icemeister]

Welcome MK. How did you become a professional member if this is your first post?...

MK signed up on HVAC-Talk back in 2004 which allowed him to be grandfathered in as a Professional Member. The new rules didn't come into play until around 2006 as I recall.

Now he has 3 posts. He just doesn't talk much.