MemberDecember 30, 2019 at 10:10 am
Cold temperature control of reach in boxes can be broken down into freezers and coolers. Many think of them as just thermostats, but they do much more. Before electronic controls we had just the simple adjustable thermostat that varied both the cut in temperature and cut out temperature.
Lets’ start with coolers. The most common was the Ranco A30 design. They normally were installed in the evaporator air stream. ussually the in bound air side. Due to varying conditions they often iced up and required a repair call or the user shutting it down for a extended period. Then the idea of a constant cut in design was developed. By bonding the thermostat sensing bulb or coil to the evaporator coil we could now use the A12 design that only changed the cut out temp. Cut in was fixed at somewhere between 36F and 45F, depending on the model number and could go down to sometimes zero at full setting. I always carried two types of sensing bulb types with a 36 to 38F cut in and mid range was about 17 to 19F. Perfect for back bar and line coolers. This meant that each cycle of the compressor, the unit had to do a defrost. No matter how cold they set it unless the capacity was acceded it would not ice. But there was no protection for condenser problems or high ambient. If the BTU’s needed to cool the box didn’t get lo the loss point or high load change it would still ice. Once there is a ice build-up the evaporator capacity is gone.
Now comes along the digital controller. It can be programmed to limit run time, monitor evaporator, box and condenser temperatures. It also can control fan operation to increase efficiency. Monitor ice build up and even learn usage of the box to use less energy. Vary common on today’s beverage coolers. They also make it possible to place a cooler in a non heated space and operate heaters to prevent freeze up of product. Common with outdoor soda vending.
Now Freezers are a different. They have to have a heated defrost system that doesn’t raise the product temp. With the mechanical thermostats we had to have a coil temp circuit that stopped the evaporator fans until the coil was freezing again. This meant a timer to stop the compressor and turn on the heaters, stop the fans and a method to sense the coil temp to stop the heat when the ice was gone. On reach in’s this was normally done with a Klixon thermostat attached to the end of the evaporator coil and was creativity wired into the defrost/fan circuit. There was a lot of problems with all the connections in the evaporator compartment, the freeze thaw cycles, and the coil temp switch. The coil temp switches failed often till they got better with encapsulation of them. If you have seen any of them you’ll note that the insulation is extra thick and a low temp material. like Hypalon.
Then came the digital controllers. Thermisters in the evaporator compartment, fans, heaters and compressor controlled by the one item. Defrost cycles shorter, less box rise and even displays tell you what is happening. The early ones had light relays, but the manufacturers have fixed that for the most part. The down side is they they can be programed in so many ways and with so many limits, one doesn’t always know what they have. The Digital s also have the added advantage of keeping the heath inspectors happy with monitoring and providing alarms when something is off.
This is why OEM controls that are programmed to a certain box is important. The controller may look the same as another, be made by the same manufacturer, but not either have all the internals or the programming for the box your working on.. The heat loss and required BTU’s to cool a box may be a lot different between brands and sizes. Thereby requiring different defrost cycles and times and even coil temp.
MemberDecember 30, 2019 at 5:07 pm
Good stuff there. I’m going to copy and save that as future reference for our HVAC/R cohorts at work. They need all the help they can get…even they won’t ask.
MemberDecember 30, 2019 at 5:55 pm
I had to upload this. A video I put together awhile back.
It’s l-o-n-g and it’s dry, but making it helped ME understand the basics of electronic controls better:
AdministratorDecember 31, 2019 at 8:46 am
Another excellent resource as always, thanks for posting!
MemberDecember 31, 2019 at 9:05 am
Over the years I’ve had to follow a lot of other techs with Iced up coolers. Many would just add a defrost timer to a case to try and solve the problem. That just drop’s the capacity even more. One should look at the cooler as a whole and make sure to understand the builders design and intention. Door gaskets, extra moisture, high load change, Condenser or evaporator capacity reduction all have to be factored in before a determination of the cause. And yes, I’ve seen cases with multiple problems.
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