Thermister Position

[coldcanadian]

Hey all,
Another rookie question directed at anyone who has an opinion.

The unit we're redesigning is a small appliance and our current thermister/refrigeration control position is on a valve where product is dispensed. It seems like this isn't ideal since we have up to 3 dispensing valves. I was thinking somewhere inside the machine wall, but we can't have any of the product go below 32F or it won't dispense. Also we currently have one of the final passes of the evaporator very close to the valve to ensure they are cooled, but again, we can't freeze them.

Anyway, I'm guessing I didn't give enough info, but feel free to ask away. I appreciate any help from my fellow refrigeration junkies.

[get-r-done]

need more information on the equipment you are working on?
ice machine? cold box? what dose it do?

[coldcanadian]

It's a cold temperature dispensing machine with a cabinet approximately 1 to 2 cubic feet inside. The product is in a plastic bag with a tube for dispensing and can never be below 32F or above 41F. The product enters the machine below 41F so all the machine has to do is keep it cold until it's empty.

Just wondering where most small refrigeration units put their thermister to control the on/off cycle of the compressor?

[get-r-done]

MOST EQUIPMENT USE A THERMOSAT WITH A ADJUSTABLE DIFFT, SETTING.
(IE) 3 TO 5 DEGREE
IT IS ATTACH TO THE PRODUCT OR CABNET.
IS THE UNIT IS A COLD PLATE OR FORCE AIR.
IS THERE A COIL,AND FAN ASSEMBLY.
THE TERMOSAT HAS 3 CONTACT'S COMMON, N/C. N/O.
IF YOU ARE WORRY ABOUT SHORT CYCLEING PUT A TIME DELAY ON MAKE OR BREAK TO PREVENT THIS.
GOOD LUCK.

[R12rules]

We cant help unless we have pix or techdrawings. And that probably isnt going to happen.

One sugestion. Why not make the thermistor probe long enough to experiment with?

Place the probe in various points in and around the product and see how that performs.

I assume by the few words you did share, this is NOT an air over evaporator. So this means the chamber walls are chilled, like a soft serve machine.

You will either need a probe well, like on many system desgins, or you will need an IR sensor mounted in close proximity to the product.

Whatever you decide, just make sure your number one concern is the serviceabilty of the system for the technician who fixes it in the future.

[durkster]

The earlier models of the liquid dispensers used a mechanical thermostat, typically a Ranco stat, with the sensing bulb located in a well. They were not always that easy to change, but a well designed bulb well in an easy to service location should accomodate a thermister. This could even have some heat conducting paste incorporated so the thermister would slide in without a struggle.
Is there a possibility that you could use a thermister for each product valve and average the temperature with the logic and control the compressor that way. This might make troubleshooting a nightmare though.
Have you checked out what the competition does? Good luck!

[R12rules]

Durkster, I love your idea. But I do not believe it would have to become a nightmare in order to diagnose trouble.

Have you serviced those slurpee systems? They use electronic DX systems.
They sit there just a tickin' away while the whole system is doing it's thing!

Three barrels (load centers).
Three thermistors.
One brain circuit board. The brain controls the exv's. An LPC controls the compressor. There would be an anti-short cycle device wired in also.


get my point? Your idea will work.

[condenseddave]

Originally posted by coldcanadian
It's a cold temperature dispensing machine with a cabinet approximately 1 to 2 cubic feet inside. The product is in a plastic bag with a tube for dispensing and can never be below 32F or above 41F. The product enters the machine below 41F so all the machine has to do is keep it cold until it's empty.

Just wondering where most small refrigeration units put their thermister to control the on/off cycle of the compressor?

Inserting the probe into a sealed bulbwell (Actually, a 6" piece of 7/8 copper capped, and drilled...) full of some type of food safe oil, like, well, vegetable oil, and putting it near the center of the storage area would give you a relatively accurate method of sensing the current product temp. This would only be inaccurate for a brief period during and after product loading. If it's always at temp during loading, then the temp would really never vary more than a couple of degrees.

32 to 41 is a really broad range to play with. Getting it to maintain 35-37 would be pretty simple.

[coldcanadian]

Durkster,
good call on the ranco set up. That's what we currently use. We're going to continue using this system, but modify it to work better and I've made the thermister super easy to replace. Instead of using three thermisters we're going to change the size of the tees so that they are balanced in relation to the temperature of the refrigerant in the line at that point.

We can't put anything in the center of the product area because the product consumes the entire refrigerated space so we are limited in our positions.

Thanks for all the input guys, I greatly appreciate it.