Absorption Fridges

[Andrew Leigh]

Hi all, my first post.

Have been doing lots of work on my caravan fridge. Have got the venting right, added a thermally controlled fan and finally a close tolerance electronic thermostat.

I know find out that the thermostat is not working out the way I thought it would. It would appear as if the heating and cooling cycle of the absorbtion fridge is such that once the element is switched on it takes a time for the ammonia to heat and therefore there is a rise in fridge temperature whilst this is happening. This is creating fluctuations that make the fridge less accurate than what I would like.

There appear to the novice a couple of ways around this;
1.) I thought of having a permanent heat source keeping the ammonia safely below boiling point. This will be done through PWM control keeping the voltage lower than full mains. Then having the thermostat kick in to do the top up buy applying full voltage to the element. This should greatly reduce the response time.

2.) As the controller is 100 times more accurate and better able to control was to increase the heater wattage.

Is this possible, is it dangerous, am i barking up the wrong tree?

Cheers
Andrew

[Brian_UK]

Welcome to the forum Andrew.

My first thought would be that you are barking up that tree.

Small fridges such as yours are not designed for, or mechanically able, close control.

I would like to be proved wrong however so will wait for those more knowledgeable than me on these little beasties.

[mad fridgie]

Absorbtion systems are slow reacting as you have gathered, why do not install a PID controller to directly control the element (may need electronic switching device) I suspect it may take a few cycles to stabalise.
I presume this is not just for your beer, but for storage of medical bits and pieces.
If I remember rightly you do not have a fan inside, so trying to keep a constant soley on natural convection is near on impossible, (an average can be obtained over time)

[Andrew Leigh]

Thanks boys,

I have made progress, originally the fridge was fitted with the old capilliary tube thermostat arrangement and that was giving about a 14.5°C range in fridge temperature. I am now down to about 3 or 4°C (but in ambients of 33°C).

The problem is that we camp in areas that can have large ambient temperature swings, this means the controller setting for daytime 45°C freezes the contents and night.

The thermally controlled fan at least takes care of some of this by automatically increasing flow as the the ambient temperature lifts.

I wrote an article for our local caravan mag on fridges, this involved recording temperatures. In December 2006 we spent 18 days in the Kruger Park. The remote sensor was on the bottom shelf of the fridge and the main sensor at the outside bottom vent to the fridge, in the tent area. The values for the period were;

Min Ambient 9.2°C, Max 51.5°C.
Max Fridge 13.0°, min -1.5°C

That holiday we froze food 5 times due to the hysteresis of the fridge.

Will look into PID.

Why can one not have the ammonia permanently heated waiting for a top up to get it to boiling?

I assume the cooling only takes place on boiling?

The best performance of the fridge, in lower ambients, is on gas. This I put down to the fact that it is because the heating source is always present keeping the ammonia at temperature. I can't see a way to control the flame according to the ambient and internal temperature and therefore would like to emulate this using the element. It must be possible?

Regards
Andrew