Absorption fridge tech details

Hi Folks, I'm working on optimising the "Electrolux type" (actually Dometic) caravan multi fuel fridge I have to operate on 12v DC. This is actually the poorest "fuel" in terms of cooling performance.

I'm looking for any books or articles on the design of these units, especially the positioning of the various heaters and details of power in to cooling delivered.

Many thanks, Ed

Can it also operate from bottled gas? That will be far more economic than 12V power generated by the car's engine. Other than that, make sure that the final heat exchanger (waste heat disposal) stage is cooled as well as possible, but giving it lots of room and a nice cooling breeze.

Edit to realize that it's multi-fuel. So of course it can run on gas.

Edited By Mark Rand on 28/01/2018 00:08:31

You can convert domestic fridges to 12 volt. You need to chnage the compressor to a 12 volt one.

Hi gents, thanks for the ideas so far. This project can only use the 12v heater, but what happens if I vary the supply to the heater, or changes its position in the pipework with that of the 240v one?

Ed

I sat down with the patent for this kind of fridge to try and understand how it operates and failed completely. But as Albert Einstein was the co-author of the patent, **LINK**, I guess I shouldn't feel too bad. I would counsel you against spending too much time trying to perform power and efficiency calculations.

As an aside, I had a colleague who used a gas powered version of this fridge on a sailing boat. The angle of heel would place the gas flame heat source appropriately or not resulting in starboard tack warm beer, port tack cold beer.

Back to your question, swapping 12V with 220V heaters should present no problem.

Hi Ed,

Usually they run without a thermostat on 12V because there's barely enough power to make them work. You need gas or 240V to make ice in one IME.

Not much choice in heater position, just make sure the vents are clear to get good air movement, just as when running on gas.

The 'fuel use' is very small, even if inefficient, the challenge is in optimising battery recharge cycles so you don't over-discharge your leisure battery or get stuck with a flat starter battery.

Hi Ed These fridges are very inefficient on 12v and will flatten your battery in no time. The one on my Bailey 620/6 supposedly draws 5 amps but will flatten a fully charged 95 amp battery to the point of not working in 9 hours. I know I have tried it.

If the 12v heater is not tight against the pipe it virtually will not work.

If a better system was available I am sure it would be fitted in caravans by now.

regards

Edited By Oldiron on 28/01/2018 17:47:17

the problem with three way fridges they need to be vertical to work efficiency. and years ago talking to old man on the market he used to mend fridges and told me how solve a particular problem some times fridges stop working when and it stands for a while if not used . he told me to turn the fridge upside down and tap the pipes it has chemicals &gases in side which separate and tapping help remix them and it does work.in the past saved me money and is it possible work 240 volt on gas re norm

Yes a 12 volt compressor fridge ! Bloody expensive though.

As a point of interest the caravan / camper compressor motors are actually 220 volt and are fed from an inverter VFD to provide the high voltage.

RE: Electrolux 3 way fridge. They use a heat source to boil a liquid which is allowed to pass through a small hole, the gas expands and adsorbes the heat from inside the fridge cabinet.

Search "Boyles Law"

"Boyle, Thompson and Joule, allowed a compressed gas, through a small hole to pass, the gas expanded and became cool" !

An old ryhme from school days.

Fun times

the old absorption type freezer it was type of plaster that used absorb the water you in a depression on the top had one many moon ago

Agreed. The Electrolux fridge in my camper-van is wired so that it will only operate on 12 V while the engine is running. When parked it's gas or 230 V. That avoids the flat battery problem.

Russell

The Einstien patent is far more complex than the modern evaporation fridge. As has been said above, the heat source evaporates the refrigerant which passes through a small orifice as vapour and goes into the fridge cooler box pipes where it condenses, drawing the heat out of the fridge as it does so, then returns via a finned cooling pipe to the evaporator Forget electric unless the engine is running, they should be set up to only work electric when driving or towing. To maximise the efficiency on gas, you need to remove the fridge, clean the gas jet, and remove all the muck and cobwebs from the back, then connect it up to the gas, light it, and make sure the gas flame bears directly on to the evaporator. what usually happens is a bit of muck in the jet directs the flame off to one side, and you loose half the heat and therefore the efficiency. when running on gas and set up properly they are good enough to keep the food fresh and the milk from souring, but they are never brilliant. I have also used a peltier effect beer cooler type fridge to keep my milk from going off at the workshop, no thermostat, and it froze my pint of milk solid overnight. but they are not efficient at all when it comes to energy consumption.

Hello Ed,

I do a lot of camping in the desert here in Namibia, and Fridge and Freezer efficiency is always important, as we run for days from 2x100watt solar panels - Fridge, Freezer, HF Radio, Sat - tracking equipment, etc.

After going through a bevy of camping fridges and freezers from many of the 'off-road' vendors, I decide to make my own...

I discovered three key elements -

You have to get rid of all the heat generated in the condensor. The evaporator must be very well insulated. Use a 12volt compressor, and the BEST I found is the Danfoss BD35. Runs from 12V and has a brushless motor with its integral DC to 3 phase converter.

Most small fridges use a capillary tube as the gas Expansion device - If you use a thermostatic Expansion valve (TXV), you can gain another 8 to 10% efficiency.

So I slowly converted a commercial camping fridge to the BD35, then substituted the TXV for the Cap-tube, and obtained large improvements. Then, of course, I built my own fridge - made my own evaporator from aluminium and wrapped with flattened copper tube, and insulated the whole thing with 100mm refrigeration foam all round. Made my own condensor, many fins with copper tubing passing through, and force cooled with a muffin fan.

The end result, for a 60liter fridge, running at 3deg Centigrade, on days that go into the low 40degC, is that in a 24hour period, the energy used is around 1.2ampere/hours, ie, 1.2amps per hour averaged over 24 hours. MUCH less than the typical 4 to 7 amps for a 12 volt heater element type system, which will NEVER hold 3deg C @ 40degC ambient...

This is the compressor - about 190mmx140mmx160mm - this was a part development of my 15liter freezer - still with capillary tube.

The freezer evaporator:

Another Freezer with shop-made condensor and cooling fan ( on the left)

View with an idea of the insulation before the covers are fitted.

The condensor and compressor piped up.

Does not really help your quest, but I can only hope to dissuade you from going the 12V heater route - It is hopelessly inefficient.

Good Luck..

Joe

Hi Gents, thanks for the details so far.

Unfortunately I am fixed on the 12v supply route, so any further tips or details of the thermodynamics of the system would be much appreciated.

Ed

Hi Ed,

well, if you insist... But it is highly unlikely you may improve on the current setup, unless they have maybe sized the heat source or condenser to small or provided poor cooling of the latter or something really obvious and silly like that.

If your system can operate from 220VAC, ie, if it has a mains driven heating element, then measure the RMS amps and volts and see how many watts it uses to do its job. You will have to provide the SAME from your 12V source.

It is almost impossible to determine how many joules are provided by the gas flame, as there are far to many influencing factors - you could determine the total heat energy in the flame by determining the gas flow rate ( burn rate) , to some extent, but as to how efficiently that flame is heating the required spot is very difficult to determine.

Perhaps you could stick a few thermocouples around the heated area, take readings and then fit a 12V heating element in the same area, and vary the input voltage ( the Watts as well then) till you get similar readings. As to improving the performance, it would be a trial and error process, moving the heater around in position, etc, but I think the returns will be subjective.. Other improvements are maybe fitting one or more 12V muffin fans to blow over the condenser - the cooler you make that, the better.

Here is a wiki on the absorption cycle - basic - if you want mathematical thermodynamics, there are many sources on the HVAC web sites. The interest in the wiki, however, are the two photos at the end of the article - one of a typical backside of the fridge, piping, location of heat source, condenser, etc, the second is a thermal image of the fridge working - you can see clearly where the heat is, where you need to place the heat source, and where you need to get rid of heat as best possible ( at the condenser).

**LINK**

Joe

edit - missing preposition..

Edited By Joseph Noci 1 on 29/01/2018 08:35:40

Agreed. The Electrolux fridge in my camper-van is wired so that it will only operate on 12 V while the engine is running. When parked it's gas or 230 V. That avoids the flat battery problem.

Russell

Hi Russell, So is mine . Years ago being very clever(in my mind lol) I thought I could put another fridge in the awning when on a 2 week holiday and use it as a drinks fridge. A very good idea unless you want to do it for more than a day. It took more effort and lugging about to charge the battery (as we were not on a mains hook up) than it was worth. In the end I went out and bought a cool box that runs on mains or 12v. It runs on our caravan 12v battery setup with solar panels a treat. We do turn it off at night so we don't have to listen to the fan humming away. We use the caravan most weekends in the spring and summer and have a 2 week holiday break. Not used a mains hook up for 7 years.

regards

Edited By Oldiron on 29/01/2018 11:40:21

Sounds as if you'd be better off just getting a bag of ice at the supermarket each time you go shopping and keep it with your food and drink in an "ice box" (the clue's in the name). Wouldn't work so well in Namibia perhaps but in the UK? Our ice box in Canada also had a Peltier cell that worked when the engine was running. That held off most of the heat when we were travelling around during the day - you weren't allowed to leave food at the camp sites due to the bears.

Murray

Hi all, have posted before on this subject, including 'resurrecting' silted up 3 way fridges, however after my experiences with our previous campervan (nobody has mentioned having to rip it out of the van to turn it upside down), when we acquired our most recent van about 18 months ago I settled for a worktop compressor fridge which cost the princely sum of £60, yes I know you have to have electric hook up, but let me ask you this, when you see a Transit size van conversion, pushing say £45 to £50,000 in price why do they not have any vent grilles in the outside? because they all have compressor fridges, thus obviating umpteen extra cutouts in the bodywork. There is a man who builds van conversions who rents a unit at the farm where my other half stables her horse & he has embarked on his latest effort this very day. So believe me when I say save yourself a lot of grief, mine hits operating temperature in around 8 minutes.

cheers, Bob H.