Have a query about a 3 way absorption fridge. Hopefully someone here with more knowledge about these things than me. I have a Dometic model RM4401. Name plate states the following.
On 240vac 135w power consumption. On 12vdc 130w power consumption. From ohms law this gives us an element resistance of 1.1076 ohms. (call it 1.11) and current draw of 10.8 amps.
Assuming a healthy battery and appropriate sized wiring (which I have) with vehicle running we are going to get around 13.8 up to 14.2 volts (alternator output) supplied to element. Being conservative
and using 13.2 volts at the element, ohms law again gives a power rating of 157 watts at 11.9 amps. From this it would seem that the 12 volt system with motorhome driving should be more
efficient than the 240vac system. So my query is why no thermostat on the 12v side. Apologies for this being so long winded just wondering if I have missed something obvious.
Probably 99.9% don't run these fridges off 12v other than when driving. Also the total cable length of + & - has too much resistance for 99.9% of installations. It's a long run there & back from the alternator to the fridge plus a few connections.
Did you measure your distance of + & - ?
__________________
Procrastination, mankind's greatest labour saving device!
50L custom fuel rack 6x20W 100/20mppt 4x26Ah gel 28L super insulated fridge TPMS 3 ARB compressors heatsink fan cooled 4L tank aftercooler Air/water OCD cleaning 4 stage car acoustic insulation.
12v is short term when refuelling as theres an open flame in petrol stations not a good idea !! Although I often wander how many turn them off ? LPG or 240v mains is their efficient supply . Some the thermostat doesnt work on DC ?
Because on 12V these fridges draw in the region of 20A and it's a fair bet most installations handle this badly and incur voltage drops due to undersize wiring, poor connections and the like thus ensuring the fridge sees significantly less energy on 12V than it does on 240V or gas. Experience has shown this assumption is correct 99% of the time.
__________________
"I beseech you in the bowels of Christ think it possible you may be mistaken"
Oliver Cromwell, 3rd August 1650 - in a letter to the General Assembly of the Kirk of Scotland
Its also the efficiency of the heater to circulate the gas !! Its a temp measure . +20a draw will flatten batteries in no time !! Why most dont have thermostat ?? Because they are NOT efficient enough to need one ! When I run mine it actually got warmer !! Most are wired up so they only work on12v when engine is running .,Full time camper ? I would fit a compressor fridge !! Yes they have pilot flame when running on LPG . If its a 3 way ?
-- Edited by Aus-Kiwi on Tuesday 2nd of August 2022 03:24:38 PM
Yes think you have hit the nail on the head. All down to saving a couple of bucks in manufacturing. (It was really just a wondering out loud type question) Similar to you I have been using this fridge now for around 4 years and have had no trouble with it freezing whilst driving. Mind you we try not to drive more than 3 or 4 hours most in a day, the less the better. Must also say that we have also been very happy with it in general use.
Cheers Leigh
-- Edited by 62supersnipe on Tuesday 2nd of August 2022 04:07:04 PM
These fridges work by gravity pulling liquid ammonia down through a small jet into a larger, low pressure pipe where it expands by pulling heat from the walls. This pipe is INSIDE the fridge and winds through the freezer and then into the top of the main compartment.
There is a small pool of water at the bottom end of the larger pipe which attracts the gaseous ammonia that dissolves into it. The pool is divided, with a small pipe at the bottom transfering the water/ammonia mix to the second part that is heated to turn the ammonia into a hot gas.
The hot ammonia rises into a finned, gently down sloped pipe at the top of the system where it cools enough to condense into liquid form. This is then pulled by gravity ....
[The actual system is a little more complicated with hydrogen gas enhansing the expansion, water vapour strippers in the up flow, corrosion inhibiter, etc. And, importantly it takes several hours to establish the separate zones in the closed system.]
Boiling the ammonia out of the water pool raises it against gravity, this can be done with a gas flame, a mains element or a 12V DC element.
To keep the system running while an RV is in motion a 12V element MAY be used, but its current draw while the alternator is not working will kill a starting battery in very short order. A rule of thumb is that a fuel stop is fine, but a lunch stop requires unplugging the 12V supply.
Also note that on a proper installation there will be 2 x 12V connections, a large cable for the element that should come DIRECTLY from the motor's alternator output and a thinner wire from the RV house battery to supply the controls.
bye.
-- Edited by bruceg on Wednesday 3rd of August 2022 02:52:16 PM
-- Edited by bruceg on Wednesday 3rd of August 2022 02:53:01 PM
__________________
Bruce & Judy pulling a 3T 23.5' Traveller Prodigy behind a Diesel Range Rover
Thanks for the detail explanation of the cooling system. I am a sparkie and understand the electrics but wasn't fully up to speed on the ammonia cycle. I have worked on compressor systems but never on absorption systems. My system consists of 2 x 140 amp hour agm batteries both solar and 240 volt charging as well as alternator charging via a manually switched relay. Agree that running on 12v when not charging will drain batteries quickly. Guess I was just surprised that no thermostat in the 12v side, as from my calculations when on 12v and motorhome charging batteries the system is as efficient if not more so than on 240vac. Please note the figures I have are for my fridge. Anyway thank you again now understand why being level is also so important. Which I guess explains why no need for thermostat on 12v as the level whilst driving is all over the shop.
12v is gasless . Your supposed to turn gas off ( or any open flame ) when approaching service stations as theres open flame when running 3 way fridge on gas . Read and comprehend! Yes Its txt !
12v is gasless . Your supposed to turn gas off ( or any open flame ) when approaching service stations as theres open flame when running 3 way fridge on gas . Read and comprehend! Yes Its txt !
Fortunately my 185L AES three way has no pilot running, as the older fridges. I assume all more recent fridges use this system.
__________________
Cheers, Richard (Dick0)
"Home is where the Den is parked, Designer Orchid Special towed by Jeep Grand Cherokee Limited"
"4x250W solar panels, Epever 80A charger and 3x135Ah Voltax Prismatic LiFePO4 Batteries".
My Dometic which is less than 1 year old, will run on gas as i drive but it states that because I could be pulling into a fuel station it will automatically turn off the gas once the vehicle motor is switched off. Then 10 minutes after restarting the vehicle it will automatically turn the gas back on and reignite.
My Dometic which is less than 1 year old, will run on gas as i drive but it states that because I could be pulling into a fuel station it will automatically turn off the gas once the vehicle motor is switched off. Then 10 minutes after restarting the vehicle it will automatically turn the gas back on and reignite.
Tim
The problem is that petrol fumes are present when you drive into servo, BOOM in an instant - Three way fridge should never run on gas whilst driving.
__________________
Possum; AKA:- Ali El-Aziz Mohamed Gundawiathan
Sent from my imperial66 typewriter using carrier pigeon, message sticks and smoke signals.
