Date:         Sun, 1 Jul 2007 23:49:21 -0700
Reply-To:     KIM BRENNAN <kimbrennan@MAC.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         KIM BRENNAN <kimbrennan@MAC.COM>
Subject:      Re: Refrigerator help-yes, I've already searched the achieves
In-Reply-To:  <C2ADE4A4.80B1%mwmiller@cwnet.com>
Content-Type: text/plain; charset=ISO-8859-1

 The Dometic fridge in our Westfalias is an absorbtion ammonia fridge. You can look up the details on how they work. There are two main types. One has 3 loops (ammonia, hydrogen and water). One has one loop (ammonia water). We have the latter. The former is probably more efficient, but is more complicated.

In the single loop system, there is an ammonia/water reservoir that gets heated up by one of the 3 heat mechanisms (propane, DC electric or AC electric.) Technically you can probably have 2 of them going at once (propane and either of the others). This won't make too much difference, other than speed up the cycle.

The key factors for efficency are the condensor portion of the cycle and the evaporation side of the cycle. The condensor side happens AFTER the liquid is heated up by the propane burner (or electric heating elements). This is where (in theory) the ammonia and the water vapor separate, with the water vapor condensing (instead of going on into the evaporation side of the cycle). If you don't have good enough air flow behind your fridge, you can disrupt this side of the cycle. Don't block the vent area at the BOTTOM of your fridge. And don't block up the vent area at the TOP of the fridge. If the inside of your van is too hot, you will disrupt this cycle.

If things are going right, however, the ammonia vapor will go up the condensor and then pool around the evaporator (that item which is inside your fridge and which has the aluminum vanes on it.) The evaporation of the (theoretically) anhydrous (i.e. without water) ammonia will absorb a LOT of heat (i.e. get cold). Meanwhile that pool of water at the boiler will be absorbing ammonia vapor. This creates a partial vacuum, which helps cause the anhydrous ammonia to evaporate in the first place. Then the ammonia/water gets heated up by the burner again and the cycle repeats.

If the condensor however is so hot that the water vapor continues over to the evaporator side, the cycle is disrupted and your fridge will not be efficient.

So keep the interior of your van cool during the day. If you have a skylight vent, keep it open. Open up the sliding windows, keep the back hatch open, sliding door open, etc. Get the interior cool and things will work so much better.

As you can see, this style of ammonia fridge DOES have limitations. A fellow by the name of Einstein (yes THAT Einstein) collaborated and invented the more efficient (but complex) 3 loop ammonia cycle. That one is used in air conditioners and industrial refrigeration units to this day. Too  bad Dometic didn't make a small model with that mechanism that could fit in our Westys.