Adsorption Chillers

Adsorption is the collection of molecules on the surface only, such as a moisture sensor which has a surface layer that collects H2O molecules which affect the electrical properties of the sensor.

Absorption is the normal sponge like way of collecting molecules into a structure.

As to how your fridge works with propane... I will let someone else explain so that I can find out!!!

John.

In the case of a propane powered refrigeration system such as those used in recreational vehicles, you are thinking of absorptive refrigeration.

An absorptive refrigeration system uses liquid amonia, which evaporates in a hydrogenated environment, cooling the system. The now gaseous amonia is then absorbed by water (dissolved into the water) which is then separated by boiling it off. This is where the propane is used as a heat source to boil off the dissolved amonia from the water. The evaporated amonia is then condensed to a liquid and the cycle repeats.

Well you live and learn!!

If that's true, and I'm not saying it isn't, I didn't know that's how the things worked!!

I've often wondered how putting heat into a system you get the cooling effect... Okay with a normal kitchen fridge its easy to understand...

John.

A little hint:The system contains hydrogen and ammonia.When mixed gases are under pressure, each respond as if they are the only gas.For instance, if ammonia is 90% of the mixture, and hydrogen is 10% of the mixture, and the pressure is 100 psi, the hydrogen only sees 10% or 10 psi.The ammonia sees 90% or 90 psi. These different vapor pressures enable the system to work.Google it to find out more.

Partial Presure would be the keyword to search for if you want more about how the hydrogen "sees" 10psi, and the ammonia "sees" 90 psi.

"The system contains hydrogen and ammonia."

Don't forget the water in the system also. There are also other working fluid combinations depending on the applicaiton.

Merkelerk is correct. The complicated thing for thermodynamics students about this cycle is that it's more chemistry than thermo. I understand that there are actually a couple underlying mechanisms that drive the changes that create a chilling effect. Absorbtion and Partial Pressures they work together to allow the cycle to work. Absorbtion is what Merkelerk described where the water wants to absorb the ammonia vapor (chemistry stuff.) And if I could find a diagram like I had in applied thermo I could show why those dang campers need to be very close to perfectly level for these things to work. Gravity is used to get the water back to the bottom so it can absorb the ammonia vapor and be boiled to lift the ammonia to its condensor. Gravity also keeps the hydrogen at the top, that's whey the evaporator (cold side) needs to be near the top, because the hydrogen partial pressure lowers the pressure on the ammonia causing it to evaporate.

In my mind the most important part is that the total pressure of all the fluids is constant throughout the system, there is no pump! The key to this is that instead of using a pressure drop through an orfice or small tube to get a vapor state, as done in typical chillers with only one working fluid, the partial pressure of the working fluid (ammonia) is droped causing it to boil which draws heat creating a chilling effect. The trick is how do we reduce the partial pressure on the ammonia? Well we get some other gas to share the 100 psi. This is where the Hydrogen comes in.

Nicely put, as you mentioned, there is no pump. This system has no moving parts (from an engineering or mechanical view) unless you want to consider the motion of the molecules or even the transfer of phonons.

I have one of these units in my old camper, it can be powered by propane or electricity. The thing is quiet and works nicely. I have not taken it apart, but I assume that when being powered by electricity, the electricity simply powers a heating element.

Interesting subject. The evaporator section operates below atmospheric pressure. Large adsorption units require vacuum pumps to remove air, so there are moving parts. Small units are hermetically sealed thus are not subject to leakage (camper unit). These units are very inefficient, and are justified only using waste, or otherwise cheap heat. Texas A&M University generates its electricity with steam turbine driven generators where low pressure steam is bled off the turbine to drive the adsorption machines. Using this cheap heat improves the overall thermal efficiency of the plant by some 10 - 15%.

G. Scott

Large Absorbtion Chillers do not use ammonia. They use Lithium Bromide. For Heat they use high pressure steam. Trane has a unit that I believe starts at 1050 Tons.

Absorption refrigeration machines re manufactured by Trane, Carrier, York and other, less commonly used units here in the US. ABsorption refrigeration machines use few moving parts and simply take advantage of the fact that a salt (in this case, Lithium Bromide) is anhydrous, i.e. ity has a strong affinity for moisture absorption (tricky name, huh?). Heat is used to eparate the water from a weak LiBr solution, thus making the solution more concentrated. The moisture that is driven off by the heat (usually steam) is condensed and cooled by an open cooling tower. The cooling tower has spray nozzles that break the cooling water (condenser water) into small droplets (more surface area, ergo better cooling) and use fans to drive air thru the water to further cool. The concentrated LiBr solution then draws liquid from the condenser section and, s we all know, whne water evaporates, it also gets cooler, thus the chiller effect. The moisture travels into the concentrated solution, thus making it more dilute..and the beat goes on....

Perfect applications for absorption refrigeration machines is to tie them to the "ass ends" of gas turbine generators...as the high temperature of the exhaust (around 1000 deg. F) normally is dumped up the exhaust...but NOW..AHA..we can recapture this heat and make chilled water!!! Ain't life grand?

Typical absorption refrigeration machines start at 100 tons (12,000 BTU/ton) and go up to 1500 tons.