Message-Id: <v01540b01b005759e3ec0@[208.201.229.59]>
Date: Wed, 30 Jul 1997 16:04:16 -0700
To: pinhole@exploratorium.edu
From: kimg@sonic.net (Kim Greco)
Subject: Gas refrigerators made simple
This explanation comes by way of, not the physics department, from from my
plumber husband.
A gas refrigerator has a generator, a condenser, an evaporator, an
absorber, and a charge of refrigerant of water and ammonio (30%
concentration) and hydrogen.
There are three fluid paths. An ammonia path uses the generator, the
condenser and the evaporator and absorber. The hydrogen path includes the
evaporator and absorber and a solution circuit that also uses the generator
and the absorber.
As heat is applied, it separates the ammonia from the solution. The ammonia
vapors flow to the condenser. Some water vapors will be carried along with
the ammonia vapors from the generator. These are removed by an analyzer
and a rectifier. In the analyzer, the ammonia filters through a strong
solution which is on its way from the absorber to the generator. This
process reduces the temperature of the generated vapor somewhat to condense
water vapor. The resulting heating of the strong solution expels some
ammonia vapor without additional heat input.
The ammonia vapor then passes through the rectifier where a residual amount
of water vapor is condensed by atmospheric cooling and draws to the
generator by way of the analyzer.
The warm ammonia vapor gooes to the condenser and is reduced to a liquid
through air cooling. The ammonia, now liquid, flows to the evaporator. A
liquid trap is in the path between the condenser and the evaporator; it's
there to prevent hydrogen from entering the condenser. The hydrogen gas
enters the lower evaporator and flows upward (this is in the opposite
direction to the downward flowing ammonia liquid). The effect of the
placing of a hydrogen atmosphere above the liquid ammonia in the evaporator
is to reduce the partial pressure of the ammonia vapor in accordance with
Dalton's law of partial pressures.
Consequently, in the evaporator, the partial ammonia pressure is less than
the total pressure. The lesser ammonia pressure results in the evaporation
of the ammonia with the absorbtion of heat from the surroundings of the
evaporator and results in the cooling of the evaporator and its
surroundings in a well insulated enclosure. The cool, heavy gas mixture of
hydrogen and ammonia vapors, formed in the evaporator, leaves the top of
the evaporator and passes downward through the gas heat exchanger to the
absorber.
Since the molecular weight of ammonia is 17 and that of hydrogen is 2, then
the specific weight of the strong gas is greater than the weak gas. This
difference alone is enough to initiate and maintain circulation between the
evaporator and the absorber. The gas heat exchanger transfers the heat
from one gas to another, saving some cooling in the evaporator by
precooling the gas.
In the absorber, a flow of weak solution (water weak in ammonia) comes in
direct contact with the strong gas. The ammonia is absorbed by water. The
hydrogen (practically insoluble) passes through another section of the gas
heat exchanger into the evaporator. The liquid and gas flow in opposite
directions. As it leaves the absorber, the strong aqua-ammonia solution
flows through the liquid heat exchanger to the analyzer and then to the
strong liquid chamber of the generator. The liquid heat exchanger precools
the liquid entering the generator. Precooling the weak solution is
obtained in the finned, air cooled loop between the liquid heat exchanger
and the absorber.
Wish I could include a drawing; it seems amazingly straightforward. If you
send me your snail mail address, I'll xerox it and mail it to you.