Peter Neilson wrote to me:

> Ammonia plus hydrogen is still used in gas-flame refrigerators,
> of which Servel is the best-known maker.  It's a curious cycle,
> lacking a mechanical compressor.  I understand a lot of weird
> things, but still don't know how gas refrigerators work.

These are referred to as "absorption" refrigerants.  There are two
common absorption cycle refrigerants -- actually, they are always
pairs.  One is ammonia/water, and the other is lithium-bromide/water.

In the case of ammonia/water, ammonia gas is separated from and
ammonia/water solution by heating.  The ammonia gas is then cooled to
produce liquid ammonia.  The liquid ammonia is allowed to boil at a low
pressure in the evaporator where it absorbs heat from the food.

This is not where the word "absorption" comes from, however.  In a
separate part of the refrigerator, cool water (I don't recall if they
actually condense water vapor) is allowed to "absorb" ammonia gas from
the evaporator.  If you recall from chemistry, ammonia _really_ likes
to be absorbed into cool water.  This is how the low pressure is
maintained so the ammonia (the refrigerant) will boil and absorb heat.

This refrigerant being absorbed into the water is where the word
"absorption" comes from.  This cycle will operate with only gravity
helping it.  There's lots of finned pipes.  ;-)  It's used mostly in
small refrigerators and freezers. (Yes, I saw a propane fired chest
freezer recently.)

The water/lithium-bromide cycle is often used in large air-conditioning
applications where there's plenty of steam available, such as any place
where there's co-generation of steam and electric power.  In this case,
water is the refrigerant.  It is allowed to boil at a low pressure (at
temperatures above 0 C., obviously).  The water vapor is "absorbed"
into a lithium-bromide brine.  The diluted brine is later concentrated
by using steam, via a heat exchanger, to boil off the unwanted water.
50 to 100 ton chillers (1 ton = 12,000 BTU/hour) built on this cycle are
not uncommon in institutional buildings.  I saw many of these at the
University of Illinois.

>
> H2S is poisonous, more so than cyanide.  It's treacherous, too,
> because it's a nasal anesthetic; you think it's gone, but it's merely
> become concentrated enough to kill off your nose.  I've never heard
> of H2S in a fridge.
>
> CO2 (Dry Ice [a trade-mark]) is a refrigerant, but it's evaporated in
> air, not in coils.
>
> Freon(tm) is used for reasons of safety.  It is far less reactive,
> under household conditions, than NH3 or SO2.
>
> Peter Neilson, straying pretty far from mechanical music.


For further reading see the web site of Ammonia Refrigeration
Technicians Association, Danville IN:

    http://www.nh3tech.org/abs.html

Jody Kravitz

 [ Would you believe: Jody is a computer engineer !!  ;)  -- Robbie