SMALL AMMONIA REFRIGERATOR
Anhydrous Ammonia (Nh3) has been used for many years to perform all types of
refrigeration and air conditioning chores. Anhydrous Ammonia means ammonia
without water, Aqua Ammonia is a solution of ammonia and water which is used
in refrigeration and air conditioners that are not a vapor compression type.
Vapor compression systems use compressors to compress the ammonia vapor.
Absorption type systems do not use a compressor but instead use a generator.
These systems shall be the main topic of the following chapters. It is assumed
by the Author that you have some knowledge of refrigeration and are aware
of the cycles and pressures of refrigeration in general. Please refer to our
available Browser Books on the subject if you are not. Ammonia Refrigeration
Basics is a good source of basic information should you require a better
understanding on the subject.
Example:# 101A
THE GENERATING CYCLE
The Intermittent absorption system uses a generator charged with water and
ammonia. A heat source, usually a kerosene flame, heats this solution in
the generator. The ammonia is vaporized and driven off.
A condenser, at the top of the system, condenses the ammonia vapor into a
liquid. The liquid flows by gravity into the liquid receiver shown above
and then into the evaporator. During the generating cycle, little or no
refrigerating effect is taking place. As the system cools, the pressure
drops, causing the liquid ammonia in the evaporator to boil (flash off)
and absorb heat. The cycle is completed when vaporized ammonia is
re-absorbed in the Aqua ammonia solution in the generator.
Example: 101A illustrates the generating cycle, in operation, the
kerosene burner tank is filled with just enough kerosene for one cycle.
This cycle is usually once a day. The burner is filled and lighted. It
heats the water and ammonia mixture (Aqua Ammonia) (Brown Mixture) in the
generator. The ammonia vapor (Tan Color) is driven off through the tube, (A)
up to the air cooled condenser. There the ammonia gas is cooled and condensed
to liquid ammonia (Red Color). The liquid flows into the receiver. Note the
one way check valve between the condenser and the liquid receiver, it serves
to restrict any back flow of liquid into the condenser. Also notice the small
restrictor port which will be covered below during the refrigeration cycle of
the system.
When the kerosene has all been burned (usually from 20 to 40 minutes), the
generating cycle ends.
THE REFRIGERATION CYCLE
Example:#101B
The pressure in the system drops as the water cools and absorbs ammonia vapor.
Liquid ammonia (Light Blue) flows into the evaporator, begins to evaporate,
and cools it. Evaporated ammonia (Aqua Blue) flows back through the tube (B).
It is again absorbed by the water in the generator. Refrigeration continues,
usually until the next firing of the kerosene burner.
The small restrictor perform two functions, it allows the liquid to enter
the evaporator at a controlled level. This protects the evaporator from
getting flooded. The restrictor also acts as a expansion valve which
greats a pressure differential. A pressure differential is essential to the
refrigeration cycle as it causes the liquid ammonia to change it's boiling
point from a high pressure flash of temperature to a lower pressure flash off
temperature. Without this change in pressures the refrigeration would not
be performed and the system would simply act as an ammonia container.
This type of refrigerating system is quite simple. The piping is welded steel
because the pressures on the generating cycle are quite high. The refrigerating
ability is quite good. Kerosene flame heated absorption refrigerators are
popular in areas where electric power is not available.
Other means of firing the burner can be propane or natural gas. Many RV unit
refrigerators use a combination of propane fired burners and add a small
12 volt fan assembly to improve cooling within the evaporator section of
the refrigerator (Ice Box).. Many types of absorption systems are on the
market these days and are not limited to small appliances. The theory can
and has been applied to very large commercial systems and perform in much
the same manner.
Most small RV type refrigerators are not field repairable due to the critical
charge ( The right amount of Aqua Ammonia ) and the nature of the system
construction. If a small unit develops a leak, the best thing to do is
replace it with a new one. Most manufacturers design the units to last for
a very long time, but a damaged unit should be replaced (Leaking). Maintenance
can and should be performed on the burner assembly and the piping that feed
the propane or kerosene. Most Aqua Ammonia leaks are easy to find as are
Anhydrous Ammonia leaks the smell is very strong and aids in detecting
even the smallest of leaks.
CONTINUOUS-CYCLE ABSORPTION SYSTEM
Example:# 101C
There are other types of absorption systems available, the continuous-cycle
absorption cooling unit is operated by the application of a limited amount
of heat. This heat is furnished by gas, electricity, or kerosene. No moving
parts are employed. The operation of the refrigerating mechanism is based
on Dalton's Law.
DALTON'S LAW
Dalton's Law of partial pressures is the foundation of the principle of
operation of one of the absorption type refrigerating systems. The Law States:
The total pressure of a confined mixture of gases is the sum of the pressures
of each of the gases in the mixture.
The total pressure of the air in a compressed air cylinder is the sum of the
oxygen, nitrogen, and the carbon dioxide gases, and the water vapor pressure.
The law further explains that each gas behaves as if it occupies the space
alone. To illustrate, the absorption refrigerator uses two gases, ammonia
and hydrogen. The ammonia, at room temperature, is absorbed by the water
in the closed sustem.
Heating this solution drives out the ammonia. (The hydrogen is not absorbed
by the water and remains as a gas.) Due to the pressure it is under, the
ammonia condenses into a liquid in the condenser. The pressure is uniform
throughout the system. Total pressure in the system is the sum of the vapor
pressure of the ammonia plus the hydrogen pressure. When the pressure of the
ammonia vapor is below the pressure corresponding to the vapor pressure
for ammonia alone, the ammonia continues to evaporate. It tries to reach
a vapor pressure corresponding to the temperature in the absorber.
This refrigeration device is widely used in domestic
refrigerators, and recreational vehicles. It is also used in year-around
air conditioning of both homes and larger buildings. The unit consists of
four main parts the boiler, condenser, evaporator and the absorber. When
the unit operates on kerosene or gas, the heat is supplied by a burner.
This element is fitted underneath the central tube (A). When operating on
electricity, the heat is supplied by an element inserted in the pocket (B).
The unit charge consists of a quantity of ammonia, water, and hydrogen. These
are at a sufficient pressure to condense ammonia at room temperature. When
heat is supplied to the boiler system, bubbles of ammonia gas are produced.
They rise and carry with them quantities of weak ammonia solution through the
siphon pump (C). This weak solution passes into tube (D), while the ammonia
vapor passes into the vapor pipe (E) and on to the water separator. Here
any water vapor is condensed and runs back into the boiler system, leaving
the dry ammonia vapor to pass to the condenser. Air circulating over the fins
of the condenser removes heat from the ammonia vapor. It condenses into liquid
ammonia and then flows into the evaporator.
The evaporator is supplied with hydrogen. The hydrogen passes across the
surface of the ammonia. It lowers the ammonia vapor pressure enough to allow
the liquid ammonia to evaporate. The evaporation of the ammonia extracts heat
from the evaporator. This, in turn, extracts heat from the food storage space,
lowering the temperature inside the refrigerator.
The mixture of ammonia and hydrogen vapor passes from the evaporator to the
absorber. A continuous trickle of weak ammonia solution enters the upper
portion of the absorber. It is fed by gravity from the tube (D). This weak
solution flows down through the absorber. It comes into contact with the mixed
ammonia and hydrogen gases. This readily absorbs the ammonia from the mixture.
The hydrogen is free to rise through the absorber coil and to return to the
evaporator. The hydrogen circulates continuously between the absorber and the
evaporator.
The strong ammonia solution produced in the absorber flows down to the absorber
vessel. It passes on to the boiler system, thus completing the full cycle
of operation.
This cycle operates continuously as long as the boiler is heated. A thermostat
which controls the heat source regulates the temperature of the refrigerated
space.
Since the refrigerant is ammonia, it can produce quite low temperatures. Most
systems require electrical devices, so both gas and electricity must be
supplied. Except for the thermostatic controls and (in some cases) fans, there
are no moving parts.
Service is usually quite simple. The burner and stack must be kept clean. The
refrigerator should be carefully leveled before being placed in operation.
The pressure differential in the type of system is created by the hydrogen
gas, it causes the ammonia to change pressure and allows it to boil off
in the evaporator (flash off).
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