In the majority of refrigerating and ice machines ammonia gas is the
substance used for producing the refrigeration, although there are
other machines in which other material is employed, one of these being
anhydrous sulphurous acid, which is also a gas. Ammonia of itself is a
colorless gas, but little more than one half as heavy as air. In its
composition ammonia consists of two gases, nitrogen and hydrogen, in
the proportion by weight of one part nitrogen and three parts
hydrogen. The gas hydrogen is one of the constituents of water and is
highly inflammable in the presence of air or oxygen, while the other
component of ammonia, nitrogen, forms the bulk or about four-fifths of
the atmosphere. Nitrogen by itself is an inert gas, colorless and
uninflammable. Ammonia, although composed of more than three-fourths
its weight of hydrogen, is not inflammable in air, on account of its
combination with the nitrogen. This combination, it will be
understood, is not a simple mixture, but the two gases are chemically
combined, forming a new substance which has characteristics and
properties entirely different from either of the gases entering into
its composition when taken alone or when simply mixed together without
chemical combustion. Ammonia cannot be produced by the direct
combination of these elements, but it has been found that it is
sometimes made or produced in a very extraordinary manner, which goes
to show that there is yet considerable to be learned in regard to the
chemistry of ammonia. Animal or vegetable substances when putrefying
or suffering destructive distillation almost invariably give rise to
an abundant production of this substance.
The common method for the manufacture of ammonia is to produce it from
the salt known as sal-ammoniac. Sal-ammoniac as a crystal is obtained
in various ways, principally from the ammoniacal liquor of gas works,
also from the condensed products of the distillation of bones and
other animal refuse in the preparation of animal charcoal, and which
is of a highly alkaline nature. This liquid is then treated with a
slight excess of muriatic acid to neutralize the free alkali, and at
the same time the carbonates and sulphides are decomposed with the
evolution of carbonic acid and sulphureted hydrogen. All animal
matter, the meat, bones, etc., contain considerable carbon, while the
nitrogen from which the ammonia is produced forms a smaller portion of
the substance. The object is then to get rid of the carbon and
sulphur, leaving the nitrogen to combine, through chemical affinity,
with a portion of the hydrogen of the water, the oxygen which is set
free going to form the carbonic acid by combining with the carbon. The
liquor after being neutralized is evaporated to dryness, leaving a
crystallized salt containing a portion of tarry matter.
The salt is then purified by sublimation, that is, it is heated in a
closed iron vessel until it is transformed into a gas which separates
and leaves, in a carbonized state, all foreign substance. After this
gas is cooled, it condenses and again forms crystals which are in a
much purer condition. If necessary to further purify it, it is again
sublimed. The iron vessels in which the sublimation takes place are
lined with clay and covered with lead. The clay lining and lead
covering are necessary, for if the gas evolved during the process of
sublimation came in contact with the iron surface, the gas would be
contaminated and the iron corroded. Sublimed sal-ammoniac has a
fibrous texture and is tough and difficult to powder. It has a sharp,
salty taste and is soluble in two and a half parts of cold and in a
much smaller quantity of hot water. During the process of sublimation
the ammonia is not decomposed. But there are several ways in which the
gas may be decomposed, and a certain portion of it is decomposed in
the ordinary use of it in refrigerating machines. If electric sparks
are passed through the gas, it suffers decomposition, the nitrogen and
hydrogen then being in the condition of a simple mixture. When
decomposed in this manner, the volume of the gas is doubled and the
proportion is found to be three measures of hydrogen to one of
nitrogen, while the weight of the two constituents is in the
proportion of three parts hydrogen to fourteen of nitrogen.
The ammonia gas may also be decomposed by passing through a red hot
tube, and the presence of heated iron causes a slight degree of
decomposition. This sal-ammoniac is powdered and mixed with moist
slaked lime and then gently heated in a flask, when a large quantity
of gaseous ammonia is disengaged. The gas must be collected over
mercury or by displacement. The gas thus produced has a strong,
pungent odor, as can easily be determined by any one working around
the ammonia ice or refrigerating machines, for as our friend, Otto
Luhr, says, "It is the worst stuff I ever smelled in my life." The gas
is highly alkaline and combines readily with acids, completely
neutralizing them, and the aqua ammonia is one of the best substances
to put on a place burned by sulphuric acid, as has been learned by
those working with that substance, for although aqua ammonia of full
strength is highly corrosive and of itself will blister the flesh, yet
when used to neutralize the effect of a burn from sulphuric acid its
great affinity for the acid prevents it from injuring the skin under
such conditions.
The distilled gas, such as has just been described, is the anhydrous
ammonia used in the compressor system of refrigeration, while it is
the aqua ammonia that is used in the absorption system of
refrigeration. Aqua ammonia or liquor ammonia is formed by dissolving
the ammonia gas in water. One volume of water will dissolve seven
hundred times its bulk of this gas, and is then known as aqua ammonia,
in contradistinction to anhydrous ammonia, the latter designating term
meaning without water, while the term aqua is the Latin word for
water.
Anhydrous ammonia, the gas, may be reduced to the liquid form at
ordinary temperatures when submitted to a pressure of about 95 pounds.
During the process of liquefaction the ammonia gives up a large amount
of heat, which if absorbed or radiated while the ammonia is in the
liquid condition, the gas when allowed to expand will absorb from its
surroundings an amount of heat equal to that radiated, producing a
very great lowering of temperature. It is this principle that is
utilized in refrigeration and ice making. In the absorption system,
where aqua ammonia is used, the liquor is contained in a retort to
which heat is applied by means of a steam coil, and a great part of
the gas which was held in solution by the water is expelled, and
carries with it a small amount of water or vapor. This passes into a
separator in the top of a condenser, from which the water returns
again to the retort, the ammonia gas, under considerable pressure,
passing into the coolers. These are large receptacles in which the gas
is permitted to expand. By such expansion heat is absorbed and the
temperature of the surroundings is lowered. From the coolers the gas
returns to the absorber, from which it is pumped, in liquid form, into
the retort, to be again heated, the gas expelled and the process
repeated. As the gas passes through the different processes, being
heated under pressure, cooled, expanded again, more or less
decomposition takes place, presumably from a combination of a small
portion of the nitrogen with vegetable, animal, or mineral matter that
finds its way into the system. Such decomposition, with the loss of
nitrogen, leaves a small portion of free hydrogen, which is the gas
that can be drawn from the top of the absorber, ignited and burned.
The presence of hydrogen gas in the absorber is not necessarily
detrimental to the effectiveness of the system, but as hydrogen does
not possess the qualities of absorbing heat in the same way and to the
same extent as ammonia, the presence of hydrogen makes the operation
of the apparatus somewhat less efficient.--_Stationary Engineer._
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The refrigerating apparatus illustrated and described in the
SCIENTIFIC AMERICAN SUPPLEMENT of June 25, No. 812, is substantially
that patented by Messrs. Erny, Subers & Hoos, of Philadelphia. The
illustration was copied from their patents of November and February
last.
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