Within the last few years considerable progress has been made in the
application of refrigerating processes to industrial purposes, and the
demand for refrigerating apparatus thus created has led to the
production of machines employing various substances as the
refrigerating agent. In a paper read by the author before the
Institution of Mechanical Engineers, in May, 1886, these systems were
shortly described, and general comparisons given as to their
respective merits, scope of application, and cost of working. In the
present paper it is proposed to deal entirely with the use of ammonia
as a refrigerating agent, and to deal with it in a more full and
comprehensive manner than was possible in a paper devoted to the
consideration of a number of different systems and apparatus. In the
United States and in Germany, as well as to some extent elsewhere,
ammonia has been very generally employed for refrigerating purposes
during the last ten years or so. In this country, however, its
application has been extremely limited; and even at the present time
there are but few ammonia machines successfully at work in Great
Britain. No doubt this is, to a large extent, due to the fact that in
the United States and in Germany there existed certain stimulating
causes, both as regards climate and manufactures, while in this
country, on the other hand, these causes were present only in a
modified degree, or were absent altogether. The consequence was that
up to a comparatively recent date the only machine manufactured on
anything like a commercial scale was the original Harrison's ether
machine, first produced by Siebe, about the year 1857--a machine
which, though answering its purpose as a refrigerator, was both costly
to make and costly to work. In 1878 the desirability of supplementing
our then existing meat supply by means of the large stocks in our
colonies and abroad led to the rapid development of the special class
of refrigerating apparatus commonly known as the dry air refrigerator,
which, in the first instance, was specially designed for use on board
ship, where it was considered undesirable to employ chemical
refrigerants. Owing to their simplicity, and perhaps also to their
novelty, these cold air machines have very frequently been applied on
land, under circumstances in which the same result could have been
obtained with much greater economy by the use of ammonia or some other
chemical agent. Recently, however, more attention has been directed to
the question of economy, and consideration is now being given to the
applicability of certain machines to certain special purposes, with
the result that ammonia--which is the agent that, in our present state
of knowledge, gives as a rule the best results for large
installations, while on land at any rate its application for all
refrigerating purposes presents no unusual difficulties--promises to
become largely adopted. It is hoped, therefore, that the following
paper respecting its use will be of interest.
In all cases where a liquid is employed, the refrigerating action is
produced by the change in physical state from the liquid to the
vaporous form. It is, of course, well known that such a change can
only be brought about by the acquirement of heat; and for the purpose
of refrigeration (by which must be understood the abstraction of heat
at temperatures below the normal) it is obvious that, other things
being equal, that liquid is the best which has the highest heat of
vaporization, because with it the least quantity has to be dealt with
in order to produce a given result. In fact, however, liquids vary,
not only in the amount of heat required to vaporize them (this amount
also varying according to the temperature or pressure at which
vaporization occurs), but also in the conditions under which such
change can be effected. For instance, water has an extremely high
latent heat, but as its boiling point at atmospheric pressure is also
high, evaporation at such temperatures as would enable it to be used
for refrigerating purposes can only be effected under an almost
perfect vacuum. The boiling point of anhydrous ammonia, on the other
hand, is 37½° below zero F. at atmospheric pressure, and therefore for
all ordinary cooling purposes its evaporation can take place at
pressures considerably above that of our atmosphere. Some other agents
used for refrigerating purposes are methylic ether, Pictet's liquid,
sulphur dioxide, and ether. In this connection it should be stated
that Pictet's liquid is a compound of carbon dioxide and sulphur
dioxide, and is said to possess the property of having vapor tensions
not only much below those of pure carbon dioxide at equal
temperatures, but even below those of pure sulphur dioxide at
temperatures above 78° F. The considerations, therefore, which chiefly
influence the selection of a liquid refrigerating agent are:
1. The amount of heat required to effect the change from the liquid to
the vaporous state, commonly called the latent heat of vaporization.
2. The temperatures and pressures at which such change can be
effected.
This latter attribute is of twofold importance; for, in order to avoid
the renewal of the agent, it is necessary to deprive it of the heat
acquired during vaporization, under such conditions as will cause it
to assume the liquid form, and thus become again available for
refrigeration. As this rejection of heat can only take place if the
temperature of the vapor is somewhat above that of the cooling body
which receives the heat, and which, for obvious reasons, is in all
cases water, the liquefying pressure at the temperature of the cooling
water, and the facility with which this pressure can be reached and
maintained, is of great importance in the practical working of any
refrigerating apparatus. Ammonia in its anhydrous form, the use of
which is specially dealt with in this paper, is a liquid having at
atmospheric pressure a latent heat of vaporization of 900, and a
boiling point at the same pressure of 37½° below zero F. Water being
unity, the specific gravity of the liquid at a temperature of 40° F.
is 0.76, and the specific gravity of its vapor is 0.59, air being
unity. In the use of ammonia, two distinct systems are employed. So
far, however, as the mere evaporating or refrigerating part of the
process is concerned, it is the same in both. The object is to
evaporate the liquid anhydrous ammonia at such tension and in such
quantity as will produce the required cooling effect. The actual
tension under which this evaporation should be effected in any
particular case depends entirely upon the temperature at which the
acquirement of heat is to take place, or, in other words, on the
temperature of the material to be cooled. The higher the temperature,
the higher may be the evaporating pressure, and therefore the higher
is the density of the vapor, the greater the weight of liquid
evaporated in a given time, and the greater the amount of heat
abstracted. On the other hand, it must be remembered that, as in the
case of water, the lower the temperature of the evaporating liquid,
the higher is the heat of vaporization. It is in the method of
securing the rejection of heat during condensation of the vapor that
the two systems diverge, and it will be convenient to consider each of
these separately.
_The Absorption Process._--The principle employed in this process is
physical rather than mechanical. Ordinary ammonia liquor of commerce
of 0.880 specific gravity, which contains about 38 per cent. by weight
of pure ammonia and 62 per cent. of water, is introduced into a vessel
named the generator. This vessel is heated by means of steam
circulating through coils of iron piping, and a mixed vapor of ammonia
and water is driven off. This mixed vapor is then passed into a second
vessel, in order to be subjected to the cooling action of water. And
here, owing to the difference between the boiling points of water and
ammonia, fractional condensation takes place, the bulk of the water,
which condenses first, being caught and run back to the generator,
while the ammonia in a nearly anhydrous state is condensed and
collected in the lower part of the vessel.
This process of fractional condensation is due to Rees Reece, and
forms an important feature in the modern absorption machine. Prior to
the introduction of this invention, the water evaporated in the
generator was condensed with the ammonia, and interfered very
seriously with the efficiency of the process by reducing the power of
the refrigerating agent by raising its boiling point. In the improved
form of apparatus, ammonia is obtained in a nearly anhydrous
condition, and in this state passes on to the refrigerator. In this
vessel, which is in communication with another vessel called the
absorber, containing cold water or very weak ammonia liquor,
evaporation takes place, owing to the readiness with which cold water
or weak liquor absorbs the ammonia, water at 59° Fahr. absorbing 727
times its volume of ammonia vapor. The heat necessary to effect this
vaporization is abstracted from brine or other liquid, which is
circulated through the refrigerator by means of a pump. Owing to the
absorption of ammonia, the weak liquor in the absorber becomes
strengthened, and it is then pumped back into the generating vessel to
be again dealt with as above described.
The absorption apparatus, as applied for cooling purposes, consists of
a generator, which is a vessel of cast iron containing coils of iron
piping to which steam at any convenient pressure is supplied; an
analyzer, in which a portion of the water vapor is condensed, and from
which it flows back immediately into the generator; a rectifier and
condenser, in the upper portion of which a further condensation of
water vapor and a little ammonia takes place, the liquid thus formed
passing back by a pipe to the analyzer and thence to the generator,
while in the lower portion the ammonia vapor is condensed and
collected; and a refrigerator or cooler, into which the nearly
anhydrous liquid obtained in the condenser is admitted by a pipe and
regulating valve, and allowed to evaporate, the upper portion being in
communication with the absorber.
Through this vessel weak liquor, which has been deprived of its
ammonia in the generator, is continually circulated, after being first
cooled in an economizer by an opposite current of strong cold liquor
passing from the absorber to the generator, while, in addition, the
liquor in the absorber, which would become heated by the liberation of
heat due to the absorption and consequent liquefaction of the ammonia
vapor, is still further cooled by the circulation of cold water. As
the pressure in the absorber is much lower than that in the generator,
the strong liquor has to be pumped into the latter vessel, and for
this purpose pumps are provided. Though of necessity the various
operations have been described separately, the process is a continuous
one, strong liquor from the absorber being constantly pumped into the
generator through the heater or economizer, while nearly anhydrous
liquid ammonia is being continually formed in the condenser, then
evaporated in the refrigerator and absorbed by the cool weak liquor
passing through the absorber.
Putting aside the effect of losses from radiation, etc., all the heat
expended in the generator will be taken up by the water passing
through the condenser, less that portion due to the condensation of
the water vapor in the analyzer, and plus the amount due to the
difference between the temperature of the liquid as it enters the
generator and the temperature at which it leaves the condenser. In the
refrigerator the liquid ammonia, in becoming vaporized, will take up
the precise quantity of heat that was given off during its cooling and
liquefaction in the condenser, plus the amount due to the difference
in heat of vaporization, owing to the lower pressure at which the
change of state takes place in the refrigerator, and less the small
amount due to the difference in temperature between the vapor entering
the condenser and that leaving the refrigerator, less also the amount
necessary to cool the liquid ammonia to the refrigerator temperature.
When the vapor enters into solution with the weak liquor in the
absorber, the heat taken up in the refrigerator is imparted to the
cooling water, subject also to corrections for differences of pressure
and temperature. The sources of loss in such an apparatus are:
a. Radiation and conduction of heat from all vessels and pipes above
normal temperature, which can, to a large extent, be prevented by
lagging.
b. Conduction of heat from without into all vessels and pipes that are
below normal temperature, which can also to a large extent be
prevented by lagging.
c. Inefficiency of economizer, by reason of which heat obtained by the
expenditure of steam in the generator is passed on to the absorber and
there uselessly imparted to the cooling water.
d. The entrance of water into the refrigerator, due to the liquid
being not perfectly anhydrous.
e. The useless evaporation of water in the generator. With regard to
the amount of heat used, it will have been seen that the whole of that
required to vaporize the ammonia, and whatever water vapor passes off
from the generator, has to be supplied from without. Owing to the fact
that the heating takes place by means of coils, the steam passed
through may be condensed, and thus each pound can be made to give up
some 950 units of heat. With the absorption process worked by an
efficient boiler, it may be taken that 200,000 thermal units per hour
may be eliminated by the consumption of about 100 lb. of coal per
hour, with a brine temperature in the refrigerator of about 20° Fahr.
_Compression Process._--In this process ammonia is used in its
anhydrous form. So far as the action of the refrigerator is concerned,
it is precisely the same as it is in the case of the absorption
apparatus, but instead of the vapor being liquefied by absorption by
water, it is drawn from the refrigerator by a pump, by means of which
it is compressed and delivered into the condenser at such pressure as
to cause its liquefaction at the temperature of the cooling water. It
must be borne in mind, however, that allowance must be made for the
rise of temperature of the water passing through the condenser, and
also for the difference in temperature necessary in order to permit
the transfer of heat from one side of the cooling surface to the
other. In a compression machine the work applied to the pump may be
accounted for as follows:
a. Friction.
b. Heat rejected during compression and discharge.
c. Heat acquired by the ammonia in passing through the pump.
d. Work expended in discharging the compressed vapor from the pump.
But against this must be set the useful mechanical work performed by
the vapor entering the pump. The heat rejected in the condenser is the
heat of vaporization taken up in the refrigerator, less the amount due
to the higher pressure at which the change in physical state occurs,
plus the heat acquired in the pump, and less the amount due to the
difference between the temperature at which the vapor is liquefied in
the condenser and that at which it entered the pump. An ammonia
compression machine, as applied to ice making, contains ice-making
tanks, in which is circulated a brine mixture, uncongealable at any
temperature likely to be reached during the process. This brine also
circulates around coils of wrought iron pipes, in which the liquid
ammonia passing from the condenser is vaporized, the heat required for
this vaporization being obtained from the brine. A pump draws off the
ammonia vapor from the refrigerator coils, and compresses it into the
condenser, where, by means of the combined action of pressure and
cooling by water, it assumes a liquid form, and is ready to be again
passed on to the refrigerator for evaporation. The ammonia compression
process is more economical than the absorption process, and with a
good boiler and engine about 240,000 thermal units per hour can be
eliminated by the expenditure of 100 lb. of coal per hour, with a
brine temperature in the refrigerator of about 20° Fahr.