A large number of scientific and other gentlemen interested in
mechanical refrigeration lately visited the works of Messrs. J. & E.
Hall, of Dartford, to inspect the working of one of their improved
horizontal dry air refrigerators!
The machine, which is illustrated below, is designed to deliver about
10,000 cubic feet of cold air per hour, when running at the rate of 100
revolutions per minute, and is capable of reducing the temperature of
the air from 90 deg. above, to about 50 deg. below zero, Fah., with an
initial temperature of cooling water of 90 deg. to 95 deg. Fah. It can,
however, be run at as high a speed as 140 revolutions per minute.
The air is compressed in a water-jacketed, double-acting compression
cylinder, to about 55 lb. per square inch --more or less according to
the temperature of the cooling water--the inlet valve being worked from
a cam on the crank shaft, to insure a full cylinder of air at each
stroke, and the outlet valves being self acting, specially constructed
to avoid noise in working and breakages, which have given rise to so
much annoyance in other cold air machines. The compressed air, still at
a high temperature, is then passed through a series of tubular coolers,
where it parts with a great deal of its heat, and is reduced to within
4 deg. or 5 deg. of the initial temperature of the cooling water. Here
also a considerable portion of the moisture, which, when fresh air
is being used, must of necessity enter the compression cylinder, is
condensed and deposited as water.
[Illustration: COMPRESSION CYLINDER. SCALE 1/60]
After being cooled, the compressed air is then admitted to the expansion
cylinder, but as it still contains a large quantity of water in
solution, which, if expansion was carried immediately to atmospheric
pressure, would, from the extreme cold, be converted into snow and ice,
with a positive certainty of causing great trouble in the valves and
passages. It is got rid of by a process invented by Mr. Lightfoot,
which is at the same time extremely simple and beautiful in action, and
efficient. Instead of reducing the compressed air at once to atmospheric
pressure, it is at first only partially expanded to such an extent that
the temperature is lowered to about 35 deg. to 40 deg. Fah., with the
result that very nearly the whole of the contained aqueous vapor is
condensed into water. The partially expanded air which now contains the
water as a thick mist is then admitted into a vessel containing a number
of grids, through which it passes, parting all the while with its
moisture, which gradually collects at the bottom and is blown off. The
surface area of the grids is so arranged that by the time the air has
passed through them it is quite free from moisture, with the exception
of the very trifling amount which it can hold in solution at about 35
deg. Fah., and 30 lb. pressure. The expansion is then continued to
atmospheric pressure and the cooled air containing only a trace of snow
is then discharged ready for use into a meat chamber or elsewhere. In
small machines the double expansion is carried out in one cylinder
containing a piston with a trunk, the annulus forming the first
expansion and the whole piston area the second, but in larger machines
two cylinders of different sizes are used, just as in an ordinary
compound engine. To compensate for the varying temperature of the
cooling water the cut-off valve to the first or primary expansion is
made adjustable; and this can either be regulated as occasion requires
by hand, or else automatically. The temperature in the depositors being
kept constant under all variations in cooling water, there is the same
abstraction of moisture in the tropics as in colder climates, and the
cold air finally discharged from the machine is also kept at a uniform
temperature.
[Illustration: Expansion Cylinder. Scale 1/60.92° F. temperature of
entering air. Cooling water entering in at 86° F.]
[Illustration: Expansion Cylinder. Scale 1/60. 68° F. temperature of
entering air. Cooling water entering in at 65° F. 125 revs. per minute,
or 312 ft. per minute per piston speed.]
The diagrams are reduced from the originals, taken from the compression
cylinder when running at the speed of 125 revolutions per minute, and
also from the expansion cylinder, the first when the cooling water
was entering the coolers at 86 deg. Fah., and the latter when this
temperature was reduced to 65 deg. Fah. In all cases the compressed
air is cooled down to within from 3 deg. to 5 deg. of the initial
temperature of the cooling water, thus showing the great efficiency
of the cooling apparatus. The machine has been run experimentally at
Dartford, under conditions perhaps more trying than can possibly occur,
even in the tropics, the air entering the compression cylinder being
artificially heated up to 85 deg. and being supersaturated at that
temperature by a jet of steam laid on for the purpose. In this case no
more snow was formed than when dealing with aircontaining a very much
less proportion of moisture. The vapor was condensed previous to final
expansion and abstracted as water in the drying apparatus. The machine
was exhibited at work in connection with a cold chamber which was
kept at a temperature of about 10 deg. Fah., besides which several
hundredweight of ice were made in the few days during which the
experiments lasted. This machine is in all respects an improvement on
the machine which we have already illustrated. In that machine Messrs.
Hall were trammeled by being compelled to work to the plans of others.
In the present case the machine has been designed by Mr. Lightfoot, and
appears to leave little to be desired. It is a new thing that a cold air
machine may be run at any speed from 32 to 120 revolutions per minute.
In its action it is perfectly steady, and the cold air chamber is kept
entirely clear of snow. The dimensions of the machine are also eminently
favorable to its use on board ship.-_The Engineer_.
[Illustration: DRY AIR REFRIGERATING MACHINE]
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