Modern mechanical re frigerating machines are radically different from those of a few decades ago. Modern design calls for mass production, especially in the smaller sizes. The large capacities, slow revolution, long stroke machines, steam engine driven, have been replaced by elec tric motors, frequently of the synchronous type direct connected to the compressor, with the compressors designed for rotative speeds up' to 400 revolutions per minute. The enclosed type of compressor, of the single-acting, vertical design, is almost uni versally used, with 2, 3, and even 4 cylinders, the latter up to Boo tons of refrigeration, on the same crank-shaft. The stroke is very short, frequently with the piston diameter 5o% greater than the stroke. Mechanically operated valves are rare, and they are usu ally of the ring plate type of design, made of chrome-vana dium steel, hardened and ground to a true surface; how ever, the light-weight, spring balanced cushioned poppet valve is still used. Lubrication is forced to every bearing, and the oil pump suction is equipped with filters. Methyl chloride and dichlorodifluoro methane as well as methylene chloride are miscible in the usual mineral oils used in re frigeration, thereby making a special design, necessary to sep arate the oil from the return suction gas. This is accom plished in the refrigerating com pressor by a special trap in the manifold of the compressor. The pistons are supplied with oil rings, to prevent oil pumping, as well as the usual compression rings. In particular, the use of automatic devices, making ef ficient operation possible with the minimum of manual control, has materially changed the design, the operating details of the re frigeration unit. The use of the halide group of refrigerants, es pecially dichlorodifluoromethane as well as methyl chloride has made possible the air-cooled compressor using fins on the cylin ders, and a considerably lighter construction of the compressor. When water-jacketed cylinders are employed, as with ammonia compressors, the jacket is confined to the neighbourhood of the head. With high rotative speeds the effect of the jacket water on the amount of superheat in the compressed vapour at the end of the compression stroke is not very great, but when used the water in the jacket is expected to keep the lubricating oil reasonably effective.
Little attention is paid to clearance in the cylinder although at one time the clearance volume in per cent of the piston dis placement was reduced to a minimum. Clearance is considered to act as a compressed spring and to have little effect except to reduce the effective pumping capacity of the refrigerant, and con trol of the refrigerating capacity for constant speed operation is obtained by the use of clearance pockets under the control of the operator or by by-passing part of the vapour to be compressed to the suction side of the compressor. A type of modern (1938) compressor can be used as a booster compressor. If designed for this purpose it would be of relatively large piston displacement, short stroke and of light construction. It would handle the low pressure stage of the load and as the suction vapour temperature would be quite low, the use of the water jacket would be dis pensed with even for ammonia.
was more important about 1913 than in 1939. This refrigeration machine requires the direct application of heat instead of mechanical work in order to increase the pressure of the gas to a point where liquefaction may occur in the condenser. The heat necessary for this action may be due to the burning of gas or it may be obtained from high pressure or exhaust steam. In the United States the absorption machine is used where exhaust steam is plentifully available and also in the small, fractional-tonnage units.
The principle of the action of the absorption machine is that a solution of ammonia and water is used in a still or generator where heat is applied. The action of the heat is to drive out some of the gas from the solution under a pressure corresponding to the condenser pressure, and the gas is condensed, in the condenser, and passes to the evaporator in the same manner as is the case in the compressor system. However, the gas from the evaporator has to be returned as a solution, or strong aqua, to the generator. This is done by sending the weak aqua, which is at the lower part of the generator, to the absorber having cooled it previously, and there bringing it into intimate contact with the gas from the evaporator. The resulting strong aqua occupies very little volume in comparison with that of the gas from the evaporating coils, and may be pumped by means of a strong aqua pump into the generator.
The advantages of the absorption machine are the use of ex haust steam at very low cost. It is used in the United States in packing plants, oil refineries and in some hotels where a combi nation of the steam-driven compressor and the absorption ma chine works out economically at times. The absorption machine will also take a low temperature of evaporation without diffi culty because the volume of the strong aqua from the absorber is not affected by the evaporator pressure in any way. However, these advantages had lost their importance by 1928 because of the economical advantage of stage compression.
The absorption machine described above has one moving part, the so-called strong aqua pump. In order to eliminate all moving parts the machine may be modified in such a way that it then will operate intermittently. In the intermittent absorption machine the still or generator is also used as an absorber. During the period when heat is applied to the gen erator the ammonia gas is distilled off to the condenser to be con densed and finally stored in a liquid receiver. When the required amount of liquid has been accumulated the steam or gas is shut off from the generator, cooling water is turned on and the expansion valve opens. As liquid passes into the evaporating coils refrigeration is ac complished and the gas proceeds to the generator now acting as an absorber, and is kept cool by the water-cooling coils. The intermittent absorption machine has less surface than the regular absorption ma chine as it eliminates the absorber, the exchanger and the weak aqua cooler as well as the strong aqua pump. It cannot, however, give continuous refrigeration. The machine operates automatically.