Absorption
is another type of machine built to eliminate the strong aqua pump. The out standing features are the elimination of the expansion valve by a device whereby the pressure in the system is made constant through out the cycle, and the addition of a charge of an inert gas, usually hydrogen, in the evaporating coils and the absorber. As the pressure in the absorber and the generator is the same, a force sufficient to overcome only the frictional resistance of the pipes is required. This unbalanced force is obtained by means of the difference in weights of two columns, one (the strong aqua) connected with the heating coils, and therefore of less density, and the other with the cooler and more dense weak aqua. The action of the hydrogen in the system is very interesting. The boiling temperature of the ammonia in the evaporat ing coils is dependent on the pressure exerted by the ammonia which in turn is determined by the proportionate amount of hydrogen pres ent. The underlying principle is that of partial pressures by which the hydrogen is charged into the evaporating coils and the absorber in such an amount as will limit the ammonia pressure to 3o or 451b. per square inch. Recently other adsorbents for the gaseous refrigerants have been developed, one of which is silica gel. Such adsorhers are solid materials composed of a large number of minute air cells, capable of adsorbing sulphur dioxide as well as water vapour.
The working medium, or refrigerant, is contained in refrigeration in a closed system and is used continuously. It is dear enough to make it desirable to keep leaks down to the lowest point possible and, with ammonia, dangerous enough so that excessive leaks are to be carefully avoided. The result is that the design of the system has the prevention of even nominal leaks as a first consideration. In consequence the pipe fittings are made specially strong, and are either of dense, air furnace, cast iron or are drop forgings. The screw joint is made up with litharge and glycerine or a soldered joint is made up by tinning the screw threads. With drop forgings the fitting may be welded to the pipe, and this has been found especially advantageous in the case of carbonic systems.
The piping may be either steel or wrought iron and usually of standard weight. The pipe is usually lap-welded in sizes tin. or over and butt-welded in smaller sizes.
Design has undergone considerable advancement since 1918. At first, safety dictated the use of pipe condensers, arranged in stands of 3o or more pipes high and with pipes croft. long, but the height of the stands has been gradually decreased until now the usual number is 12 pipes. In these pipe condensers the gas from the compressor enters at one end of the condenser, and the refrigerant is condensed during its passage through the 200 or more feet of pipe composing the condenser. Attempts to achieve counter-flow of the ammonia and the condensing water and more control in the velocity of the water have resulted in the following types of condensers: the submerged, the common atmos pheric, the double pipe and the flooded designs. The more recent
designs are the shell and tube single-pass and the shell and tube multi-pass condensers. In the multi-pass type the water fills the tube whereas in the vertical shell and tube condenser the water is made to flow as a film on the inside of the tubes.
Research on heat transfer in ammonia condensers has developed so that there is no advantage in the counter-flow principle, whereas it is highly desirable to remove the liquid condensate as quickly as possible from the condensing surfaces. It has been found im possible to subcool the liquid unless the liquid is free of its own vapour and so the elaborate attempts to achieve the counter-flow arrangement are not worth the trouble. On the other hand, the ordinary condenser has 5o% of all liquefaction occurring in a small portion of the condenser where the condensing water enters the condenser. This part of the condenser is the upper 3ft. of the shell and tube and the lower two pipes of the double pipe con denser.
There does not appear to be any definite rule in regard to the use of brine or direct ex pansion. The use of direct expansion means a lesser cost of operation inasmuch as the suction pressure required will be less when cooling brine in order that the latter may be used to cool air, or other substances to be refrigerated than when the evaporat ing coils are used directly for the desired cooling. Such a double cooling means that more horse-power per ton of useful refrigera tion is required than with direct expansion. However, brine makes it possible to have considerable flexibility and more safety. A brine spray may be used for cooling air, or brine storage may be utilized for the purpose of storing up refrigeration to be used to carry the load during a "peak" or during the intermittent operation of the refrigerating machine. The meat-packing organ izations use brine in the majority of cases, whereas cold storage plants frequently use direct expansion if the system is self con tained. District cooling plants, as found in certain localities in large cities, prefer brine because of the reduced danger due to leaks, loss of refrigerant and excessive pressure drop in the long suction lines.
The kind of brine is confined to calcium and sodium chloride, and the former is used where temperatures of io° F or lower are required, because of the ability of calcium chloride to take low temperatures without freezing. The corrosive action of brine with respect to metals is a very moot point. As a rule there is little tendency for "corrosion" if the brine has a specific gravity of 1.2 and is kept neutral or slightly alkaline. The use of mixtures of several salts is very unwise as this tends to corrosion. It has been found also that any brine which is unstable in character and tends to become acid will be bound to cause excessive "corrosion." The best means of preventing corrosion in the open or closed brine systems is to use sodium dichromate in sufficient quantities to make the brine slightly alkaline. Zinc dust may be used to ad vantage in open calcium chloride brine circuits.