to be built with vertical compressors as shown in fig. 33 and to be arranged for either belt- or steam-driving; and for the larger sized machines to be horizontal as shown, with simple, compound, or triple expansion steam cylinders Evaporator Guage Patent Safety Valve Separator Compressor Patent hollow Connecting Rod Driving pulley Insulated division FIG. 33. Crank shaft Brine circulating pump arranged tandem with the compressors, which are driven by tail-rods from them. The general design will be readily gathered from fig. 34 which illustrates a 120-ton refrigerating CO, machine. 2 The steam and CO2 condensers are sometimes arranged in the bed-plate and sometimes separately, and the compressors, when in duplicate, are so arranged that they can either be run together on the same work, or can be put on to separate duties, so that each compressor and steam cylinder to all intents and purposes comprises a separate machine. This feature is useful in guarding against breakdown, and affords an additional safeguard if the whole machinery is not installed in duplicate. 2 The Compressor.-Reference to the curve giving the temperatures and corresponding pressures of CO2 on page 90 will show the high pressures to be dealt with. In consequence, great soundness and strength of the various parts is essential. To meet this, the compressors are, for the larger machines, bored out of a solid steel forging, cast-iron or cast-steel being unsuitable on account of its porosity. This precaution is taken for the double purpose of securing a sound and reliable material, and of providing a perfectly smooth bore, as it is plain that any unevenness or inequalities in the cylinder would, with the pressures to be dealt with, be a very serious matter. For small machines the compressors, fig. 35, are of bronze (CO, has no deleterious action on copper). The suction and delivery valves are all made to the same design and are interchangeable. The compressor piston is packed with cup leathers similar to the ram of a hydraulic press, which makes the most gas-tight packing possible. The lantern gland is also made gas-tight by cup leathers as shown, and sealing-oil, at a higher pressure than the highest in the compressor, is forced into it between these cup leathers, keeping them tight against the piston-rod. The superior pressure is maintained on the oil by means of a patent differential piston, having one side in contact with CO2 at the con 2 denser pressure, and the other in contact with the oil which has been forced in against it by a hand-pump. As the area of the piston in contact with CO, is greater than that in contact with the oil by the area of the pistonrod, a greater pressure per square inch is maintained on the latter than on the former. The oil is connected to the compressor gland, and as any oil leaks past the cup leathers either into the compressor or to the atmosphere, it is at once replaced by oil from this cylinder. When the cylinder becomes empty, oil is again forced into it by the small hand FIG. 35. pump. A small leakage of oil into the compressor is an advantage, as it lubricates the piston and fills up the clearance spaces; in ordinary working, however, the amount of leakage in this way is small, and only a few strokes of the hand-pump are required every four or five hours to replenish the reservoir. The gas discharged from the delivery valves carries with it a certain amount of oil, which is separated before the gas passes to the condenser. The compressor is fitted with a patent safety valve formed of a copper disc, which is designed to give way at a pressure of some 1950 lbs. on the square inch. In conjunction with this disc, fig. 36 is an ordinary spring safety valve, which closes as soon as the excessive pressure has been relieved. This safety valve is, of course, provided to guard against accident due to a careless driver starting up the machine without opening the valves on the delivery side. In the case of rupture, the CO2, being cheap and harmless, is allowed to escape to the atmosphere. Safety valves have been fitted on some ammonia machines, but in that case, 62,8. FIG. 36. the gas, being more valuable and more pungent, is blown off and passed into the suction side of the plant. The Condenser.-The condenser is made of both the submerged and atmospheric type. Owing to the pressure, special hydraulic piping, usually of 13/16 bore, is employed, the lengths being electrically welded together, so as to minimize the number of joints generally, and to render it unnecessary to have any joints in an inaccessible position. These condenser coils for land work are usually of wrought-iron, but, for marine work or for any special purpose, the coils, both in the condenser and evaporator, can be made of copper either plain or tinned. The Evaporator. The evaporator is, in general design, similar to the condenser. For cold storage work and icemaking, separate brine-coolers are almost invariably employed. In this case the evaporating coils are nested one inside the other. In cold stores employing the air-blast system the air is blown directly over brine coils in much the same way as that described in Chapter XII., or in some cases over larger metal surfaces wetted and consequently cooled by the cold brine from the evaporator. General Considerations. It has frequently been urged that CO2 machines cannot work successfully under conditions found in tropical and semi-tropical countries, where the temperature of the condensing water is high. The incorrectness of this contention will be apparent from a careful examination of the entropy diagrams in Chapter II., and in this case theory is fully borne out by the results which have been obtained in practice. The compactness of the CO2 machine makes it especially suitable for work in positions where space is of especial value, such, for instance, as on board ship. The smallness of the charge and the working parts, and also the existence of the safety valve, renders a serious explosion practically impossible. The harmlessness of the gas prevents any escape that may occur from having serious consequences. 2 |