Document RaRjxrgod4QGpM4dRVYmR02g7

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 A diagrammatic representation of a typical steam ejector water cooling system is shown in Fig. 3. The figures correspond to an average repre sentative system. The water to be cooled enters the evaporator and is cooled to a temperature corresponding to the vacuum maintained. Because of the high vacuum, a small amount of the water introduced in the evaporator is flashed into steam. As this requires heat, and the only source of heat is the rest of the water in the evaporator tank, this other water is almost instantly cooled to a temperature corresponding to the boiling point determined by the vacuum maintained. The amount of water flashed into steam is a small percentage of the total water circu lated through the evaporator, amounting to approximately 11 lb per hour' per ton of refrigeration developed. The remainder of the water at the CHAPTER 24. REFRIGERATION 5 to 200 lb per square inch and condenser water temperatures as high as 90 F. The steam consumption in pounds per hour per ton of refrigeration increases rapidly as the booster steam pressure is lowered. For example, the lowering of the booster steam pressure from 200 to 90 Jb per square inch results in an increase in steam consumption of approximately 5 per cent whereas a further decrease in booster steam pressure to 10 lb per square inch increases the steam consumption by approximately 72 per cent over that required at 200 lb per square inch. The capacity of a steam jet system is usually controlled by controlling the number of boosters in use since the unit usually has several boosters operating on the same evaporator. Usually one booster is automatically Fig. 3. Diagrammatic Arrangement of Steam Jet Vacuum Cooling Unit desired low temperature is pumped out of the evaporator and used at the point where it is required. The ejector compresses the vapor which has been flashed in the evapor ator, plus any entrained air taken from the circulated water, to a some what higher absolute pressure and the vapor and air mix with the impel ling steam on the discharge side of the jet. The total mixture then passes from the ejector into the condenser. The slight amount of air which may be entrained in the cooled water is removed by a small secondary ejector which raises the pressure suffi ciently so that the air can be discharged to the atmosphere. A secondary condenser is necessary to condense the steam in the secondary jet. Although steam jet vacuum cooling units have been built for as small as 5 to 6 tons capacity, a single booster of smaller than 15 tons capacity is difficult to build. They can readily be built for steam pressures of from 452 controlled whereas the others are manually operated. The capacity is dependent, as for all compressors, upon the evaporator temperature, or in other words, the suction pressure. For example, the capacity is lowered approximately 17 per cent if the evaporator or chilled water temperature is lowered from 50 to 45 F. The capacity therefore can be controlled to some extent by regulating the evaporator temperature. CHARACTERISTICS OF COMPRESSION SYSTEMS The various types of compression systems have quite different charac teristics of capacity and power with varying evaporator and condenser temperatures, as may be noted from curves in Figs. 4 and 5. 453