Document q2MEpYJZL6wVKdEv00dqMJKx

874 CHAPTER 37 1955 Guidep Vg? Evaporation with the vapors leaving slightly superheated. In flooded^ evaporators not all of the refrigerant is evaporated, the liquid-vapor mix- ;, ture leaving the evaporator flows into a surge drum from which the vapors - are drawn into the compressor suction line, and the liquid is recirculated through the evaporator. ^. The types of coolers used in connection with air conditioning work fall into three general groups: (1) direct water coolers, (2) direct air coolers; and (3) brine coolers for circulation of the brine in a closed system, and thus cooling indirectly either water or air. 1. Water coolers. One method of the direct cooling of water is to install direct expansion coils in the spray chamber so that the water sprayed into the air comes in direct contact with the cooling coils. Another common and efficient method of cooling spray water is to use a Baudelot type of heat absorber where the water flows over direct expansion coils at a rate sufficiently high to give efficient heat transfer from water to refrigerant. Another type of spray water cooler is the shell and tube heat exchanger in which the refrigerant is expanded into a shell enclosing the tubes through which the water flows. The velocity of the water in the tubes affects the rate of heat transfer, and as the refrigerant is in the shell completely surrounding the tubes at all times, good contact and a high rate of heat transfer are insured. The disadvantage of such a system is that with the falling off of load on the compressor, the suction tempera ture or the temperature in the evaporator drops, and there is a possibility of freezing the water in the tubes, which, of course, might split the tubes and allow the re frigerant to escape into the water passage. This danger can be eliminated by auto matic safety devices. Another system of cooling spray water is to submerge coils in the spray collecting tank, or in a separate tank used for storage. The heat transmission through the walls of the coils, however, is low and a great deal more surface is required than for any other type of cooler. However, with large storage tanks this type of cooling can be utilized to advantage. 2. Air coolers. When direct cooling of air is employed, the refrigerant is inside the coil and the air passes over it. Cooling depends upon convection and con duction for removing the heat from the air. The type of coil used can be either smooth or finned, the finned coil being more economical in space requirement than the smooth coil. The fins, however, must be far enough apart so as not to retain the moisture which condenses out of the air. When refrigeration evaporators are used for cooling air or other gases by frcj^ convection, they are usually termed blast coils or unit coolers. A blast coil may.. placed in a duct or in an assembled unit, and the air forced across the coil and dis charged through distributing ducts or directly into the space to be conditioned. Unit coolers, designed much like unit heaters, consist of a finned coil, propeller fan, and controls suspended directly in the space to be cooled. 3. Indirect brine coolers. The indirect cooler, where brine is cooled by the re frigerant and the resulting cold brine is used to cool either air or water, introduces several other considerations. It is not the most economical from a power consump tion standpoint, as it is necessary to cool the brine to a temperature sufficiently so that there is an appreciable difference between the average brine temperature and that of the substance being cooled. This requires that the temperature of tne refrigerant must be still lower, and consequently the amount of power required to produce a given amount of refrigeration increases due to the higher compression ratio. There are other considerations which make such a system desirable. In tne first place, where a toxic refrigerant is undesirable or cannot be used because of nre or other risks, especially in densely populated areas, the brine can be cooled m an isolated room or building and can then be circulated through the air conditioning equipment. This arrangement eliminates any possibility of direct contact betwee the air and refrigerant. REFRIGERATION CONTROL In addition to the compressor, evaporator, and condenser, several auxih' aries are required for proper operation of a refrigeration system. Some device must be supplied for the controlled expansion of the refrigerant fronJ Refrigeration 875 the high condenser pressure to the low evaporator pressure; controls are required for the on-off operation of the compressor, the flow of the con densing medium, and for safety devices; proper piping is required for connecting the various portions of the systems. Where refrigerating appa ratus is used for the cooling of rooms, additional controls are required. Expansion Devices Some form of expansion device must be provided to control the rate of flow of the liquid refrigerant between the high and low side pressures of the system. This device is usually an expansion valve and m&y be either manual or automatic; however, with few exceptions, manual valves are obsolete and no longer used. Automatic Expansion Valves. An automatic or pressure controlled expansion valve operates to maintain a constant pressure in the evaporator. The liquid re frigerant passes through an orifice, the opening size of which is controlled by means Fig. 13. Typical Thermostatic Expansion Valve ^'t\Deet^e y^ve connected to a flexible bellows. This bellows expands or contracts variations in the evaporator pressure transmitted to the expansion chamber through the refrigerant outlet from the evaporator. The position of this needle valve is controlled by the degree of compression in an adjustable spring, balanced against the bellows, and these two forces operate to maintain a constant pressure in tne evaporator by increasing or decreasing the flow of liquid refrigerant. Such an xpansion valve is usually applied to evaporators of the direct expansion type, but is not satisfactory for fluctuating loads such as are encountered in air conditioning installations. ' ^vh7\WSlaiic Expansion Valves. A thermostatic expansion valve controls the of hquid refrigerant to the evaporator so as to maintain the entire coil filled 1 evaPorabng refrigerant, and to keep a constant superheat iu the refrigerant gas to t{,nf * co^' The construction of such a valve is shown in Fig. 13 and is similar elem t *or an automatic expansion valve but incorporates, in addition, a power Ieav'6n LeS*)-n8^ve Ganges in the degree of superheat of the refrigerant gas toe coil. This power element consists of a bellows connected by means of a The hit. to a feeler bulb fastened to the suction line from the evaporator, used k bellows, and tube are usually charged with the same liquid refrigerant great10 evaP.rator itself. A starved condition in the evaporator results in a the r>Gr Superheat in the gas leaving the evaporator, and this in turn operates through redur^k^61?1611^ increase toe flow of liquid refrigerant. A flooded evaporator frj es toe discharge superheat, and thus tends to reduce the flow of liquid re- load8^' u.h an expansion valve is satisfactory for operation with fluctuating all tin?6 contool tends to keep the evaporator filled with refrigerant Float Valves. A liquid refrigerant control of the low-side float valve onsists of a ball float located in either a receiver or the evaporator itself on