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944 CHAPTER 36 1958 Guide units use only a small portion of the water required for a waste water cooled con denser. The water is vaporized by the heat of the refrigerant so that each pound of water used extracts approximately 1000 Btu from the refrigerant, whereas under standard rating conditions where the water temperature rise is 20 F, each pound of water extracts only 20 Btu from the refrigerant. The water used, including the loss by entrainment in the discharge air, by blow down and the stand-by evaporation, amounts to about 3 to 5 percent of that which would be required for a water cooled condenser. The evaporative condenser requires more maintenance, occupies greater space (must be located where air is available), and has a higher first cost than the water cooled condenser, but where the use of water is restricted or expensive, the evapora tive condenser has become widely accepted. Compared with a water cooled con denser and cooling tower, which combination uses about the same quantity of water, the evaporative condenser has the advantage of lower cost and smaller space re quirements. Evaporators and Coolers Refrigeration evaporators must be designed for efficient removal of heat from the medium being cooled, as well as effective boiling of the refrigerant and a minimum drop of pressure through the coil. There are two general types of evaporators, dry and flooded. In the dry evaporator the re frigerant enters in the liquid state, and the design provides for complete 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 frc^ convection, they are usually termed blast coils or unit coolers. A blast coil mayo 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- Refrigeration 945 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 low 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 the 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 the first place, where a toxic refrigerant is undesirable or cannot be used because of fire or other risks, especially in densely populated areas, the brine can be cooled in an isolated room or building and can then be circulated through the. air conditioning equipment. This arrangement eliminates any possibility of direct contact between the air and refrigerant. REFRIGERATION CONTROL \ Expansion Devices In addition to means of controlling the compressor capacity and the necessary limit and safety controls as outlined under the section describing Fig. 38. Typical Thebmostatic Expansion Valve the compressors, reciprocating compressor refrigeration systems require a device for controlling the expansion of the refrigerant from the high liquid pressure to the low evaporator pressure. Automatic Expansion Valves. An automatic or pressure controlled expansion valve operates to maintain a constant pressure in the evaporator. The liquid rerigerant passes through an orifice, the opening size of which is controlled by means *needle valve connected to a flexible bellows. This bellows expands or contracts tli v?n?*'ins i.n the evaporator pressure transmitted to the expansion chamber 1U the refrigerant outlet from the evaporator. The position of this needle a Ve Is controlled by the degree of compression in an adjustable spring, balanced thpUnSt *' Allows, and these two forces operate to maintain a constant pressure in evaporat,or by increasing or decreasing the flow of liquid refrigerant. Such an is n?SIn- va^ve i usually applied to evaporators of the direct expansion type, but insUllat'SfaCt0ry ^r uctuating loads such as are encountered in air conditioning '^static Expansion Valves. A thermostatic expansion valve controls the w;t. 01 2"ffuid refrigerant to the evaporator so as to maintain the entire coil filled leavinV tLra^-n refrigerant, and to keep a constant superheat in the refrigerant gas to th t f 6 co*'- ^he construction of such a valve is shown in Fig. 17 and is similar clemet *r an a.utmatic expansion valve but incorporates, in addition, a power leavin tLeS*>ons've to changes in the degree of superheat of the refrigerant gas g the coil. This power element consists of a bellows connected by means of a