Document Mo75rEEZYLr0jXq4mvD9mVw4z

HEATINC VENTILATINC AIR CONDITIONING GUIDE 1944 obtain, as, for instance, in railway air conditioning. Even on fractional tonnage installations, air is used as the condensing medium only, where water is expensive or where simplicity of installation warrants the higher condensing pressure, and consequent higher power costs than would be obtained using water as the condensing medium. The conventional air cooled condenser consists of an extended surface coil across which air is blown by a fan. The hot discharge gas enters the coil at the top and, as it is condensed, flows to a receiver located below the condenser. Air cooled condensers should always be located in a well ventilated space so that the heated air may escape and be replaced by cooled air. The principal disadvantages of air cooled condensers are the power required to move the air and the reduction of capacity on hot days. This loss of capacity due to high condensing pressures on hot days requires that equipment of increased capacity be selected to meet the peak load. Thus at normal loads the equipment is oversized. Water Cooled Water cooled condensers are of the double pipe type, the shell and tube type, or the shell and coil type. Double pipe condensers are arranged so that water passes through the inner of two concentric pipes and refrig erant circulates through' the annular space between the pipes. `Where possible, there should be counter-flow of the refrigerant and the con-' densing water to obtain maximum temperature differences. This type is usually used only with small condensing units. The amount and temperature of the condensing water determine the condensing temperature and pressure, and indirectly the power required for compression. It is therefore necessary to determine a balance so that the quantity of water insures economical compressor operation. Because there is a decided tendency to conserve the water in city mains and because most large cities are restricting the use,of water for air con ditioning and refrigeration equipment, it is often, necessary to install cooling towers or evaporative condensers. Cooling towers; unfortunately, produce the warmest condensing water at the time when the load on the system is greatest, so that the refrigeration equipment must be designed to meet the maximum load at abnormal condensing water temperatures. If properly designed, this makes little difference in the efficiency of operation throughout the year except at those times when the condensing water temperature is highest. As this occurs only for 5 per cent of the entire cooling period it can be disregarded as a factor in establishing yearly operating costs. The cooling tower has a certain advantage over the use of Water from the city mains. Economies are possible when a cooling tower is used, which cannot be achieved by the use of condensing water from city mains. In certain localities, the lowest city water temperature met during the summer months is from 65 to 70 F. This temperature range takes place for the entire cooling period, regardless of the outdoor temperature. With a cooling tower, the temperature of the condensing water may rise to 80 or 85 F under maximum conditions, but under less than maximum, conditions the temperature of the water leaving the cooling tower drops 480 CHAPTER 25. REFRICERATION considerably. It has been established that in these localities during 50 per cent of the time, the outdoor wet-bulb temperature varies from 60 to 70 F and the cooling tower water, for the same periods, varies from 65 to 75 F. When the outdoor wet-bulb temperature drops below 60 F, which occurs approximately 30 per cent of the.time, the condensing water temperature is still lower. The cost of water used for condensing is small as the only water required is that used to make up the loss by evaporation Fig. 8. Schematic View of an Evaporative Condenser in the cooling tower itself. Refer to the section on Cooling Towers in Chapter 27. , Shell and coil condensers are in general use for medium sized condensing units, and consist of a coil of tubing mounted inside a shell. The cooling water passes: through the coil. Evaporative Condensers; Due to the high cost of city water for condenser purposes, and due to ordinances in some localities prohibiting the discharge of large quantities of such water into the sewage systems, there has been developed a con denser which uses a minimum amount of water on a finned surface; cooling 481