Document bOBbb85nvGEBE2pb3qBxg87Vy

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 An average ice-activated system for such capacities uses about 500 pounds of ice and 1.2 kw per hour. The increase in car weight due to such a system is approximately 8500 lb. The same service from a steam jet system is obtained with the expendi ture of 230 lb of steam and 3.3 kw per hour, with an added weight per car of 11,000 lb. The mechanical compression systems, all of which use dichlorodifluoromethane as a refrigerant, may be classified by several types depending on the method of driving the compressor. The source of power for driving the compressor (approximately 10 hp) is complicated by the necessity of obtaining this power at all times whether the car is in motion or standing still on the right-of-way or in a terminal where auxiliary power plug-ins are available. In those cases where compressors are driven from car axles, additional refinements in the drive are necessary in order that a nearly constant cooling capacity may be obtained from a variable speed power source. Numerous combinations of electrical generating schemes for generating sufficient electrical energy from the car axle for lighting, ventilation, and summer air conditioning are in use, and their operation is closely interlocked with compressor demands, need for pre-cooling, battery charging, etc. It is difficult therefore to state the additional weight imposed on a car because of such a compression air conditioning system, but it is probably id the vicinity of 6000 lb. These systems, depending mostly upon the locomotive for supplying power for operation, impose a load which may amount to 10 per cent of the capacity of the locomotive. . Several schemes for relieving the locomotive of this compression load are used. Some of the articulated trains, which run as unit equipment-- the same cars always in the same train--employ a head-end, enginegenerator combination for. supplying power to compressor motors. In other cases engine-alternators on individual cars are used to supply alternating current power to compressor motors, as well as to supply all power for car lighting and auxiliaries. Engine-compressor combinations on individual cars provide attractive low weight equipment where con tinuous engine operation is permissible under all circumstances. Diesel engines and propane engines are used for these purposes, and such enginedriven units have the additional advantage of being able to use waste engine heat either for modulating refrigeration with a reheat cycle or for car heating purposes. railway Passenger car humidity and temperature control The temperature to be maintained in a car depends upon'the outside temperature and the humidity desired inside the car. With a low hu midity it is necessary to maintain a higher temperature to establish a desirable comfort condition'. Little humidity control has been attempted on cars up to the present time. A certain degree of automatic humidity control is secured with cooling, but the relative humidity obtained depends largely upon the temperature of the evaporator, which should be below the dew-point temperature of the air. With certain outside atmospheric conditions it may not be possible to operate the conventional equipment with a sufficiently low evaporator temperature to reduce the humidity 712 CHAPTER 38. TRANSPORTATION AIR CONDITIONING without dropping the temperature too low. One method has been developed whereby the evaporator temperature is carried below the dew point a sufficient amount to insure dehumidification and then the cold air is heated to the proper temperature by passing it over coils through which part of the high temperature liquid from the condenser is by-passed. Such a system is costly and has not been generally applied. The reheat cycle obtainable from waste engine heat may be used to good advantage in reducing the humidity without reducing the dry-bulb temperature. During the heating season humidification is desirable from a comfort standpoint, but unless properly controlled, condensation will appear on the windows. A steam or water spray controlled by a humidistat will provide the necessary moisture for humidification. There are several cars with this feature now in use. Temperature control for the most part obtained by rugged thermostats and relays capable of withstanding vibrations attendant with mobile service is usual equipment. Manual zone control for varying outdoor conditions, as well as controls which regulate the car temperature automatically in accordance with outdoor conditions, are employed. Simplified controls from the standpoint of operation by train crews and especially from the servicing viewpoint are very desirable. The control of summer temperatures is accomplished mainly by cycling the complete cooling system; however, modulation is being effected by using multiple evaporators in which a fixed portion may be cut out of the system. In the engine-driven equipments, modulation is obtained by changing engine speed. For further information on controls, see Chapter 34. PASSENGER BUS SUMMER AIR CONDITIONING AND VENTILATION The highways in the United States are now traveled by about 1000 summer air conditioned passenger buses. Many of the facts stressed in connection with the design and installation of summer conditioning equipment in railway cars are even more important in these newer vehicles. Weight and space limitations are more stringent, and the problem of circulating from 900 to 1200 cfm of air in coaches carrying from 25 to 40 passengers with about 35 cu ft of space per passenger without drafts is no easy one. Some bulkhead delivery systems have been used, and while the over head package racks have served to break up drafts to some extent, these installations are not gaining in popularity. Longitudinal ducts in the corners above the package racks are sometimes used to carry conditioned air to a series of outlet louvers along the top of the windows. Other designs provide for false spaces below the package racks which serve as ducts to distribute air to either entrainment grilles in the bottom of the racks or distributing slots at the edges of the package racks. Some coaches employ a false ceiling to provide a duct, with delivery taking place from numerous perforations in the ceiling. Return air grilles and filters are usually located near the rear ceiling where the evaporator is placed. Outside air intakes and filters are located 713