Document O1R175JYE4QXGXJLYwmykOo2v

964 CHAPTER 47 1950 Guide from the air by mechanical separation; finer materials, by electrostatic- action. \ Activated carbon units sometimes are used in addition to the regular filters for absorbing odors and other impurities, thus reducing the amount' of outside air necessary for ventilation. Temperature and Humidity Control ' Controls in a passenger car should be as automatic as possible. The regular train crew cannot be relied on to make adjustments for the comfort of passengers. . For this* reason the latest Systems of temperature control have only an off-on switch to be operated by the train crew. When the system is in operation, heating or-cooling is provided automatically as required. When heating, it is important that floor-heat finned tubing be controlled at stable temperatures. Wide fluctuation in its temperature is highly ob jectionable because of location close to the passengers. Stable operation maybe secured by controlling the floor heat on the basis of outside condi tions (see Chapter 29 on zoning), and using an overhead air circulating system to maintain final car temperatures. . Because of window condensation and other problems, usually no attempt is made to raise relative humidity ini a railroad car in winter time. : When cooling, the steam jet refrigeration system is controlled in an on-off; manner. Some means are ordinarily, provided for operating mechanical compressor systems at partial capacity. In this case split evaporators are used, so that evaporator surface and compressor capacity can be reduced together under light load conditions. ; Under very light cooling loads, the relative humidity in a car tends to increase because of long off periods of refrigeration equipment. This can be prevented by starting refrigeration equipment on low capacity at an' established outdoor air temperature, operating it continuously, and using a heating coil in the overhead system to re-heat sufficiently to maintain de sired car temperatures. Under higher load conditions the heating coil becomes inoperative, and compressor and evaporator capacity are increased as needed. Room type sleeper cars introduce a further problem of providing indi vidual adjustment of room temperatures. Sometimes this individual control is secured by adjusting air volume, but such adjustment is unsatis factory for overall comfort, and may affeet the air supply to other rooms. Another method is to use the heaters at the floor to control the room ternperature, but this tends to cause unstable and improper floor heat tempera tures which may be objectionable to the passengers. A simpler and basically more satisfactory system is to use a small booster heater in individual overhead air supply ducts to each room under manual control of the occupant. In this case, the floor heat and basic overhead systems are automatically adjusted for varying load conditions just as in a simple coach type car. A fixed amount of heat regulated by the occupant can be added by the room booster heater to maintain desired individual room, temperature. Temperature lag is reduced in the case of room boosters over use of gravity floor heating surfaces. Any adjustment of the booster, will give the occupant immediate change, in space conditions. The use of floor heat surfaces for room control may also cause low or excessive surface temperatures close to the passenger, with resulting discomfort. Transportation Air Conditioning 965 STREETCAR AHD TROLLEY COACH HEATING AND VENTILATING Streetcars and trolley coaches present a special problem in the main tenance of satisfactory comfort conditions because of the frequent opening of the doors and highly fluctuating passenger load. Space limitations for ducts, and a desire to keep outlet grilles well above the floor to facilitate car cleaning, add further problems to the distribution system. Maintain ing comfort conditions at the driver's station cannot be overlooked, since his term of occupancy is considerably longer than that of any passenger, and because he is usually dressed more lightly than passengers. A separate source of heat is usually provided for the operator, and is under his control. Heating and Ventilating Recently-built streetcars and trolley coaches obtain heat from air blown over the main accelerating resistors and track switch resistors, to heat the passenger space. In the modem streamlined streetcars, designated P.C.C.; approximately 2400 cfm are drawn from the car and blown over these re sistors to dissipate their heat. In trolley coaches, an amount of 800 cfm is customary. The heated air is then delivered to the passenger space or diverted to the outside by means of dampers, as required. If available heat from this source is insufficient, auxiliary electric heaters in the supply ducts may be cut in. The air distribution system is. 100 per cent recirculat ing when the maximum heating requirement is being met. At conditions other than maximum heating demand, part of the air drawn from the car is exhausted to the atmosphere. Outside air enters the car, under these con- ditions, by infiltration at all cracks and through doors when opened at stops. The most recently-built streetcars have added ventilating fans in the roof structure to introduce outside air through ceiling diffusing grilles. By governing the outside air volume introduced through these roof fans in coor dination with the heated air distributing system, it is possible to maintain a slight pressurization of the passenger space and avoid inrush of air when the doors are opened to load passengers. The roof fans also provide an effec tive means of maintaining lower inside temperature during slimmer opera tion. Ventilation tests on P.C.C. streetcars indicate that with 90 F outside temperature and above, 12,000 cfm are required to provide sufficient air change to keep inside temperature within a few degrees of outside air tem perature, and to provide enough air movement over passengers for comfort. The best results have been obtained by operating ventilating fans with windows closed. New cars provided with adequate ventilating capacity have been built with fixed sash. ; Control . .: Temperature control, consisting of equipment especially designed to with stand the vibration present on transportation equipment, is used; " Auto* matically operated dampers are used to control flow-of heated air to the passenger space, or to direct heated air to the atmosphere. An automat ically operated rheostat or multi-point switch is used to vary the speed of the ventilating fans. Recent control system applications employ one thermostat to operate both heating dampers and ventilating fans in a modu lating or graduated manner, with a compensating thermostat in the heat supply duct to correct for wide fluctuations in temperature of air leaving the resistors. Ventilating fans are usually stopped or operated at lowest speed during the heating cycle, and then their speed is gradually increased as the car temperature rises above the heating cycle control point. :