Document VjNmBxE8nOnD386MEEEykgvGj

664 CHAPTER 48 1959 Guide drive the Air.^ntitinning compressor, the electric power source is a problem. If the power source is an axle-driven generator, there is an appreciable increase in the drag on the locomotive. The power available for refrigeration, when train is stopped at a station, it limited to storage batteries. One solution to this problem is to use a direct-current generator driven by a gas engine. Another is to use a Diesel-driven alternator in a special head-end car to furnish power to the entire train. Recently there have been in stallations in which a Diesel-driven alternator is mounted on an individual passenger car to supply the power re quirements of the car. The attractiveness of this. type of ingtAllftfinn can be increased by utilizing spare alternator capacity in winter for electric heating. If this capacity is supplemented by exhaust heat from the Diesel engine, there is sufficient capacity to heat the entire car and to provide hot water for washrooms when outdoor tem peratures are above approximately 30 F. This feature is important on trains lining Diesel locomotives, since it eliminates the need for operating the steam heating boiler in the locomotive during a portion of the year. Air Distribution and Geaning Railway cars present critical problems in air distribution because air space per passenger is small (60 to 190 cu ft), and the sun load is great. An average passenger car con tains approximately 5000 cu ft of air, and may seat as many as 80 passengers. The occupants are continually liberating heat, carbon dioxide, moisture, odors, and some organic matter from their breath, skin and clothing. The heat and moisture can be removed by cooling and de- humidification, but the other constituents can be success fully handled only by proper ventilation and air cleansing. In an average car, from 2000 to 2500 cfm are circulated by the air-conditioning unit. Some of this air may be re circulated, but a portion of it should always be brought in from outdoors. The amount of outdoor air desirable depends upon the type of car, number of passengers, air temperature, humidity, odors,' and whether or not the occupants are smoking. It will vary from 15 to 90 percent of the total air circulated. Careful attention must be exercised in specifying the rate of outdoor air taken in so as to fit the type of service adequately, and yet not supply more ventilation than is' necessary. Conditioning of this outdoor air is a major factor in determining size of both summer and winter equipment. For normal conditions, 10 cfm of outdoor air per pas senger are provided. When smoking is permitted, at least 15 cfm should be admitted. In some dining cars, and deluxe sleeping cars, outdoor air rates as high as 20 to 30 cfm per occupant are used. A ceiling duct lengthwise along the i center of the car is usually used to distribute .the air to the interior by fans or blowers. A perforated ceiling supplied from an overhead duct, or delivery grilles and plaques designed to give considerable entrainment and mixing, .are used to deliver air to the car space. Smoking rooms present a special problem. The cloud of smoke that usually hangs near the ceiling can be broken up by directing incoming air along the ceiling at a velocity somewhat higher than that used for the rest of car. The air is exhausted .through the washroom or lavatory. For compartments, provision is made in the door or partition for removal of used air. Lower berths are provided with a low-velocity air outlet. Recirculating air grilles are usually of straight-flow types. Outdoor air intAltea are usually located in the vestibule, on the side of the car, or on the roof, depending upon location of the cooling coils. On many air-conditioned cars, there are no dampers or shutters at the outdoor air in takes; the percentage of the outdoor air is controlled by adjusting flow through the recirculating grille. Many coach cars are now being equipped with return-air ducts fitted in the structure of baggage racks. Part of the air circulated is returned to the blower unit through these ducts, and part through the car body. This arrangement reduces quantity and velocity of air returning through the car body, and removes smoke fumes at the source. This, and any other design features aimed at taking recirculated air at the floor and adjacent to both end doors (rather than drawing all recirculated air to one end of the car) also reduces infiltration of cold air in ankle-height strata, when doors are opened during the heating season. All air circulated by the blower is filtered before passing over the cooling atiH heating coils. In some cars outdoor air and recirculated air are filtered separately before mixing, while in others air from the two sources is mixed before passing through a common filter. Filters in use are combinations of metal, wool, cloth, spun glass, hemp, paper, hair, and wire screen. Most filters have a viscous coating of oil for greater cleaning efficiency. Some types may be cleaned, re-treated, and re-used, while other types are dis carded' when dirty. Applications are also being made of electric precipitation for air cleaning. In this system the coarser particles arc removed from the air by mechanical separation; finer materials, by electrostatic action. Activated carbon units sometimes are used in addition to the regular filters for adsorbing odors and other impur ities, thus reducing the amount of outdoor 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 auto matically as required. When heating, it 13 important that floor-heat finned tubing be controlled at stable temperatures. Wide fluctu ation in its temperature is highly objectionable because of location close to the passengers. Stable operation may be secured by cycling the floor heat on the basis of indoor con ditions in conjunction with an overhead air-circulating system to maintain final car temperatures. Because of window condensation and other problems, no attempt is usually made to raise relative humidity in a railroad car in winter time. When cooling,- the steam-jet refrigeration system is controlled in an on-off manner. Some means are ordi narily provided for operating mechanical compressor systems at partial capacity. In this case split evaporators are used, so that evaporator surface and compressor ca pacity can be reduced together under light load conditions.. Attempts have been made in the past, largely on an experimental basis, to control the relative humidity in railway cars. This was done by operating the air-con ditioning equipment according to outdoor' temperatures Transportation Air Conditioning 665 and then re-heating the air to an acceptable temperature. 100 percent when the heating or cooling load diminishes. This was found to be an expensive method of operating The distribution ducts and diversion-damper arrangement and the results did cot justify the cost. A common method of. this system makn available two supply ducts and one of controlling heating and cooling is to provide a thermo return duct for heating and for cooling, with a change stat to control heating only and a separate thermostat set over to all three ducts to supply air during the inter at a slightly higher temperature to control the cooling mediate ventilating cycle. This system permits utilisation equipment. This obtains heating control, ventilation, and of atmospheric cooling and ventilation "to the greatest cooling control and the reverse, as the case may be. A car degree when it can be most economically employed in in service can go from heating into cooling in a very short the interval between the heating and cooling demand. time. An example of this is found on the- full-dome cars Conventional throw-away type filters or renewable fil in service on several railroads in the country. In the early ters are used in intake air ducts for many -vehicles. Electro morning hours before sunrise, the car may be requiring static filters have been successfully used in some instal a considerable amount of heat. Shortly after sunrise, the lations. The need for elimination of dirt is great, but the car may go into the ventilating cycle and as the sun load problem is complicated by space limitations and limited increases, cooling may be required. On the majority of power. car heating systems, frequent cycling of the compressor is prevented by the one degree difference between the heating Refrigeration and cooling control points and by lag imposed on the system by applying artificial heat to the heating ther mostat when cooling is required. On some car heating systems, a degree of modulation is obtained by the use of split evaporation. This requires two cooling thermostats with one of them set at a somewhat higher temperature than the other. The lower temperature cooling thermostat controls a portion of the cooling capacity and, if unable to maintain the car temperature, the higher thermostat adds the remaining capacity. Summer conditioning systems for inter-urban vehicles range in cooling capacity from 36,000 to 48,000 Btu per hour. Mechanical. compression systems **ng refrigerants are used, and are powered by water-cooled gasoline engines of approximately 14 hp. Complete systems add from 800 to 1300 lb to the weight of the coach. Sometimes an auxiliary generator driven by the refrigeration system engine is used and serves to help charge the bus battery, thereby offsetting power drain imposed by the ventilating blower. Belted recip PASSENGER BUS AIR CONDITIONING rocating compressors and direct-driven V-type and rotary compressors are used, with engine speeds up to about 1800 The passenger bus designed for urban transportation rpm. Air-cooled condensers for this service require about operation presents a greater problem to the designer of 5000 cfm of outdoor air, and this is provided by either cen Wring systems than does the inter-urban"-bus. More trifugal or propeller type fans belted or direct-driven by the frequent stops, and rapidly changing passenger load cre air-conditioning engine. Preventing noise and vibration from ate this problem on urban vehicles. Provision of heat affecting passengers is of vital importance. Installations for the driver independent of the passenger heating sys must be made for quick daily engine servicing. In all cases tem, is a further problem. The inter-urban bios, howeyer,''' fuel is obtained from the main bus tanks, and in some, the is usually a deluxe vehicle and may require a comfort main engine cooling system cools the air-conditioning en cooling system. Space and weight limitations and vibration gine. must be considered. Heating .. Control Recent designs of bus heating systems obtain improved air distribution. Heat in the engine coolant liquid is used to warm air by means of suitable finned coils and t.hi heated air is distributed throughout the passenger space by ducts and outlets directed toward the floor. Some designs include finned surface near the floor in an appli cation similar to that in railway passenger cars. Forced air circulation over this finned floor-beating surface has been provided to increase its effectiveness. Oil-burning booster heaters have been applied to many Dieselpowered buses to raise the temperature of the engine coolant for muTimnm engine operating efficiency, and to provide sufficient heat for the passenger space. The simplest control system for heating of buses consists of a single thermostat, located in the passenger section to start and stop the blower of the Wring unit. This method of control is not generally satisfactory because, without con tinuous air circulation, temperature gradients from floor to ceiling and from front.to rear are quite pronounced. A more satisfactory method- of control provides for continuous fan operation whenever heating is required. In this method a thermostat, usually in the return air to the heater, operates a modulating valve in the supply line to the heater to throttle the flow of engine coolant to the beater coil in accordance with bus interior temperature. On some systems a means of remotely adjusting the control point of the thermostat is provided at the operator's location. Automatic means are Ventilation available for starting and stopping booster beaters in the engine coolant system, under control of the thermostat. Also, Air for ventilation is usually brought into a bus at the some systems provide for automatically stopping the Mower front, and distributed throughout the length of the pas of the heater unit and the booster coolant pumps when no senger space by a duct or ducts near the ceiling. Except beat is required in the bus. Outdoor air for ventilation, drawn for a few designs employing 100 percent outdoor air into the bus by the heating unit blower, is usually not auto for heating, no heating of the ventilating air has been matically controlled but is preset, either by means of a man provided. One type of distribution system for an inter- ual damper or by sizing the outdoor-air intake to provide the urban bus provides for a fixed minimum of outdoor air, and desired amount of outdoor air. is arranged to increase the percentage of outdoor air to On buses equipped with mechanical refrigeration for air X'