Document aBJVG3DVYXndd6MEpJbDm1g7e

220 CHAPTER 17 1962 Guide And Data Book generator and 600 volts when taking power from the third rail. Power supplied from diesel or gas (propane) generators is usu ally 220 volts, 3-phase, 60 cycle or 440 volts, 3-phase, 60 cycle. Where direct current is obtained from axle driven generators, large batteries must be provided to provide power at train stops. For precooling the cars in the yards and for keeping up the charge on the batteries, the usual 25 KW generators have as a.c. motor uu the same shaft which operates the generator from a standby power source of 220/3/60 power. The operating requirements of the cars to be air conditioned will to some extent determine the type of equipment to be used. Passenger cars that will always operate in the same train can receive their power from a head end source by train-lining down through the cars. In this case 440/3/60 power is used and this permits light weight units using hermetic compres sors. However, if a train has to be made up of cars from differ ent railroads, each car must have its own power plant and can then be of any type design. Similarly, commuter cars using 600 v d.c. power from the third rail will normally be designed to use the 600 v d.c. power for the air conditioning equipment. Railroads using steam ejector type air conditioning systems usually keep these cars in the same train. Therefore it is neces sary to know in advance where and what type trains the car will operate in before deciding on a specific design. Unlike a building which has a fixed geographical location, a railroad car can operate in any part of the country. This is true in the case of coaches and sleepers which are leased to other railroads at certain times of the year. In this case the maximum outside air dry bulb and wet bulb design tempera tures for any part of the country should be considered. In many cases such ss commuter cars and subway cars they will operate in a relatively small geographic area and only the local design temperatures must be considered. As discussed above the question of interchangeability of the cambetween different railroads mustbe considered atthe time of selecting both the type power and the specific design of the air conditioning equipment. EQUIPMENT DESIGN LIMITATIONS Space both underneath and inside a railroad car is at a pre mium. A typical cross section of a passenger car is shown in Fig. 10. This lack of space for the air conditioning equipment is even more pronounced on subway cars and commuter cars where maximum carrying capacity of passengers is required during rush hours. This problem generally rules out the use of floor mounted units. The usual case is to build the components of the system to meet the configuration of the space that is available. The curvature of the roof and the clearance line be neath the car are two of the major factors that determine the shape and size of the equipment. The weight of the equipment is more a problem today then formerly because the trend is toward lighter and lighter cars to reduce the power required for faster acceleration. Unlike the aircraft industry otherconsiderations than weightalone havea greater bearing on the design. Present equipment will vary from 1100 lb for a self-contained unit of 7 tons capacity to 3000 lb for a steam-jet system not including all the piping and controls. Lighter equipment can be designed using additional aluminum and other light weight materials but the added cost does not justify it. The major factor determining weight is the type of power available. With alternating current, hermetic compressors, light weight motors and controls can be used which greatly re duces the weight over that of a system using direct current power. With the event of the light weight railroad car, noise and vibration has become a greater design consideration then pre- Fig. 10.... Typical Cross-section of Railroad Passenger Car viously. This is particularly true on sleepers, reclining chair cars and business cars. All the air conditioning equipment components are resiliently mounted on the car structure. Where a package unit is used the complete unit is generally resilient mounted as well as the internal components such as fans, motors and compressors to reduce vibration from being transmitted to the car structure. Air noises and other sounds can be reduced by the proper useofsounddeadening materials of the blanket type or spray on type. On a railroad car the problem of dirt and corrosion is an im portant design factor. This holds true especially if the equip ment is beneath the car floor where it is subject to all types of weather both summer and winter as well as the most severe dirt conditions as can be found. For this reason corrosion re sistant materials and coatings must be used wherever pos sible. Copper silicon has been more widely used than stainless steel for casings because of its easy working qualities. Alumi num has not proven durable enough when used under the severe conditions encountered. The sand blasting effect de stroys any type of surface treatment on aluminum when used beneath the car. Since the dirt problem cannot be eliminated, provisions must be designed into the equipment to quickly and easily clean the dirt out. This requires access doors where needed to get inside or behind coils for blowing out the dirt. The dirt problem enters into the design of the fin spacing on condenser and evaporator coils and limits the fin spacing to 8 to 10 fins per inch compared to 18 fins per inch on 'commercial and in dustrial type equipment. The closer the fin spacing the more rapid the dirt build up and the more costly it is to dean the coils. The dirt problem as well as the Bevere environmental conditions must be considered in the type of motors and con trols used. Considerably more attention must be given to the problem of maintenance and servicing on railroad equipment then on stationary units. With a modern railroad car having sealed Surface Transportation Air Conditioning 221 Table I ... - Approximate Capacity Required for Railroad Cars Table 2 .... Typical Coding Load for a Standard 85 Ft Passenger Car Car Typo Passenger Commuter Gallery Commuter Bubw*y Pullman Type Seeping j^rtial Dome Type Full Dome Type Business Military ImgrfS, 85 85 85 51-61 85 85 85 85 85 51-85 No. of fatmigwt Approximate Capacity, Toni of Refrigeration 65-85 130 165 125-200 26-38 8 8-0 12-16 9-J2 6-8 40-60 40-60 60-80 2-12 12 6-8 12-16 16-20 4-8 4-8 windows and a well insulated structure an air conditioning failure makes the car practically unusable in hot weather. Many additional items are designed into the equipment to permit quick determination of the trouble and similarly allow s fast correction of the item causing the failure. All items such as motors, compressors, valves, etc. must be easily accessible for inspection or repair- For quick checking of the amount of refrigerant in the system, eight glasses on the liquid receiver should be provided. likewise, a readilyaccessible liquid charg ing valve should be used. Pressure gages and test switches are used for quickly checking the operation of the system while the train is stopped at intermediate stations enroute. . Safety precautions must be given consideration on a railroad car, especially whereequipment is located underneath the car. In designing supports they should be made to fail safe', should a mounting bolt shear off or a nut become loose. Should a piece of equipment hang down or drop off, a train wreck could occur. All belt drives or other revolving items must be protected .with a safety guard. High voltage controls and equipment must be so designated by approved danger signs. All pressure vessels and coils must meet ASME test specifica tions for protection of the passengers and maintenance people. EQUIPMENT DESIGN The cooling load on a standard 85 ft passenger car can be calculated approximately. In the early days of air conditioning railroad cars, each new type car to be built was considered separately and a cooling load estimate -was made. The total cooling load usually was 6-8 tons and the capacity was usually selected for the calculated load. In recent years the practice has been to standardize on an eight ton system for an 85 ft coach operating in United States, Canada or Mexico. Table 1 shows in a general way, the capacity in tons of re frigeration, normally used for several types of cars operating in North America. A cooling load calculation for a typical 85 ft standard pas senger car would show a load distribution as in Table 2. Item 1--Transmission Load. This load is based on tbe use of three inches of glass fiber insulation on all interior walls, ceilings and floor of the car; double glass sealed windows and an outside temperature of 100 F dry bulb and 78 F wet bulb; made 78 F dry bulb and 50 percent relative humidity. Item 9--Sun Load. A railroad car while in motion will be ex posed to the sun first on one tide and then on the other. How ever, the above figure considers the roof and one tide. Item 9--People. Although the number of people will vary, to average figure of eighty people was used in this example. Item 4--Electrical (Lights and Motors). .The sensible heat. Min No. 1 2 3 4 5 Source of Hoot Gam Transmission Solar Radiation People (80) Electrical (Lights and Motors) Outdoor Air (Ventilation) Total Hoot (Senable, or SenaWe and lofeof). BhA 18.960 (Sensible) 6,460 (Sensible) 32,000 (Sensibleand Latent) 12,690 (Sensible) 21,600 (Sensible and Latent) Total Load =91,710 from the lights is used here; however in bright sunshine the lights may not be on and if used, will be a safety factor. Item 9--Outdoor Air (Ventilation). It is standard practice. to take in not less than 25 percent of the total supply air for ventilation. The usual eight ton system uses 2400 cfm total' supply air and 25 percent of this is 600 cfm. There are many other factors that could be considered but experience has shown that by allowing in advance some extra capacity in the equipment, comfortable conditions can be at tained with present day equipment. To prevent infiltration into the car, the usual practice is to pressurize the car at 0.02 in. water pressure. This is controlled by using exhaust fans in of the toilets, less air thn that taken in for ventilation. HEATING LOAD Most cars are equipped with what are called overhead heat and floor heal. The overhead heal is usually the primary heat source, with thefloor heat used to maintain comfortable condi tions on the floor. The indoor winter design temperature is usually set at 69 F. All three types heating media are used in both the overhead heating coil and floor heat; steam, electric and hot water. The latter is not used to any great extent be cause of the freezing problem, requiring the use of an anti freeze. The outside design temperatures will vary from 0 F in the East to --40 F in the North Central States and Canada. The overhead- beat steam coils are usually designed for 100,000 Btuh while some electric heater coils will vary from 17 kw to 24 kw. AIR DISTRIBUTION The most common type air distribution system is a center line supply duct running the length of the car and located in the space between the ceiling and roof. The air outlets are either of the longitudinal type, individual circular type, or perforated wiling type. Fig. 11 shows a typical air distribu tion system As can be seen, no return air duct system is used. The main supply duct must be insulated on three sides be cause of the high temperatures in the space above the ceiling. The ventilation air should be taken in'from both tides of the roof Line to overcome the wind effect. Adequate mow and rain louvers must be used on the outdoor air intakes. Separate fresh air filters are usually used in conjunction with a return air filter. The air filters are of the wire mesh cleanable type and must be cleaned often. PIPING DESIGN REQUIREMENTS Where the component system is used, standard refrigerant piping practice is followed. The average length of suction line