Document 5kGKYQV13XGX05MZnokj5qyqN

224 CHAPTER 17 1962 Guide And Data Book Table 3 .... Relative Cost Comparison for Railroad AirConditioning Systems T/p* SyOtm Steam Ejector Ice Activated 1>A Component firrf Corf* Highest Lowest High Openrfwe Corf* Low Medium Mcrinteooaco Corf* Medium DX Component (AC Power) 1>A Package Unit (iXJ Power) UX Package Unit (AC Power) Medium High Low Low Medium Low Medium Lowest flnt eflt U based oa relative coat at air **<m1it{"nirir equifimest and IM mm and does not taciude pi*a eouipaeav Opeatiac coat i* baaed oa power coeaumed oniy. Maintenance coat does not include maintenance coat of power equipment. proximately 500 lb less than a comparable d.c. unit! The motor starters and controls are also much smaller and lighter. The light weight and smaller size permits installation of 8 too units in the ceiling on double deck or gallery type com muter cars. The self-contained unit greatly reduces the amount of work that the carbuilder previously had to do when a component system was used. In theory, the package unit lends itself to a complete change out of a unit for major re pairs, however, the weight of 1100-1500 lb is usually too heavy for yard handling. It is therefore necessary to provide com plete access to all parts for servicing or replacement. COST CONSIDERATIONS From a standpoint of initial cost of the air-conditioning equipment to the railroad or carbuilder, the component sys tem is usually cheaper than the unitary system. However, the installed cost of the component system is much higher, since the cost of piping, wiring, charging, and testing must be added to the initial cost of the equipment. By its very nature, direct-current equipment will cost approximately 20 percent more than the alternating-current unit. Operating cost for the alternating-current self-contained unit is generally lower as far as the air-conditioning unit itself is concerned. The higher efficiency of the hermetic com pressor and other motors results in less power. If the cost of producing the power such as individual diesel or propane generators is taken into account, the savings is more equa lised. A gingk head end power plant would be more efficient than several individual power units. The cost of power such as on direct current axle driven generators is difficult to de termine accurately because the power comes from the diesel locomotive. A comparison of the service and maintenance coat between the different systems would show the a_c. self-coutained unit the lowest. With the self-contained units above the floor the cost of cleaning condenser coils is reduced, likewise, the problem of damage from flying ballast, corrosion from snow and ice is practically eliminated. All the equipment is lo cated in one place on the car which cuts down on the tim? of cheeking or servicing the equipment. At this point it should be mentioned that because of the compact design of the uni tary system, it is not always as easy to replace defective items as it would be where more working room is available such as on the component systems. Table 3 shows a comparison of the first cost, operating cost and maintenance cost of 6 different systems presently in service. The axle driven compressor has been purposely omitted because very few, if any, are being applied. PART III: BUS AIR CONDITIONING This section will discuss the design considerations, capacity requirements, air distribution systems, drives, and controls for air-conditioning of passenger buses. DESIGN CONSIDERATIONS The requirements tor bus air-conditioning equipment in volve considerations which in other air-conditioning applica tions are of less importance. One of the first of these special considerations is space limitations imposed by tire overall bus design. Normally, it is very difficult to find a large enough volume of the correct shape to accommodate a self-contained air-conditioning unit. For this reason, most systems are installed with components located to make optimum use of available space. Tied in with equipment location is the matter of weight. Of course, it is desirable for all components to be as light in weight as economically practical. However, special considera tion must be given to some components, depending upon their location, in relation to wheel loading. As a result, present day bus air-condidioning systems use compressors with alum inum crankcases, and some use all aluminum evaporators and condensers. Also, to save weight and fit limited space, high speed compressors, sometimes using Refrigerant 22, are be coming more prevalent. Applications using high speed com pressors are usually those where the compressor is driven di rectly from the main bus engine without the use of an auto matic speed changing device between the engine and the compressor. The power requirements for this appliestion are somewhat dictated by the space and weight limitations. In many cases, for example, the optimum fan selection for a bus air-condition ing system would be considered as requiring excessive power compared to a conventional stationary system. Likewise, the power requirement Cot the compressor drive is usually greater than normal because provision must be made for extreme operating conditions. Special consideration must be given to design for reliable operation even under the most severe conditions. The equip ment is installed in revenue producing vehicles, and un scheduled stops for repairs due to a design deficiency cannot be tolerated. Factors such as road shock, dust, rain, snow, hail, sleet, and unusually high temperatures all must be con sidered. Even such items as location of condenser air openings ' can prove to be troublesome if not adequately considered. For example, a high velocity discharge from the condenser can cause annoying dust at stops if not properly directed. Tied in with reliability is design, for ease of maintenance. All points requiring regular servicing or adjustments must be readily accessible (or maintenance to be accomplished in a minimum time. Wearing part components must be so located mid attached as to be readily replaceable with minimum dis assembly of other parts. To provide for these factors, some systemshave swing-out condensers for ready access to the rest of the system. Electrical controls on some installations are located in separate compartments, accessible from the out side of the bus. However, probably the most important feature to keep in mind is design simplicity to allow trouble shooting and main tenance time to be kept to a minimum. Temptation to design in features or gadgets for improved efficiency at the expense of added complication must be avoided. CAPACITY REQUIREMENTS For the urban bus, the cooling requirements are quite severe. The cooling load is made up of three parts: (1) the Surface Transportation Air Conditioning 225 Table 4 .... Text Data for Door Operation of a City Bus (Taken during the rush period on a 35 passenger bus in San Antonio, Texas) Door Open Times per mile Times per hour % of operating time Longest time open Shortest time open Avenge time open Door 7M 70 35 55 a 13.5 see Doer 5K 44 15 34 see 3 see 12.5 sec paggpngp-r load, (2) tiie load due to ventilation air and in filtration, and (3) the load due to heat transmission. The passenger load is extremely severe because a city bus may seat up to 50 people and cany an equal number as standees. The fresh air mid infiltration load is also severe be cause of the great number of door openings. Table 4 shows a record of door openings from actual test. It should be realiced that a city bus may stop once in each city block and may open both front and rear doors to take on and discharge passengers. The amount of transmission load depends on whether the bus is fitted with single or double glass and the amount of in sulation. However, the transmission load is usually a relatively small part of the total. The maximum cooling capacity required for the average urban bus is in the order of 7-10 tons of refrigeration. How ever, the equipment must be flexible enough to range from tero to maximum promptly as required. This is necessary be cause the passenger load on a city bus can vary greatly, de pending on location and time of day. The heating of a present day bus equipped with air con ditioning is usually accomplished by means of a heating coil located in series with the evaporatory. Traction engine coolant is piped to this heating coil, and in some cases individual sup plementary heaters are provided for windshield defrosting. It is usual that the cooling capacity need for the main engine is sufficient to heat the bus even under the most severe weather conditions. With this type system, ventilation air is usually taken in through fixed openings in the side of the bus. These openings and duct-work to the return air passage should be so sized to admit outside air in a ratio of approximately 20 percent out door air to 80 percent recirculated air. The inter-urban bus hs a somewhat different problem in cooling and heating th*o the urban bus. The cooling load is less because the maximum passenger load is less and there are fewer door openings. However, the quality of cooling must be better because the passengers remain on board for longer periods. For this reason, placement of the main heater coil in senes with the evaporator to provide reheat is probably much more important in this service. Ventilation requirements for the inter-city bus are also more important. Normally, openings are provided in the side of the bus to bring outside air into the return air duct. Also, in ad dition, some Bystems provide manually operated dampers for introduction of outside air directly into the conditioned spae*. Since smoking may be allowed on an inter-urban bus, it is desirable to admit about 15 cfm per person. The heating system normally used on an inter-urban bus is essentially the same as that described above for a city bus. AIR DISTRIBUTION SYSTEMS Where the conditioned air is distributed overhead, various systems of outlets have been used. Some buses have been fitted with a perforated ceiling air distributor system. This system gives good air distribution, but uses up valuable head room which in many cases is not available. Some systems employ individual air outlets located in the center of the celling. This system alleviates the headroom problem and furnishes reasonably even distribution throughout the bus. However, serious draft problems are usually encountered which are specially disturbing to standees. Use of overhead distribution also precludes the possibility of using this system for heating. It is important to provide heat at the floor level. Side wall outlets above the windows have been used with favorable results. However, on modern buses it is verydifficult to provide duct work of sufficient size to serve these outlets. Most popular are side wall outlets located just below the windows. The duct system for such outlets is more practical, headroom is not affected, and cooling air is delivered close,to the heat gain from the side walls and windows. The main dis advantage of this system is the possibility of drafts affecting the passenger in the window seat. This is usually overcome by making outlets as long as possible to minimize velocity. Floor outlets are not desirable for cooling and therefore are normally used only for heating. This is because it is most desirable to keep the passengers' feet warm. The simplest way to heat the bus is by means of small fan coil units located at intervals and provided with hot water from the main engine cooling system. These recirculate the air locally and involve the least amount ofduct work. In any air conditioning system, the air return must be given careful consideration. The most satisfactory arrange ment of air supply and return is for the air to be delivered near the ceiling and removed near the floor. It may also be delivered near the ceiling and removed at a different part of the ceiling, depending upon the location of the cooling coils. EQUIPMENT LOCATION The cooling equipment which consists of the high ode com pressor and condenser assembly and the low side evaporating coil and air circulating fans may be located all under the floor, all in the roof, or in split arrangements where the high side is under the floor and the low side is mounted in the ceil ing or on the roof on the bus. In certain city buses, part of the high side is on the roof, and part of it is under the floor. The under-floor system offers many advantages for the high speed inter-urban bus. Among other thingB, it keeps the center of gravity low. It also reduces the amount of duet woik neces sary. It is actually quieter than the overhead system. It does have the problem of using up valuable luggage space, partic ularly on a long distance bus. The split level offers a numberofadvantages because it uses up less luggage space by putting the evaporator unit upstairs. The roof mounting of the condenser of a split system is of interest. This has been done on many urban bus installations. This makes available to the condenser, air which is much cooler than that which can be drawn off the road. It also eliminates a great deal of dirt which otherwise would tend to clog the condenser surface. If the appearance of a bump on the roof can be accepted, it apparently does not exceed the clearance of obstruction such as bridges, and therefore, oc cupies space which otherwise would not be used. In-these installations, the compressor itself is usually mounted under the floor. DRIVE One of the earliest and most commonly used drives for the air conditioning system is by means of a separate engine. This