1、11.1CHAPTER 11 MASS TRANSITVentilation and Thermal Comfort . 11.1Thermal Load Analysis . 11.2Bus Air Conditioning. 11.2Rail Car Air Conditioning. 11.5Fixed-Guideway Vehicle Air Conditioning. 11.7HIS chapter describes air-conditioning and heating systems forTbuses, rail cars, and fixed-guideway vehic
2、les that transport largenumbers of people, often in crowded conditions. Air-conditioningsystems for these vehicles generally use commercial components,but are packaged specifically for each application, often integralwith the styling. Weight, envelope, power consumption, maintain-ability, and reliab
3、ility are important factors. Power sources may beelectrical (ac or dc), engine crankshaft, compressed air, or hydraulic.These sources are often limited, variable, and interruptible. Charac-teristics specific to each application are discussed in the followingsections. Design aspects common to all mas
4、s-transit HVAC systemsinclude passenger comfort (ventilation, thermal comfort, air quality,expectation) and thermal load analysis (passenger dynamic meta-bolic rate, solar loading, infiltration, multiple climates, vehiclevelocity, and, in urban applications, rapid interior load change).1. VENTILATIO
5、N AND THERMAL COMFORTThe requirements of ASHRAE Standards 55 and 62.1 apply fortransportation applications, with special considerations, becausepassengers in transit have different perceptions and expectationsthan typical building occupants. These considerations involve lengthof occupancy, occupancy
6、 turnover, infiltration, outdoor air quality,frequency and duration of door openings, personal preference, inte-rior contamination sources such as smoking, and exterior contami-nation sources such as engine exhaust.Historically, in nonsmoking air-conditioning and heating applica-tions, outdoor air h
7、as been supplied to the vehicle interior by fans at5 to 10 cfm per passenger at a predetermined nominal passengerloading. Nominal passenger load is based on the number of seats andmay include a number of standees, up to the maximum number ofstandees possible if this type of loading is frequent. Ther
8、e are a fewexamples of no outdoor air being supplied by fans, but they are onshort-duration trips such as people movers or urban buses with fre-quent door openings. Besides providing for survival, ventilation pro-vides odor and contamination control. The amount needed forsurvival is less than the la
9、tter. Contamination control from interiorsources is a factor in building design, but is less of a factor in vehicledesign because of the ratio of people to furnishings and the lack ofinterior processes such as copy machines. Exterior contamination,such as from tunnel fumes, can be a problem, however
10、. Door open-ings, if frequent enough, provide some additional intermittent venti-lation, although this infiltration should be minimized for thermalcomfort. Ventilation from doors may not be effective in controllingodors away from the doors. Fan-supplied outdoor air must be distrib-uted equally in th
11、e vehicle for effective ventilation. Symptoms ofinadequate ventilation are odors noticeable to passengers initiallyentering an occupied vehicle or when moving from section to sec-tion. Passengers on board who are exposed to slowly increasing odorlevels may not be aware of them.Based on ASHRAE resear
12、ch, ASHRAE Standard 161 establisheda ventilation rate for aircraft passengers at 7.5 cfm per passenger.This rate was based in part on the consideration that not all spaces inthe enclosed area achieve 100% ventilation effectiveness. The min-imum effective ventilation rate for several crowded but larg
13、er-volume spaces, as defined in ASHRAE Standard 62.1, is 5 cfm perperson. It is recommended that ground mass transit applications use7.5 cfm of outdoor air per passenger for most transit applications.Emergency ventilation, such as windows or exits that can beopened or battery-powered ventilators, sh
14、ould be provided in caseother systems fail. For example, a power interruption or a propulsionsystem failure may strand passengers in a situation where exit is notpossible. Emergency situations include overtemperature, oxygendepletion, smoke, or toxic fumes. Operator-controlled dampers arenow provide
15、d on some vehicles to close off fresh air when smoke ortoxic fumes are encountered in tunnels. The duration that the damp-ers remain closed must be limited to avoid oxygen depletion, eventhough the air-conditioning system remains in operation. Fresh-airsupply alone or battery-powered ventilators wil
16、l not prevent over-temperatures when a full passenger load is present and/or a solarload exists in combination with high ambient temperature. Eachemergency situation requires an independent solution.The nature of the transit service may be roughly categorized byaverage journey time per passenger and
17、 interval between stationstops, and this service type affects the necessary interior conditionsin the vehicle. For example, a commuter rail or intercity bus passen-ger may have a journey time of an hour or more, with few stops; pas-sengers may remove heavy outer clothing before being seated. Incontr
18、ast, a subway or transit urban bus rider typically does notremove heavy clothing during a 10 min ride. Clothing and the envi-ronment from which passengers come, including how long theywere exposed to those conditions and what they were doing (e.g.,waiting for the train outdoors in winter), are impor
19、tant factors intransit comfort. At the opposite extreme, many subway stations arenot climate controlled, and often reach dry-bulb temperatures over100F in the summer. Thus, when boarding a climate-controlledvehicle, these passengers immediately perceive a significant in-crease in comfort. However, a
20、 passenger adjusts to a new environ-ment in about 10 to 20 min; after that, the traditional comfort indicesbegin to apply, and the same interior conditions that were perceivedas comfortable may now be perceived as less than comfortable.Before stabilization, a passenger may prefer higher-velocity air
21、 orcooler or warmer temperatures, depending to some extent on cloth-ing. At the same time, other passengers may already have stabilizedand have completely different comfort control desires. Therefore,the transit system designer is presented with a number of unusualrequirements in providing comfort f
22、or all.Jones et al. (1994) evaluated the heat load imposed by peopleunder transient weather and activity conditions as opposed to tra-ditional steady-state metabolic rates. An application program,TRANMOD, was developed that allows a designer to predict the ther-mal loads imposed by passengers (Jones
23、 and He 1993). Variables areactivity, clothing, wet- and dry-bulb temperatures, and precipitation.The preparation of this chapter is assigned to TC 9.3, Transportation AirConditioning.11.2 2015 ASHRAE HandbookHVAC ApplicationsEuropean Committee for Standardization (CEN) Standard EN13129-1 provides g
24、uidance in the area of railroad passenger com-fort. Although this standard does not apply to countries outside theCEN, the information is valuable and may not be readily availableelsewhere.2. THERMAL LOAD ANALYSISCooling Design ConsiderationsThermal load analysis for transit applications differs fro
25、m station-ary, building-based systems because vehicle orientation and occu-pant density change regularly on street-level and subway vehiclesand, to a lesser degree, on commuter and long-distance transporta-tion. Summer operation is particularly affected because cooling loadis affected more by solar
26、and passenger heat gain than by outdoor airconditions. ASHRAE Standard 55 design parameters for occupantcomfort may not always apply. Vehicle construction does not allowthe low thermal conductivity levels of buildings, and fenestrationmaterial must have safety features not necessary in other applica
27、-tions. For these reasons, thermal loads must be calculated differently.Because main-line passenger rail cars and buses must operate in var-ious parts of the country, the air conditioning must be designed tohandle the national seasonal extreme design days. Commuter andlocal transit vehicles operate
28、in a small geographical area, so onlylocal design ambient conditions need be considered.The following cooling load components should be considered:Ambient air conditions for locations in North America and world-wide are given in Chapter 14 of the 2013 ASHRAE HandbookFundamentals. For vehicles operat
29、ing in an urban area, the heatisland effect should be considered if the Handbook design valuesare derived from remote reporting stations. For subway car oper-ation, tunnel temperatures should be considered. In humidregions, consider the wet-bulb temperature coincident with dry-bulb temperature relat
30、ive to fresh-air loads.For vehicle interior comfort conditions, consult Figure 5 in Chap-ter 9 of the 2013 ASHRAE HandbookFundamentals. Total heatgain from passengers depends on passenger activity before board-ing the vehicle, waiting time, journey time, and whether they arestanding or seated during
31、 the journey. Representative values aregiven in Table 1 in Chapter 18 of the 2013 ASHRAE HandbookFundamentals.Ventilation air loads should be calculated using the method inChapter 18 of the 2013 ASHRAE HandbookFundamentals, inthe section on Infiltration and Moisture Migration Heat Gains. Airleakage
32、and air entering during door dwell time should be takeninto account.Interior heat includes that produced by the evaporator fan motor,indoor lighting, and electrical controls.The vehicles conductivity, in Btu/hF, should be provided by thevehicle designers. For outdoor skin temperature guidance, use t
33、hevalues in Table 1 in Chapter 29 of the 1997 ASHRAE HandbookFundamentals; however, consider that air over a vehicle in motionreduces these temperatures The car design dry bulb should beused as the interior temperature.The instantaneous solar gain through the glazing should be calcu-lated using summ
34、er midafternoon data listed in Chapter 29 of the1997 ASHRAE HandbookFundamentals, and the glass shadingcoefficient. The glass shading coefficient must be obtained fromthe window supplier. Adjustments for frequent change in vehicledirection or intermittent solar exposure may be justified. Addi-tional
35、 information is shown in Chapter 15 of the 2013 ASHRAEHandbookFundamentalsThe summer cooling analysis should be completed for differenttimes of the day and different passenger densities to verify a reliableresult. Cooling equipment capacity should consider fouling andeventual deterioration of heat t
36、ransfer surfaces.Heating Design ConsiderationsWinter outdoor design conditions can be taken from Chapter 14of the 2013 ASHRAE HandbookFundamentals. Interior tempera-tures can be taken from Figure 5 in Chapter 9 of the 2013 ASHRAEHandbookFundamentals. During winter, conductivity is the majorheat loss
37、. The heat required to temper ventilation air and to counter-act infiltration through the body and during door openings must alsobe considered.Other ConsiderationsHarsh environments and the incursion of dirt and dust inhibit theefficiency of HVAC units. Specifications should include precisemaintenan
38、ce instructions to avoid capacity loss and compromisedpassenger comfort.3. BUS AIR CONDITIONINGIn general, bus air-conditioning systems can be classified as inter-urban, urban, or small/shuttle bus systems. Bus air-conditioningdesign differs from other air-conditioning applications because ofclimati
39、c conditions in which the bus operates, equipment size limi-tations, vehicle engine, electrical generator, and compressor rpm.Providing a comfortable climate inside a bus passenger compartmentis challenging because the occupancy rate per unit of surface and airrecirculation volume is high, glazed ar
40、ea is very large, and outdoorconditions are highly variable. Factors such as high ambient temper-atures, dust, rain, snow, road shocks, hail, and sleet should be con-sidered in the design. Units should operate satisfactorily in ambientconditions from 22 to 122F.Ambient air quality must also be consi
41、dered. Air intakes areusually subjected to thermal contamination from road surfaces, con-denser air recirculation, or vehicle engine radiator air discharge.Vehicle motion also introduces pressure variables that affect con-denser fan performance. In addition, engine speed governs com-pressor speed, w
42、hich affects compressor capacity. R-134a is thecurrent refrigerant of choice, but some units operate with refriger-ants such as R-22 (pre-2010 production) and R-407C.Bus air conditioners are initially performance-tested as units in aclimate-controlled test cell. Performance tests encompass unit oper
43、-ation at different compressor speeds to make sure the compressor per-formance parameters e.g., unit operation at maximum and minimumambient conditions, thermostatic expansion valve (TXV) sizing, oilreturn, and vibration/shock are within boundaries. In addition, indi-vidual components should be qual
44、ified before use. Larger test cellsthat can hold a bus are commonly used to verify installed unit perfor-mance. These tests are to measure the amount of time required toreduce the vehicles interior temperature to a specified value, andthey vary in performance and time requirements. Some commonlyacce
45、pted tests include the Houston pulldown (extreme heat or perfor-mance when using higher-pressure refrigerant gas such as R-407C),modified pulldown (mild to hot climates with R-134a or equivalent),white book pulldown (mild to hot climates), and the profile test (mildto hot climates, 95 and 115F ambie
46、nt). All these tests are describedin American Public Transportation Association (APTA) standard busprocurement and recommended practices for transit bus HVAC sys-tem instrumentation and performance testing.Reliability and ease of maintenance are also important designconsiderations. All parts requiri
47、ng service or regular maintenanceshould be readily accessible, and repairs should be achievable withoutremoving any additional components and within a minimum time.Heat LoadThe main parameters that must be considered in bus air-conditioning system design includeMass Transit 11.3Occupancy data (numbe
48、r of passengers, distance traveled, dis-tance traveled between stops, typical permanence time)Dimensions and optical properties of glassOutdoor weather conditions (temperature, relative humidity, solarradiation)Dimensions and thermal properties of materials in bus bodyIndoor design conditions (tempe
49、rature, humidity, air velocity)Power and torque limitations of bus engineThe heating or cooling load in a passenger bus may be estimated bysumming the heat flux from the following loads:Solid walls (side panels, roof, floor)Glass (side, front, and rear windows)PassengersEngine and ventilation (difference in enthalpy between outdoorand indoor air)Evaporator fan motorExtreme loads for both summer and winter should be calculated.The cooling load is the most difficult load to handle; the heatingload is normally handled by heat recovered from the engine, exter-nal heater, or ele
