ASHRAE HVAC APPLICATIONS SI CH 11-2015 MASS TRANSIT.pdf

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. Mass, envelope, power consumption, maintainabil-ity, and reliabil

3、ity are important factors. Power sources may be elec-trical (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 at2.5 to 5 L/s 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. The

8、re 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 l

9、atter. 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, howeve

10、r. 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 t

11、he 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 resea

12、rch, ASHRAE Standard 161 establisheda ventilation rate for aircraft passengers at 3.5 L/s 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 lar

13、ger-volume spaces, as defined in ASHRAE Standard 62.1, is 2.5 L/s perperson. It is recommended that ground mass transit applications use3.5 L/s of outdoor air per passenger for most transit applications.Emergency ventilation, such as windows or exits that can beopened or battery-powered ventilators,

14、 should 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 prov

15、ided 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

16、will 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

17、and 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. Inco

18、ntrast, 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 im

19、portant factors intransit comfort. At the opposite extreme, many subway stations arenot climate controlled, and often reach dry-bulb temperatures over38C in the summer. Thus, when boarding a climate-controlled vehi-cle, these passengers immediately perceive a significant in-crease incomfort. However

20、, a passenger adjusts to a new environment inabout 10 to 20 min; after that, the traditional comfort indices beginto apply, and the same interior conditions that were perceived ascomfortable may now be perceived as less than comfortable. Beforestabilization, a passenger may prefer higher-velocity ai

21、r or cooler orwarmer temperatures, depending to some extent on clothing. At thesame time, other passengers may already have stabilized and havecompletely different comfort control desires. Therefore, the transitsystem designer is presented with a number of unusual requirementsin providing comfort fo

22、r 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 Applications (SI)European Committee for Standardization (CEN) Standard EN13129-1 provid

24、es guidance 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

25、 from 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 so

26、lar 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 app

27、lica-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 oper

28、ate 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 op

29、erating 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 r

30、elative 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 du

31、ring 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. Airleak

32、age 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 W/K, should be provided by thevehicle designers. For outdoor skin temperature guidance, use

33、thevalues 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 sum

34、mer 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-tiona

35、l 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

36、transfer 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 los

37、s. 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 precisemaintena

38、nce 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 ofclimat

39、ic conditions in which the bus operates, equipment size limi-tations, vehicle engine, electrical generator, and compressor rev/s.Providing a comfortable climate inside a bus passenger compartmentis challenging because the occupancy rate per unit of surface and airrecirculation volume is high, glazed

40、 area 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 30 to 50C.Ambient air quality must also be con

41、sidered. 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,

42、 which 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 op

43、er-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 qu

44、alified 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 commonlyac

45、cepted 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, 35 and 46.1C am

46、bient). 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 requ

47、iring 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 (nu

48、mber 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 (te

49、mperature, 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 el