ASHRAE HVAC APPLICATIONS IP CH 4-2015 TALL BUILDINGS.pdf

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1、4.1CHAPTER 4TALL BUILDINGSStack Effect. 4.1Typical HVAC Design Process. 4.8Systems. 4.9System Selection Considerations . 4.9Displacement Ventilation . 4.11Central Mechanical Equipment Room Versus Floor-by-Floor Fan Rooms 4.11Central Heating and Cooling Plants 4.14Water Distribution Systems 4.17Verti

2、cal Transportation 4.19Life Safety in Tall Buildings . 4.20ALL buildings have existed for more than 100 years and haveTbeen built in cities worldwide. Great heights only became pos-sible after the invention of the elevator safety braking system in1853; subsequent population and economic growth in ci

3、ties madethese taller buildings very popular. This chapter focuses on the spe-cific HVAC system requirements unique to tall buildings.ASHRAE Technical Committee (TC) 9.12, Tall Buildings, de-fines a tall building as one whose height is greater than 300 ft. TheCouncil on Tall Buildings and Urban Habi

4、tat (CTBUH 2014) de-fines a tall building as one in which the height strongly influencesplanning, design, or use; they classify recently constructed tallbuildings as supertall (buildings taller than 1968 ft) and megatall(buildings taller than 2600 ft).Traditionally, model codes in the United States

5、were adopted ona regional basis, but recently the three leading code associationsunited to form the International Code Council (ICC 2012), whichpublishes the unified International Building Code(IBC). Anotherimportant national code, developed by the National Fire ProtectionAssociation (NFPA), is NFPA

6、 Standard 5000.The overall cost of a tall building is affected by the floor-to-floorheight. A small difference in this height, when multiplied by thenumber of floors and the area of the perimeter length of the building,results in an increase in the area that must be added to the exteriorskin of the

7、building. The final floor-to-floor height of the officeoccupancy floors of any building is jointly determined by the owner,architect, and structural, HVAC, and electrical engineers.There are increasing numbers of tall buildings in the world (eitherplanned or built) that will have a much greater heig

8、ht than 300 ft.There is also a trend that most of the new tall buildings today are ofthe mixed-use type: for example, many will have a combination ofcommercial offices, hotel, apartments, observation deck, club floor,etc., stacked on top of each other. Tall buildings with these heightsand mixed uses

9、 will significantly affect HVAC system design.Much of the material in this chapter derives from Ross (2004).1. STACK EFFECTStack effect occurs in tall buildings when the outdoor temperatureis lower than the temperature of the spaces inside. A tall building actslike a chimney in cold weather, with na

10、tural convection of air enteringat the lower floors, flowing through the building, and exiting from theupper floors. It results from the difference in density between thecold, denser air outside the building and the warm, less dense airinside the building. The pressure differential created by stack

11、effect isdirectly proportional to building height as well as to the differencebetween the warm inside and cold outdoor temperatures.When the temperature outside the building is warmer than thetemperature inside the building, the stack effect phenomenon is re-versed. This means that, in very warm cli

12、mates, air enters the build-ing at the upper floors, flows through the building, and exits at thelower floors. The cause of reverse stack effect is the same in that itis caused by the differences in density between the air in the buildingand the air outside the building, but in this case the heavier

13、, denser airis inside the building.Reverse stack effect is not as significant a problem in tall build-ings in warm climates because the difference in temperature betweeninside and outside the building is significantly less than the tem-peratures difference in very cold climates. Accordingly, this se

14、ctionfocuses on the problems caused by stack effect in cold climates.Note that these measures can be very different than those in hot andhumid climates.TheoryFor a theoretical discussion of stack effect, see Chapter 16 in the2013 ASHRAE HandbookFundamentals. That chapter describescalculation of the

15、theoretical total stack effect for temperature dif-ferences between the inside and outside of the building. It also pointsout that every building has a neutral pressure level (NPL): the pointat which interior and exterior pressures are equal at a given temper-ature differential. The location of the

16、NPL is governed by the actualbuilding, the permeability of its exterior wall, the internal partitions,and the construction and permeability of stairs and shafts, includingthe elevator shafts and shafts for ducts and pipes. Other factorsinclude the air-conditioning systems: exhaust systems that exten

17、dthrough the entire height of the building tend to raise the NPL,thereby increasing the total pressure differential experienced at thebase of the building. This also increases infiltration of outdoor air,which tends to lower the NPL, thus decreasing the total pressure dif-ferential experienced at th

18、e base of the building. Finally, wind pres-sure, which typically increases with elevations and is stronger at theupper floors of a building, also can shift the neutral plane, and shouldbe considered as an additional pressure to stack effect when locatingthe neutral plane.Figure 1 depicts airflow int

19、o and out of a building when the out-door temperature is cold (stack effect) and hot (reverse stack effect).Not shown is the movement of air up or down in the building as aThe preparation of this chapter is assigned to TC 9.12, Tall Buildings.Fig. 1 Airflow from Stack Effect and Reverse Stack Effect

20、(Ross 2004)4.2 2015 ASHRAE HandbookHVAC Applicationsfunction of stack effect. Assuming there are no openings in the build-ing, the NPL is the point in the building elevation where air neitherenters nor leaves the building. Vertical movement of air in the build-ing occurs at the paths of least resist

21、ance, including but not limited toshafts and stairs in the building as well as any other openings at theslab edge or in vertical piping sleeves that are less than totally sealed.Figure 1 also indicates that air movement into and out of the buildingincreases as the distance from the NPL increases. El

22、evator shafts,especially ones that connect the top and bottom of a tall building(e.g., a fire lift), are likely paths of least resistance for airflow. Thetotal theoretical pressure differential can be calculated for a buildingof a given height and at various differences in temperature betweenindoor

23、and outdoor air.The theoretical stack effect pressure gradient for alternative tem-perature differences and building heights is shown in Figure 2. Thediagram illustrates the potential maximum differentials that canoccur (which are significant), but these plotted values are based on abuilding with no

24、 internal subdivisions in the form of slabs and par-titions. The plot, therefore, includes no provisions for resistance toairflow in the building. Further, the outer walls permeability influ-ences the values on the diagram and, as noted previously, the windeffect and operation of the building air-ha

25、ndling systems and fansalso affect this theoretical value. Thus, the diagram should be con-sidered an illustration of the possible magnitude of stack effect, notas an actual set of values for any building. The actual stack effect andlocation of the NPL in any building are difficult (if not in a prac

26、ticalsense impossible) to determine. Nevertheless, stack effect can betroublesome, and its possible effects must be recognized in the de-sign documentation for a project.Practical ConsiderationsStack effect in tall buildings often presents major problems:Elevator doors may fail to close properly bec

27、ause of the pressuredifferential across the doors, which causes the door to bind in itsguideway enough that the closing mechanism does not generatesufficient force to overcome it.Manual doors may be difficult to open and close because ofstrong pressure created by stack effect.Smoke and odor propagat

28、ion through the air path of stack effectcan also occur.Heating problems can occur in lower areas of the building thatmay be difficult to heat because of a substantial influx of cold airthrough entrances and across the buildings outer wall (caused byhigher-than-anticipated wall permeability). Heating

29、 problems canbe so severe as to freeze water in sprinkler system piping, coolingcoils, and other water systems on lower floors. Generallyaccepted standards suggest a maximum leakage per unit of exte-rior wall area of 0.06 cfm/ft2at a pressure difference of 0.30 in. ofwater exclusive of leakage throu

30、gh operable windows. In reality,tall buildings in cold climates can exceed this pressure differencethrough a combination of stack, wind, and HVAC system pres-sure. Even when leakage similar to the NAAMM criterion isincluded in project specification, it is not always met in actualconstruction, thereb

31、y causing potential operational problems.CalculationUncontrolled infiltration and ventilation is caused by climate,wind pressure, and stack effect; environmental factors associatedwith stack effect include wind pressure, stack pressure difference,airflow rate, outdoor and indoor temperature, buildin

32、g height, andbuilding construction.Wind creates a distribution of static pressure on the buildingenvelope that depends on wind direction and velocity against thebuilding envelope. The basic formula to determine this pressure canbe expressed asPW= Po+ CCpVw2/2 (1)wherePW= wind pressure above outdoor

33、air (OA) pressure, in. of waterC = unit conversion factor, 0.0129Cp= surface (location on building envelope) pressure coefficient, dimensionless = air density, lbm/ft3(about 0.075)Vw= wind speed, mpho = outdoorWhen using this equation, wind pressure is 0.1 in. of water at15 mph on the windward side.

34、Air density varies with temperature. In cold weather, low-densityair infiltrates the high-rise building and rises in the buildings verti-cal shafts as it warms, creating stack effect pressure. The basic stackeffect theory is expressed asPs= C2ig(h hneutral)(Ti To)/To(2)wherePs= stack pressure differ

35、ence (indoor outdoor), in. of waterC2ig = air density and gravity constant, 0.01444h = building height, fthneutral= height of neutral pressure level, fti = indoorT = temperature, FWhen using Equation (2), the stack pressure is 1.1 in. of water fora 60-story building with 10F OA temperature.Once the

36、wind pressure PWand stack pressure difference Psare calculated, total pressure Ptotalcan be found, based on indoorand outdoor pressure difference, and used to calculate the airflowrate:Ptotal= (Po Pi) + PW+ Ps(3)The resultant pressure difference will cause air flow based on thefollowing expression f

37、or flow through an orifice:Fig. 2 Theoretical Stack Effect Pressure Gradient for Various Building Heights at Alternative Temperature Differences(Ross 2004)Tall Buildings 4.3Q = Cf A (4)whereQ =airflow, cfmCf= flow coefficientA = cross-sectional area of opening, ft2Ptotal= total pressure difference a

38、cross flow area, in. of water = air density, lbm/ft3Temperature, air density, and pressure can also be calculated, asfollows:T = TG 0.00356h (5) = 2.7 Pabs/(T + 459.7) (6)whereT = temperature, FTG= ground temperature, FPabs= absolute pressure, psiP = pressure, psiCalculation Examples. For the calcul

39、ation examples, four citieswere selected, each with its own unique climate characteristics: Bei-jing (cold, dry), Bangkok (hot, humid), Dubai (hot, arid), andCopenhagen (cold, humid). Because stack effect has different influ-ences on buildings of different heights, in each city, buildings of var-iou

40、s heights were calculated.For each city, climate data (acquired from a climate consultant)show the average winter and summer temperature and humidity,which can be used to calculate stack effect in different ASHRAEclimate zones (Figure 3). The lighter shade indicates the averages,and the darker shade

41、 shows the summer and winter temperatureextremes.The building type is residential. Outdoor temperature andhumidity levels are equal to the heating or cooling seasons averagetemperature. Indoor temperature is 70F, height above sea level is0 ft, wind speed is 5 mph, typical building floor height is 14

42、 ft, airpressure is 29.92 in. Hg, and results are per square foot of envelopearea.Beijing. Table 1 gives the example parameters, and Table 2shows the calculation results. As shown in Figure 4, the biggest dif-ference between internal pressure occurs in winter, when internal2 PtotalTable 1 Parameters

43、 for Beijing Example BuildingSummer WinterOutdoor temperature, F 94.9 15.6Indoor temperature, F 75 68Relative humidity, % 54 15Height above sea level, ft 180 180Wind speed, mph 14.9 14.9Air pressure, in. Hg 29.77 29.77Fig. 3 Climate Data for (A) Bangkok, (B) Beijing, (C) Dubai, and (D) Copenhagen4.4

44、 2015 ASHRAE HandbookHVAC Applicationspressure increases along the building height; in summer, it decreasesalong the height. In addition, when the building gets taller, its NPLon the windward side rises: the extreme is for a building height of1969 ft, for which the NPL on the windward side is almost

45、 on the topof the building. For a 328 ft building, the windward-side pressure isalmost the same as the stack pressure.For the climate in Beijing, which is cold and dry in the winter andwarm and humid in summer, stack effect is much more intense thanin warmer climates. Stack effect during cold outdoo

46、r conditionsmay cause problems, such as elevator doors not closing properlybecause of the pressure differential across the doors, causing thedoors to stick in their guideways enough that the closing mechanismcannot overcome it.Another difference of Beijing compared with other cities is thatthe winte

47、rtime NPL is slightly lower, or below the middle of thebuilding. During winter in a 1969 ft building in Beijing, the NPL isslightly below 984 ft, which means the indoor air pressure is muchhigher at the upper level of the building than in the following cities.Therefore, for upper levels of the build

48、ing, air leakage is muchgreater in Beijing than other, warmer cities, the airflow rate ishigher, the building function is significantly affected. Address stackeffect during design. Architects and engineers must pay close atten-tion to solving the problems associated with stack effect, which areexace

49、rbated in extremely cold climates.Bangkok. Bangkok has a hot and humid climate throughout theyear (Table 3); therefore, in winter, the leeward- and windward-sidepressures on the Bangkok building change more from the bottom tothe top than those on the Beijing building (Figure 5 and Table 4).Also, because even in winter outdoor air is warmer than the indoorair, internal pressure decreases along the building height.Because Bangkoks climate is humid and hot throughout theyear, winter and summer temperatures are similar (average wintertemperature is