ASHRAE SL-08-059-2008 Design Charts for Combined Chilled Ceiling Displacement Ventilation System.pdf

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1、574 2008 ASHRAE This paper is based on findings resulting from ASHRAE Research Project RP-1438.ABSTRACT This paper proposes operational design charts forcombined chilled ceiling (CC) displacement ventilation (DV)hybrid air conditioning system (CC/DV). The design chartswere developed by performing a

2、large number of simulationsusing a simplified transport plume multi-layer model of theCC/DV conditioned space. The simulation model results werevalidated by conducting a series of experiments that showedgood agreement with the predictions of the simplified model ofthe stratification height, the CC l

3、oad, and the room air verticaltemperature gradient. The proposed design charts, like any chart in the litera-ture, are based on the important parameter of the ratio of theCC cooling load to the total load (R), the thermal comfortrepresented by the temperature gradient (dT/dZ), and theamount of displ

4、aced air parameter ( ). The designchart parameters include the temperature ranges of the supplyair temperature and the chilled ceiling for any R in the feasibledesign regions where dT/dZ is less than 2.5 K/m and with thefacility to read off the stratification height H and insure that itis above 1.2

5、m. The stratification height was selected as anadditional criterion to the thermal comfort because in humidwarm climates, designing the CC/DV system for a high strat-ification height might be quite costly. The CC/DV design chartshave shown that R can be met by different fordifferent air supply tempe

6、rature and chilled ceiling tempera-tures and that the stratification height is strongly correlatedto compared to air and ceiling temperatures. Twocorrelations were developed at high predictability for the strat-ification height and vertical temperature gradient dependenceon room height and system op

7、erational parameters. INTRODUCTIONIn displacement ventilation, the cooler air entering theroom at the floor level displaces the warmer room air that risesdue to its natural buoyancy effect. Consequently, the bottomoccupied zone contains the fresh cool air with no recirculationflow while the heat and

8、 contaminants produced by the roomactivities rise to the ceiling level where they are exhaustedJiang et al. (1995) and Yuan et al. (2001). Yuan et al. (2001)indicated that displacement ventilation cannot maintainacceptable comfort for cooling load above the recommended40 W/m2unless the air supply vo

9、lume is increased or addi-tional heat removal capacity is provided through the use ofcooled ceiling panels. The relatively small cooling capacity ofthe DV system is dictated by the constraint of thermal comfortin tolerating cold air draft in the occupied zone. However, theDV system load has been rep

10、orted to tolerate a maximum loadof 120 W/m2if the ventilation rate is increased, the ceiling ishigh, and there is sufficient space for installing large diffusers.It was pointed out by Yauan et al. (2001) that if the cooling loadis high, then energy consumption significantly increases. Forhigher cool

11、ing capacities in warmer climates, a combinedchilled ceiling and displacement ventilation is recommended.Behne (1999) design diagram sets the cooling load limit ofdisplacement ventilation when combined with chilled ceilingto 100 W/m2of floor area. The chilled ceiling carries a portionof the sensible

12、 cooling load and the DV system carries the restof the sensible load in addition to the latent cooling loads. PQm()=PQm()=PQm()=Design Charts for Combined Chilled Ceiling Displacement Ventilation SystemNesreen Ghaddar, PhD Kamel Ghali, PhDMember ASHRAERalph Saadeh Amer KeblawiStudent Member ASHRAE S

13、tudent Member ASHRAENesreen Ghaddar is Endowed Qatar Chair and a professor of energy studies and Ralph Saadeh and Amer Keblawi are graduate studentsin the Department of Mechanical Engineering, American University of Beirut, Lebanon. Kamel Ghali is an associate professor of mechanicalengineering at B

14、eirut Arab University, Lebanon.SL-08-059 (RP-1438) 2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions Vol. 114, Part 2. For personal use only. Additional reproduction, distribution, or transmission in either print

15、or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 575The design of the CC/DV air conditioning systeminvolves design of two subsystems: the displacement ventila-tion cooling subsystem and the chilled ceiling subsystem. Aproper design of such a system is not

16、 straight forward as thecase when one system is used in meeting the cooling load. Thecomplexity of the design stems from the fact that two systemshave contradicting advantages and disadvantages. Increasingthe proportion removed by the chilled ceiling will enhance thethermal comfort by decreasing the

17、 vertical temperature gradi-ent, but indoor air quality will be reduced due to a loweramount of supply air to displace the contaminated air from theoccupied zone. On the other hand, if the load removed by theDV system is increased, then the indoor air quality willimprove but the thermal comfort will

18、 decrease Behne, 1999.The proportions of the load removed by the two systemsshould be carefully selected to meet the two basic requirementof any air-conditioning system: the provision of thermalcomfort and of good indoor air quality (IAQ). Previous researchers have recognized such a need indesigning

19、 charts for the DV/CC system Tan et al. (1998),Behne (1999). Tan et al. (1998) design chart for CC/DVsystems is based on the fraction of the load removed by the DVsystem (QDV) to the total load (Q) and the ratio of the total loadQ to supply flow rate to meet thermal comfort. The indoorair quality re

20、quirements for the CC/DV system were presentedin a different chart by Behne (1999). The Behne (1999) and Tanet al. (1998) design charts do not insure the attainment of thestratification height (the elevation at which the density gradi-ents disappear in the rising air and its plume spreads horizon-ta

21、lly) at a minimum level for acceptable indoor air quality in theoccupied zone. Those literature charts did not present the ther-mal comfort and the indoor air quality in a single chart and alsodid not provide the practicing engineer a feasible design rangeof air temperatures and chilled ceiling temp

22、erature that couldsatisfy the loads removed by the CC and DV subsystems. Areview of the design guidelines of CC/DV systems by Yaun etal. (2001) and Novoselac and Srebric (2002) indicated thatfurther research is needed to develop a universal but simplemethod for determining vertical temperature gradi

23、ent andimposing a desired minimum stratification height in the space. The objective of this work is to provide engineers with auseful and convenient tool to assess the CC/DV system designparameters. The proposed design chart for the CC/DV systemwill be based on the CC system load ratio to total load

24、 andcorresponding load to supply flow rate ratio at various chilledceiling and supply air temperatures. The stratification heightand regions of thermal comfort where the air vertical temper-ature gradient (dT/dZ) is less than 2.5 K/m will also be indi-cated in the chart. The following sections will

25、provide themethodology used to develop the design chart, description ofthe design charts, and the evaluation tools used to validate thechart results by experimentation and 3-D simulations.CC/DV DESIGN CHART DEVELOPMENT METHODOLOGYA successful design of CC/DV system would be robust ifthe stratificati

26、on height in the room, the vertical distribution ofair temperature, and humidity within the enclosure can accu-rately be predicted. The wall plume multi-layer model Ayoubet al. (2006) (plume-multilayer model) of spaces cooled by theCC/DV system is used in the development of the design chartsthat are

27、 also validated by experiments due to inherent modelassumptions. Ayoub et al. (2006) model predicts the stratifi-cation height and the vertical air and wall temperature gradi-ents as a function of supply air conditions, and the chilledceiling temperature. The stratification height is determinedfrom

28、mass balances at the height when air supply flow rate isequal to the thermal plumes Mundt, 1996. Unlike othermodels which assume the conditions of adiabatic walls orisothermal walls, the wall plume-multilayer takes into accountthe non-uniformity of the wall temperature and the depen-dence of this te

29、mperature distribution on the internal andexternal conditions. This feature is important when evaluatingexposed surfaces to solar radiation. The model has limitationsbased on inherent assumptions. The heat sources are suffi-ciently distant to prevent plume interaction. Point heat sourcesare located

30、at their actual positions and heat sources withdimensions are located at equivalent virtual point source posi-tion Goodfellow and Esko (2002). The model also assumesthat the equilibrium height of each plume is not less than the3/4 of the room height where the equilibrium height is theelevation at wh

31、ich the density gradients disappear and theplume spreads horizontally as reported by Goodfellow andEsko (2002). The main input parameters to the wall-plume-multilayermodel are the chilled ceiling temperature (Tc), supply airtemperature (Ts), supply air flow rate ( ), supply air humidityratio (ws), r

32、oom area (A) and height (HR), the room total cool-ing load (Q). For a given load (Q), we need to assist the engi-neer to select the supply conditions and the chilled ceilingtemperature to see the effect of the selected parameters on dT/dZ and stratification height. With the exception of humidityrati

33、o, all operational parameters, stratification height andacceptable thermal comfort conditions are either included orwill be possible to deduce from the design charts that aredeveloped for two load ranges, three standard room heights of2.5 m, 3 m and 3.5 m, and three room areas (area to perimeterrati

34、os are 1, 1.25, and 1.5) to account for geometric effectChen and Glicksman, 2006. The wall-plume-multilayermodel Ayoub et al., 2006 facilitated performing the simula-tions. More than 12,000 design scenarios of the CC/DVsystem are generated and evaluated to check how these scenar-ios meet the system

35、constraints and to identify ranges ofdesign parameters when both indoor air quality and thermalcomfort are satisfied. The data on the room and CC/DV systemparameters for the considered cases are summarized inTable 1. Note that the first step in design of the system, thespace sensible load is determi

36、ned to assess the need for the usemm 2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions Vol. 114, Part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital for

37、m is not permitted without ASHRAEs prior written permission.576 ASHRAE Transactionsof the combined CC/DV system. The need for the combinedsystem is justified when the sensible cooling load is larger than40 W/m2Rees and Haves, 2001. The considered range ofvalues of supply air temperature conditions i

38、s between 288 Kand 293 K. The constraint on the supply air temperature arisesfrom the risk of cold drafts in the occupied region and frommaintaining sufficient density gradient to keep the fresh air inthe occupied zone. The supply mass flow rate is constrained bythe fresh air requirement for accepta

39、ble IAQ with the condi-tion that air velocity is less than 0.15 m/s to reduce draft forthermal comfort ASHRAE, 2005.After careful examination of the interdependence of thevarious system parameters, it was found that the stratificationheight is strongly dependent on supply air flow rate and isweakly

40、dependent on supply temperature Tsand chilled ceil-ing temperature Tc. The chilled ceiling load ratio R = Qcc/Qcan be attained by many combinations of , Q/, Tsand Tc. A sample design chart is shown in Figure 1 where thechilled ceiling temperature is plotted as a function of R forconstant-P lines ( )

41、 at 90-100 W/m2load and aroom geometry (A = 25 m2and HR = 3m). The load in theselected room is based on the presence of nine persons in addi-tion to internal and solar external load. The constant-P linesvary from 2 to 14 kJ/kg and are segmented by horizontal lines.Each zone between the horizontal li

42、nes represents a supplytemperature Tswithin 0.5 K of the shown value. In whatfollows, we will show how the chart shown in Figure 1 isproduced. The input design parameters to the wall-plume-multi-layer model are the ceiling temperature, the outdoordesign conditions, internal loads, and the supply air

43、 flow rateand temperature. The outputs of the model are the convectiveand the radiative heat loads on the ceiling panel, the verticalwall and air temperature distributions and gradients, and strat-ification height. The load is selected such that it is within therange of 90-100 W/m2in this example. T

44、he decision on feasi-ble and acceptable design values are checked against desiredconditions for stratification height and for dT/dZ to be smallerthan 2.5 K/m. At constant Parameter , 225 simulationswere performed starting at fixed ceiling temperature andincrementing the supply temperature by 0.5 K f

45、or each runfrom 288 K to 295 K to find the corresponding chilled ceilingload ratio R and recording all other related parameters for theroom including H and the room air vertical temperature distri-bution. The procedure is repeated for all chilled ceilingtemperature in the range of 288 to 295 with in

46、crements of 1 K.Figure 2 shows the result of these runs at (a) P = 14 kJ/kg and(b) P = 6 kJ/kg where Tcis plotted against R. The solid lineconnecting the points will represents the averaged constant-Pline. Each vertical segment will be assigned to a given supplytemperature within 0.5 K starting with

47、 the upper segmentrepresenting the lowest supply temperature 288 K and thelowest segment representing the highest supply temperature at298 K. The constant-P line gives low value of R at high Tcandlow Tsand gives high value of R at low Tcand high Ts. Togenerate all the constant P-lines, several diagr

48、ams wereproduced for different constant-P values and they werecombined in the single design chart for the given load. Eachconstant P-line needed around 1500 simulations using the 1-D model to identify feasible operational regions for the givenload range. Table 1. Summary of CC/DV Operational/Geometr

49、ic Parameters andTheir Ranges for Production of the Design ChartsParameter Symbol unit Values and RangesRoom height HRm 2.5 m, 3.0 m, and 3.5 mRoom area (perimeter per unit area) A m2 (m) 16 (1) - 25 (1.25) - 36 (1.5)Thermal load/unit area Q W/m2 60-70 and 90-100 Supply mass flow rate kg/sTotal load/supply mass flow rate kJ/kg 4 14Ratio of chilled ceiling load to total loadR = Qcc /Q 0 1Chilled ceiling temperature Tc K288 295 K, The ceiling temperature must be about 1.5 C above the dew point to avoid the risk of condensation.Supply tempera