1、428 2010 ASHRAEABSTRACTAs for a variable air volume (VAV) system, the supply fanspeed typically modulate to maintain a duct static pressuresetpoint. Traditionally, this setpoint is a constant based onsystem characteristics at design condition and the pressuresensor location. The static pressure is t
2、he summation of theduct pressure loss downstream of the sensor and box inlet staticpressure. Under partial load conditions, the pressure loss inthe duct is much less than the design value due to reducedairflow. Thus, the static pressure set point can be reset lower.This can reduce fan power, avoid n
3、oise at terminal box damp-ers and prevent box damper malfunction due to excessive pres-sure. This paper develops theoretical models to demonstratethe impacts of static pressure reset on air leakage, fan powerand thermal energy for both pressure independent and pres-sure dependent terminal boxes.INTR
4、ODUCTIONFor a variable air volume system, it is typical to modulatesupply fan speed to maintain a duct static pressure set point.Traditionally, this set point is a constant aiming to ensureproper air distribution under design load (ASHRAE 1995).This set point is the summation of the total pressure l
5、oss alongthe air duct downstream of the sensor and the terminal boxpressure required by the manufacturer under design condi-tions. However, under partial load conditions, the terminal boxdampers will close to reduce airflow. Since the pressure loss isproportional to the square of the airflow ratio,
6、the requiredpressure set point can be reduced. Using constant pressure setpoint at partial load condition leads to more fan powerconsumption due to higher fan head. Whats more, with higheraverage duct pressure, air leakage in ducts will increase. Thisalso results in more fan power consumption.The su
7、pply fan control method using static pressure resetcan significantly reduce fan power. Liu (2007a) has demon-strated a simple fan power savings model by comparingconstant static pressure set point and static pressure reset. It isalso demonstrated that without static pressure reset, the designminimum
8、 airflow for pressure dependent box can not beachieved due to higher pressure before the terminal box damp-ers. Liu et al. (1997b) studied the impact of low static pressurein dual-duct systems on fan energy consumption.Besides the theoretical research, fan power savings due tostatic pressure reset i
9、s demonstrated by experiments and casestudies. The impact of static air pressure on the fan power wasrecognized by Warren and Norford (1993). The static air pres-sure reset schedule was investigated by Rose and Kopko(1994). Significant energy savings and improved indoorcomfort conditions have been m
10、easured and presented byClaridge et al. (1996). Liu et al. (1995) presented the impactsof VFD and static pressure reduction on energy consumption.Even though the fan power savings potential due to staticpressure reset has been widely acknowledged, no mathemat-ical model has been developed to quantit
11、atively analyze itsimpacts, especially with the consideration of air leakage. Also,the resulted thermal energy savings have not yet been demon-strated. An air leakage rate at 10 to 20% of the inlet airflow ratein commercial building is not uncommon (Fisk et al. 2000),and it has great impacts on both
12、 fan power and thermal energyconsumption. Wray (2003) has demonstrated the increase inannual fan energy is estimated to be 40 to 50% for a systemwith a total leakage of 19% at design conditions compared toa tight system with 5% leakage rate. Annual cooling plantImpacts of Static Pressure Reset on VA
13、V System Air Leakage, Fan Power, and Thermal EnergyMingsheng Liu, PhD, PE Jingjuan Feng Zhan WangMember ASHRAE Student Member ASHRAE Student Member ASHRAELixia Wu Keke Zheng Xiufeng Pang, PhDStudent Member ASHRAE Student Member ASHRAE Student Member ASHRAEMingsheng Liu is professor and Jingjuan Feng
14、, Zhan Wang, Lixia Wu, Keke Zheng, and Xiufeng Pang are graduate research assistantsin the Department of Architectural Engineering, University of Nebraska Lincoln, NE.OR-10-046 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE
15、 Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 429energy also increases by about 7 to 10%. However, his studyin a VAV sy
16、stem only consider the case with constant staticpressure setpoint.This paper presents mathematical models to demonstratethe impacts of static pressure reset on air leakage, fan powerand thermal energy for both pressure independent and pres-sure dependent boxes. Air leakage reduction is considered as
17、an important factor contributing to the fan power and thermalenergy savings.MODELIn this part, mathematical models are developed for calcu-lating the reduction of duct air leakage, fan power and thermalenergy due to static pressure reset. The mathematical modelsare developed based on a single duct V
18、AV system, and its sche-matic diagram is shown in Figure 1. The supply fan speedmodulates to maintain a static pressure set point. The terminalboxes consist of a modulation damper and a reheat coil.Major assumptions are: (a) boxes are evenly distributedalong the supply duct, and so does the leakage.
19、 This assump-tion implies that if we assume the ratio of the length of ductdownstream of the pressure sensor to the total duct length is y,then the ratio of the airflow diffused by downstream terminalboxes to the total system airflow is y, and the ratio of the airleakage at downstream to the total i
20、s y. (b) Load ratio at allboxes are the same. This means the static pressure will be resetusing Equation (1) at partial load.(1)Duct Leakage ModelAccording to ASHRAE Handbook (2001) the air leakagerate, CFM, can be calculated byCFMleak= Psn(2)Where, = Constant, reflecting area characteristics ofleak
21、age path; Psis static pressure differential from the ductinterior to exterior, in. of water; n = pressure exponent, usually0.5 n 1, corresponding to fully developed or laminar flow,here we assume fully developed flow with n = 0.5.Assuming zero building pressure, then the ratio of leak-age in the duc
22、t downstream to design is Equation (3):(3)For the main duct, the upstream of the pressure sensor, theleakage ratio is calculated by:(4)Where Hfan,desand Hfanare fan head at design load and atpartial load respectively, in. of water.(5)Where upis the ratio of upstream duct average airflow to fandesign
23、 airflow, calculated by:(6)Introduce = Ps,des/Hf,des, the duct leakage ratio de-fined by CFMleak/CFMleak,descan be calculated below:(7)See Appendix I for calculation of up.The duct leakage ratio at partial load isWhere = ratio of total leakage to fan airflow at design condi-tion.For air duct leakage
24、 class from 3 to 48, when the staticpressure is at 1 in. of water, the system percent leakage canvaries from 0.6 to 24%. (ASHRAE 2001). For demonstration,Figure 1 Typical single duct VAV systems.PsetpointPsetpoint des,-CFMboxCFMbox des,-2box2=CFMleak down,CFMleak down des,-Psdes,Psdes,- 1 no reset,=
25、box2Psdes,Psdes,- boxreset,=CFMleak up,CFMleak up des,-CFMleak up,CFMfan des,-Psdes,Hfan+Psdes,Hfan des,+-=HfanPsdes,2up1 y+-2Hfan des,Psdes,()no reset,+box2Psdes,2up1 y+-2Hfan des,Psdes,()resent,+=upCFMfanCFMleak down,yCFMbox+2CFMfan des,-=y 1 y()22up1 y+-21 ()+ 1+- no reset,+ybox1 y()+ 1 box2+()2u
26、p1 y+-21 ()+ 1+- reset,=leakageCFMleakCFMfan des,- 1-= 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in eit
27、her print or digital form is not permitted without ASHRAEs prior written permission. 430 ASHRAE TransactionsFigure 2 shows the duct leakage ratio, leakage, reduction withstatic pressure reset with 15% duct leakage at design airflow.Other values of key parameters for deriving Figure 2 are listedin Ta
28、ble 1.We can see that with the assumption that the sensor islocated at 2/3 of the distance from the first to the most remoteterminal, the leakage ratio decrease from 14 to 7%.Fan Power Savings ModelThe fan power calculation is used to determine the numer-ical savings achieved by static pressure rese
29、t. In this section,two basic types of terminal boxes, pressure dependent andpressure independent box are considered separately. Themodel also examines the effect of pressure reset on terminalbox minimum airflow and the resulting savings for a pressuredependent box. Figure 3 shows the typical working
30、 points ofa centrifugal fan at both design and partial load.As we can see that the working point at partial load with-out reset has higher fan head than the case with reset, then thisin turn increases the fan power input, which is given by Equa-tion (8)(8)Where, C = conversion factor, 1/8507; CFMfan
31、 = fanairflow rate, CFM; f= fan efficiency; and Hf= fan head, in.water, given by Equation (5). From Equation (8), we can alsosee that reduced air leakage can also contribute to fan powerreduction.Defining fan power consumption without static pressurereset as the base case (Efan,b), and with static p
32、ressure reset asimproved case Efan,i. The fan power saving ratio is given by:(9)Comparison of Pressure Dependent Box and Pressure Independent BoxVAV terminal boxes are available in many combinations.Pressure dependent and pressure independent boxes are themost common two for a single duct VAV system
33、. A pressuredependent terminal box is essentially a pressure reducing boxwith a motorized damper that is controlled by a room thermo-stat. These boxes do not regulate the airflow, but simply posi-tion the damper in response to the signal from the thermostat.Because the airflow to these boxes is in d
34、irect relation to thebox inlet static pressure, it is possible for the boxes with higherpressure upstream to get more airflow than the boxes withlower pressure upstream even if they are positioned the same.Pressure independent boxes can maintain airflow at any pointbetween maximum and minimum regard
35、less of box inlet staticpressure, as long as the pressure is within the design operatingrange. The sensing devices regulate the flow rate through thebox in response to the room thermostats call for cooling orheating.The pressure dependent boxes (PDB) and pressure inde-pendent boxes (PIB) considered
36、in this paper are bothequipped with a reheat coil and the following control algo-rithms are used:When the building cooling load is large, the reheat coilvalves are closed and the damper in PDB or airflow rate in PIBis modulated to maintain room comfort level. For PDB, a mini-mum damper position is p
37、reset, while for PIB a minimumairflow rate is preset. When building cooling load decreases tothe ratio that the damper (PDB) or the airflow rate (PIB)reaches their minimum value set point, the damper (PDB) orairflow rate (PIB) shall be maintained at the minimum valueTable 1. Parameter AssumptionsPar
38、ameter min, boxDownstream Duct Ratio y = Ps,desHfan,desValue 0.3 0.3 0.3Figure 2 Compare leakage ratio reduction with pressurereset and without reset vs. building load.EfanKW()C CFMfan Hfanfan-=EfanEfan b,Efan i,Efan des,-=Figure 3 Fan working point. 2010, American Society of Heating, Refrigerating
39、and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions
40、 431and the heating coil valves start to modulate to maintain roomcomfort level.Pressure Dependent BoxFor a pressure dependent box, with the load ratio () de-creasing, the damper position will close to a minimum posi-tion. The minimum damper position is determined based onthe required minimum airflo
41、w rate delivered by the box at thedesign condition, and at that time the pressure right at up-stream of the box is low due to high pressure drop in the ductwith high airflow rate. However, at partial load the pressuredrop in the duct decrease dramatically, and if the static pres-sure set point remai
42、ns constant, the actual minimum airflowratio min, boxis higher than the design minimum airflow ratio,min, box, due to excessive pressure on terminal damper com-paring to pressure at the design condition. If optimal staticpressure reset is used, the design minimum airflow ratio can beachieved and, th
43、erefore, fan power consumption in the lattercase can be reduced due to reduced airflow. See Figure 4 forrelationship of design, actual minimum airflow and the corre-sponding building load ratio. Without static pressure reset,when the building load decreases to minthe damper reachesminimum position.
44、And since at this time the pressure beforethe damper is higher than design pressure, the actual minimumairflow ratio box, minwill be higher than the design valuebox,min. Appendix II demonstrates the calculation of actualminimum airflow. Figure 5 shows the relation of simulatedactual minimum airflow
45、and the design value under differentratios of the static pressure set value and the design terminalbox static pressure value, = Ps, des/Pbox, des. We can see thatthe actual minimum airflow can be much higher than the ex-pected design value.The following part in this section numerically studies thefa
46、n power savings at different building load ratios. Threescenarios are considered: (a) when the load ratio is higher thanthe ratio corresponding to actual terminal box minimumairflow; (b) the load ratio is lower than the load ratio corre-sponding to actual terminal box minimum airflow, whilehigher th
47、an the design value; (c) the load ratio is lower than theload ratio corresponding to design minimum airflow; Assum-ing constant fan efficiency, integrate Equations (5), (6), (8),and (9), the fan power saving ratio is given by:(10)Where the subscript min = the corresponding value withminimum damper position, i and b means improved and basecase respectively fanis the ratio of partial load fan airflow rateto design value which i
