1、824 2009 ASHRAEABSTRACTTraditional constant air volume systems consume signif-icantly more energy than variable air volume (VAV) systems because a constant amount of air is supplied to each zone regardless of the zone load. Due to seasonal and daily load variations, variable frequency drives (VFDs)
2、can be installed on these constant air volume systems to reduce system energy consumption without needing to retrofit the terminal box. This paper presents the procedures for supply fan speed control and the results of its application in an office building. The results show electricity savings of 23
3、% and gas savings of 19% over a six-month period. INTRODUCTIONMany constant air volume (CAV) systems were installed in the 1950s and 1960s. Unfortunately, many of these systems are still in use due to the high cost of converting them to VAV systems. Constant air volume systems, like VAV systems, are
4、 often oversized and incur a significant energy penalty due to their incapability of modulating airflow. Zone loads are often much lower than the design load due to partial occupancy and maintain cooler air temperatures than design weather condi-tions. Since a constant supply air temperature is requ
5、ired for humidity control, more energy may be used during moderate weather conditions. CAV systems consume more fan power, more heating energy, and more cooling energy. The conven-tional method of reducing energy consumption is to install variable frequency drives (VFDs) on the supply and return air
6、 fans and convert the terminal box. Retrofitting the existing terminal box requires significant investment and interrupts the buildings normal occupancy pattern.Liu and Claridge (1999) presented a method to convert dual-duct constant volume systems without having to retrofit the terminal boxes to a
7、VAV system. A damper is installed in the main hot duct. During summer, the hot air is shut off. This method reduces both fan and thermal energy. For single-duct constant volume systems, Liu et al. (2002) suggested install-ing VFDs on both supply and return air fans to reduce airflow at night and on
8、weekends. This practice can be extended to normal operating hours with use of a proper control sequence for the CAV system.The objective of this study is to develop a method for VFD control through a case implementation. This paper presents facility information, procedures for supply fan speed contr
9、ol, and the results of the case study application.FACILITY INFORMATIONThe test facility is a 12-story office building with 489,000 ft2in Omaha, Nebraska. It was built in the late 1960s. The typical office hours are 8:00 a.m. to 5:00 p.m. during the weekdays. Two typical air-handling units (AHUs) loc
10、ated in the east interior zone and exterior zone were chosen for the case study. Each AHU is a typical single-duct CAV system with terminal boxes. The interior zone unitshave one 60 hp supply fan, and the exterior zone units have one 25 hp supply fan. To apply the method for VFD control, VFDs were i
11、nstalled in the supply fan of each unit. The AHU schematic diagram is shown in Figure 1. Figure 2 shows a schematic diagram of the section plan.VFD CONTROL METHODThe building load varies significantly with outside air conditions and internal occupancy conditions. In constant air VFD Application for
12、Constant Volume Air-Handling UnitsYoung-Hum Cho Mingsheng Liu, PhD, PE Gang Wang, PhDStudent Member ASHRAE Member ASHRAE Member ASHRAEJinrong Wang, PE Timothy Rauscher, PEMember ASHRAE Associate Member ASHRAEYoung-Hum Cho is a student and Mingsheng Liu is a professor in the Architectural Engineering
13、 Department at the University of Nebraska-Lincoln, Omaha, NE. Gang Wang is an assistant professor of Civil and Architectural Engineering at the University of Texas A&M-Kingsville, TX. Jinrong Wang and Timothy Rauscher are senior technical analysis engineers for Omaha Public Power District, Omaha, NE
14、.LO-09-080 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted w
15、ithout ASHRAEs prior written permission.ASHRAE Transactions 825Figure 1 Schematic diagram of AHU.Figure 2 Section plan of the building.826 ASHRAE Transactionsvolume systems, a significant amount of energy is consumed unnecessarily. Most of this energy waste can be reduced by simply installing a VFD
16、on the supply fan without a major retrofit effort if there are no retrofit funds available. The following is the procedure for identifying the VFD speed control. Step 1: Trend the Zone TemperaturesTo identify the building load profile for an AHU, room air temperature (Tr), supply air temperature, (T
17、SA) and zone discharge air temperature (TDA) should be measured daily. Figure 3 shows trended data of zone temperatures with highest TDA(zone A) and lowest TDA(zone B). The graphic shows that the supply air temperature and room air temperature have similar trends. The difference between zone dischar
18、ge air temperatures is about 10F. Hence, each zone has a different load profile and wastes reheating energy. Step 2: Calculate Zone Load ProfileThe interior zone load profile in an office building depends on occupancy only. The building load ratio can be calculated by the following equation based on
19、 the previous measurement data:(1)Figure 4 shows the zone load profile calculated by Equa-tion 1. The graph shows that zones A and B have different load profiles. Although zone B has the maximum load, it still wastes reheat energy because of the high airflow. The supply air temperature (TSA) from th
20、e central units was maintained at design value (55F) for the humidity control, but the discharge air temperature increased to maintain the room temperature setpoint. To reduce the reheat energy, the supply air Figure 3 Measured data of zone temperatures.Figure 4 Calculated zone load profile.TrTDATrT
21、SA-=ASHRAE Transactions 827temperature (TSA) can be increased, or the airflow can be decreased using a VFD control. Step 3: Pick-Up Maximum Load ProfileIn this case study building, there were load profiles for 12 zones between maximum and minimum, according to the previous calculated zone load profi
22、le. To control the supply fan speed of the CAV system, the maximum load profile zone, zone B, was chosen as shown in Figure 4.Step 4 : Set-Up VFD Speed as the Profile Interior Zone Unit. Develop the supply fan speed setpoint based on the previous picked up maximum load profile and modulate the VFD s
23、peed to maintain its setpoint. The VFD speed of the interior zone unit is a function of the time of day, which is determined by Equation 2. The exam-ple in Figure 5 shows the VFD speed setpoint of the interior zone unit as 55% from 8:00 a.m. to 9:00 a.m. and 65% from 9:00 a.m. to 11:00 a.m. (2)Exter
24、ior Zone Unit. The exterior zone load office building depends on both the outside air conditions and internal heat gains. The VFD speed of the exterior zone unit is a function of time of day and outside air temperature (Equation 3). During the heating period, the supply fan has minimum heating requi
25、red speed. If the system works continuously 24 hours a day, 7 days per week, the fan speed will have three different modes: (1) occupied hours at weekday daytime, (2) unoccupied hours at weekend daytime, and (3) unoccupied hours at week-day night time. Figure 6 shows the determined supply fan VFD sp
26、eed of the exterior zone unit with three different modes.(3)RESULTS AND DISCUSSIONAfter implementing the VFD control, the fan speed, fan power, discharge air temperature, and energy consumption were all reduced. Comparison of Fan SpeedInterior Zone Unit. Figure 7 shows the trended data for the suppl
27、y fan speed of the interior zone unit. The supply fan speed was reduced significantly. The reduction resulted in major fan power and zone reheating energy savings. The fan speed reduction was 35% after implementing the VFD control. The system can be operated continuously without frequent speed adjus
28、tment by the building operator.Exterior Zone Unit. Implement the supply fan speed of the exterior zone unit by the previous determined setpoint. Figure 8 shows the trended data for the supply fan speed and outside air temperature of the exterior zone unit. The supply fan speed modulated its setpoint
29、 based on the outside air temperature and time of day.Comparison of Fan Power Interior Zone Unit. The supply fan power consumption was 2534 kWh per week before installing the VFD. After implementing the VFD control method, the supply fan power Figure 5 VFD speed of interior zone unit.VFD_SPEED f TOD
30、()=VFD_SPEED f TOD OAT,()=828 ASHRAE Transactionsconsumption was 1013 kWh per week. The fan power savings is 60% after VFD control. Exterior Zone Unit. Figure 9 shows the trend data for supply fan power of the exterior zone unit. Before installing VFD, the supply fan power consumption was 5796 kWh p
31、er week. After implementing the VFD control method, the supply fan power consumption was 1426 kWh per week. The fan power savings is 75% after VFD control. Comparison of Thermal EnergyTo compare energy consumption, we measured the supply airflow, supply air temperature, and zone discharge air temper
32、ature of the interior zone unit during typical occupied hours on weekdays in June 2006. The reheat energy consump-tion can be determined using Equation 4.(4)Before implementing the VFD control method, the reheat energy consumption was 6947 kBtu/h per day, as shown in Table 1. The reheat energy consu
33、mption was reduced to 3896kBtu/h after implementing the VFD control method. The reheat energy savings is 44%. The reheat energy consumptioncan be reduced more by rebalancing the airflow of zones.Comparison of Whole Building Energy Consumption After implementing the VFDcontrol method in this building
34、, the whole building energy consumption was reduced. The VFD retrofit area comprised 57% of the entire campus. Figure 10 compares the whole campus electricity consumption over a six-month period. The electricity consumption savings was 4,425,946 kWh, a reduction of 23%. Figure 11 compares gas consum
35、ption over a six-month period. The whole campus gas consumption savings was 44,490 therms, a reduction of 19%. The electricity and gas energy consumption and cost savings are shown in Table 2.Evaluation of Thermal ComfortTo verify the room thermal performance of the interior zone after implementing
36、the VFD control method, we measured the room temperature at the furthest zone (11thfloor) from the AHUs and the highest load zone (8th floor).We chose the furthest zone (11th floor) for the exterior zone. The room air temperatures varied from 72.4F to 73.8FFigure 6 VFD speed of the exterior zone uni
37、t.Figure 7 Supply fan speed of the interior zone unit.Figure 8 Supply fan speed of the exterior zone unit.Erh CpmTDATSA()=ASHRAE Transactions 829during occupied hours. The room air temperature was main-tained at its setpoint.During the noneconomizer cycle, the measured average CO2level on each floor
38、 varied from 400 to 530 ppm when the average outdoor air concentration was 320 ppm. CONCLUSIONSA procedure to use VFD to control the supply fan in CAV systems was developed and demonstrated in an office building. The measured reheat energy saving was 44%. The fan energy savings was 60% for the inter
39、ior unit and 75% for the exterior unit. The measured energy savings are comparable to savings from a converted VAV retrofit. The cost of the VFD control method is less than 20% of the conventional retrofit.ACKNOWLEDGMENT The cooperation and support from the building owner and Omaha Public Power Dist
40、rict are greatly appreciated.NOMENCLATURE= specific heat capacity, Btu/lbm F= mass flow rate, lbm/hr= room dry bulb temperature, F= supply dry bulb temperature, F= Discharge dry bulb temperature, F= Building load ratio, %= reheating coil energy consumption, Btu/hREFERENCESASHRAE. 2008. 2008 ASHRAE H
41、andbookSystems and Equipment. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.ASHRAE. 2004. ASHRAE Standard 55-2004, Thermal Envi-ronmental Conditions for Human Occupancy. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta
42、.Liu, M., D. Claridge, and D. Turner. 2002. Continuous Com-missioning Guidebook: Maximize Building Energy Efficiency and Comfort. Reston, VA: American Society of Civil Engineers.Liu, M., and D. Claridge. 1999. Converting dual-duct con-stant-volume systems to variable volume systems with-out retrofit
43、ting the terminals. ASHRAE Transactions101(1):6670.Hartman, T. 2003. Improving VAV zone control. ASHRAE Journal 45(6):2431.Zheng, B., M. Liu, and P. Xiufeng. 2005. Continuous com-missioning of an office building. Proceedings of the 5th International Conference for Enhanced Building Operations, Pitts
44、burgh, Pennsylvania.Phillips, J. 2004. Side-by-side test program verifies vari-able-frequency energy savings. HPAC Engineering76(4):1821. Sellers, D. 2006. Specifying variable-frequency drives. HPAC Engineering 78(10):5455.Figure 9 Supply fan power of the exterior zone unit.Figure 10 Comparison of e
45、lectricity consumption.Figure 11 Comparison of gas consumption.CpmTrTSATDAErh830 ASHRAE TransactionsTable 1. Comparison Data of Heating Energy ConsumptionFloor No VFD Control, Btu/h VFD Control, Btu/h Energy Savings, %PLF 784,891 502,611 36.02nd 680,100 350,993 48.43rd 527,329 163,780 68.94th 799,42
46、0 551,056 31.15th 693,341 344,032 50.46th 720,195 467,998 35.07th 624,738 429,354 31.38th 458,972 81,093 82.310th 822,660 599,766 27.111th 836,246 405,810 51.5Total 6,947,892 3,896,493 43.9Table 2. Energy Consumption and Cost savingsSavingsConsumption CostElectricity 4,425,946 kWh $123,526Gas 44,490 therms $32,957
