ASHRAE LO-09-071-2009 Improving Control and Operation of a Single Duct VAV System through CCLEP《通过CCLEP提高控制与操作单管可变空气量系统》.pdf

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1、760 2009 ASHRAEABSTRACTThis paper discusses the implementation of new innova-tive technologies in a Continuous Commissioning Leading Energy Project (CCLEP). Results of a case study show that CCLEP implementation can improve building operations and reduce energy costs. Energy consumption is compared

2、in detailbefore and after CCLEP. Results show average electricity savings of 26.8% and gas savings of 47.8%. INTRODUCTIONWith the energy crisis of the early 1970s came the realization that buildings could be made much more efficient without sacrificing comfort. Over the last 30 years, use of variabl

3、e air volume systems has become common practice.Many variable air volume (VAV) systems with pneumatic controls were installed in the 1980s and are still in use. However, these systems often have outdated control strategies and deficient mechanical systems are deficient, which may cause occupant disc

4、omfort and excess energy consumption. An ASHRAE committee proposed building commission-ing in 1988 to ensure that system performance met design specifications. Continuous Commissioning (CC) technologywas developed and implemented in 1992. CC is an ongoing process to resolve operating problems, impro

5、ve comfort, opti-mize energy use and identify retrofits for existing commercial and institutional buildings and central plant facilities 1-5. Since 1999, the Energy Systems Laboratory (ESL) at the University of Nebraska has conducted extensive research to implement optimal system control during the

6、design phase and finalize the optimal setpoints after system installation. ESL researchers have developed and implemented the Continuous Commissioning Leading Energy Project (CCLEP) process with federal and industry support. The CCLEP process has two stages: the contracting stage and the implementat

7、ion stage. During the contracting stage, a comprehensive technical evaluation is performed. The CCLEP implementation stage involves planning, retrofit and trouble shooting, and optimization and follow-up. The CCLEP process, procedures and seven case study results are presented in 6. This paper prese

8、nts information on the case study facility, existing and improved control sequences, and building perfor-mance improvement and energy consumption measures before and after CCLEPimplementation.FACILITY INFORMATIONThe case study building is a 5-story office building in Omaha, Nebraska, which was built

9、 in 1988 with a total floor area of 91,200 square feet (8,472 square meters). The plant consists of one chiller and three gas boilers. There are two single-duct VAV air handling units (AHU) supplying condi-tioned air to 106 VAV terminal boxes. The typical office hours are from 8:00 a.m. to 5:00 p.m.

10、 during the weekdays.Terminal BoxesThere are two types of pressure independent VAV t e r m i -nal boxes. One is a VAV box with reheat coil for exterior zones and the other is a VAV box without reheat coil for interior zones. All of these terminal boxes are controlled by pneumatic controllers. Air-Ha

11、ndling UnitsThere are two similar single-duct VAV air-handling units serving this building. AHU 1 serves the basement, 1st and 2ndfloors. AHU 2 serves the 3rd, 4th and 5thfloors. Both use similar Improving Control and Operation of a Single Duct VAV System through CCLEPYoung-Hum Cho Mingsheng Liu, Ph

12、D, PE Xiufeng PangStudent Member ASHRAE Member ASHRAE Student Member ASHRAEJinrong Wang Thomas G. LewisMember ASHRAE Member ASHRAEYoung-Hum Cho and Xiufeng Pang are doctoral students and Mingsheng Liu is a professor in the Department of Architectural Engineering, University of NebraskaLincoln, Linco

13、ln, NE. Jinrong Wang is a senior technical analysis engineer and Thomas G. Lewis is a technical anal-ysis engineer in Omaha Public Power District (OPPD), Omaha, NE.LO-09-071 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Tr

14、ansactions 2009, vol. 115, part 2. 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 761control sequences but have different parameter settings. The AHUs include

15、one 50 HP (37.3 kW) supply fan and one 20 HP (14.9 kW) relief fan. The supply fans and relief fans have their respective variable frequency driver (VFD) controls. A typical single-duct AHU system is shown in Figure 1.Chilled Water SystemThe chilled water system includes one 250-ton chiller, one 250-

16、ton cooling tower with one 40 HP (29.8 kW) two-speed tower fan, two 25 HP (18.6 kW) chilled water pumps and two 20 HP (14.9 kW) condenser water pumps. The chilled water system is shown in Figure 2.Hot Water SystemThe hot water system consists of 2800 MBH (820 kW) three gas boilers, seven main loop c

17、onstant-speed pumps and 10 HP (7.46 kW) two constant-speed pumps for case study building. The hot water is supplied to the case study building through a heat exchanger. The hot water system also provides heating to an adjacent building. The system is shown in Figure 3.SYSTEM UPGRADETo realize the op

18、timal control and better building energy operation and energy performance, three CCLEP retrofits were made: 1) the pneumatic controllers for HVAC system were upgraded to DDC, 2) the Energy Management System (EMS) was installed, and 3) a VFD was installed on the chilled water pump. SYSTEM IMPROVEMENT

19、SThe existing control sequences were verified based on analyzing trending data and taking measurements for selected parameters under the existing operating schedules. Based on the actual site conditions, recommendations for Continuous Commissioning (CC) were developed. Table 1 summarizes the major r

20、ecommendations and actions taken. Comparison of the major control schedules before and after CCLEP, as well as other CCLEP activities, are presented in the subsec-tions below.Terminal BoxesFigure 1 Single-duct AHU system (original design).Figure 2 Chilled water system with installed VFD.762 ASHRAE T

21、ransactionsThe major CCLEP measures for the terminal boxes involve resetting the minimum primary airflow and fixing thebox mechanical problems to achieve building thermal comfort. Existing Schedule. The terminal boxes were originally tested and balanced to provide a minimum primary airflow of 25% to

22、 50%. The minimum primary airflow of 25% - 50% in the existing control caused a majority of the rooms to become too cold when the boiler was not operating during the summer months. Also, the lack of primary airflow at the maximum primary airflow setting caused several rooms to be too hot.Improved Sc

23、hedule. To solve these issues, adjustments to the minimum and maximum primary airflows were made. For VAV reheat terminal boxes that serve exterior zones, the minimum airflow setpoint is typically selected to be the largest of the following: 1) the airflow required by the room design heating load, o

24、r 2) the minimum required for ventilation. Where applicable, the minimum primary airflow in interior zone was set to 0% and the exterior zone was set to 20%. In this building, multiple zones serve an open office area. The open office plan allows some VAV boxes serving a space to go to zero airflow,

25、provided other boxes serving that space are controlled to provide sufficient minimum ventilation for the entire space. This eliminates the need to operate the boiler during the summer months while maintaining the room temperatures at normal levels, resulting in significant fan power and reheat energ

26、y savings. By doing this in conjunction with proper static pressure control from the air-handling units, the rooms receive adequate cooling airflow and do not over-whelm the fan. Results. After adjustment of the minimum and maximum primary airflows, hot and cold complaints were significantly reduced

27、 throughout the building. Moreover, fan power and gas consumption also decreased. When the outside air tempera-ture is 55F (12.7 C), the energy consumption is 8,177 Btu/hr (2,396 W) when there is conventional minimum airflow (56%). On the other hand, the energy consumption is 4,623 Btu/hr (1,354 W)

28、when there is improved minimum airflow (22%). The thermal energy consumption of improved mini-mum airflow is less than that of the conventional minimum airflow by 43%. By adjusting the minimum airflow, the termi-nal box in the exterior zone can reduce thermal energy. Indoor air quality meters were u

29、sed to verify indoor air quality in occupied spaces after reducing minimum air flow. The aver-age CO2level in each floor was in the range of 350 550 ppmwhen the average outdoor air concentration was 300 ppm. VAV AIR-HANDLING UNITSThe major CCLEP measures for the VAV air-handling units are 1) resetti

30、ng the duct static pressure, 2) resetting thesupply air temperature, 3) improving the economizer, 4) improving the pre-heat valve control, 5) implementing the relief fan and building pressure control, and 6) improving theoperation schedule. Operation ScheduleExisting Schedule. There was no operation

31、 schedule. The operator manually turns on/off the AHUs daily based on empirical knowledge.Improved Schedule. The AHUs are turned on during occupied hours. The AHU operation schedule is shown in Table 2.Results. Automatically starting and stopping the AHUseliminates the operators manual startup tasks

32、. Static Pressure ControlExisting Schedule. The static pressure schedule was maintained at a constant setpoint under normal operating conditions. Static setpoints were 1.25 in. W.C. during the summer and 0.75 in. W.C during the winter. Improved Schedule. The static pressure setpoint resets are based

33、 on the airflow ratio. The VFD is modulated to main-tain the supply duct static pressure at its setpoint of 0.4 1.25 in.W.C. The fan static pressure setpoint should be within the Figure 3 Hot water system schematic diagram (original design).ASHRAE Transactions 763high and low limits, which are calcu

34、lated by the following equation:(1)Results. Comparison of the trend data for the existing and improved control schedules reveals a reduction in fan speed.The decline in fan speed results in a reduction in noise at partial load conditions and major power savings.Supply Air Temperature ControlExisting

35、 Schedule. The supply air temperature setpoint was maintained at 55F (12.7 C) during the summer and 61F(16.1 C) during the winter. Improved Schedule. The supply air temperature setpoint is based on building load. Before the supply air flow reaches its minimum limit, which is 30% of maximum flow, the

36、 constant supply air temperature setpoint is adjusted to 55F(12.7 C). When the supply air flow reaches the minimum airflow limit, the supply air temperature resets to maintain minimum airflow when the outside air humidity ratio is low. In addition, the upper limit of the supply air temperature is 68

37、F (20 C). Results. Comparison of the trend data for the existing and improved control schedules reveals a reduction in supply airflow ratio from the supply air temperature reset. The decline in supply airflow results in major fan power savings.Preheating coilExisting Schedule. The existing control m

38、odulated the pre-heat valve to maintain the mixed air temperature setpoint. Improved Schedule. The pre-heat valve remains closed unless the mixed air temperature reaches the lower limit temperature. Then, the pre-heat valve modulates to maintain the mixed air temperature setpoint.Results. The new co

39、ntrols eliminate the use of preheat under most operating conditions. With the improved schedule, the pre-heat valve did not open to maintain mixed air temperature, according to trend data.Economizer and Outside Air ControlExisting Schedule. The outside air economizer was enabled when the outside air

40、 temperature was less than 55F(12.7 C). The minimum outside air damper position was maintained at 30% open during both occupied and unoccupied hours. Table 1. Summary of System ImprovementsFacility System ImprovementsTerminal Boxes Reset the minimum primary airflowAHUsReset the duct static pressureI

41、mprove the supply air temperature controlImprove the outside air damper control (economizer)Improve the preheating valve controlImplement the relief fan and building pressure controlImprove the operation scheduleChilled Water SystemReset the chilled/condenser water temperature setpointImplement the

42、chilled water pump speed controlImprove the operation scheduleHot Water SystemReset the heating water temperature setpointImprove the operation scheduleTable 2. AHU Operation ScheduleAHUsMonday Tuesday Friday Saturday SundayON OFF ON OFF ON OFFPre - CC Operator manually starting and stoppingPost - C

43、C 3:30am 6:00pm 4:30am 6:00pm 5:30am 1:00pm OFFPSetMin Max PLLimit,PHLimit,()Pset Max,=where PHLimit,Max 1 ()QQd-2,+Pset Max,=PLLimit,Max PMinPset Max,QQd-2,=764 ASHRAE TransactionsImproved Schedule. When the outside air temperature is lower than 68F (20 C), the economizer is enabled. Table 3 shows

44、the outside air damper control sequence of the AHUsunder economizer mode. The minimum outside air damper position corresponds to the occupied and unoccupied periods. The minimum outside air damper position is maintained at 10% open during occupied hours and at 0% during unoccupied hours. Results. Th

45、e existing control schedule does not take full advantage of the free cooling, while the improved schedule maximizes use of free cooling. This may reduce cooling energy consumption.Relief Fan ControlExisting Schedule. The existing control modulated the relief fan VFD to maintain the building pressure

46、 at its setpoint (0.02 in. W.C). The relief damper was modulated according to the relief fan VFD speed. Building pressure was negative and sometimes could not fully close the main entrance door.Improved Schedule. When the outside air damper oper-ates between its minimum and maximum position, the fan

47、 airflow station is applied to the relief fan speed control. The relief fan VFD is modulated to maintain a relief airflow setpoint. The relief damper will be fully open when the relief fan is enabled. When the outside air temperature is higher than 68F (20 C), the relief fan is disabled. The relief

48、fan VFD is modulated to maintain a relief airflow setpoint, which is calcu-lated by Equation (2) below. The outside air intake can be obtained from Table 4. (2)Results. Figure 4 shows the comparison of trend data for supply and relief airflows. The new control method eliminates the original building

49、 pressure control issue by maintaining a positive pressure (0.05 in. W.C.) in the building.Chilled Water SystemThe major CCLEP measures for the chilled water system are 1) resetting the chilled water temperature setpoint, 2) reset-ting the condenser water temperature setpoint, 3) implementation of chilled water pump speed controls, and 4) an improved operation schedule. Operation ScheduleExisting Schedule. To control the chiller, the operator manually turns on/off the chilled water system. The chiller is typically enabled from