Supply Fan Control for Constant Air Volume Air Handling Units
YoungHum Cho Gang Wang Mingsheng Liu
Professor of Architectural Graduate Student Research Associate Professor Engineering, PhD, PE
Energy Systems Laboratory, University of Nebraska-Lincoln 1110 So. 67th St, Omaha, NE 68182
Abstract required design size when the HVAC systems design. Since terminal boxes do not have a modulation This results in major fan power waste. damper in constant volume (CV) air handling unit The following is the possible solution for how to (AHU) systems, zone reheat coils have to be solve these issues and control supply fan in the current modulated to maintain the space temperature with CV systems. The best way to control supply fan is an constant supply airflow. This conventional control installation of terminal box and modulate airflow with sequence causes a significant amount of reheat and VFDs on AHUs. In this way, the system should be constant fan power under partial load conditions. changed from CV system to VAV system. This method Variable Frequency Drives (VFDs) can be installed on will be able to control adequate supply fan speed under these constant air volume systems. The fan speed can partial load conditions. However, this method needs be modulated based on the maximum zone load. This high installation and labor fees. Another way is an paper presents the procedure to control the supply fan installation of the VFD on CV system and set the fan speed and analyzes the thermal performance and major speed. In this method, the system will not change and fan energy and thermal energy savings without does not need high installation and labor fees. The cost expensive VAV retrofit through the actual system of using VFD is much higher than changing the pulley; operation. however, it can allow accurate airflow adjustment. Consequently, using a VFD is a potential solution to Introduction control supply fan without system change and any In constant volume (CV) systems, all delivered air retrofit fee. Fan energy and both the heating and is cooled to satisfy the maximum zone cooling load cooling energy savings can potentially increase. with constant speed fans. Air delivered to other zones Few engineers recognize the advantage and is then reheated with heating coils in individual zone requirement of VFD. Phillips (2004) tested the VFD ducts. In ASHRAE Handbook, CV systems are energy usage. The data showed a 72 % energy savings generally limited to applications with fixed ventilation and the payback on the VFD is 1.44 year and VFDs needs, such as hospitals and special processes or can help lower maintenance costs. Miller (2002) laboratories; nevertheless, CV systems are widely used showed the minimum application requirement and said in a lot of old office buildings because these systems contemporary VFDs are more reliable, cost effective can be easily controlled and have simple mechanical and more capabilities. Currently, in CV system, the equipment. supply fan controls with a fixed VFD speed by a rule However, there are issues in current CV systems. of thumb. But, VFD speed can not adjust under partial One issue is each zone has variable zone load. The CV load conditions due to lack of the capability of system uses conditioned air from AHU generally at a changing the airflow. Therefore, dynamic airflow fixed cold air temperature to meet the maximum adjustment using VFD on CV system should be studied. cooling load. Reheat is added to the discharge air in The objective of this study is that existing CV each zone to match the cooling capacity to the current system will not change VAV system and how to get the zone load. The result is very high thermal energy use. largest benefits from minimum investments by only Another issue is the reheat energy can be reduced if the installing VFD. This paper presents the issues of supply air temperature from the AHU can be varied. applied building with conventional CV systems, However, the supply air temperature has to be develops a procedure how to control supply fan using maintained at design value for the humidity control VFD on current CV system to solve these issues, and when the outside air humidity ratio is high. Significant applies to the actual building. The energy consumption reheat energy is consumed unnecessarily due to of conventional and improved supply fan controls were humidity control requirement. The other issue is compared using measured data. normally the supply fan chooses bigger size than
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Building and Facility information Figure 2 shows a schematic diagram for the section plan of Building III. Each AHU serves each The applied building complex, located in Omaha, floor. There is no terminal box and modulating damper. Nebraska, was built in the late 1960s and composed of Each zone has reheat coils to maintain room air building III, II and I. The 12-story building III, 8-story temperature for unequal loading. The AHU schematic building II and 4-story building I are used as an office diagram is shown in Figure 3. building complex with total area of 489,000 square feet, which is served by AHUs with VFD. Each system Return Mechanic is a typical multi-zone CV system without terminal box. Air room The typical office hour is from 8:00am to 5:00pm during the weekdays. Figure 1 shows a schematic T Filter diagram of the AHUs serving area layout. Two typical T T P T Outside T C.C. SF Supply AHUs, located at the interior zone (AHU 7A) and Air Air VFD exterior zone (AHU 8), were chosen in this study. Figure 3. Schematic diagram of AHU
AHU 24 Procedure for supply fan control using VFD Building III AHU 6 AHU 13 Most of this energy waste can be avoided by simply installing a VFD on the fan without a major AHU 14A AHU 7A AHU 8 AHU
AHU 15 AHU retrofit effort. The following is the procedure for the
AHU 14B AHU 7B optimal supply fan speed.
Building II AHU 2A AHU 3A Interior zone Interior zone load profile in an office building is AHU 2B AHU 3B normally not too variable because there is no influence AHU 4A AHU AHU 4B AHU AHU 2C AHU 3C on the outside air conditions and similar heat gains. Building I Cooling load for interior zones considers total sensible AHU 1A cooling load due to heat gains by people, lighting and office equipment conditioned spaces. The supply fan speed for interior zone is typically AHU 1B optimized to be the following:
Figure 1. AHUs serving area layout Identify the maximum load ratio for the AHU
To identify the building load ratio for an AHU, Relief Fan Relief air room air temperature, supply air temperature and
H H H H discharge air temperature should be measured daily. LEVEL 11 After measuring room conditions of each floor, choose H H H H LEVEL 10 the highest (zone B) and lowest load zone (zone A) of
H H H H LEVEL 9 each zone as shown in Figure 4. The building load ratio
H H H H can be calculated by the following equation: LEVEL 8
H H H H − TT DAr LEVEL 7 α = (1)
H H H H − TT LEVEL 6 SAr
H H H H 80 LEVEL 5
Return airReturn shaft Tr (zone A ) H H H H 75 LEVEL 4 Tr (zone B) H H H H 70 LEVEL 3 TDA (zone A) H H H H 65 LEVEL 2
H H H H 60
LEVEL 1 (F) Temperature TDA (zone B) 55 PLAZA H H H H LEVEL TSA 50 Return LOWER AHU 6 AHU 7A AHU 7B AHU 8 LEVEL air 8:00 9:30 11:00 12:30 14:00 15:30 17:00 Time Figure 2. Section plan of the East of Building III Figure 4. Measured data for building load ratio
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Figure 5 shows the calculated load ratio graph by Exterior zone equation (1) to determine the supply fan speed. The graph shows zone A and B have different load ratios. Exterior zone load profile in office building will be Even though zone B has maximum load, it still wastes variable by the outside air conditions but internal heat reheat energy because of high airflow ratio. The supply gains are similar to interior zone loads. air temperature from central units provided to be The supply fan speed for the exterior zone is maintained at design value (55 °F) for the humidity determined by the following: control, but discharge air temperature increased to maintain room conditions by reheat coil. Therefore, the Calculate design condition and determine the reheat energy will be able to be reduced by increasing optimal fan speed load ratio. There are two ways to increase the load ratio. Calculate the design airflow based on the real heat One way is an increase of the supply air temperature gains data such as U value of the wall and window, (TSA). However, this method may have a humidity solar heat gain, lights, appliances and people. All issue. Another way is a decrease of the discharge air referenced data are based on the ASHRAE temperature (TDA) by reducing airflow ratio. Fundamentals Handbook. 1.0 Maximum load ratio (zone B) Determine the optimal fan speed 0.8 Based on the above calculation, the supply fan speed ratio was calculated by different outside air 0.6 temperatures from 40°F to 100°F. During heating period, the fan speed has minimum fan speed like 0.4 Load ratio Load terminal box minimum airflow ratio. If the system works continuously 24 hours and 7 0.2 Minimum load ratio (zone A) days, the fan speed will have 3 different modes: occupied hours at weekday daytime, unoccupied hours 0.0 8:00 9:30 11:00 12:30 14:00 15:30 17:00 at weekend daytime and unoccupied hours at weekday Time night time. VFD speed was modulated to maintain the
Figure 5. Calculated zone load ratio graph supply fan speed at its set point. Figure 7 shows the determined supply fan speed ratio with 3 different Develop/implement the optimal fan speed modes. Develop the supply fan speed set point based on 100 the previous maximum load ratio and modulate the 80 VFD to maintain the supply fan speed at its set point. Figure 6 shows the determined fan speed ratio based on 60 the maximum load ratio. To verify this control, select critical zones like furthest zone and highest load zone 40 .
from AHU and measure the room air condition. Supply fan speed(%) 20 weekday occupied load ratio 1.0 Weekday unoccupied and weekend night load ratio Fan speed set point weekend daytime load ratio 0 0.8 30 50 70 90 110 Outside air temperature(F) 0.6 Figure 7. Supply fan speed ratio for dynamic VFD speed
Load ratio Load 0.4 Implement the optimal fan speed 0.2 Implement the supply fan speed set point based on the pervious determined set point. Also, measure 0.0 the room air condition to verify this control. Figure 8 8:00 9:30 11:00 12:30 14:00 15:30 17:00 shows the trending data for the supply fan speed and Time outside air temperature. Figure 6. Determined fan speed ratio
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100 75.0 90 ) 72.5 PLF 80 2F
70 ) 70.0 3F 60 67.5 4F 50 5F 65.0 40 6F 30 62.5 7F
Supply fan speed(% fan Supply 20 Temperature(F 8F 60.0 10 10F 0 57.5 11F 50 60 70 80 90 100 Outside air temperature(F) 55.0 6:00 8:00 10:00 12:00 14:00 16:00 18:00 Figure 8. Supply fan speed and outside air temperature Time trend Figure 10. Discharge temperature graph of optimal supply fan control Results and discussions After applying the optimal supply fan control The reheat energy savings can be considered as the sequence in real buildings, the energy consumption and reheat energy consumption required to heat the reduced room conditions were measured and analyzed. airflow from the discharge air temperature to the room temperature. The reheating coil energy consumption Energy consumption in the actual system can be estimated based on the equation (2). Interior zone Room, supply and discharge air temperature were • CmE ( αα −⋅−⋅⋅=∆ TT )() (2) measured with a data-logger to compare the thermal rh p fixed dynamic rDA energy consumption. The results show room and supply air temperature are similar but discharge air From previous measured room condition data, the temperature is different for each area served. Figures 9 reheat energy consumption is 6,947,892 Btu/hr when and 10 show the discharge air temperature graphs of there is fixed VFD speed, as shown in Table 1 and existing and optimal supply fan control. Two figures Figure 11. On the other hand, the energy consumption show each floor has a similar trend but different is 3,896,493 Btu/hr when there is dynamic VFD speed. discharge temperature to maintain room air The thermal energy consumption of dynamic VFD temperature by adjusting reheating coil. Each floor speed is less than that of the fixed VFD speed by 44 %. discharge temperature shows that there is unnecessary Therefore, when the supply fan speed control is reheat energy from supply air condition to maintain optimized, thermal energy can be reduced. room air temperature, even the 8th floor, and also wastes a fan power energy. After implementing a Table 1. Comparison data of thermal energy dynamic VFD speed, the discharge air temperature has consumption Fixed VFD Dynamic VFD Energy a lower than fixed VFD speed as shown in Figure 9 Floor speed (Btu/hr) speed (Btu/hr) saving (%) and 10. The discharge air temperature is reduced and reheating energy should also be saved by dynamic PLF 784,891 502,611 36.0 VFD speed. 2nd 680,100 350,993 48.4 75.0 3rd 527,329 163,780 68.9 72.5 PLF 4th 799,420 551,056 31.1 2F
) 70.0 th 3F 5 693,341 344,032 50.4 67.5 4F th 5F 6 720,195 467,998 35.0 65.0 6F 7th 624,738 429,354 31.3 62.5 7F th
Temperature(F 8F 8 458,972 81,093 82.3 60.0 10F 10th 822,660 599,766 27.1 57.5 11F 11th 836,246 405,810 51.5 55.0 6:00 8:00 10:00 12:00 14:00 16:00 18:00 Total 6,947,892 3,896,493 43.9 Time Figure 9. Discharge temperature graph of existing supply fan control
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