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Supply Fan Control for Constant Air Volume Air Handling Units

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Supply Fan Control for Constant Air Volume Air Handling Units 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 1 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 2 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.
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