1、4722 Integrated Damper and Pressure Reset for VAV Supply Air Fan Control Guanghua Wei, P.E. Associate Member ASHRAE Mingsheng Liu, Ph.D., P.E Member ASHRAE David E. Claridge, Ph.D., P.E. Member ASHRAE ABSTRACT This paper presents an integratgd damper and pressure reset (IDPR) method for variable air
2、 volume (VAV) system fan control. The IDPR method controls the static pressure at a minimum required level while maintaining at least one terrni- na1 box damper at full open position. When the entire system is flawless, the fan speed is controlled in a way similar to the terminal regulated air volum
3、e (TRAV) method to maintain at least one terminal box damper full open. When system faults exist, however, this controlstrategy uses less fan power than the TRAV method. The IDPR method can be implemented in both full DDC (direct digital control) systems and hybrid systems, where only the space temp
4、erature readings are communicated to the DDCcontroller at the air-handler level and the terminal boxes are controlled by pneumatic controllers. INTRODUCTION In a VAV system, the total supply airflow rate decreases as the building load decreases until the airflow reaches the minimum value. To ensure
5、adequate airflow under all load conditions, the supply air fan is often required to provide enough static pressure in a preselected duct location. When the duct static pressure is not satisfied, the control system can modulate the volume control device, such as a variable frequency drive (VFD), on t
6、he fan motor to maintain the static pressure at its setpoint. The duct static pressure sensor is typically located two- thirds of the way downstream in the main trunk of the supply air duct. Although this old rule of thumb is no longer recom- mended by ASHRAE (ASHRAE 1999), it is quite common in man
7、y existing systems. The fan usually maintains a constant static pressure setpoint. The setpoint is selected such that it provides enough static to terminal VAV boxes under design full-load conditions. However, under partial-load conditions, as we will discuss later, the static pressure required at t
8、he terminal VAV boxes may be far less than this setpoint. To improve partial-load operations, static pressure can be decreased to save fan power (Hartman 1989; Englander and Norford 1992; Warren 1993; Liu et al. 1997). A popular method is to reset the static pressure setpoint as a function of the ou
9、tside air temperature. This is reasonably accurate in envelope-dominated buildings (i.e., buildings with large ratios of exterior zones). Other methods reset the static pressure based on total supply airflow rate or supply air fan speed, since they are also indicators of the building load. The selec
10、tion of the reset schedule depends on a number of factors, such as internal load, envelope, and occupancy schedule. Expert judg- ment is required to set up the reset schedule. Due to the complex nature of building systems, even those control experts are sometimes very conservative in setting up the
11、reset schedules. The terminal regulated air volume (TRAV) control method integrates terminal box operation with supply air fan control (Hartman 1989). The TRAV method controls the supply fan based on real-time terminal box airflow require- ments (often calculated by the DDC controller) rather than m
12、eeting a duct static pressure setpoint. When the AHU and terminal boxes are flawless, the TRAV method minimizes fan power consumption. Similar control algorithms have been proposed by others. Englander and Norford (1992) used primary airflow error signals from one or more zones to modu- late the sta
13、tic pressure or fan speed. Warren (1993) presented a control strategy that resets the fan static pressure setpoint based on terminal box flow requirements. The reset signal Guanghua Wei is an assistant research engineer at the Energy Systems Laboratory and David E. Claridge is a professor in the Dep
14、artment of Mechanical Engineering, Texas A otherwise, the system operation could be unpredictable. For some hybrid systems, the DDC system only has indi- vidual zone space temperature input information. The actua- tors (dampers and/or reheat valves) at the terminal VAV boxes are controlled by pneuma
15、tic controllers. These boxes interface with the DDC system through EP (electric to pneumatic) transducers. Since the DDC system does not have information on airflows at the box level, the above-mentioned control strat- egies are not applicable. Therefore, an alternative or improve- ments for these c
16、ontrol strategies are needed. This paper discusses an improved control strategy that can be used with both full DDC systems and hybrid systems. In order to provide building comfort with low fan power consumption, the fan can be modulated based on the maxi- mum VAV box damper position (damper control
17、 output signal), combined with a predefined static pressure reset schedule. We call this control strategy the integrated damper and pressure reset (IDPR) method. It requires modulating the fan speed to maintain the maximum VAV box damper position at 95% open, as long as the static pressure is mainta
18、ined below the calculated setpoint based on a predefined reset schedule. THE IDPR CONTROL METHOD Theory We introduce the IDPR method by discussing the static pressure control method for a VAV system. To simplify the discussion, we assume the VAV system is equipped with a VFD. The static pressure sen
19、sor is typically located two-thirds of the way downstream in the main supply air duct for many existing systems. The static pressure control method main- tains the setpoint of the static pressure by modulating the fan speed. When the static pressure is lower than the setpoint, the fan speeds up to p
20、rovide more airflow (static) to meet the box needs, and vice-versa. A constant setpoint value specified by the design engineer is often used regardless of the building load conditions. Under partial-load conditions, however, the static pres- sure required at the terminal VAV boxes may be far less th
21、an this constant setpoint since the pressure drop across the box is proportional to the square of the airflow rate, as Equation 1 illustrates. AP=CvxQ* (1) where A = pressure drop across VAV box volume damper, Pa (in. Q = airflow, L/min (CFM) Cv = flow coefficient, Pa x min2 / L2 (in. H20 / CFM2) Fo
22、r example, at 70% design flow rate, the static pressure required at the VAV box is approximately 50% of the static pressure corresponding to the design flow rate (assuming the VAV box damper is wide open in both cases; that is, Cv is the same). Under this partial-load condition, the static pressure
23、loss between the locatiop of the static pressure sensor and the terminal box is also much less than that under full-load condi- tion. due to reduced airflow. This pressure loss reduction increases the static pressure in front of the terminal VAV box when a constant static pressure setpoint is used,
24、and the pres- sure drop (AP) across the VAV box volume damper increases. To maintain space condition, the VAV box damper has to move toward the closed position (to increase Cv) in order to reduce the airflow to 70% of design value. Consequently, a significant amount of fan power is wasted through th
25、rottling at the termi- nal VAV box. To illustrate the interaction between the supply fan speed, duct static pressure, and the terminal box damper positions, refer to the schematic of a single-duct VAV system with termi- nal reheat, as shown in Figure 1. The supply fan is equipped with a VFD. Assume
26、that (1) there is only one VAV box, (2) the system has a constant static pressure setpoint of249 Pa (1 .O in. H20), (3) the supply air temperature is set at a constant of 123C (55“F), and (4) at maximum cooling load, the terminal box is fully open and the fan is running at 100% speed, and the static
27、 pressure is maintained at 249 Pa (1 .O in. H20). We now examine the sequence of operation under this conventional control strategy as the load changes from maxi- mum cooling to heating. The process is illustrated on the fan curve, as shown in Figure 2. As the cooling load decreases, the terminal bo
28、x damper will start to modulate toward the mini- H2O) Figure 1 Schematic of a VAV system. 31 O ASHRAE Transactions: Research Static Pressure & Box damper closes Fan slows Box damper opens, fan slows N3 I t Flow Figure 2 Operating point changes in a VAVsystem. mum position. This causes the static pre
29、ssure to increase (A+B) due to increased system resistance. The controller senses the increase in static pressure and slows down the fan speed from Ni to N2 in order to maintain the constant static pressure setpoint (B+C). When it settles down at the new speed, N, the static pressure is maintained a
30、t the setpoint of 249 Pa (1 .O in. H,O). However, the VAV box damper position is partially open at this moment, which means that the unit is operating under increased system resistance. Therefore, the static pressure setpoint is higher than necessary for this oper- ating airflow. To maintain the sam
31、e airflow rate under this condition, the fan speed can be further reduced to N3 by opening up the damper to its original position, as shown in process C+D. This eliminates the wasted fan power due to the extra pressure head (P, - Pd) that the fan has to overcome. The fan has the same efficiency when
32、 the fan operating point is located on the initial system curve. The damper position remains fully open. The damper control method requires modulating the fan speed to maintain the VAV box damper at fully open position. The damper control method is essentially the same as the air volume control meth
33、od. However, it uses the damper posi- tion control output instead of measured airflow. This method can be readily applied to a system with multiple terminal VAV boxes. The control system selects the maximum damper posi- tion from all of the VAV box control output signals. Fan speed will be modulated
34、 based on this maximum VAV box damper position. By controlling the maximum terminal box damper position at or near the fully open position, the fan speed is kept at minimum and the power consumption is minimized. Implementation Issues Since any VAV box damper can end up controlling the fan speed as
35、along as its position is the highest among all the boxes, care must be taken in applying this damper control strategy. This control method may not work if any one of the following conditions occurs: 1. 2. 3. 4. 5. 6. 7. 8. 9. One terminal box reheat control valve leaked hot water and caused unwanted
36、 reheats, forcing the box damper to be fully open to provide more cooling. One terminal box damper was stuck at a partially open posi- tion due to linkage or actuatorproblems, causing the zone to call for maximum cooling. One terminal box damper actuator was out of calibration. One space temperature
37、 sensor read too high due to sensor calibration problem or bad location (e.g., sensor reading was 3C 5.4“F higher than the actual value), causing the zone to call for maximum cooling. One terminal box was undersized due to changes in space functional use and/or increased internal load. One section o
38、f the flex duct was twisted. One zone could not receive enough airflow due to air balance problem. One occupant used a foot heater since the room temperature setpoint was lower than what helshe wanted. Occupants could adjust the room temperature setpoint in a wide range, resulting in some rooms that
39、 always call for full cooling (e.g., room cooling temperature setpoint is 20C 68“F instead of 23C 73.S0F). Based on the authors experiences, the above-mentioned nine problems are very common in the buildings. When any of these potential problems occurs, the fan speed could be out of control. For exa
40、mple, a terminal box damper may be stuck in a partially open position. The room temperature cannot be maintained at a comfortable level unless excessive static pres- sure is provided. This can result in the fan running at full speed regardless of the actual building load. Not only does it waste fan
41、power, but also heating and cooling energy. The Solution Realistically speaking, it is impossible to avoid all of the problems mentioned above. The chances for such problems to occur are very high for a large system with a number of termi- nal boxes. Compared with the constant static pressure contro
42、l strategy, the actual fan power consumption by using this damper position control strategy alone could be much higher if the system experiences any of these problems. To prevent these kinds of problems from happening, a high static pressure limit is required to avoid full-speed oper- ation under pa
43、rtial-load conditions. Integrating the damper control method with static pressure reset, the IDPR method is developed. Figure 3 is a schematic of the control logic for this IDPR method. Under this IDPR control strategy, each terminal VAV box receives its damper control signal from the DDC controller
44、. The control system selects the maximum damper position (%) from all of the VAV box control output signals and feeds the value into the fan speed PID (proportional, integral, and deriv- ative) control loop. The control loop then compares this maxi- mum damper position value with the setpoint of 95%
45、. When ASHRAE Transactions: Research 31 1 Maximum Damper Maximum Damper Position Position Set Point Outside Air Temperature -1.1 OC (30F) 32.2“C (90F) Outside Air Temperature I Static Pressure Setpoint 149 Pa (0.6 in. H20) 249 Pa (1.0 in. H70) Fan VFD Duct Static Pressure Static Pressure Set Point F
46、igure 3 Schematic of the IDPR method control logic. the maximumVAV box damper position is less than 95% open, the control loop output will slow down the fan speed. If the maximum VAV box damper position is more than 95% open, the control loop output will ramp up the fan speed until the damper is les
47、s than 95% open. Meanwhile, a parallel PID control loop modulates the fan speed to maintain the duct static pressure at the setpoint, which is reset based on one of the following parameters: outside air temperature, total airflow, supply air fan speed, or time of the day. The lower output from these
48、 two control loops is sent to the fan VFD to control the fan speed. Note that the maximum box damper position does not necessarily belong to one particular box. Depending on zone orientation, internal load pattern, and time of day, the box or boxes that experienced the maximum damper position may va
49、ry. EXPERIMENTAL APPROACH AND RESULTS System Description To demonstrate the benefit of the IDPR method for fan speed control, an experiment was conducted using a single- duct VAV hot water reheat system in an ofice building in central Texas. The air-handling unit (AHU) serves one floor of the building. Total floor area served is approximately 776 m2 (8350 fi2). The longest duct length is about 30 m (98 fi). Fan speed is modulated by a VFD to maintain a constant duct static pressure setpoint. The VFD has a low speed limit of 40% to provide minimum ventilation. The hybrid contr
