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  CONTROLLING VARIABLE-VOLUME SYSTEMS   by Douglas C. Hittle, Ph.D., Fellow ASHRAE    Variable-volume (VAV) systems provide multizone control with a single duct. The supply air is maintained at a constant temperature, and individual zone thermostats vary the supply air quantity to the zone to maintain the desired space temperature. Many VAV systems do not include any heating function in the main air handling unit, although preheat may be required for systems with large amounts of outside air. Baseboard heating or reheat coils are used in exterior zones.  Varying the airflow to the space benefits system efficiency in several ways. One obvious  benefit is that the total cooling load on the cooling coil diminishes more or less linearly with diminishing room loads. This is in sharp contrast to the conventional reheat system and to other constant-volume systems that add heat as the space cooling load goes down. The reduction in required cooling as the loads diminish persists until the VAV volume controls reach their minimum setting. If the room VAV boxes reach their minimum  position, reheating is required to keep the space comfortable. However, in contrast to a reheat system, heating is required on only a fraction of the maximum airflow rate. Because only part of the air is heated, the reheat energy requirements are substantially reduced. Finally, the reduced airflow rates that occur when cooling loads are low or when heating is required result in substantial fan power savings. Although reduced energy consumption is possible, VAV systems demand robust control schemes if they are to be as efficient as expected. Single-Duct Variable-Volume System    Figure 1  shows a typical single-duct VAV system. Each zone thermostat controls its zone damper to reduce the air supply to the zone as the space temperature decreases. In zones that require heat (exterior zones or zones that would overcool because of high minimum air volumes), reheat coils or baseboard heaters are energized on a further decrease in temperature. Supply air temperature is controlled by a discharge temperature controller. Proportional only control, proportional plus integral control, and cold deck reset schemes are all possible. However, the control scheme must perform under varying air volume flow rates, so stability is a concern. For example, if the air volume flow rate over the cooling coil is only half the design maximum, then the change in valve position required to produce a given change in Page 1of 5ASHRAE Journal4/5/2004  discharge temperature will be about half that needed at full flow. This has the effect of doubling the process gain. If a proportional only controller was used to control the cooling coil, and if the gain of the controller was set to provide stable control at full flow, the control system might be completely unstable at half flow because the overall loop gain would have nearly doubled. This problem is compounded by the more sluggish response of the temperature sensor under low-flow conditions. The higher gains and longer time constants that result from lower flow rates require lower  proportional gain settings for the coil controller. Lower gain settings mean bigger throttling ranges and a much larger droop or steady state error if proportional only control is used. A large droop does not cause so drastic an increase in energy consumption with a VAV system as it does with constant-volume systems. However, the penalty can be significant, especially if the minimum flow settings for the VAV boxes are relatively high. Proportional plus integral control for the cooling coil provides a good way to eliminate droop. Given the wider proportional band that may be required with VAV systems, PI control could become a necessity; proportional only control might work little better than no control at all. Economy cycle control of the outdoor and return air dampers is often used with VAV systems. A separate PI controller may be appropriate for the outdoor/return air damper system. It is possible to reset the supply air temperature controller and the outdoor/return air damper controller from the warmest zone. Again, careful commissioning is required. The reset schedule must be set up so that the upward adjustment of the supply air temperature set  point from a room thermostat takes place with the VAV box fully open and the reheat coil valve closed. Typically, this requires dividing the room thermostat throttling range into three  parts. The upper part is used for resetting the supply air temperature controller, the middle section modulates the VAV dampers, and the lower section modulates the reheat coil. It may  be impractical to implement supply air reset without using digital or high-quality electronic controls. The benefits of supply air temperature reset depend strongly on the application. If the minimum VAV box settings must be high (50% of full flow, for example) and the requirements for cooling are not dominated by zones with constant large cooling loads, then supply air temperature reset may be justified in spite of is complexity. The variation in total airflow caused by modulating VAV boxes usually requires controlling supply fan capacity. The goal of a supply fan control system is to keep the pressure at some representative point in the duct system at the appropriate value. This is done to keep from  bursting ductwork as the fan discharge pressure rises with decreasing flow, to keep relatively uniform pressures at the inlet to each VAV box, and to reduce the energy required  by the fan. To control the fan, a pressure sensor is placed toward the end of the main duct. Its signal is compared to the set point of a controller, and the output controls fan capacity. The three most common types of fan capacity control are discharge dampers, inlet guide vanes, and variable speed drives. Taking full advantage of inlet guide vane or variable speed drive systems requires PI control. Page 2of 5ASHRAE Journal4/5/2004    Return/Exhaust Air Volume Control  Both constant and variable volume systems require return and exhaust (relief) air volume control. Return air quantity is equal to supply air quantity less any fixed exhaust and exfiltration requirements. On variable volume systems with an outside air economizer, the air handling system return and exhaust air quantities constantly change. If gravity relief dampers can be provided in appropriate locations, then neither return nor exhaust fans are needed. In systems with a ceiling return air plenum, the pressure drop through the ceiling and out through the relief dampers may be low enough to avoid the need for fans. The first alternative, then, is to avoid either return or exhaust fans. Page 3of 5ASHRAE Journal4/5/2004  The second approach is to use an exhaust fan (see Avery, 1986 and Avery, 1989).  Figure 4  shows the controls needed for this system. The supply fan is controlled to maintain a constant duct pressure near the end of the main duct. In constant volume systems, the exhaust fan modulates to maintain a constant pressure upstream of the return air damper. However, in VAV systems the fan must modulate to maintain a constant pressure ratio across the return air dampers. The reason for this difference is best explained by an example. Suppose the system shown in  Figure 4  has the return dampers open and a mixed air plenum  pressure of -1 inch water gage at design flow. Also suppose the pressure upstream of the return air damper is -0.75 inch water gage. If the outdoor air damper opens fully and the return air damper closes, the exhaust fan will have to produce -0.75 inch upstream of the return air damper to provide the required relief volume. If the total system volume goes down to fifty percent of full flow, only -0.25 inch will be required in the mixed air plenum to bring in the required amount of air, and only one-fourth of -0.75 or -0.1875 inch of water gage will be required to exhaust half of the design return air volume. Another feature of the control system of  Figure 4  is the use of an outside airflow sensor and reverse-acting PI controller. This subsystem is used to provide the desired minimum outside air quantity. If the outside air volume drops below the set point, the output from the flow controller will become the high signal and take charge of the outdoor, return, and relief dampers. Large air handling systems with lengthy return air ducts may have a significant pressure drop in the return air duct. Designers have used return air fans in the arrangement shown in  Figure 2  to overcome this pressure drop. Return fans are only necessary in any system if the  pressure drop in the return duct is high. The return fan arrangement of  Figure 2  can introduce control problems. The mixing box will normally be at a slight negative pressure in order to suck in outside air. Depending on the pressure drop in the outside air duct work (if any), the pressure upstream of the outside air damper will be near atmospheric pressure. However, the pressure upstream of the return dampers will have to be slightly positive if relief air is to escape the building. In both VAV and constant volume systems, the positive pressure upstream of the return air dampers caused by the need to push air through the relief dampers can make damper selection difficult. It also complicates and destabilizes the mixed air control system. In many cases, poor commissioning leads to a significant positive pressure upstream of the return damper, meaning that little or no outdoor air is introduced unless the return dampers are closed and do not leak. The relief fan configuration of  Figure 3  solves this problem. In this design, the supply fan maintains enough negative pressure in the mixing box to overcome the pressure drop in the return duct and to bring return air through the wide open return air damper. As the outdoor, return, and relief air dampers modulate, the exhaust fan is controlled to maintain the  pressure upstream of the return dampers at a level less negative than that of the mixing box  by the amount of the wide open pressure drop through the return air damper. This maintains full flow through the return duct no matter what the damper position. Recall that return or exhaust fans are needed only when economy cycles are used and when the introduction of 100% outside air would over pressurize the building. Nevertheless, return air fans are common in existing VAV systems, and several schemes have been used to control them. One scheme uses airflow measurement devices. The return fan volume is controlled to maintain an airflow rate that is less than the supply airflow rate by a constant Page 4of 5ASHRAE Journal4/5/2004


Jul 23, 2017

Bach Invention 9

Jul 23, 2017
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