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Select & Control Economizer Dampers in VAV Systems

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Select & Control Economizer Dampers in VAV Systems COLUMN ENGINEER’S NOTEBOOK This article was published in ASHRAE Journal, November 2014. Copyright 2014 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org. Steven T. Taylor Select & Control Economizer Dampers in VAV Systems BY STEVEN T. TAYLOR, P.E., FELLOW ASHRAE Last month’s column1 discussed how to control return fans in VAV systems. This month, we will discuss a closely related subject: how to select and control economizer dampers in VAV systems. Damper type, sizing, and control strategies vary further below). The damper sequencing shown in Figure depending on the type of building pressure control sys- 5 can be achieved by using separate control system tem. The three common types are: outputs to the two dampers (which may also be needed 1. Relief Dampers (aka non-powered or barometric for minimum outdoor air control, to be discussed in a relief) as shown in Figure 1;*,2 future column) or by sequencing the active range of the 2. Relief Fans (aka powered exhaust fans) as shown in actuators, e.g., 0 to 5 VDC for one and 5 to 10 VDC for the Figure 2; and other. 3. Return Fans (aka return/relief fans) as shown with Conventional logic is also not optimum for systems Airflow Tracking in Figure 3A and with Direct Pressure with Return Fans (Figure 3A and Figure 3B). Not only does Control in Figure 3B. it increase pressure drop and supply fan energy, it can Figures 1 to 3B also show three common options to con- cause reverse outdoor airflow through the relief damper trol minimum ventilation outdoor air. The advantages when Airflow Tracking control is used.3 This can result and disadvantages of these options, and others, will be in excess outdoor air intake, depending on minimum the subject of a future Engineer’s Notebook column. ventilation control logic, and also may cause contami- nated air to be drawn in through the relief outlet since Control Logic its location may not meet the code and Standard 62.1 Conventional economizer control logic calls for the separation distances to pollutant sources required of return air damper to close as the outdoor air damper outdoor air intakes. opens. For systems with return fans, conventional logic A better economizer control sequence for systems with calls for the relief air damper to track the outdoor air Return Fans and Airflow Tracking control is shown in damper. This is shown in Figure 4. But this logic is not Figure 6 and summarized as follows: optimum for multiple-zone VAV system applications. • Simply open the outdoor air damper fully. This is For VAV systems using Relief Dampers (Figure 1) or true even when the economizer is disabled, i.e., open Relief Fans (Figure 2), pressure drop and supply fan the outdoor air damper whenever the air handler is on energy can be reduced by sequencing the outdoor air in occupied mode. (It is closed when the air handler is and return air dampers, as shown in Figure 5. The pres- off or when operating in non-occupancy modes such as sure drop savings is readily apparent at the 50% outdoor night setback, warm-up, etc.) air signal: the two dampers are both wide open rather • Modulate the return air damper off the supply air than both half closed. This substantially reduces pres- temperature loop in sequence with the mechanical cool- sure drop through the mixing plenum, particularly ing per standard economizer control logic. if the dampers have opposed blade action (discussed *The figure shows direct pressure control with motorized relief dampers. Gravity type barometric Steven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a mem- dampers are also an option. See Reference 2 for a comparison of these two options. ber of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration. 50 ASHRAE JOURNAL ashrae.org NOVEMBER 2014 COLUMN ENGINEER’S NOTEBOOK • The relief damper modulates as FIGURE 1 Relief damper. the inverse of the return damper. For Return Fan systems using P-2 Direct Pressure control, the M sequence is the same except the Return/Relief relief damper is controlled indepen- Relief Air Air Plenum Return Air dently off of building pressure. Relief Damper PL-1 D-1 It may be counterintuitive, but economizer outdoor air dampers [A] [B] Atmosphere MIN T-2 T-1 Building on air handlers with return fans (Outdoors) Return Air P-3 Supply have no impact on the outdoor air- M Damper D-2 Fan M M flow rate into the mixing plenum. Mixed Air Supply Air Outdoor Air Plenum Instead the return fan and return Economizer OA PL-2 Filter Cooling damper controls dictate outdoor Damper D-3 Coil Speed Outdoor Air Control airflow. From ASHRAE Guideline 4 Minimum OA M2 P-1 16: “The flow of outdoor air … is Damper D-4 always the difference between the supply airflow and the return air- flow through path PL-1 to PL-2, and, FIGURE 2 Relief fan. therefore, the [outdoor air] damper P-2 size and type have no effect on the Speed amount of outdoor air.” Fully open- M-2 Control Relief Air Return/Relief ing the outdoor air damper rather Air Plenum Return Air than tracking the other dampers Relief PL-1 Damper Relief reduces pressure drop and fan D-1 Fan energy, and it ensures there is no [A] [B] backflow through the relief damper Atmosphere MIN T-2 T-1 Building (Outdoors) for systems using Airflow Tracking Return Air Supply V-1 M Damper D-2 control (see Reference 3 for more M M Fan Mixed Air Supply Air details). Outdoor Air Plenum Economizer OA PL-2 Filter Cooling Damper D-3 Coil Speed Damper Selection Outdoor Air Airflow Sensor Control Table 1 is a summary of recom- Minimum OA M P-1 mendations for damper action Damper D-4 and sizing, adapted from ASHRAE Guideline 16. Recommended damper action is referred to as either parallel blade Relief Damper System damper (PBD) or opposed blade damper (OBD) (Figure 7 ). For VAV systems using Relief Dampers (Figure 1), the One important point in selecting economizer most important damper from a sizing perspective is the outdoor air and return air dampers is that the con- relief damper (D-1). The relief system relies on build- cept of “damper authority”5,6 does not apply. The ing pressurization to cause excess outdoor air to relieve damper authority concept applies to dampers that to atmosphere. The relief damper and entire relief throttle airflow but economizer dampers do not; path (Path B to A in Figure 1) must be sized large enough they are mixing dampers and are not intended to to prevent overpressurization of the building, i.e., no vary overall airflow. This fact, and other factors, are more than about 0.05 to 0.10 in. w.c. (12 to 25 Pa), when considered in Guideline 16 and Table 1 damper sizing the VAV system is supplying maximum outdoor air. recommendations. The resulting face velocity will therefore be very low, NOVEMBER 2014 ashrae.org ASHRAE JOURNAL 51 typically 200 to 500 fpm (1 to 2.5 FIGURE 3A Return fan with airflow tracking control. m/s). The economizer outdoor air damper (D-3) should be selected for Relief Damper Return Airflow Fan low pressure drop to minimize sup- D-1 Return/Relief Sensor ply fan energy. Damper face veloci- Relief Air Air Plenum Return Air M PL-1 ties from 400 to 1,500 fpm (2 to 8 Speed m/s) at design flow are typical; the Control [A] range is a function of space avail- V-1 [B] Atmosphere MIN T-2 T-1 Building ability and cost. From a functional- (Outdoors) Return Air Supply V-2 M Damper D-2 ity standpoint, damper size does M2 M Fan not matter. Often, because of space Airflow Mixed Air Supply Air Outdoor Air Sensor Plenum limitations, this damper is the same Economizer OA PL-2 Filter Cooling Airflow Sensor Speed size as the outdoor intake louver and Damper D-3 Coil Control thus has very low velocity to avoid P-1 rain entrainment. Similarly, the return air damper (D-2) pressure drop should gen- erally be low to minimize supply FIGURE 3B Return fan with direct pressure control. fan energy. Damper face velocities from 800 to 1,500 fpm (4 to 8 m/s) Return at design flow are typical. In cold M Fan Return/Relief climates where coil freezing is pos- Relief Air Air Plenum Return Air sible and no other mixing device Relief Damper PL-1 D-1 Speed (e.g., air blender) is provided, the P-3 Control design velocity should be on the [A] [B] Atmosphere MIN T-2 T-1 Building high end of this range to encour- (Outdoors) Return Air Supply age mixing (see ASHRAE Research V-1 M Damper D-2 M2 Fan Project 10457). Mixed Air M Outdoor Air Airflow Supply Air Where outdoor air and return air Sensor Plenum Economizer OA PL-2 Filter Cooling dampers are controlled so that one Speed Damper D-3 Coil Control opens when the other closes (Figure 4), both outdoor air and return air P-1 dampers should be PBDs to minimize pressure drop and reduce fan energy. Figure 8 shows why: the pressure drop across the mixing improvement in mixing efficiency vs. dampers that plenum is almost constant for PBDs and rises significantly were rotated 90°, but their tests were limited, and ear- when dampers are 50% open for OBDs.
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