Utilizing mixed-air-plenum pressure to control minimum-outside-air quantity: How to do it and why it works
Much has been written about controlling the minimum-outside-air quantity in variable-air-volume (VAV) air-handling systems. However, instead of debating the merits of various control methods, this article will discuss how to apply the mixed-air-plenum pressure method and why it works.
Although most buildings require outside air to make up exhausts and meet ventilation needs, other buildings require a constant minimum quantity of outside air. In constant-volume systems, the minimum required outside-air quantity is a constant percentage of the constant supply-air quantity. Therefore, a fixed outside-air damper position provides the fixed minimum-outside-air quantity.
In VAV systems, the supply-air quantity varies with load, but the minimum required outside-air quantity does not change. Exhausts, such as toilet exhausts, might be constant regardless of load. Design occupancy and the resulting outside-air quantity required for ventilation also might be constant. If the minimum required outside-air quantity remains constant, but the supply-air quantity decreases, the minimum required outside-air quantity increases as a percentage of the supply-air quantity. When a system operates with minimum outside air (and an economizer is not active), a fixed minimum-outside-air damper position will result in smaller amounts of outside air as the VAV supply-air quantity decreases.
The system needs a way to control the outside-air quantity to maintain the desired minimum. Several methods have been employed, such as:
Outside-air quantity can be measured directly by pitot or electronic device. The outside-air damper modulates as required to deliver the minimum outside-air quantity. The technique measures outside air directly, independent of and unaffected by other control loops.
The method has been criticized because it can be difficult to measure the low velocities involved accurately, but some of the difficulties have been overcome by advancements in hot-wire anemometers and related technologies. The method also does not address other control considerations, such as controlling the return fan. A return fan that runs too fast can depressurize a building. A return fan that runs too slow can result in outside air being sucked in through the relief/spill damper (Damper 1 in Figure 1).
Fan tracking measures supply and return airflows. The return-fan capacity is controlled to maintain a constant difference between supply and return airflow.
The method is appealing because it measures the quantities of interest more or less directly. As previously mentioned, the direct-measurement method does not always measure low air velocities accurately; however, fan tracking reduces this risk by measuring airflow at duct velocity.
Nevertheless, the method has been criticized because inaccurate outside-air-quantity measurements are the result of the cumulative inaccuracy of two independent measurements (supply and return airflow). Additionally, the method applies only to systems that have return fans.
Gravity dampers or an exhaust fan can be used to control building pressure. An engineer can subtract the amount of exhausts (such as toilet exhausts) from the minimum-outside-air quantity and estimate the extent to which the remaining outside air could pressurize the building. During commissioning, pressurization set points can be determined by taking building-pressure field measurements with known outside-air quantities.
One drawback is that the method does not truly control minimum outside air. The method uses a surrogate, such as the supply-fan capacity signal, as an approximation of total supply airflow. The controls use the surrogate signal to open the outside-air damper further as the supply fan slows, maintaining a nearly constant minimum outside airflow rather than a constant percentage of supply airflow.
Return-fan capacity can be controlled by pressure in the mixed-air plenum (Point C in Figure 1). The rest of this article will discuss how to apply the mixed-air-plenum pressure-control method.
A package rooftop air-handling unit (AHU) from an actual project will illustrate how mixed-air-plenum-pressure control can be implemented. The unit's characteristics include:
A draw-through airfoil supply fan providing 20,000 cfm at 5.32 in. wg of total static pressure.
An airfoil return fan providing 17,000 cfm at 1.50 in. wg of total static pressure.
A chilled-water cooling coil.
A 100-percent-outside-air economizer.
Supply- and return-fan variable-speed drives.
A minimum outside-air quantity of 4,000 cfm.
A winter indoor design temperature of 70°F.
A winter outdoor design temperature of 0°F.
Summer indoor design conditions of 75°F dry-bulb temperature and 55-percent relative humidity.
Summer outdoor design conditions of 91°F dry-bulb temperature and 73°F wet-bulb temperature.
A duct static-pressure sensor (Point A, Figure 1) that regulates the supply-fan variable-speed drive and controls supply-fan capacity.
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The return-fan speed is controlled to maintain a constant negative pressure in the mixing box. When commissioning the controls, it is important to know how to select the constant negative-pressure set point in the mixing box to control the return-fan speed. The appropriate set point can be selected by:
Temporarily set up the unit for 100-percent-outside-air operation: Shut off the return fan, close the return damper, and open the outside-air damper.
Set the outside-air damper to the selected minimum position. (An arbitrary adjustment, the damper could be anywhere from 10- to 25-percent open.) The minimum position should be high enough to avoid excessive pressure drop, but low enough to leave sufficient stroke for the outside-air dampers to modulate open during the economizer cycle. The example AHU's initial position was 15-percent open (1.5 v on a 0- to 10-v signal to the damper motor).
Start the supply fan and adjust the variable-speed drive to deliver the desired minimum outside-air quantity. The example unit had an initial setting of 20-percent speed. If the setting were correct, the desired 4,000 cfm would move at 1,778 fpm in the unit's 18-in.-by-18-in. supply duct.
The example's measured duct velocity (by pitot traverse) was only 1,000 fpm (2,250 cfm). Pressure in the mixed-air plenum (Point C, Figure 1) was -0.06 in. wg. The airflow was less than the desired minimum, and the mixed-air-plenum pressure was too low to be a workable control set point. After several iterations of increasing fan speed and adjusting outside-air-damper position, a fan speed of 60 percent and a damper position of 25 percent produced a duct velocity of 1,697 fpm (3,818 cfm). The mixed-air-plenum pressure was -0.28 in. wg.
The minimum outside-air-damper-position set point was left at 25 percent. The mixed-air-plenum-pressure set point was -0.30 in. wg. (The square root of 0.30/0.28 multiplied by the measured 3,818 cfm equals 3,952 cfm, which was acceptably close to the desired 4,000-cfm minimum outside-air quantity.)
Finally, return the unit to normal control. The example AHU's static-pressure profile and airflow measurements were taken at full load and compared with the fan curve as part of the unit's commissioning. The entire process took about 5 to 6 hr.
After the constant negative-pressure set point has been selected and the AHU has been returned to normal control, the relief/spill damper (Damper 1, Figure 1) modulates and tracks the outside-air damper (Damper 3, Figure 1). The relief damper should be closed when the outside-air damper is at minimum.
The return damper (Damper 2, Figure 1) and outside-air damper (Damper 3, Figure 1) are controlled via discharge air or mixed-air temperature as part of the economizer cycle. As the outside-air damper opens beyond minimum, the return damper moves toward a closed position.
Maintaining the minimum outside-air quantity is not the only concern when controlling a VAV unit in a freezing climate. At a full load of 20,000 cfm that included 4,000 cfm of 0°F outside air and 16,000 cfm of 70°F return air, the example AHU's mixed-air temperature was 56°F. Therefore, the mixed-air temperature matched the design supply-air temperature.
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The example unit served some perimeter rooms that had hot-water baseboard heat. The baseboard heat canceled the heat loss through the walls and windows, making the rooms work like interior spaces for winter cooling-load purposes; therefore, their winter cooling load did not decrease to zero. However, the transmission portion of the cooling load was zero, and the solar gain sometimes was quite low (particularly for the rooms that did not face south). Also, at various times, some rooms were empty with their lights off. Even though the 20,000-cfm design supply airflow represented the peak cooling load, a winter cooling load of 8,000 cfm was not unreasonable or unrealistic for this system.
Introducing a minimum outside-air quantity of 4,000 cfm at 0°F with a total airflow of only 8,000 cfm would have resulted in a mixed-air temperature of 35°F, which was too cold to send down the duct and well below the usual 38°F set point for the freezestat. To solve the problem, a mixed-air low-limit override control was incorporated into the system, limiting the outside-air damper position to prevent the mixed-air temperature from going below a desired minimum, such as 50°F. If the override is activated, the building would have been short of outside air. Because the conditions occur only a few hours per year (probably during unoccupied hours at night when toilet and other exhausts could be shut off), the expense of installing a preheat coil in the AHU could not be justified.
A damper open to a fixed position acts like a fixed orifice. The pressure drop through an orifice (or fixed damper) varies with the square of the flow. As long as the damper does not move, a constant pressure drop across the damper translates to a constant flow (Point D to Point C in Figure 1). Regardless of the total airflow through the mixing box, if the pressure drop across the outside-air damper (Damper 3, Figure 1) is constant, the outside-air quantity will be constant as well.
In the mixed-air-plenum-pressure control scheme, the minimum outside-air control drives only the return-fan speed or other fan-capacity control, such as inlet vanes or a discharge damper. The return-air-damper position is a function of mixed-air temperature, not building pressure or return-fan speed. The return-air damper moves in conjunction with the outside-air damper in the economizer cycle. If the outdoor temperature is within a range in which the economizer cycle is not active, the return-air damper will remain in one position, regardless of system airflow.
If the supply fan slows, the airflow through the mixing box is reduced, and the mixing-box pressure becomes “less negative.” A smaller pressure drop across the outside-air damper means the system has less than the desired minimum outside airflow. Supply air is the total combination of return and outside air. If outside air is too low, return air must be too high. Slowing the return fan in response to the “less negative” pressure in the mixing box reduces the amount of return airflow. Outside airflow then increases to make up the difference. The pressure drop across the outside-air damper (at its minimum position) increases accordingly, so the negative pressure in the mixing box and the minimum outside airflow returns to set point.
The mixed-air-plenum-pressure concept works for systems with return fans. The sole function of a return fan is to overcome pressure in the return duct. For minimum-outside-air and building-pressure control, return fans are problems to be solved, not solutions to a problem. Avoiding return fans saves costs and often makes sense.
The concept also works for systems with low return-duct pressure drop that do not have return fans and still need minimum outside-air control. If no return fan exists, mixed-air-plenum pressure overrides the economizer control and limits the opening of the return-air damper to maintain the desired negative pressure in the mixing box. Without a return fan, building pressure can be controlled via a gravity damper or exhaust fan to relieve excess outside air introduced by the economizer cycle.
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Controlling minimum outside air from mixed-air-plenum pressure offers a few advantages. For example, the method is simple. Static pressure is easy to measure with reasonable accuracy.
Additionally, small errors in static-pressure measurements affect minimum-outside-air quantity by the square root. If a set point of -0.25 in. wg is selected to achieve the desired minimum outside-air quantity, but the “real” pressure in the mixing box is 0.20 in. wg (a fairly gross error), outside airflow still is 89 percent of design.
Finally, unlike control schemes that use the return and spill/relief dampers to control minimum outside air, the risk that the return fan will “run away” and depressurize the building is not as great.
This article has described ways to maintain the minimum required outside-air quantity as supply airflow changes. If a system has an outside-air economizer, the outside-air quantity will exceed the minimum requirement. While maintaining the minimum outside-air quantity no longer is a concern, the return fan still needs to be controlled. The mixed-air-plenum-pressure control method also works when the economizer is active.
Figure 2 graphs flow across a parallel-blade damper at various damper positions. (Parallel-blade dampers can provide better mixing than opposed-blade dampers and, therefore, are preferred for mixing-box applications.) The graph represents a damper with 50-percent authority at constant pressure drop. Damper authority is the ratio of pressure drop across the damper to total pressure drop for the flow path that the damper controls.1 The outside-airflow path runs from the outdoors to the mixed-air plenum. The intake louver and the outside-air damper usually are the only significant pressure drops in the flow path. If pressure drop across the wide-open damper is about the same as that through the louver (a not unlikely scenario), the pressure drop across the damper is 50 percent of the total and, therefore, damper authority is 50 percent.
When mixed-air-plenum-pressure control is utilized, the pressure in the mixed-air plenum is constant. A damper has nearly linear performance under these conditions (Figure 2). This linear relationship is important because it means the airflow relationships also remain roughly linear.
If the supply-fan speed increases at any given outdoor-air temperature (and resulting outside-air percentage), the airflow through the outside-air damper increases. The mixed-air-plenum pressure then becomes “more negative,” calling for the return fan to speed up and bring more return air into the mixed-air plenum. The reverse occurs as the supply fan slows. In other words, controlling the return fan via mixed-air-plenum pressure in the economizer cycle causes the return fan to track supply airflow.
When the outside-air temperature is between the supply-air temperature (typically 55°F to 60°F) and the economizer changeover temperature (when return air is less expensive to cool than outside air, typically 70°F to 75°F), the unit will operate on 100-percent outside air. The return-air damper will be closed, so controlling the return fan from mixed-air-plenum pressure no longer has feedback, meaning changes in return-fan speed will not change mixed-air-plenum pressure. Also, the wide-open outside-air damper is a fixed orifice. Pressure drop across it and the resulting pressure in the mixed-air plenum no longer will be constant and instead will vary with the square of supply airflow.
In 100-percent-outside-air mode, the return fan becomes a building exhaust fan. If the return fan is too slow, the building will be pressurized more than is desired. (This is not a terrible problem, but it could cause annoyances, such as doors that remain open against their closers.) If the return fan runs too fast, it can depressurize the building.
Depressurizing the building is a problem because it increases infiltration of unconditioned air and can draw rain water into the walls. One way to avoid this problem is to incorporate a building-pressure limit on the return-fan speed. At low supply airflow with a wide-open outside-air damper, the pressure in the mixed-air plenum will be “less negative” than the set point. The return fan, functioning as a building exhaust fan, ordinarily would speed up. The building pressure limit will keep the fan from speeding up so much that it pulls the building pressure below the desired set point (typically 0.02 to 0.05 in. wg).
This article explained how to use mixed-air-plenum pressure to control a VAV system's minimum outside air and why it works. The goal was to educate readers about constant mixed-air-plenum-pressure control, not advocate for or against it.
Lizardos, E., & Elovitz, K. (2000). Damper sizing using damper authority. ASHRAE Journal, 42, 37-43.
A member of HPAC Engineering's Editorial Advisory Board, Kenneth M. Elovitz is an engineer and in-house counsel for Energy Economics Inc. His knowledge and experience with HVAC, electrical, and life-safety systems allow him to understand system function and performance, including interactions among disciplines.