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Reverse relief airflow prevention and building pressurization with a decoupled relief air damper in air-handling units.

INTRODUCTION

The purpose of an air-handling unit (AHU) is to create a comfortable and healthy indoor environment in buildings. To achieve its purpose, a conventional single-duct AHU with economizer operation and with high return air pressure drop (Taylor 2014a) normally consists of a cooling coil, a supply fan (SF), a return fan (RF), and a set of three dampers: an outdoor air damper (OAD), a recirculating air damper (RCD), and a relief air damper (RLD). Typically, both the SF and RF have a variable-frequency drive (VFD) in variable-air-volume (VAV) systems. Supply air temperature is thermally controlled at a set point, typically 13[degrees]C (55[degrees]F), to ensure that supply air is cool and dry enough to remove sensible and latent cooling load in spaces.

In addition to thermal control, supply air duct static pressure, building static pressure, and outdoor airflow rates need to be controlled. The supply air duct static pressure is controlled at a set point, which ensures that all terminal boxes will receive the required supply air to maintain comfortable space conditions. Historically, the set point was a constant, typically 374 Pa (1.5 in. [H.sub.2]O) (Montgomery and McDowall 2008). However, since a constant set point usually uses more energy than necessary, currently many energy standards, such as ASHRAE/IES Standard 90.1 (ASHRAE 2013b) and California Title 24 (CEC 2013), require automatically adjusting duct static pressure set point based on zone demand. A slightly positive building static pressure (1 to 20 Pa [0.005 to 0.08 in. [H.sub.2]O) is generally desired to reduce outdoor air infiltration to conditioned spaces (ASHRAE 2015). Moreover, the outdoor airflow intake is maintained at a desirable level depending on outdoor air conditions. The outdoor airflow rate is either set at a minimum level for indoor air quality (IAQ) control required by ASHRAE Standard 62.1(2013a) or adjusted up to 100% for the economizer operation required by ASHRAE/IES Standard 90.1 (ASHRAE 2013b).

Therefore, in pressure-airflow control, five control inputs--the speed of the SF and RF and the positions of the OAD, RCD, and RLD--are available to control three controlled variables: the supply air duct static pressure, the building static pressure, and the outdoor airflow rate. The three dampers are interlinked completely or partially to reduce the number of independent control inputs needed to match the number of controlled variables.

In the traditional control, the SF speed is modulated to maintain the desired supply air duct static pressure, the RF speed is modulated to maintain the desired building static pressure, and the three dampers are completely interlinked and modulated for outdoor air control (Levenhagen 1998; Underwood 1999; McQuiston et al. 2005). The traditional control is a simplistic form of control. It has better outdoor airflow control performance with almost constant system gain, the derivative of the outdoor airflow rate to the damper command, and it has lower disturbance gains, the derivative of the outdoor airflow rate to the SF and RF speeds (Wang et al. 2015). However, in this type of control reverse airflow through the RLD might occur as the OAD faultily approaches the closed position, in addition to poor fan energy performance (Seem et al. 2000). As a result, IAQ could be jeopardized if the relief air louver is located near a pollution source.

ASHRAE Guideline 16 (2010), ASHRAE Research Project RP-1455 (Hydeman and Eubanks 2015), and Pennsylvania State University's PSU Standard Sequences of Operation Guideline (PSU 2010) propose two decoupled RLD controls to avoid reverse relief airflow. The RLD is decoupled from the interlinked OAD and RCD to form a new control input, while the relief air plenum static pressure forms a new controlled variable. The static pressure set point at the relief air plenum is determined to overcome the losses through the RLD and associated ductwork. ASHRAE Guideline 16 provides a normal set point range from 25 to 75 Pa (0.1 to 0.3 in. [H.sub.2]O) without any requirement of set point reset, while ASHRAE RP-1455 proposes a set point reset scheme. In both the decoupled RLD controls, the SF speed is modulated to maintain the desired supply air duct static pressure while the interlinked OAD and RCD are modulated to maintain the desired outdoor airflow rate with either an overlapping action or a sequenced action. The overlapping control has better control performance with more constant system gain and lower disturbance gains (Wang et al. 2015) and less change in mixing air plenum static pressure, which can stabilize terminal box operation (ASHRAE 2010). The sequenced control has better fan energy performance (ASHRAE 2010; Taylor 2014b; CEC 2003; Wang et al. 2015). On the other hand, the control loop design for the space and relief air plenum static pressure is different between the two controls. ASHRAE Guideline 16 (2010) and ASHRAE RP-1455 (Hydeman and Eubanks 2015) propose the RF speed to maintain the desired relief air plenum static pressure and the decoupled RLD to maintain the desired building static pressure. Meanwhile, the PSU Standard Sequences of Operation Guideline (2010) proposes the RF speed to maintain the desired building static pressure and the decoupled RLD to maintain the desired relief air plenum static pressure.

Theoretically, the controllability of the damper and fan are quite different. The controlled variable can always be maintained by modulating the fan speed if the fan is sized appropriately. On the other hand, the controlled variable associated with the damper control is not only determined by the damper position but also limited by the fan speed. For example, the outdoor airflow cannot be higher than the supply airflow, which is determined by the supply fan speed, even though the OAD is fully open. Therefore, the damper control is called a passive control while the fan speed control is called an active control (Trane 2002). The fan speed control always achieves its control purpose. However, the controlled variable of the damper control may not be maintained at its set point when the damper reaches either the fully open position or the fully closed position. In this paper, the ASHRAE Guideline 16 control will be referred to as building pressure passive (BPP) control and the PSU control as building pressure active (BPA) control.

Since the BPP control uses the RLD to control the building static pressure and the BPA control uses the RLD to control the relief air plenum static pressure, the building static pressure may lose control with the BPP control and reverse relief airflow may occur with the BPA control once the RLD reaches its limits, either the fully closed position or the fully open position. The aim of this work is to evaluate the system performance of both controls with both constant and reset relief air plenum static pressure setpoints. Because the relief airflow increases as the outdoor air increases and vice versa, the RLD may reach the fully closed position at low outdoor airflow ratio and may reach the fully open position at high outdoor airflow ratio.

In this paper, a nonlinear network solution is discussed first in order to mathematically describe the nonlinear network that consists of the SF, RF, and three air dampers in an AHU, along with the air distribution ductwork and conditioned space. Then, the system performance with both BPP and BPA controls is simulated using the nonlinear network solution at both low and high outdoor airflow ratios. Finally, conclusions are drawn based on the simulation results.

MODELING

Figure 1 presents a schematic of a single-duct VAV AHU with economizer function as well as its air distribution ductwork and conditioned space, which is equipped with a constant-speed exhaust fan (EF). Each OAD, RCD, and RLD has its own actuator so that it can be interlinked or decoupled easily. The AHU system network has seven paths: supply air (sa), return air (ra), relief air (rel), recirculating air (rec), outdoor air (oa), exhaust air (ex), and building envelope infiltration (inf). Infiltration and exfiltration are unintentional airflow through the building envelope driven by the pressure difference between the indoor and outdoor air. Negative building pressure generates infiltration and positive building pressure generates exfiltration. Because building static pressure is a controlled variable, infiltration generally indicates both entering (negative) and leaving (positive) airflow through envelopes in this paper. For building pressurization, entering unconditioned outdoor airflow should be avoided. The AHU system network also has three pressure nodes: a relief air plenum (RE), a mixed air plenum (MA), and a space or room (RM). The outdoor air static pressure is normalized to zero and all pressures are measured with respect to it.

A nonlinear network solution is applied to simulate the system performance. First, the interlinked relationship between the damper positions and the damper command is assigned for both BPP and BPA controls, then the path pressure-airflow characteristic functions are discussed, followed by the node mass conservation discussion.

Damper Position-Command Relationship

As discussed previously, the three dampers can be interlinked either completely or partially based on different controls. With the traditional control, all the three dampers are controlled by one OAD command (D). With the BPP and BPA controls, only the interlinked OAD and RCD are controlled by one OAD command (D) while the RLD is controlled by the RLD command (R). The OAD and RCD can be interlinked with either an overlapping action or a sequenced action. As discussed previously, the overlapping control has better control performance and the sequenced control has better fan energy performance. Since the goal of this paper is to evaluate reverse relief airflow and building static pressure, to simplify the simulations, only the overlapping action between the OAD and RCD is discussed in the paper. For comparison purposes, the traditional control is also simulated along with the BPP and BPA controls.

[FIGURE 1 OMITTED]

The damper blade position ([theta]) is presented as a ratio of the fully open position. The damper is fully open when [theta] = 1 and the damper is fully closed when [theta] = 0. The relationships between the damper positions and damper commands are associated with a control. Table 1 summarizes each control loop defined by its controlled variable as well as its corresponding control input for the BPP and BPA controls and the traditional control.

For the traditional control, all three dampers are interlinked completely:

[[theta].sub.OAD] = D (1a)

[[theta].sub.RCD] = 1 - D (1b)

[[theta].sub.RLD] = D (1c)

For the BPP and BPA controls, the OAD and RCD are interlinked and controlled by the damper command (D) while the RLD has a direct correlation with the RLD command (R).

[[theta].sub.OAD] = D (2a)

[[theta].sub.RCD] = 1 - D (2b)

[[theta].sub.RLD] = R (2c)

Since the OAD has a direct correlation with the OAD command for all three damper controls, numerically the OAD command is the same as the OAD position.

Path Characteristic Function

The pressure-airflow characteristic function for each path relates the airflow rate through each path to differential static pressures at two connected nodes. The three node static pressures are named [P.sub.RE], [P.sub.MA], and [P.sub.RM]. Since a path has a static pressure drop along the airflow direction, the path functions depend on the path airflow direction. In this paper, the path airflow direction convention is shown in Figure 1. A reverse pressure drop causes a reverse airflow. An airflow sign is used in the path functions to represent this relationship.

Three different types of paths are treated in the AHU network: a simple path, a damper path, and a fan path. Building envelope infiltration can be treated as a simple path because it does not possess any dampers or fans. Infiltration airflow ([Q.sub.inf]) relates to the differential static pressure between the space node and the outdoor air, which is equal to the space static pressure since the outdoor air static pressure is zero:

[P.sub.RM] = [S.sub.inf] x [Q.sup.n.sub.inf] x sign ([Q.sub.inf]) (3)

The resistance factor ([S.sub.inf]) and exponent (n) depend on the nature of infiltration. The exponent (n) ranges between 1 and 2.5 (McQuiston et al. 2005). For a given building, the resistance factor ([S.sub.inf]) remains constant.

A damper path has an air duct as well as a modulation damper. The damper paths include the relief air (rel), recirculating air (rec), and outdoor air (oa) paths. The resistance factors include the fixed resistance factors of the air ducts ([S.sub.rel], [S.sub.oa], and [S.sub.rec]) and adjustable resistance factors of the dampers ([S.sub.reld], [S.sub.oad], and [S.sub.recd]), which vary with damper position ([[theta].sub.RLD], [[theta].sub.OAD], and [[theta].sub.RCD]). The nodal differential static pressure can be expressed as a function of the resistance factors and airflow for each damper path:

[P.sub.RE] = [[S.sub.rel] + [S.sub.reld] ([[theta].sub.RLD])] x [Q.sup.2.sub.rel] x sign([Q.sub.rel]) (4)

-[P.sub.MA] = [[S.sub.oa] + [S.sub.oad] ([[theta].sub.OAD])] x [Q.sup.2.sub.oa] x sign ([Q.sub.oa]) (5)

[P.sub.RE] - [P.sub.MA] = [[S.sub.rec] + [S.sub.recd] ([[theta].sub.RCD])] x [Q.sup.2.sub.rec] x sign([Q.sub.rec]) (6)

The damper resistance factor depends on the damper type (opposed blade or parallel blade damper) and can be expressed as a function of the damper blade position [theta] for a given damper:

S([theta]) = f([theta])[S.sub.d] (7)

The resistance factor when the damper is fully open ([S.sub.d]) is a design parameter for damper selection. The damper correction factor (f) is a function of the damper blade position ([theta]) and can be found in different references (ASHRAE 2010; Felker and Felker 2010; Brandemuel et al. 1993).

A fan path has an air duct as well as a fan, which overcomes the pressure drop in the path. The differential static pressure or fan static head (H) is a function of relative fan speed ([omega]) and fan airflow rate (Q) for a given fan. The fan paths include the return air duct (ra), supply air duct (sa), and exhaust air duct (ex). The nodal differential static pressure is related to the duct resistance factor and airflow as well as fan head curve for each fan path, as shown in Equations 8-10. The duct in the supply fan path includes an air distribution section (ad) and a terminal box section (TB) separated at the supply air duct static pressure sensor. The TB pressure loss is always equal to the supply air duct static pressure ([P.sub.TB]).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

[P.sub.RM] - [P.sub.RE] = [S.sub.ra] x [Q.sup.2.sub.ra] x sign([Q.sub.ra]) - [H.sub.RF]([Q.sub.ra], [[omega].sub.RF]) (9)

[P.sub.RM] = [S.sub.ex] x [Q.sup.2.sub.ex] x sign([Q.sub.ex]) - [H.sub.EF]([Q.sub.ex]) (10)

The fan head curve at the design speed for the SF, RF, and EF can be regressed as

[H.sub.d](Q) = a[Q.sup.2] + bQ + c (11)

The fan head curve of the SF and RF under partial speeds can be deduced using the fan laws. At a relative fan speed ([omega]), a ratio of actual fan speed to design fan speed, the fan head curves can be expressed as

H (Q,[omega]) = a[Q.sup.2] + b[omega]Q + c[[omega].sup.2] (12)

Node Mass Conservation Function

The mass flow rate through all seven paths can be determined implicitly with given node static pressures, fan speeds, and damper commands using Equations 3 to 6 and 8 to 10. Then the mass conservation can be applied at three nodes, including the relief air plenum, the mixed air plenum, and the room:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13)

In summary, ten governing equations are available, including seven path characteristic functions as defined by Equations 3 to 6 and 8 to 10 and three node mass conservation functions as defined by Equation 13. Meanwhile, the system performance variables includes seven path airflows, [Q.sub.sa], [Q.sub.ra], [Q.sub.rel], [Q.sub.rec], [Q.sub.oa], [Q.sub.ex], and [Q.sub.inf] and three node pressures plus one supply air duct static pressure, [P.sub.RE], [P.sub.MA], [P.sub.RM], and [P.sub.TB].

The system also has three independent control inputs, the SF speed ([[omega].sub.SF]), RF speed ([[omega].sub.SF]), and OAD command (D) for the traditional control and an independent control input, the RLD command (R), for the BPP and BPA controls. The supply airflow rate is maintained by TB dampers based on the space cooling load conditions and is treated as an independent variable in this paper. Therefore, three constraints for the traditional control and four constraints for the BPP and BPA controls need to be applied to determine the system performance. The constraints can be either controlled variables or given control inputs.

APPLICATION

A hypothetical AHU system is designed based on design space cooling load, minimum outdoor airflow rate, and supply and return air duct static pressure drops. To evaluate the performance of the BPP and BPA controls at the potential RLD fully closed and fully open positions, the system performance on this AHU is simulated at both low and high outdoor air ratios.

AHU System Design

A single-duct VAV AHU with economizer function was designed using the equipment selection software of one AHU manufacturer (Trane 2014). The design supply airflow rate is 18.8 [m.sup.3]/s (40,000 cfm) and the design return airflow rate is 17.9 [m.sup.3]/s (38,000 cfm). The design airflow rate difference between the supply air and return air is 0.9 [m.sup.3]/s (2000 cfm) for space pressurization at 12 Pa (0.05 in. [H.sub.2]O) to balance with the design exhaust airflow rate of 0.7 [m.sup.3]/s (1500 cfm) and the infiltration airflow rate of 0.2 [m.sup.3]/s (500 cfm).

The characteristic of each path is represented by the static pressure drop at a given airflow rate, which may not be the same as the design airflow rate of the path; the characteristics are listed in Table 2. The damper characteristic is for the fully open position. High-velocity dampers are selected to avoid stratification in the mixed-air plenum. Terminal boxes always remain at a constant static pressure drop, which is equal to the supply air duct static pressure set point regardless of its airflow rate, under perfect control conditions.

The resistance factor for a partially open damper depends on the damper type and is given based on the inherent damper characteristic curves. ASHRAE Guideline 16 (2010) and Taylor (2014b) state that the RLD should be an opposed blade damper and sized for a pressure drop equal to 7% to 15% of the relief path total pressure drop to provide a fairly linear control response. The OAD can be either a parallel or an opposed blade damper and the RCD is a parallel blade damper. Because high-velocity dampers are selected by the selection program (Trane 2014), the damper authority is relatively high for all three dampers. Figure 2 compares the inherent damper characteristic curves of parallel and opposed blade dampers, provided by ASHRAE (2010), as well as installed damper curves for the OAD, RCD, and RLD based on their authorities. It can be seen that the installed damper curves for parallel blade dampers have approximately linear relationships for all three dampers. To achieve a fairly linear control response (ASHRAE 2010; Lizardos and Elovitz 2000), parallel blade dampers were chosen for all three dampers in the simulations.

Three fans--an SF, an RF, and an EF--are installed in the AHU and the building. The SF and RF have a VFD while the EF always operates at design speed. The fan head and power curves of the SF and RF at the design speed were provided by the same equipment selection software used to design the AHU (Trane 2014) and are shown in Figure 3. The EF is selected separately, and the fan head curve for the EF is shown in Table 3 (Greenheck 2011). The EF fan curve is applied to simulate the system performance when the building static pressure loses control.

System Performance at Low Outdoor Airflow Ratio

In simulation 1, the required OAD command is first determined to maintain a minimum outdoor airflow rate at given supply airflow rates. Then, the OAD is forced to reduce below the required position to 40% of the required position due to faults, such as fixed OAD controls, faulty airflow sensors, and malfunctioning dampers. The aim of this simulation is to evaluate the system performance when the OAD faultily approaches the closed position.

Required OAD Command to Maintain Constant Minimum Outdoor Airflow. In the traditional control, three control inputs, including the OAD command (D), the SF speed, and the RF speed, are determined based on three controlled variable setpoints. A constant duct static pressure set point of 374 Pa (1.5 in. [H.sub.2]O) is chosen to simplify the simulation. Moreover, the space static pressure set point is 12 Pa (0.05 in. [H.sub.2]O) and the outdoor airflow rate set point is 1.88 [m.sup.3]/s (4000 cfm); the supply airflow rate varies from 7.5 to 18.8 [m.sup.3]/s (16,000 to 40,000 cfm).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

For the BPP and BPA controls, the relief air plenum static pressure control is added in order to determine the fourth control input, the RLD command (R). Since the relief air demand is the lowest at a minimum outdoor airflow when the economizer is off, the relief air plenum static pressure set point is selected as 25 Pa (0.1 in. [H.sub.2]O), the low limit of the set point range recommended by ASHRAE Guideline 16 (2010), in order to reduce RF power.

Since the minimum outdoor airflow rate remains constant for IAQ control while the supply airflow rate varies with space cooling load, the outdoor airflow ratio and damper command should increase as the supply airflow rate decreases. Figure 4 demonstrates that the required minimum OAD commands with square markers for the traditional control and diamond markers for the two decoupled RLD controls increase as the supply airflow ratio, the ratio of the supply airflow rate to the design supply airflow rate (18.8 [m.sup.3]/s [40,000 cfm]), decreases from 1 to 0.4 in order to maintain constant outdoor airflow rate. The two decoupled RLD controls have the same OAD command (D) curve since the dampers are within their control range between the fully open and the fully closed positions.

For AHUs without reliable airflow sensors, it is common practice to use a fixed minimum OAD position for minimum outdoor airflow control in AHUs (Apte 2006). The fixed minimum OAD position, which is calibrated at the design supply airflow, will cause the actual OAD position to be less than required as the supply airflow decreases. In addition to the fixed OAD position control, malfunctioning outdoor airflow sensors and stuck dampers also can cause the OAD to be below the required OAD position. In such conditions, the actual outdoor airflow rate is less than needed and IAQ is severely affected. Moreover, less-than-needed outdoor air intake may also potentially lead to reverse relief airflow and negative building static pressure. To evaluate the system's ability to handle these faulty operations, it is worth investigating the system performance when the OAD commands are less than the required minimum OAD commands as defined in Figure 4.

[FIGURE 4 OMITTED]

System Performance with the OAD Command at Less than the Required Damper Command. System performance is simulated with the OAD command varying from 40% to 100% of the required minimum OAD command. In practice, the system only operates at the design condition for a short period in a year; therefore, the supply airflow rate is kept at a lower value than the design airflow rate, and the simulation is conducted when the supply airflow ratio is 80%--in this case a supply airflow rate of 15.0 [m.sup.3]/s (32,000 cfm). The required OAD command for the traditional control is 0.172, and that for the BPP and BPA controls is 0.173. In the simulation case illustrated in Figure 4, the required OAD commands were calculated to maintain a constant outdoor airflow rate. In this section, the system performance, including the outdoor airflow, is evaluated when the OAD is kept at a position lower than the required OAD position.

In the traditional (interlinked dampers) control, precise control over the duct static pressure and building static pressure is achieved and respective setpoints are met as these controlled variables are controlled by active components, i.e., the SF and RF.

The relief plenum pressure in BPP control and the building static pressure in BPA control always meet their set point along with the duct static pressure through active controls by modulating fan speeds. However, the building static pressure in the BPP control and the relief air plenum static pressure in the BPA control are controlled by the decoupled RLD and may not always meet the set point if the RLD reaches limits of its operation, i.e., either the fully open or the fully closed position. In this case, the limit of the RLD position acts as a constraint forcing the building static pressure to deviate from the set point in the BPP control and the relief air plenum static pressure in the BPA control.

Figure 5 demonstrates the simulated outdoor airflow rate ratio (a), which is defined as the ratio of the outdoor airflow rate to the actual supply airflow rate (15.0 [m.sup.3]/s [32,000 cfm]), RLD position (b), building static pressure (c), and relief air plenum static pressure (d) versus the relative OAD command, which is normalized by the required OAD command shown in Figure 4. Based on the actual supply airflow rate, the required outdoor air ratio is 0.125 for IAQ control. As shown in Figure 5a, outdoor airflow is less than the required for the three controls due to the OAD position being less than the required. It is obvious that the IAQ will not be guaranteed in this case.

With the traditional control (shown with square markers in Figure 4), the outdoor airflow decreases (Figure 5a) and the RLD is closed (Figure 5b) continuously as the OAD is closed. Since the building static pressure is always maintained at its set point by the RF speed (Figure 5c), the relief airflow decreases as the outdoor airflow decreases and finally flows in a reverse direction to make up the insufficient outdoor airflow when the outdoor airflow is below the summation of the building infiltration and exhaust airflow. As a result, the relief air plenum static pressure gradually decreases to a negative value as the OAD is closed below 60% of the required OAD position (Figure 5d). Therefore, negative pressure and reverse relief airflow are the problems associated with tradition control when the OAD is operated below the required damper position.

In Figure 5, system performance is represented by diamond markers for the BPP control and circle markers for the BPA control. The RLD in the decoupled RLD controls is impacted by the relief air plenum static pressure directly in the BPP control and indirectly in the BPA control. From Figure 5b it can be observed that the RLD approaches closing position much faster as compared to the OAD command when the RLD is decoupled from the control, and as a result the RLD is fully closed when the OAD command is below 60% of the required command. From Figures 5c and 5d it can be observed that both the relief air plenum and the building static pressure are well controlled at their setpoints before the RLD is fully closed or when OAD command is above 60% of the required command. Since the BPP and BPA controls share the same governing equations along with the same controlled variable setpoints, the system performance, including the RLD position as shown in Figure 5b, is the same before the RLD is fully closed. However, the system performance tends to be different after the OAD drops below 60% of the required position and the RLD is fully closed. The building static pressure loses control and eventually becomes negative with BPP control (Figure 5 c) while the relief air plenum static pressure loses control and eventually becomes negative with BPA control (Figure 5d).

[FIGURE 5 OMITTED]

With BPP control, the RF always can maintain positive relief air plenum static pressure (Figure 5d). Since negative building static pressure demands a fully closed RLD, the relief airflow is positive and insignificant. However, the negative building static pressure results in uncontrolled outdoor air infiltration, which is balanced with the insufficient outdoor airflow (Figure 5a) when the RLD is fully closed.

With BPA control, negative relief air plenum static pressure demands a fully closed RLD and consequently avoids reverse relief airflow. Since building static pressure is maintained at its set point by the RF, the building does not have outdoor air infiltration. Figure 5a also shows a constant outdoor airflow when the RLD is fully closed that is balanced with the building exhaust and infiltration airflow.

In summary, the fixed minimum OAD control results in less outdoor airflow intake and reverse relief airflow and is no longer allowed by ASHRAE Standard 62.1 (2013a) or California Title 24 (CEC 2013). On the other hand, when malfunctioning airflow sensors and stuck dampers result in the OAD approaching the closed position, both the BPP and BPA controls can well prevent reverse relief airflow. However, the BPP control results in negative building static pressure while the BPA control can well maintain the positive building static pressure. It is suggested that the BPP controls reset the relief air plenum static pressure with the output of the building pressure control loop when the RLD is fully closed to avoid negative building static pressure.

System Performance with High Outdoor Airflow Ratio

A previous section discussed the simulated case where OAD command is less than the required command with low outdoor air ratio. This section co e system performance with the outdoor airflow ratio up to 100%.

The relief airflow should increase as the outdoor airflow increases in order to maintain the building static pressure set point. With BPP and BPA control, the maximum capacity of the relief air path with the fully open RLD increases as the relief air plenum static pressure set point increases. The lower static pressure set point may limit the relief air capacity and result in excessive positive building pressure when the outdoor airflow ratio is close to 100%. On the other hand, the higher static pressure set point will lead to higher RF energy consumption. Therefore, the system performance, including the RF speed and the building static pressure, was simulated at a controlled outdoor airflow ratio from 0.1 to 1.0 with a supply airflow of 15.0 [m.sup.3]/s (32,000 cfm), or 80% of the design supply airflow. Besides the outdoor airflow ratio and relief air plenum static pressure, other two controlled variables, the duct static pressure and the building static pressure, have the same setpoints as in the previous simulation. The duct static pressure set point is 374 Pa (1.5 in. [H.sub.2]O) and the building static pressure set point is 12 Pa (0.05 in. [H.sub.2]O).

ASHRAE Guideline 16 (2010) gives the normal range of relief air plenum static pressure setpoints from 25 to 75 Pa (0.1 to 0.3 in. [H.sub.2]O) without the requirement of the pressure set point reset. Therefore, the system performance is first simulated using these two constant setpoints then followed by the simulation with reset pressure set point proposed by Taylor (2014a).

With Constant Relief Air Plenum Static Pressure Setpoints. Figure 6 shows the RLD position (a), RF speed (b), building static pressure (c), and relief air plenum static pressure (d) versus the outdoor airflow ratio with BPP and BPA controls.

To maintain the building static pressure, the relief airflow increases as the outdoor airflow increases. As a result, the RLD is open more as the outdoor airflow ratio increases, as shown in Figure 6a. As a control input, the RLD has an identical position with four identical controlled setpoints between the BPP and BPA controls. Therefore, two controls share the same RLD position curve under the same relief air plenum static pressure set point. On the other hand, the lower relief air plenum static pressure forces the RLD to be open more in order to maintain the relief airflow required by the outdoor airflow. The RLD position under a relief air plenum static pressure of 25 Pa (0.1 in. [H.sub.2]O), indicated by circle markers in Figure 6, is more open than the curve under the relief air plenum static pressure of 75 Pa (0.3 in. [H.sub.2]O), indicated by cross markers. As a result, the lower relief air plenum static pressure forces the RLD to quickly reach the fully open position at the outdoor airflow ratio of 0.6 and finally lose control of its controlled variable when the outdoor airflow ratio is above 0.6.

According to Equation 9, the RF head increases as the relief air plenum static pressure increases. Therefore, the RF speed under the relief air plenum static pressure of 75 Pa (0.3 in. [H.sub.2]O), indicated by cross markers in Figure 6, is higher than that under the static pressure of 25 Pa (0.1 in. [H.sub.2]O), as shown in Figure 6b. Since the RLD is entirely within the controllable range under the higher relief air plenum static pressure and the return airflow is constant at a supply airflow of 15.0 [m.sup.3]/s (32,000 cfm), the BPP and BPA controls share a higher constant RF speed curve. With the same principle, the RF runs at a lower constant speed with the lower static pressure set point below the outdoor airflow ratio of 0.6 when the RLD is still within the controllable range. However, the RF speed curve splits into curves, with square markers for the BPP control and circle markers for the BPA control, when the outdoor airflow ratio is above 0.6.

With higher relief air plenum static pressure, the RLD is always within the controllable range in both controls. Therefore, the building static pressure is maintained at its set point of 12 Pa (0.05 in. [H.sub.2]O), represented with cross markers in Figure 6c, and the relief air plenum static pressure is maintained at its set point of 75 Pa (0.3 in. [H.sub.2]O), represented with cross markers in Figure 6d. With lower relief air plenum static pressure, the building static pressure and relief air plenum static pressure are also maintained at their setpoints before the outdoor airflow ratio reaches 0.6 and the RLD reaches the fully open position. However, when the RLD reaches the fully open position, its controlled variable loses control. With BPP control, the relief air plenum static pressure, indicated by square markers in Figure 6d, is well controlled by the RF speed. On the other hand, the relief airflow remains constant because of the fully opened RLD as the outdoor airflow increases. As a result, the building static pressure, indicated by square markers in Figure 6c, significantly increases. With BPA control, the building static pressure, indicated by square markers in Figure 6c, is well controlled by the RF speed. On the other hand, the RLD cannot open more to reduce the relief air plenum static pressure. As a result, the relief air plenum static pressure, indicated by circle markers in Figure 6d, increases from 25 to almost 75 Pa (0.1 to almost 0.3 in. [H.sub.2]O).

In summary, with constant setpoints, the BPP control will cause high RF speed as well as high RF power under the high relief plenum pressure set point and lose building static pressure control under the low relief plenum pressure set point when the outdoor airflow ratio is close to 100%. The BPA control can well control the building static pressure with low RF power by setting a low relief air plenum static pressure set point even though the actual relief air plenum static pressure is higher than its set point.

With Reset Relief Air Plenum Static Pressure Set point. To avoid the issues of constant setpoints with BPP control, Taylor (2014a) proposed that the plenum pressure set point should be reset with the output of the building pressure control loop. The loop output is applied to first open the relief dampers with the low constant plenum pressure set point, 25 Pa (0.1 in. [H.sub.2]O), until the relief damper is fully open and then to raise the relief plenum pressure set point and maintain the relief damper at the fully open position.

[FIGURE 6 OMITTED]

Figure 7 shows the simulated RLD position and reset relief air plenum static pressure with the BPP control. When the outdoor airflow ratio varies from 0.1 to 0.6, the relief air plenum static pressure is set at 25 Pa (0.1 in. [H.sub.2]O) and the RLD opens from 0.1 to 1. When the outdoor air ratio varies from 0.6 to 1.0, the RLD approaches the fully open position and the relief air plenum static pressure set point increases from 25 to 71 Pa (0.1 to 0.285 in. [H.sub.2]O). The reset relief air plenum static pressure setpoints make the building static pressure always at its set point of 12 Pa (0.05 in. [H.sub.2]O). Compared with Figures 6a and 6b, the system performance of the BPP control with reset pressure setpoints is exactly the same as that of the BPA control with a constant low-pressure set point.

[FIGURE 7 OMITTED]

In fact, for the BPP control with the fully open RLD, the building static pressure control loop determines the relief air plenum pressure set point in the relief air plenum static pressure control loop, which determines the RF speed. Therefore, the building static pressure is actually controlled by the RF speed through a cascade control. As a result, the BPP control becomes the BPA control.

CONCLUSION

The relief airflow and building static pressure of an AHU are simulated at different outdoor airflow rates from lower outdoor airflow ratio to 100% using a nonlinear network solution for both building pressure passive (BPP) and building pressure active (BPA) controls with a decoupled RLD.

The simulation results show that both controls can well prevent reverse relief airflow by fully closing the decoupled relief air damper when the OAD faultily approaches the closed position. However, the BPP control may result in negative building static pressure. It is suggested that the BPP control reset the relief air plenum static pressure with the output of the building pressure control loop the when the OAD approaches the fully closed position.

Moreover, the BPA control well maintains the building static pressure and has better RF energy performance even with a constant low plenum static pressure set point when the outdoor air ratio approaches 100% during economizer mode. On the other hand, the BPP control may result in excessively positive building static pressure under a low constant relief air plenum static pressure set point and excessive RF energy consumption with a high constant relief air plenum static pressure set point. It is suggested that the BPP control reset the relief air plenum static pressure with the output of the building pressure control loop when the OAD approaches the fully open position.
NOMENCLATURE

a, b, c   = constants in regressed fan head curve at design speed
D         = damper control signal
f         = damper resistance factor
H         = fan static head, Pa (in. [H.sub.2]O)
P         = static pressure at each node, Pa (in. [H.sub.2]O)
Q         = airflow through each path, [m.sup.3]/s (cfm)
R         = relief air damper control signal for BPP and BPA
            damper controls
S         = path resistance factor
[omega]   = relative fan speed based on design speed
[theta]   = damper blade position (1 for fully open and 0 for
            fully closed)

Subscripts

d     = design
EF    = exhaust air fan
ex    = exhaust air (path)
inf   = envelope infiltration (path)
MA    = mixed air plenum (node)
oa    = outdoor air (path)
OAD   = outdoor air damper
ra    = return air (path)
RCD   = recirculating air damper
RE    = relief or return air plenum (node)
rec   = recirculating air (path)
rel   = relief air (path)
RF    = return air fan
RLD   = relief air damper
RM    = room air or space (node)
sa    = supply air (path)
SF    = supply air fan
TB    = terminal box


REFERENCES

Apte, M.G. 2006. A review of demand control ventilation (LBNL-60170). Technical report under DOE contract DE-AC03-76SF00098. Berkeley, CA: Lawrence Berkeley National Laboratory.

ASHRAE. 2010. ASHRAE Guideline 16-2010, Selecting outdoor, return, and relief dampers for air-side economizer systems. Atlanta: ASHRAE.

ASHRAE. 2013a. ANSI/ASHRAE Standard 62.1-2013, Ventilation for acceptable indoor air quality. Atlanta: ASHRAE.

ASHRAE. 2013b. ANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard for Buildings Except Low-Rise Residential Buildings. Atlanta: ASHRAE.

ASHRAE. 2015. ASHRAE handbook--HVAC applications. Atlanta: ASHRAE.

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CEC. 2003. Advanced variable air volume system design guide. Sacramento, CA: California Energy Commission.

CEC. 2013. 2013 Building energy efficiency standards for residential and nonresidential buildings. California Title 24. Sacramento, CA: California Energy Commission.

Felker, L.G., and T.L. Felker. 2010. Dampers and airflow control. Atlanta: ASHRAE.

Greenheck. 2011. Centrifugal roof downblast exhaust fans. Schofield, WI: Greenheck Fan Corp.

Hydeman, M., and B. Eubanks. 2015. ASHRAE RP-1455: Advanced control sequences for HVAC systems; Phase I air distribution and terminal systems. Atlanta: ASHRAE.

Levenhagen, J.I. 1998. HVAC control system design diagrams, 1st ed. New York: McGraw-Hill.

Lizardos, E., and K.M. Elovitz. 2000. Damper sizing using damper authority. ASHRAE Journal, April.

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Montgomery, R., and R. McDowall. 2008. Fundamentals of HVAC control systems. Atlanta: ASHRAE.

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Gang Wang, PhD, PE Member ASHRAE

Kaustubh Phalak

Gang Wang is an assistant professor and Kaustubh Phalak is a doctoral student in the Department of Civil, Architectural and Environmental Engineering at University of Miami, Coral Gables, FL.
Table 1. Control Loops in a Single-Duct VAV AHU

                                Control Variables

#   Control Method           Duct Static    Building Static
                             Pressure       Pressure

1   Traditional control      SF speed          RF speed
2   BPP control              SF speed          RLD (R)
3   BPA control              SF speed          RF speed

                   Control Variables

#   Outdoor Airflow Rate     Relief Plenum Static Pressure

1   OAD, RCD, and RLD (D)    --
2   OAD and RCD (D)          RF speed
3   OAD and RCD (D)          RLD (R)

Table 2. Characteristics of an AHU Network

Path            Component      Static Pressure   Airflow,
                               Drop, Pa (in.     [m.sup.3]/s
                               [H.sub.2]O)       (cfm)

Infiltration    Envelope       12 (0.05)         0.2 (500)

Exhaust air     Duct           124 (0.5)         0.7 (1500)

Supply air      Distribution   946 (3.8)         18.8 (40,000)
                Terminal box   374 (1.5)         --

Return air      Duct           473 (1.9)         17.9 (38,000)

Relief air      Duct           50 (0.2)          17.9 (38,000)
                Damper         71 (0.287)        17.9 (38,000)

Outdoor air     Duct           50 (0.20)         18.8 (40,000)
                Damper         79 (0.318)        18.8 (40,000)

Recirculating   Duct           90 (0.363)        18.8 (38,000)
air             Damper         71 (0.287)        18.8 (38,000)

Table 3. EF Head Data

   Airflow Rate         Fan Head

[m.sup.3]/s   cfm     Pa   in. [H.sub.2]O

0.523         1108   163      0.652
0.629         1334   139      0.559
0.699         1482   116      0.466
0.755         1599    93      0.373
0.802         1699    70      0.280
0.842         1785    46      0.186
0.862         1826    35      0.140
0.882         1868    23      0.093
0.902         1911    12      0.047
0.922         1954     0      0.000
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Author:Wang, Gang; Phalak, Kaustubh
Publication:ASHRAE Transactions
Article Type:Report
Date:Jan 1, 2016
Words:7577
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