Air handling unit fan control systems and methods

Abstract

A method and system of controlling a supply fan and a return fan of an air handling unit (AHU). The supply fan has an associated design supply airflow, and the return fan has an associated design return airflow. In one embodiment, the method includes calculating an actual supply airflow, generating a supply fan speed set point based on the design supply airflow and the actual supply airflow, and modulating the speed of the supply fan based on the supply fan speed set point. The method also includes calculating an actual return airflow, generating a return airflow set point based on the actual supply airflow, and modulating the speed of the return fan based on a comparison of the return airflow set point and the actual return airflow.

Description

BACKGROUND

Various types of facilities, such as industrial facilities, medical buildings, manufacturing assemblies, and laboratories, often use air handling units (“AHUs”) to control indoor temperatures. An AHU generally uses fans to supply air to and remove air from different areas, zones, or rooms at designated airflow rates. Fluctuations in the amount of air that is supplied to or removed from the different areas, zones, or rooms can result in excessive energy consumption, as well as moisture-related issues, including mold and structural damage.

SUMMARY

In an embodiment, a method of controlling a supply fan of an AHU having an associated design supply airflow and a return fan of an AHU having an associated design return airflow includes calculating an actual supply airflow, generating a supply fan speed set point based on the design supply airflow and the actual supply airflow, and modulating the speed of the supply fan based on the supply fan speed set point. The method also includes calculating an actual return airflow, generating a return airflow set point based on the actual supply airflow, and modulating the speed of the return fan based on a comparison of the return airflow set point and the actual return airflow.

In another embodiment, a fan speed control system that controls a supply fan and a return fan associated with an air handling unit includes a supply fan sensor, a return fan sensor, and a controller. The supply fan head sensor measures airflow pressure proximate to the supply fan. The return fan head sensor measures airflow pressure proximate to the return fan. The controller communicates with the supply fan head sensor and the return fan head sensor, calculates a supply airflow value based at least partially on the air pressure proximate to the supply fan and a return airflow value based at least partially on the air pressure proximate to the return fan, and modulates the speed of the supply fan and the speed of the return fan based at least partially on the supply airflow value and the return airflow value, respectively.

In yet another embodiment, a method of retrofitting an air handling unit with a fan control system includes integrating a controller into the air handling unit such that the controller is in communication with a supply fan and a return fan. The method also includes modulating the speed of the supply fan based at least partially on a comparison of a calculated supply airflow value and a design supply airflow value, and the speed of the return fan based at least partially on a comparison of a calculated return airflow value and a design return airflow value. The design supply airflow value and the design return airflow value correspond to theoretical maximum efficiency airflow values.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of an air handling unit (“AHU”) according to an embodiment of the invention.

FIG. 2 is a side view of a fan that can be implemented in the AHU of FIG. 1 according to an embodiment of the invention.

FIG. 3 is a front view of the fan of FIG. 2.

FIG. 4 illustrates a process for controlling the speed of a supply fan and the speed of a return fan according to an embodiment of the invention.

FIG. 5 illustrates a process for adding a fan control system to an air handling unit according to an embodiment of the invention.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and“coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and“coupled” are not restricted to physical or mechanical connections or couplings.

FIG. 1 is a schematic diagram of a portion of an air handling unit (“AHU”) 100 that can supply airflow to, and recover or return airflow from, one or more zones (e.g., an area or room that is serviced by the AHU 100). The AHU 100 generally includes a supply fan 105 having a supply fan variable frequency drive (“VFD”) 110 and a supply fan head sensor 115, and a return fan 120 having a return fan VFD 125 and a return fan head sensor 130. The AHU 100 also includes a controller 135 that is in communication with the supply fan VFD 110, the supply fan head sensor 115, the return fan VFD 125, and the return fan head sensor 130, as described in greater detail below.

In other embodiments, the AHU 100 may include additional components. For example, the AHU 100 may include a terminal box having an airflow cooling device and/or an airflow heating device. The AHU 100 may also include an economizer (e.g., a supply damper, a return damper, a release damper, an exhaust fan, etc.), as well as other components typically associated with an AHU.

The supply fan 105 draws airflow in through a suction chamber or supply fan inlet 140 and discharges the airflow through a discharge chamber or supply fan outlet 145. The airflow being supplied to the supply fan 105 is typically outside air, or air that has been recycled through a return loop in the AHU 100. The supply fan VFD 110 can turn the supply fan 105 at a variable rate, and thus produce a variable airflow out of the discharge chamber 145. In some embodiments, the supply fan VFD 110 receives commands from the controller 135, as described in greater detail below. The supply fan head sensor 115 measures or senses a difference between a fan inlet pressure near the supply fan inlet 140 and a supply fan outlet pressure near the supply fan outlet 145 (i.e., a pressure differential). In some embodiments, the supply fan head sensor 115 generates a supply fan head signal indicative of the pressure differential, and transmits the supply fan head signal to the controller 135.

Similar to the supply fan 105, the return fan 120 draws airflow in through a suction chamber or return fan inlet 150 and discharges airflow through a discharge chamber or return fan outlet 155. The airflow being supplied to the return fan 120 is typically air from the zones of the AHU 100. The return fan VFD 125 can turn the return fan 120, which varies the airflow being drawn out of the zones of the AHU 100. In some embodiments, the rate at which the return fan VFD 125 turns the return fan 120 can be linked, either directly or indirectly, to the rate at which the supply fan VFD 110 turns the supply fan 105. The return fan head sensor 130 measures the pressure differential between the return fan inlet 150 and the return fan outlet 155, and can generate a signal indicative of the pressure differential, similar to the supply fan head sensor 115.

Generally, the controller 135 can be a suitable electronic device, such as, for example, a programmable logic controller (“PLC”), a personal computer (“PC”), and/or other industrial/personal computing device. As such, the controller 135 may include both hardware and software components, and is meant to broadly encompass the combination of such components. In the embodiment shown in FIG. 1, the controller 135 receives signals from the supply fan VFD 110, the supply fan head sensor 115, the return fan VFD 125, and the return fan head sensor 130, and transmits signals to the supply fan VFD 110 and the return fan VFD 125. In some embodiments, the signals received by the controller 135 are used to generate the signals that are transmitted from the controller 135. For example, as described in greater detail below, the controller 135 may receive speed signals and/or power usage signals from the VFDs 110 and 125, as well as pressure differential signals from the head sensors 115 and 130, and use those signals to generate speed control signals that are transmitted to the VFDs 110 and 125. In other embodiments, the controller 135 may gather speed and power usage data from other components. For example, a tachometer (not shown) in communication with the controller 135 can be used to directly measure a fan speed, while fan power can be measured using true power meters. Additionally, in other embodiments, the controller 135 may be in communication with other components of the AHU 100 (e.g., other controllers, terminal boxes, damper actuators, etc.).

In some embodiments, prior to receiving signals from the various components of the AHU 100, the controller 135 polls or requests the signals from the components. Additionally, in order to perform functions (e.g., transmitting a speed control signal to the supply fan VFD 110), the controller 135 includes a set of commands and/or parameters, or a program. The set of commands can be stored, accessed, and/or changed, and can be created using a variety of computer programming languages (e.g., ladder logic, C++ commands, etc.). For example, in one embodiment, the controller 135 includes a program that can be used to carry out the process described below with respect to FIG. 4.

FIGS. 2 and 3 respectively show a side view and a front view of a fan system 200 that can be used to implement the supply fan 105 or the return fan 120 of FIG. 1, wherein like parts are referred to with like numerals. The supply fan 105 and the return fan 120 each have a respective fan system 200. To represent the respective supply-side and return-side fan systems, FIGS. 2 and 3 depict one fan system 200 with alternative reference numerals for supply-side and return-side components (e.g., 145, 155; 115, 130; etc.). In each fan system, two total pressure traverses 205, 210 are incorporated at a fan inlet and a fan outlet, respectively. Thus, in the embodiment shown in FIG. 1, four total pressure traverses are included (e.g., a total pressure traverse incorporated at each of the supply fan inlet 140, the supply fan outlet 145, the return fan inlet 150, and the return fan outlet 155). In some embodiments, the total pressure traverses 205 at the fan inlets 140, 150 are respectively linked to a negative input 215 of the fan head sensors 115, 130, while the total pressure traverses 210 at the fan outlets 145, 155 are respectively linked to a positive input 220 of the fan head sensors 115, 130. The traverses may be replaced by two tubes that are located in the respective suction and discharge chambers.

FIG. 4 illustrates a process 400 for controlling the speed of a supply fan and the speed of a return fan according to an embodiment of the invention. Several steps of the process 400 may be carried out by components of the AHU 100 shown in FIG. 1. For example, the controller 135 is often referred to below when steps of the process 400 are being executed or completed. However, it should be understood that the process 400 can be applied to a variety of AHUs, and is not limited in implementation to the specific AHU 100 or the components described in conjunction with the AHU 100 shown in FIG. 1.

The first step in the process 400 is to calculate or otherwise generate a set of design parameters (step 405). The design parameters can include, for example, a design fan speed (N_d), a design airflow (Q_d), a design fan head (H_d), and a design fan power usage (w_d). These design parameters may correspond to ideal values, or values that allow the AHU 100 to operate most efficiently. The design parameters may also be determined from the results of testing or trials, according to manufacturer specifications (e.g., a fan manufacturer's design fan speed), or in an alternative manner. In the embodiment shown in FIG. 4, specific equations for determining the fan head and power usage are used, and depend on the type of fan curve associated with the fans 105 and 120. Typically, there are a number of types of fan curves, such as a steep fan curve and a flat fan curve. Fans with a steep fan curve include fans whose differential pressure or fan head increases as a result of decreasing airflow rates (Q) at the same fan speed (N), as described below. Alternatively, fans with a flat fan curve include fans whose differential pressure or fan head remains generally constant when the fan airflow rate (Q) changes. For such fans, the fan power varies significantly when the fan airflow rate (Q) changes at the same fan speed (N). Thus, the design fan head and the design fan power can be calculated using Equations (1) and (2), respectively. H_d=a₀+a₁Q_d+a₂Q_d²  (1) W_d=b₀+b₁Q_d+b₂Q_d  (2) In Equations (1) and (2), a₀, a₁, a₂, b₀, b₁, and b₂ are fan curve coefficients obtained from the fan curve, typically provided by the manufacturers of the fans 105 and 120. Additionally, the design airflow Q_d is determined according to the configuration of the AHU 100 and the zones that the AHU 100 supplies with airflow.

The next step in the process 400 is to measure the fan head of the supply fan (H_s), the fan head of the return fan (H_r), the power usage of the supply fan (w_s), and the power usage of the return fan (w_r) (step 410). These values can be determined using data from the supply fan head sensor 115, the return fan head sensor 130, the supply fan VFD 110, and the return fan VFD 125, respectively. In alternative embodiments, the fan heads and fan power usages may be determined and/or calculated differently. For example, the fan power usage may be determined using a true power meter.

The next step in the process 400 is to determine the supply airflow (Q_S) and the return airflow (Q_R) (step 415). For example, the controller 135 can use Equation (3) shown below to determine the fan airflow rate (Q) for both the supply fan 105 and the return fan 120, which is measured in cubic-feet-per-minute (“CFM”), for fans with a steep fan curve. Equation (3) is based on the measured fan head (H) (see step 410), and a ratio (ω) between the fan speed (N) that is measured in revolutions-per-minute (“RPM”) and a design fan speed (N_d) that is also measured in RPM. $\begin{array}{cc}Q=\left(\frac{-a_1-\sqrt{a_1^2-4a_2\left(a_0-\frac{H}{{\omega }^2}\right)}}{2a_2}\right)\omega & \left(3\right)\end{array}$ Similar to Equation (1) shown above, a₀, a₁, and a₂ are fan curve coefficients obtained from the fan curve, typically provided by manufacturers of the fans 105 and 120.

Further, the controller 135 can also use Equation (4) to determine the fan airflow rate (Q) for fans with a flat fan curve. Equation (4) is based on the speed ratio (ω), and the measured fan power (W_f) (see step 410). $\begin{array}{cc}Q=\frac{-b_1{\omega }^2-\sqrt{b_1^2{\omega }^4-4b_2\omega \left(b_0{\omega }^3-w_f\right)}}{2b_2\omega }& \left(4\right)\end{array}$ Similar to Equation (2) shown above, a₀, a₁, and a₂ are fan power curve coefficients, also provided by manufacturers of the fans 105 and 120. In this way, the process 400 can determine the fan airflow rate (Q) for the supply fan 105 and the return fan 120 using either of the above equations as appropriate.

The process 400 continues by determining a supply fan speed set point (ω_set) (step 420). The supply fan speed set point (ω_set) can be determined using Equation (5) below. The supply fan speed set point (ω_set) is based on the speed ratio (ω) and a ratio of the supply airflow (Q_S) to the design airflow (Q_d). $\begin{array}{cc}{\omega }_{\mathrm{set}}=\max \left({\omega }_{\min },\frac{Q}{Q_d}\right)& \left(5\right)\end{array}$ More specifically, the supply fan speed set point (ω_set) is chosen between a minimum speed ratio (ω_min) and the ratio of the calculated supply airflow (Q_S) to the design airflow (Q_d).

After the supply fan speed set point (ω_set) has been determined, the next step in the process 400 is to compare the speed difference between the speed ratio (ω) and the supply fan speed set point (ω_set) to a preset value (s) (step 425). For example, Equation (6) compares the absolute value of the difference between the speed ratio (ω) and the supply fan speed set point (ω_set) to the preset value (s), which is approximately 5% to 15% of the design fan speed (N_d). In other embodiments, the present value (s) can be determined or adjusted differently. |ω−ω_set|<s  (6) This comparison is then used to modulate the speed ratio (ω) (and therefore the fan speed N) of the supply fan 105. For example, if the difference between the speed ratio (ω) and the supply fan speed set point (ω_set) is less than the present value (s), the supply fan speed set point (ω_set) is set equal to the speed ratio (ω) (i.e., the speed of the supply fan 105 is not changed) (step 430). However, if the difference between the speed ratio (ω) and the supply fan speed set point (ω_set) is greater than the present value (s), the supply fan speed set point (ω_set) is set to the speed ratio (ω) plus the present value (s) (i.e., the speed of the supply fan 105 is increased by an amount equivalent to the present value (s)) (step 435).

After the speed of the supply fan 105 has been adjusted, the process 400 continues by determining a return airflow set point (Q_r,set) (step 440). As described below, the return airflow set point (Q_r,set) can be used as a basis for modulating the speed of the return fan 120. The return airflow set point (Q_r,set) can be determined, for example, using Equation (7) below. Q_r,set=Q_S−Q_EX−cQ_d  (7) In Equation (7), the supply airflow (Q_S) is calculated, for example, using Equation (3) or Equation (4) above. The exhaust airflow (Q_EX) corresponds to airflow from exhaust fans of the AHU 100. The exhaust airflow (Q_EX) can be measured near the exhaust fans or obtained directly from exhaust fan design data. The variable (c) corresponds to airflow losses from the envelope of the area or zones that are being supplied with airflow from the AHU 100 (e.g., walls, windows, doors, etc.), and is typically approximately 0.03 to 0.05. Thus, Equation (7) also factors in losses by multiplying the loss variable (c) by the design airflow (Q_d). In other embodiments, Equation (7) may be adjusted according to the design of the AHU 100 (e.g., the number of exhaust fans, the makeup of the zones, etc.).

After the return airflow set point (Q_r,set) has been determined, the next step in the process is to compare the return airflow (Q_R) (e.g., calculated using Equation (3) or Equation (4) above) to the return airflow set point (Q_r,set) (step 445). If the return airflow (Q_R) is less than the return airflow set point (Q_r,set), the controller 135 increases the speed of the return fan 120 using the return fan VFD 125 (step 450). If, however, the return airflow (Q_R) is less than the return airflow set point (Q_r,set), the controller 135 decreases the speed of the return fan 120 using the return fan VFD 125 (step 455). Thus, the speed of the return fan 120 is modulated to maintain the return airflow set point (Q_r,set). In some embodiments, if the return airflow set point (Q_r,set) is less than 30% of a design return airflow (Q_R,d), the return fan 120 is stopped completely.

After the speed of the return fan 120 has been modulated according to the return airflow set point (Q_r,set), the process 400 begins again with step 410. In some embodiments, the process 400 is repeated on a continual basis, such that the supply fan 105 and return fan 120 are perpetually attempting to attain an optimal speed. In other embodiments, the process 400 is executed on a certain cyclical basis (e.g., every 30 seconds, every 10 minutes, every hour, etc.) that depends on other components of the AHU 100. For example, in embodiments where the AHU 100 includes a terminal box, an execution cycle or interval for the process 400 is set to at least 1.5 times the time constant of the terminal box.

In other embodiments, the steps of the process 400 may be carried out in an alternative order. For example, in one embodiment, the supply air flow (Q_S) and the return airflow (Q_R) need not be calculated concurrently (e.g., the return airflow (Q_R) can be calculated immediately prior to step 445). Additionally or alternatively, in other embodiments, the process 400 may have more or fewer steps than those shown (e.g., step 405 may be removed).

FIG. 5 illustrates a process 500 for adding a fan control system to an air handling unit according to an embodiment of the invention. The fan control system can include, for example, a program or logic commands that are used to modulate the speed of a supply fan and/or the speed of a return fan included in an AHU to maintain optimal fan speeds. The first step in the process 500 is to integrate a fan control system controller into an AHU (step 505). For example, the controller 135 (shown in FIG. 1) can be added to an AHU that includes existing control components. Alternatively, the program or logic in the controller can be added to existing control components. The next step in the process 500 is to modulate the speeds of the supply fan and the return fan using the controller that was integrated into the AHU (see step 505) according to design airflow values (step 510). This can be performed, for example, using the process 400 described with respect to FIG. 4. In some embodiments, the controller (e.g., device, program, or logic) that was integrated into the AHU can be disabled, such that existing control components assert control of the AHU (e.g., in an override mode).

Various features and advantages of the invention are set forth in the following claims.

Claims

1. A method of controlling a supply fan having an associated design supply airflow and a return fan having an associated design return airflow, the supply fan and the return fan being associated with an air handling unit, the method comprising: calculating an actual supply airflow; generating a supply fan speed set point based on the design supply airflow and the actual supply airflow; modulating the speed of the supply fan based on the supply fan speed set point; calculating an actual return airflow; generating a return airflow set point based on the actual supply airflow; and modulating the speed of the return fan based on a comparison of the return airflow set point and the actual return airflow.

2. The method of claim 1, wherein modulating the speed of the return fan includes stopping the return fan if the return airflow set point is less than approximately 30 percent of the design return airflow.

3. The method of claim 1, wherein modulating the speed of the return fan includes increasing the speed of the return fan if the actual return airflow is less than the return airflow set point.

4. The method of claim 1, wherein modulating the speed of the return fan includes decreasing the speed of the return fan if the actual return airflow is greater than the return airflow set point.

5. The method of claim 1, wherein the actual supply airflow is calculated utilizing supply fan speed data and supply fan power usage data, and wherein the actual return airflow is calculated utilizing return fan speed data and return fan power usage data.

6. The method of claim 1, wherein the actual supply airflow is calculated utilizing supply fan speed data and supply fan pressure data, and wherein the actual return airflow is calculated utilizing return fan speed data and return fan pressure data.

7. The method of claim 1, wherein generating the supply fan speed set point includes calculating a supply fan speed comparison value by multiplying a supply fan design speed by a ratio of the actual supply airflow and the design supply airflow, wherein the supply fan design speed corresponds to an optimal supply fan speed and the design supply airflow corresponds to an optimal supply airflow.

8. The method of claim 7, wherein generating the supply fan speed set point further includes choosing between a minimum supply fan speed and the supply fan speed comparison value.

9. The method of claim 8, wherein modulating the speed of the supply fan includes comparing a preset speed value equivalent to approximately 5 to 15 percent of the supply fan design speed to a difference between the supply fan speed and the supply fan speed set point.

10. The method of claim 9, further comprising maintaining the supply fan speed if the difference between the supply fan speed and the supply fan speed set point is less than the preset speed value.

11. The method of claim 9, further comprising increasing the supply fan speed if the difference between the supply fan speed and the supply fan speed set point is greater than the preset speed value.

12. The method of claim 1, wherein generating the return airflow set point includes calculating an exhaust airflow corresponding to unrecoverable airflow, calculating a loss airflow corresponding to airflow losses within the air handling unit, and subtracting the exhaust airflow and the loss airflow from the actual supply airflow.

13. A fan speed control system configured to control a supply fan and a return fan associated with an air handling unit, the fan speed control system comprising: a supply fan head sensor configured to measure airflow pressure proximate to the supply fan; a return fan head sensor configured to measure airflow pressure proximate to the return fan; and a controller configured to communicate with the supply fan head sensor and the return fan head sensor, to calculate a supply airflow value based at least partially on the air pressure proximate to the supply fan and a return airflow value based at least partially on the air pressure proximate to the return fan, and to modulate the speed of the supply fan and the speed of the return fan based at least partially on the supply airflow value and the return airflow value, respectively.

14. The fan speed control system of claim 13, wherein the controller is further configured to measure the speed of the supply fan and the speed of the return fan, and wherein the respective speeds of the supply fan and the return fan are also used to calculate the supply airflow value and the return airflow value.

15. The fan speed control system of 14, further comprising a supply variable frequency drive configured to drive the supply fan and a return variable frequency drive configured to drive the return fan, wherein the supply variable frequency drive and the return variable frequency drive are in communication with the controller.

16. The fan speed control system of 15, wherein the controller is further configured to measure a supply fan power value using data from the supply variable frequency drive and a return fan power value using data from the return variable frequency drive, and the respective power values from the supply fan and the return fan are also used to calculate the supply airflow value and the return airflow value.

17. The fan speed control system of claim 13, wherein the controller is further configured to calculate a supply fan speed set point using the supply airflow value, and to modulate the speed of the supply fan based at least partially on a comparison between the supply fan speed and the supply fan speed set point.

18. The fan speed control system of claim 13, wherein the controller is further configured to calculate a return airflow set point using the supply airflow value, and to modulate the speed of the return fan based at least partially on a comparison between the return airflow value and the return airflow set point.

19. A method of retrofitting an air handling unit with a fan control system, the air handling unit having existing control components, the method comprising: integrating a controller into the air handling unit such that the controller is in communication with a supply fan and a return fan; and by the controller, modulating the speed of the supply fan based at least partially on a comparison of a calculated supply airflow value and a design supply airflow value, and the speed of the return fan based at least partially on a comparison of a calculated return airflow value and a design return airflow value, wherein the design supply airflow value and the design return airflow value correspond to theoretical maximum efficiency airflow values.

20. The method of claim 19, further comprising disabling the fan control system such that the air handling unit can utilize the existing control components.