US20070028632A1

US20070028632A1 - Chiller control system and method

Info

Publication number: US20070028632A1
Application number: US11/498,538
Authority: US
Inventors: Mingsheng Liu
Original assignee: Individual
Current assignee: University of Nebraska System
Priority date: 2005-08-03
Filing date: 2006-08-03
Publication date: 2007-02-08

Abstract

A method and system of dynamically controlling a water pump system that includes a water pump. The method includes determining a pressure differential at the water pump, determining a water flow rate based on the pressure differential, and determining a speed set point for the water pump. The method also includes sensing a water flow speed at the water pump, generating a pump speed control signal based on the speed set point and the water flow speed, and modulating a speed of the water pump based on the pump speed control signal.

Description

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/705,110, filed on Aug. 3, 2005, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments of the invention relate to temperature control systems and methods for buildings and other structures.

BACKGROUND

Various types of facilities, such as buildings, industrial production facilities, medical buildings, manufacturing assemblies, and laboratories, often employ chiller systems to chill water for use by other temperature control systems, such as air handling units (“AHUs”). A typical chiller system uses one or more flow meters to measure a water flow rate through one or more water pumps in the chiller system.
Measurements from typical flow meters can be unreliable for various reasons. For example, because flow meters have sensitivity limits, water flow rates below predetermined levels cannot be accurately measured. Further, dirt (e.g., debris, deposits, or other contaminants) accumulating on moving parts of flow meters impedes movement of such parts, reducing accuracy of measurements. In addition, environmental exposure may corrode flow meter parts, leading to inaccurate measurements.
Other chiller systems incorporate ultrasound flow meters external to supply and return conduits in order to measure water flow rates. While ultrasound flow meters are often more accurate, they are generally more expensive than typical water flow meters. In chiller systems that require flow rate measurements at multiple system points, implementation costs of ultrasound flow meters can be substantial.
Additionally, a constant loop pressure differential is typically used to control the speed of a water pump. Under partial load conditions, more power is supplied to the pump than designed for. As such, the pump is overburdened, and control valve(s) at the pump often experience excessive pressure differences, which can cause water leakage and valve damage.

SUMMARY

Embodiments of the invention provide energy-efficient control systems and methods that can be retrofitted in existing temperature control systems, or can be incorporated in new systems.
In one embodiment, the invention provides a method of dynamically controlling a water pump system that includes a water pump. The method includes determining a pressure differential at the water pump, determining a water flow rate based on the pressure differential, and determining a speed set point for the water pump. The method also includes sensing a water flow speed at the water pump, generating a pump speed control signal based on the speed set point and the water flow speed, and modulating a speed of the water pump based on the pump speed control signal.
In another embodiment, the invention provides a controller to dynamically control a water pump system that includes a variable speed water pump, a pump head sensor operable to sense a pressure differential at the water pump, and a water pump speed sensor operable to sense a water flow speed of the water pump. The controller includes a flow rate module, a speed set point module, a speed control module, and a modulator. The flow rate module determines a water flow rate based on the pressure differential. The speed set point module determines a speed set point for the water pump. The speed control module generates a pump speed control signal based on the speed set point and the water flow speed. The modulator modulates the water pump based on the pump speed control signal.
Various embodiments of the invention herein can identify an optimal pump speed and can control a pump based on the optimal pump speed for maximum pump efficiency and minimal loop resistance. Embodiments can also provide a required minimum loop flow for reliable chiller and boiler operations. Furthermore, a controller can provide a plurality of pump efficiency and consumption reports based on the optimal pump speed.
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 temperature system according to an embodiment of the invention.
FIG. 2 is a block diagram of a controller that can be implemented in the temperature system of FIG. 1 according to an embodiment of the invention.
FIG. 3 is a flow chart illustrating an exemplary process carried out by the controller of FIG. 2 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.
As should also be apparent to one of ordinary skill in the art, the systems shown in the figures are models of what actual systems might be like. Many of the modules and logical structures described are capable of being implemented in software executed by a microprocessor or a similar device or of being implemented in hardware using a variety of components including, for example, application specific integrated circuits (“ASICs”). Terms like “controller” may include or refer to both hardware and/or software. Furthermore, throughout the specification capitalized terms are used. Such terms are used to conform to common practices and to help correlate the description with the coding examples, equations, and/or drawings. However, no specific meaning is implied or should be inferred simply due to the use of capitalization. Thus, the claims should not be limited to the specific examples or terminology or to any specific hardware or software implementation or combination of software or hardware.
Embodiments of the invention provide control systems and methods that can be retrofitted in existing temperature control systems (e.g., including air handling units and/or a chiller unit), or can be incorporated in new systems. In one particular embodiment, a controller in a chiller system determines a pump pressure differential and a pump flow rate. Based on the pump pressure differential and the pump flow rate, the controller determines an optimal pump speed and pump efficiency. Although some embodiments herein refer to a chiller unit, it is to be appreciated that other embodiments need not be implemented with a chiller unit.
FIG. 1 shows a temperature system 100 that includes a controller 104 controlling a chiller unit 108. The chiller unit 108 supplies chilled medium such as water through supply conduits 112, 113, and receives return medium through return conduits 116, 117. In some embodiments, the conduits 112 and 116 constitute a primary flow loop, and the conduits 113 and 117 constitute another flow loop. A supply medium temperature sensor 118 positioned at supply conduit 112 measures or senses a temperature of the medium supplied by the chiller unit 108. Similarly, a return medium temperature sensor 119 positioned at the return conduit 116 measures or senses a temperature of the medium returning to the chiller unit 108. The chiller unit 108 includes a chiller controller 120.
The temperature system 100 also includes a primary medium pump 124 to pump the return medium to the chiller unit 108 in the primary loop, and another medium pump 125 in the condenser water loop. Although the following discussion describes controlling the primary medium pump 124, it is to be appreciated that the discussion can also be applied to the condenser pump 125. A pump head sensor 128 measures a pressure differential or pump head at the pump 124 between an inlet chamber (not shown) and an outlet or discharge chamber (not shown). In some embodiments, pressure probes are installed on the pump inlet and the pump outlet, respectively. The pressure probe on the inlet is linked to a negative side of the pump head sensor 128. Similarly, the pressure probe on the outlet is linked to a positive side of the pump head sensor 128. The controller 104 generates a pump speed signal to a variable frequency drive (“VFD”) 132, which drives the pump 124 to pump at a certain pump speed.
To determine the pump speed at which the VFD 132 drives the pump 124, the controller 104 receives supply power signals from the compressor 120, the temperature measured at the supply temperature sensor 118, the temperature measured at the return temperature sensor 119, the pump head from the pump head sensor 128, an actual pump speed generated by the VFD 132 or the pump 124, and the like. Operations for controlling the pump speed are detailed below. It is to be appreciated that the temperature system 100 can include one or more pumps similar to the pump 124.
FIG. 2 is a block diagram of a controller 200 that can be implemented in the controller 104 of the temperature system 100 of FIG. 1. The controller 200 includes an interface module 204 that interfaces its inputs and outputs. In some embodiments, the controller 200 receives the supply medium temperature from the supply temperature sensor 118, the return medium temperature from the return temperature sensor 119, the pump head from the pump head sensor 128, and the like. Similarly, the controller 200 also outputs data and signals from the interface module 204, such as a plurality of VFD signals to the VFD 132 to modulate the pump 124, one or more medium flow rates, a supply medium temperature, a return medium temperature, a thermal energy consumption by the pump 124 such as coefficient of performance (“COP”), a supply medium temperature set point to the chiller unit 108 or a mixing valve (not shown), and a pump enable signal to enable the pump 124.
The controller 200 also includes a memory module 208 that stores data, such as pump curve coefficients, a design pump speed, a design flow rate, a predetermined pump speed threshold, a minimum pump speed ratio, a design pump head or pressure differential, a time interval, a design pump power value, and the like. Based on the data stored in the memory module 208 and received at the interface module 204, the controller 200 uses a pump power module 212 to determine an amount of power consumed by the pump 124, and a flow rate module 216 to determine a pump flow rate generated by the pump 124. Based on the pump flow rate and other data, a pump speed set point module 220 determines a pump speed set point, which indicates an optimal speed at which the pump 124 is to be run.
A comparator module 224 compares an actual pump speed with the pump speed set point and the pump speed threshold. When the actual pump speed is greater than the pump speed set point plus the pump speed threshold, the controller 200 uses a pump speed control module 228 to generate a pump speed control signal to reduce the speed generated by the pump 124. When the actual pump speed is less than the pump speed set point plus the pump speed threshold, the controller 200 uses the pump speed control module 228 to generate a pump speed control signal to increase the speed generated by the pump 124. After receiving the pump speed control signal, a modulator or VFD module 232 sends a modulating signal based on the pump speed control signal to the VFD 132 of FIG. 1 to modulate a speed of the pump 124. However, when the actual pump speed is equal to the pump speed set point plus the pump speed threshold, the controller 200 uses a pump efficiency module 236 to determine a pump efficiency.
FIG. 3 is a flow chart illustrating a speed set point identifying process 300 carried out by the controller 200 of FIG. 2. At block 304, the process 300 inputs or initializes system parameters, such as the pump curve coefficients, the design pump speed, the design flow rate, the predetermined pump speed threshold, the minimum pump speed ratio, the design pump head or pressure differential, the time interval, and the design pump power value. At block 308, the process 300 determines or retrieves from the pump head sensor 128 the pressure differential or pump head (“Δp”). In some embodiments, the process 300 determines the pump power (“w_pump”) based on the modulating signals supplied to the VFD 132 of FIG. 1.
At block 312, the process 300 determines a flow rate (“Q”) of the pump 124 of FIG. 1 as follows. A specific equation for determining the flow rate is used depending on a type of pump curve associated with the pump 124. Typically, there are a number of types of pump curves, such as a steep pump curve and a flat pump curve. Pumps with a steep pump curve include pumps whose differential pressure or pump head increases as a result of decreasing flow rates (“Q”) at the same pump speed (“N”). Pumps with a flat pump curve include pumps whose differential pressure or pump head remains generally constant when the pump flow rate (“Q”) changes. For such pumps, the pump power varies significantly when the pump flow rate changes at the same pump speed.
For example, the process 300 can use EQN. (1) to determine the pump flow rate (“Q”), which is measured in gallons-per-minute (“GPM”), for pumps with a steep pump curve. EQN. (1) is based on a measured pump head (“H”), and a ratio (“ω”) between the actual pump speed (“N”) that is measured in revolutions-per-minute (“RPM”) and the design pump speed (“N_d”) that is also measured in RPM. $\begin{matrix} Q = (\frac{- a_{1} - \sqrt{a_{1}^{2} - 4 a_{2} (a_{0} - \frac{H}{ω^{2}})}}{2 a_{2}}) ω & (1) \end{matrix}$
In EQN. (1), a₀, a₁, and a₂are the pump curve coefficients obtained from the pump curve, typically provided by manufacturers of the pump 124. In some embodiments, the design fan speed (“N_d”) is about 1,450 RPM. Furthermore, the pump curve coefficients (“a₀, a₁, and a₂”) also relate the design flow rate (“Q_d”) to a design pump head (“H_d”), as shown in EQN. (2).
H_d=a₀+a₁Q_d+a₂Q_d ² (2)
The design pump head (“H_d”) is the pressure differential determined when the pump 124 is run at a design pump speed (“N_d”).
Further, the process 300 can also use EQN. (3) to determine the pump flow rate (“Q”) for pumps with a flat pump curve. EQN. (3) is based on the ratio (“ω”), and a fan power (“ω_f”). $\begin{matrix} Q = \frac{- b_{1} ω^{2} - \sqrt{b_{1}^{2} ω^{4} - 4 b_{2} ω (b_{0} ω^{3} - w_{f})}}{2 b_{2} ω} & (3) \end{matrix}$
In EQN. (2), b₀, b₁, and b₂are pump power curve coefficients, also provided by manufacturers of the pump 124. In this way, the process 300 can determine the pump flow rate (“Q”) using either of the above flow rate equations as appropriate. In embodiments where multiple medium pumps 124 are used, a total flow rate (“Q_total”) is determined as a sum of all flow rates (“Q”) of all individual pumps.
In block 316, the process 300 uses the flow rate (“Q”) determined in block 312 to determine an optimal pump speed set point (“N_set”) with EQN. (4). $\begin{matrix} N_{set} = N_{d} \max (ω_{\min,} \frac{Q}{Q_{d}}) & (4) \end{matrix}$
In EQN. (4), the pump speed set point (“N_set”) is determined as a function of the design pump speed (“N_d”), a ratio between the actual pump speed (“N”) and a minimum pump speed (“N_min”) allowed by the pump 124 (“ω_min”), the flow rate (“Q”), and the design flow rate (“Q_d”). In other embodiments, the pump speed set point (“N_set”) is determined from an optimal pump speed set point ratio (“ω_set”) which is a function of the ratio between the actual pump speed (“N”) and a minimum pump speed (“N_min”) allowed by the pump 124 (“ω_min”), the flow rate (“Q”), and the design flow rate (“Q_d”). In some embodiments, the value of ω_minvaries from about 0 percent to about 60 percent. In still other embodiments, the pump speed set point (“N_set”) is determined as a function of the design pump speed (“N_d”), a ratio between the actual pump speed (“N”) and a minimum pump speed (“N_min”) allowed by the pump 124 (“ω_min”) the flow rate (“Q”), the design flow rate (“Q_d”), an energy consumed by the pump 124 (“E”), and a design energy consumed by the pump 124 (“E_d”) in EQN. (5). $\begin{matrix} N_{set} = N_{d} \max (ω_{\min,} \frac{Q}{Q_{d}}, \frac{E}{E_{d}}) & (5) \end{matrix}$
At block 320, the process 300 compares the actual pump speed (“N”) with the pump speed set point (“N_set”). Specifically, the process 300 determines at block 320 a relationship between the actual pump speed (“N”) and the pump speed set point (“N_set”) plus a portion of the design pump speed (“N_d”) (“δ”). That is, the process 300 determines a relationship between N and N_set+δ. In some embodiments, the value of δ is between about 0 percent and 15 percent of the design pump speed. For example, if the design pump speed (“N_d”) is 1,450 RPM, the value of δ is chosen from between about 0 RPM and about 217.5 RPM. More particularly, at block 320, the process 300 determines if the actual pump speed (“N”) is within a predetermined range of the pump speed set point (“N_set”).
In the embodiment shown, if the process 300 determines that the actual pump speed (“N”) is not within the predetermined range of the pump speed set point (“N_set”), the process 300 adjusts the pump speed (“N”). For example, when N is greater than N_set+δ, or the actual pump speed (“N”) is above the predetermined range of the pump speed set point (“N_set”), the process 300 reduces the pump speed (“N”) by a predetermined amount (“Δ”) at block 324, and repeats block 304. However, if N is less than N_set-δ, or the actual pump speed (“N”) is below the predetermined range of the pump speed set point (“N_set”), the process 300 proceeds to increase the pump speed (“N”) by the predetermined amount (“Δ”) at block 328, and repeats block 304. In some embodiments, the predetermined amount (“Δ”) ranges from about 3 percent to about 5 percent of the design speed (“N_d”). For example, when N is greater than N_set+δ by about 10 percent, and if the predetermined amount (“Δ”) is 5 percent, the process 300 proceeds to decrease the pump speed (“N”) by about 5 percent. In this way, the process 300 can adjust the actual pump speed (“N”) to converge to within the predetermined range of the pump speed set point (“N_set”).
On the other hand, if the actual pump speed (“N”) is within the predetermined range of the pump speed set point (“N_set”), the process 300 will keep or maintain the current pump speed (“N”). Once the actual pump speed (“N”) is within the predetermined range of the pump speed set point (“N_set”), the process 300 will proceed to determine an efficiency of the pump 124 (“η”) at block 332. In some embodiments, energy consumption and thus an efficiency of the pump 124 (“η”) are calculated as a product of the flow rate (“Q”), and a difference between the supply temperature and the return temperature. In other embodiments, an efficiency of the pump 124 is determined with EQN. (6). $\begin{matrix} η = c \frac{Δ pQ}{w_{pump}} & (6) \end{matrix}$
Depending on the specific controller 200 used in the temperature system, different sampling intervals can be used. In some embodiments, a minimum time interval is used. The minimum time interval can be defined as the time required for water (or another medium) to flow through a loop. The minimum time interval can be determined based on the loop constant, which indicates a time required for the medium to flow through the loop (e.g., the primary loop) of FIG. 1.
Various features and advantages of the invention are set forth in the following claims.

Claims

1. A method of dynamically controlling a water pump system including a water pump, the method comprising:

determining a pressure differential at the water pump;

determining a water flow rate based on the pressure differential;

determining a speed set point for the water pump;

sensing a water flow speed at the water pump;

generating a pump speed control signal based on the pump speed set point and the water flow speed; and

modulating a speed of the water pump based on the pump speed control signal.

2. The method of claim 1, further comprising determining an efficiency of the pump based on the modulated speed.

3. The method of claim 1, wherein determining a water flow rate comprises determining a pump power value based on a pump curve of the pump.

4. The method of claim 1, wherein generating a pump speed control signal comprises converging the pump speed to within a predetermined range of the pump speed set point.

5. The method of claim 4, wherein the pump has an associated design pump speed, and wherein the predetermined range is between about 0 percent and 15 percent of the design pump speed.

6. The method of claim 1, wherein the water pump system further includes a chiller unit.

7. A controller to dynamically control a water pump system including a variable speed water pump, a pump head sensor operable to sense a pressure differential at the water pump, and a water pump speed sensor operable to sense a water flow speed of the water pump, the controller comprising:

a flow rate module configured to determine a water flow rate based on the pressure differential;

a speed set point module configured to determine a speed set point for the water pump;

a speed control module configured to generate a pump speed control signal based on the speed set point and the water flow speed; and

a modulator configured to modulate the water pump based on the pump speed control signal.

8. The controller of claim 7, further comprising a pump efficiency module configured to determine an efficiency of the pump based on the modulated speed.

9. The controller of claim 7, further comprising a pump power module configured to determine a pump power value based on a pump curve of the pump.

10. The controller of claim 7, wherein the speed control module is further configured to converge the pump speed to within a predetermined range of the pump speed set point.

11. The controller of claim 10, wherein the pump has an associated design pump speed, and wherein the predetermined range is between about 0 percent and 15 percent of the design pump speed.

12. The controller of claim 7, wherein the water pump system further includes a chiller unit.

13. A chiller system comprising:

a chiller unit configured to chill a medium;

a pump coupled to the chiller unit and configured to return a medium to the chiller unit;

a variable frequency drive coupled to the pump and configured to run the pump at a selected speed; and

a controller coupled to the variable frequency drive, the controller configured to determine a flow rate generated by the pump, to determine a pump speed set point based on the flow rate, and to modulate, with the variable frequency drive, the pump speed such that the pump speed converges to within a predetermined range of the pump speed set point.

14. The chiller system of claim 13, further comprising:

a supply conduit coupled to the chiller unit and configured to supply chilled medium from the chiller unit;

a temperature sensor coupled to the supply conduit and configured to sense a temperature of the chilled medium;

a return conduit coupled to the chiller unit and configured to return medium to the chiller unit; and

a second temperature sensor coupled to the return conduit and configured to sense a temperature of the returned medium.

15. The chiller system of claim 14, wherein the controller is further configured to determine a pump power value based on a difference of temperatures sensed by the respective temperature sensors.

16. The chiller system of claim 13, wherein the controller is further configured to determine an efficiency of the pump based on the modulated speed.

17. The chiller system of claim 13, wherein the controller is further configured to determine a pump power value based on a pump curve of the pump.

18. The chiller system of claim 13, wherein the pump has an associated design pump speed, and wherein the predetermined range is between about 0 percent and 15 percent of the design pump speed.