U.S. patent application number 11/001369 was filed with the patent office on 2006-06-01 for method and apparatus for determining critical pressure of variable air volume heating, ventilating, and air-conditioning systems.
Invention is credited to Clifford Conrad Federspiel.
Application Number | 20060116067 11/001369 |
Document ID | / |
Family ID | 36567957 |
Filed Date | 2006-06-01 |
United States Patent
Application |
20060116067 |
Kind Code |
A1 |
Federspiel; Clifford
Conrad |
June 1, 2006 |
Method and apparatus for determining critical pressure of variable
air volume heating, ventilating, and air-conditioning systems
Abstract
A strategy for determining the critical supply duct pressure in
variable-air-volume heating, ventilating, and air-conditioning
systems that compensates for duct leakage and variable loads,
enabling its use during normal system operation. The strategy
consists of a static pressure sensor, an airflow sensor, a supply
fan, a fan modulating device, a controller coupled to the static
pressure sensor and the airflow sensor, and a data processing
algorithm for analyzing results from a function test using these
components. The functional test involves changing the supply duct
pressure setpoint, waiting for equilibrium, recording pressure,
flow, and time, then changing the supply duct pressure setpoint to
the next setting in the sequence.
Inventors: |
Federspiel; Clifford Conrad;
(El Cerrito, CA) |
Correspondence
Address: |
Clifford Conrad Federspiel
1764 Wesley Avenue
El Cerrito
CA
94530
US
|
Family ID: |
36567957 |
Appl. No.: |
11/001369 |
Filed: |
December 1, 2004 |
Current U.S.
Class: |
454/256 |
Current CPC
Class: |
F24F 2110/40 20180101;
F24F 2110/30 20180101; F24F 11/0001 20130101 |
Class at
Publication: |
454/256 |
International
Class: |
F24F 7/00 20060101
F24F007/00; F24F 11/00 20060101 F24F011/00 |
Goverment Interests
[0001] "This invention was made with State of California support
under California Energy Commission Grant number 02-03. The Energy
Commission has certain rights to this invention."
Claims
1. An apparatus for determining a critical supply duct pressure of
a variable-air-volume heating, ventilating, and air-conditioning
systems comprising in combination: said supply fan; a fan
modulating device coupled to said supply fan; a static pressure
sensor; at least one airflow sensor; a supply fan controller
coupled to said static pressure sensor and said fan modulating
device, said supply fan controller configured to regulate static
pressure to a static pressure setpoint; a calculator configured to
determine a critical supply duct pressure.
2. The apparatus of claim 1 wherein said airflow sensor is located
upstream of said supply fan.
3. The apparatus of claim 1 wherein said airflow sensor is located
downstream of said supply fan.
4. The apparatus of claim 1 wherein said calculator uses data from
said airflow sensor to compute said critical supply duct
pressure.
5. The apparatus of claim 1 wherein said calculator uses data from
said static pressure sensor to compute said critical supply duct
pressure.
6. The apparatus of claim 1 wherein said calculator uses said
static pressure setpoint to compute said critical supply duct
pressure.
7. The apparatus of claim 1 wherein said calculator is configured
to use a controlling-mode model.
8. The apparatus of claim 7 wherein said model of the controlling
system behavior includes a leakage term.
9. The apparatus of claim 7 wherein said controlling-mode model
includes a time-dependent term.
10. The apparatus of claim 1 wherein said calculator is configured
to use a starved-mode model.
11. The apparatus of claim 1 wherein said calculator uses a least
squares data fitting procedure.
12. A method for determining a critical supply duct pressure of a
variable-air-volume heating, ventilating, and air-conditioning
system, the method including the steps of: commanding a supply duct
setpoint to a setting; waiting for equilibrium; measuring a supply
airflow rate after equilibrium is reached; repeating said
commanding, waiting, and measuring steps for a sequence of said
supply duct setpoint settings; processing measured data with a
calculator configured to determine a critical supply duct pressure
from said data acquired.
13. The method of claim 12 further including the step of measuring
a supply duct static pressure after equilibrium is reached and
including said supply duct static pressure in said processing
step.
14. The method of claim 12 further including the step of measuring
a time at which equilibrium is reached and including said time at
which equilibrium is reached in said processing step.
15. The method of claim 12 wherein said calculating step uses a
controlling-mode model.
16. The method of claim 15 wherein said controlling-mode model
includes a leakage term.
17. The method of claim 15 wherein said controlling mode model
includes a time-dependent term.
18. The method of claim 12 wherein said calculating step uses a
starved-mode model.
19. The method of claim 12 wherein said calculating step uses a
least squares data fitting procedure.
Description
BACKGROUND
[0002] 1. Field of the Invention
[0003] The following invention relates to controls for
variable-air-volume heating, ventilating, and air-conditioning
(HVAC) systems, specifically to supply duct static pressure
control.
[0004] 2. Description of Prior Art
[0005] Modern buildings typically have complex heating,
ventilating, and air-conditioning systems to control indoor
temperature, pressure, ventilation rate, and other variables in a
way that makes efficient use of energy. One way to conserve energy
in these systems is to use a so-called variable-air-volume design.
Key components of a variable-air-volume system are a supply fan and
terminal units. The supply fan is a prime mover that causes air to
move. A terminal unit contains a throttling damper that regulates
an amount of air supplied to a space in a building that it controls
in order to regulate temperature and ventilation in that space.
[0006] In a variable-air-volume system, a flow rate of conditioned
air supplied to a building is adjusted so that no more air than
necessary is used. Variable flow is achieved using controls on or
near the supply fan and by the use of controls on the terminals.
The supply fan controls adjust the speed of the fan, an angle of
the fan blades, an angle of guide vane at an inlet or outlet of the
fan, or by adjusting a damper upstream or downstream of the fan
that throttles the flow. The controls on the terminals determine
how much air flows through each terminal.
[0007] The most common control strategy for the supply fan of
variable-air-volume systems is to regulate a static pressure in a
supply duct at a point downstream of the supply fan. This strategy
seeks to keep the static pressure at a measurement point constant
at all times. Control strategies based on a constant static
pressure in the supply duct have been proposed in U.S. Pat. No.
4,437,608 to Smith (1984) and U.S. Pat. No. 6,227,961 to Moore et
al. (2001). U.S. Pat. No. 4,836,095 to Wright (1989) describes a
variant of this strategy for systems that have multi-speed fans
rather than fans in which the speed is continuously variable. A
rule of thumb for this strategy is to locate the pressure sensor
two-thirds of the distance from the supply fan to the end of the
supply duct. A problem with this strategy is that it is inefficient
at part-load conditions, when the supply flow rate is significantly
lower than a design flow rate, which is the flow rate at which the
system should operate when the fan is running at full speed.
[0008] A control strategy that overcomes the problem of constant
static pressure control is one in which a static pressure setpoint
is reset based on a position of a terminal damper that is most
open. Control strategies that reset the static pressure based on
the position of the terminal damper that is most open have been
proposed in U.S. Pat. No. 4,630,670 to Wellman and Clark (1986) and
U.S. Pat. No. 5,863,246 to Bujak (1999). An objective is to keep
this damper nearly open or completely open. Doing so keeps the
supply duct pressure near the critical pressure, reducing
throttling losses at part-load conditions. The critical pressure is
the lowest supply duct pressure at which all of the terminal
dampers are still controlling. When the supply duct pressure is
below the critical pressure one or more terminal dampers will be
fully open yet unable to get enough air.
[0009] A report published by the California Energy Commission (CEC
publication number P500-03-052F, 2003) showed that the critical
supply duct pressure is correlated with the supply airflow rate.
This fact is exploited in U.S. Pat. No. 6,719,625 to Federspiel
(2004), which describes a static pressure reset strategy that
adjusts the static pressure setpoint based on the supply airflow
rate. This strategy overcomes many of the problems of static
pressure reset strategies that rely on terminal damper position
measurements. However, it requires some knowledge of how the
critical supply duct pressure is related to the supply airflow
rate.
[0010] Accordingly, a need exists for a strategy that will allow
the relationship between critical supply duct pressure and a supply
flow rate to be determined so that the static pressure reset
strategy based on supply airflow rate can be optimized.
SUMMARY OF THE INVENTION
[0011] In accordance with the present invention, a strategy for
determining the critical supply duct pressure of a
variable-air-volume heating, ventilating, and air-conditioning
system comprises the supply fan, a fan modulating device, a static
pressure sensor, an airflow sensor, and a controller coupled to the
static pressure sensor. The controller is commanded to a sequence
of static pressures. Supply airflow at each static pressure
setpoint is recorded, and the data are processed using a
model-based analysis technique that determines the critical supply
duct pressure, the leakage coefficient, and the rate of change of
the load at the test condition.
OBJECTS OF THE INVENTION
[0012] Accordingly, a primary object of the present invention is to
provide a strategy for determining the critical supply duct
pressure of variable-air-volume heating, ventilating, and
air-conditioning systems so that a static pressure reset strategy
can be configured and optimized.
[0013] Another object of the present invention is to provide a
strategy for determining the critical supply duct pressure of
variable-air-volume heating, ventilating, and air-conditioning
systems that can be implemented during normal system operation.
[0014] Another object of the present invention is determine the
leakage rate of the supply air duct in variable-air-volume heating,
ventilating, and air-conditioning systems.
[0015] Another object of the present invention is to provide a
strategy for determining the critical supply duct pressure of
variable-air-volume heating, ventilating, and air-conditioning
systems that can compensate for load changes that occur while the
strategy is implemented.
[0016] Other further objects of the present invention will become
apparent from a careful reading of the included drawing figures,
the claims, and detailed description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic diagram of a portion of a
variable-air-volume (VAV) heating, ventilating, and
air-conditioning (HVAC) system.
[0018] FIG. 2 is a graph of supply duct pressure versus supply
airflow. The points in the graph show the measured pressures and
flows from a laboratory test. The curves in the graph show the
starved and controlling models. The pressure at the intersection of
the curves is the critical pressure.
REFERENCE NUMERALS IN DRAWINGS
[0019] TABLE-US-00001 11 supply fan 12 fan modulating device 13
supply duct 14 terminal duct 15 terminal unit 16 terminal unit
controller 17 static pressure sensor 18 airflow sensor 19 supply
fan controller 20 terminal damper 21 starved-mode model 22
controlling-mode model 23 supply duct critical pressure
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] FIG. 1 shows the components of a variable-air-volume
heating, ventilating, and air-conditioning system that are relevant
to the critical pressure determination strategy. These components
include a supply fan 11, a fan modulating device 12, a supply duct
13, two or more terminal ducts 14, two or more terminal units 15,
two or more terminal unit controllers 16, a static pressure sensor
17, an airflow sensor 18, and a supply fan controller 19. The
system also contains other components such as heat exchangers and
filters not shown in FIG. 1, which are used for other functions
such as heating, cooling, and cleaning air. Supply fan 11 could be
a centrifugal fan or an axial fan. Fan modulating device 12 could
be a variable-speed drive, variable inlet guide vanes, a throttling
device such as a damper, or a device to adjust the pitch of the fan
blades. Supply duct 13 is an elongate sheet metal structure with
rectangular cross-section used to transport air. Each terminal duct
14 is also an elongate sheet metal structure used to transport air.
Each terminal duct 14 contains a terminal unit 15, which contains
at least one terminal damper 20 used to regulate a flow rate of air
in the terminal duct 14 in response to commands from the terminal
unit controller 16. Static pressure sensor 17 is located downstream
of supply fan 11. Static pressure sensor 17 indicates the static
pressure in supply duct 13. Airflow sensor 18 indicates a flow rate
of air in supply duct 13. Airflow sensor 18 may be located either
upstream or downstream of supply fan 11. Alternatively, the airflow
readings from terminal units 15 may be added together to measure
the supply airflow rate. Supply fan controller 19 may be an
electronic device with a microprocessor and memory, an analog
electrical circuit, or a pneumatic device.
[0021] Determination of the critical supply duct pressure involves
implementing a functional test on the air-handling unit, then
processing the data from the functional test using a model-based
procedure. The data processing uses a dual-mode model of a
variable-air-volume air-handling system. The two modes are
"controlling" and "starved". The supply fan in most
variable-air-volume air-handling systems is used to regulate the
static pressure at a point in the supply duct. The static pressure
should be sufficiently high that all terminals served by the
air-handling unit get enough air to meet their load. If it is too
high, then even the most-open variable-air-volume terminal will be
throttling considerably, and energy will be wasted. The critical
supply duct pressure occurs when the most-open variable-air-volume
terminal is 100% open and just meeting the load because this
condition minimizes throttling losses while keeping the system in
control. When the supply duct pressure is high enough that all of
the terminals are meeting the load, the system is operating in the
controlling mode. When one or more terminal dampers are 100% open
and not meeting the load, the system is in the starved mode. The
lowest supply duct pressure that keeps all the terminals in control
is called the critical pressure.
[0022] The controlling-mode model contains three terms. The first
is a constant term that represents the cumulative flow rate through
the dampers at the beginning of the functional test used to
determine the critical pressure. The second is a term to account
for duct leakage, which can be very significant in some systems.
The third is a time-dependent term that accounts for the fact that
the loads, and therefore the supply flow, may change over the
course of the functional test if it is conducted during normal
operation. Mathematically, the controlling model is as follows:
Q.sub.c=Q.sub.0+C.sub.pP.sup.n+C.sub.tT (1) where Q.sub.c is the
total supply airflow rate when the system is in control, C.sub.p is
the leakage coefficient, and C.sub.t is the rate of change of the
supply airflow rate due to changing load conditions. The first term
on the right-hand side of Equation 1 is the controlled cumulative
terminal flow (cumulative flow through the terminal dampers) at the
start of the functional test. The second term is leakage flow, and
the third is the time-varying component of the controlled
cumulative terminal flow.
[0023] When C.sub.t=0, Q.sub.0 is held constant as long as the
terminal dampers can change the system flow coefficient according
to the following relation: C Q = Q 0 P n ( 2 ) ##EQU1## where
C.sub.Q is the system flow coefficient.
[0024] When the supply duct pressure drops below the critical
pressure (starved mode), the relationship between flow coefficient
and pressure in Equation 2 no longer holds. The flow coefficient
becomes less that that of Equation 2, and Q.sub.0 becomes a
function of the pressure. In the starved mode, the flow coefficient
is modeled a quadratic function of pressure as follows:
C.sub.Q=c.sub.0+c.sub.1P+c.sub.2P.sup.2 (3) where the polynomial
coefficients c.sub.0, c.sub.1, and c.sub.2 must be determined
empirically. The starved-mode model is as follows: Q s = ( c 0
.times. P n + c 1 .times. P 1 + n + c 2 .times. P 2 + n ) .times. (
1 + C t .times. T Q 0 ) + C p .times. P n ( 4 ) ##EQU2##
[0025] The starved-mode model has three additional parameters
besides the three parameters of the controlling-mode model
(Equation 1). The term C.sub.tT/Q.sub.0 compensates for the fact
that only a fraction of the terminal flows (those of unstarved
terminals) may be changing with time in response to changing
loads.
[0026] The preferred functional test procedure for determining the
critical supply duct pressure involves the following sequence of
operations: [0027] 1. start at a sufficiently high supply duct
pressure setpoint [0028] 2. wait for the terminals to reach
equilibrium (e.g., 2 minutes for laboratory experiments, 15 minutes
for field experiments) [0029] 3. take a reading of supply flow,
static pressure, and time [0030] 4. reduce the supply duct static
pressure setpoint by a small amount (e.g., 0.1 in. w.c.) [0031] 5.
wait for the terminals to reach equilibrium again [0032] 6. take a
reading of supply flow, static pressure, and time [0033] 7. repeat
steps 4-6 until the supply flow is less than a pre-determined limit
(e.g., 70% of the starting flow) [0034] 8. increase the pressure by
a small amount (e.g., 0.1 in. w.c.) [0035] 9. wait for terminals to
reach equilibrium again [0036] 10. take a reading of supply flow,
static pressure, and time [0037] 11. repeat steps 8-10 until the
pressure equals the starting pressure
[0038] The preferred analysis procedure for determining the
critical pressure from the functional test data is as follows:
[0039] A. Assign the first N high-pressure points at the beginning
of the test and the M low-pressure points at the end of the test to
the controlling-mode model. Estimate the coefficients of the
controlling-mode model with least squares. Determine if the
time-dependent term can be dropped from the model using a t-test
with a decision probability of 0.02. [0040] B. Assign the remaining
data points to the starved-mode model. Estimate the three
coefficients of the starved-mode model using the coefficients
determined from the controlling model. [0041] C. Compute the
variance of the combined residuals. [0042] D. Repeat steps 1-3 for
all allowable values of N and M. [0043] E. Choose the values of N
and M that produce the lowest variance [0044] F. Determine the
pressure at which the flow predicted by the starved model equals
the flow predicted by the controlling model. FIG. 2 is a graph
showing the functional test data and results of the analysis
procedure for a test conducted on a laboratory air-handling unit.
The gradual slope of controlling-mode model 22 is due to duct
leakage. Starved-mode model 21 shows a rapid decrease in pressure
below the airflow rate corresponding to critical supply duct
pressure 23, which is caused by terminals becoming starved.
Experiments conducted over a wide range of conditions demonstrate
that the standard error of values from the preferred embodiment is
6% of the true critical supply duct pressure. Conclusion,
Ramifications, and Scope
[0045] Accordingly, the reader will see that the critical pressure
determination strategy of this invention has a number of advantages
including the following: [0046] (a) It can be implemented during
normal system operation, [0047] (b) It can determine duct leakage,
[0048] (c) It can compensate for time-varying loads.
[0049] This disclosure is provided to reveal a preferred embodiment
of the invention and a best mode for practicing the invention.
Having thus described the invention in this way, it should be
apparent that various different modifications can be made to the
preferred embodiment without departing from the scope and spirit of
this disclosure. Thus the scope of the invention should be
determined by the appended claims and their legal equivalents,
rather than by the examples given.
* * * * *