U.S. patent application number 11/014603 was filed with the patent office on 2006-06-22 for virtual controller for mixed air low temperature protection of hvac systems.
Invention is credited to Larry E. Weber, Richard A. Wruck.
Application Number | 20060130502 11/014603 |
Document ID | / |
Family ID | 36593991 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060130502 |
Kind Code |
A1 |
Wruck; Richard A. ; et
al. |
June 22, 2006 |
Virtual controller for mixed air low temperature protection of HVAC
systems
Abstract
A method for protecting an HVAC system of an air handling unit
from low temperature outdoor ventilation air. The air handling unit
may include an outdoor fresh air region, a return air region, a
supply air region, and a damper situated in or adjacent to the
fresh air region to regulate the flow of outdoor air into the air
handling unit. The air handling unit mixes the outdoor fresh air
and the return air to provide a mixed air stream to the HVAC
system. In one illustrative embodiment, one or more sensors are
used to measure the temperature and flow rate of the air entering
or passing through the outdoor fresh air region, the temperature of
the air entering or passing through the return air region and the
flow rate of the air passing through the supply air region. The
temperature of the mixed air stream, which is provided to the HVAC
system, is then calculated using a controller or the like. In some
cases, when the temperature of the mixed air stream falls below a
threshold value, the controller may instruct the damper to close
and reduce the fresh outdoor air that is brought into the air
handler unit. Various other embodiments and algorithms are
disclosed.
Inventors: |
Wruck; Richard A.; (Mount
Prospect, IL) ; Weber; Larry E.; (New Brighton,
MN) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Family ID: |
36593991 |
Appl. No.: |
11/014603 |
Filed: |
December 16, 2004 |
Current U.S.
Class: |
62/186 ;
62/179 |
Current CPC
Class: |
F24F 11/30 20180101;
F24F 2110/20 20180101; F24F 2110/70 20180101; F24F 2011/0006
20130101; F24F 2110/10 20180101; F24F 2110/00 20180101; Y02B 30/70
20130101; F24F 7/08 20130101; F24F 13/08 20130101 |
Class at
Publication: |
062/186 ;
062/179 |
International
Class: |
F25B 49/00 20060101
F25B049/00; F25D 17/00 20060101 F25D017/00; F25D 17/04 20060101
F25D017/04 |
Claims
1. A method for determining a characteristic of a mixed airflow
stream in a air handling unit (AHU), wherein the AHU includes at
least two flow inputs that are mixed to provide the mixed airflow
stream, and a flow output, the method comprising: providing a
measure of a first characteristic and a second characteristic of
air in a first flow input; providing a measure of the first
characteristic of air in a second flow input; providing a measure
of the second characteristic of air in the flow output; and
calculating the first characteristic of the mixed air stream in the
AHU as a function of the first and second characteristics of the
first flow input, the first characteristic of the second flow
input, and the second characteristic of the flow output.
2. The method of claim 1 wherein the first characteristic is
temperature and the second characteristic is air flow.
3. The method of claim 1 wherein the first characteristic is
moisture content and the second characteristic is air flow.
4. The method of claim 1 wherein the first characteristic is carbon
dioxide concentration and the second characteristic is air
flow.
5. The method of claim 2 wherein the first input is a fresh outdoor
air input.
6. The method of claim 5 wherein the second input is a return air
input.
7. The method of claim 6 wherein the flow output is a supply air
output.
8. The method of claim 1 wherein the AHU includes a heating,
ventilation, and air conditioning (HVAC) system.
9. The method of claim 1 wherein the measure of the first
characteristic and the second characteristic are provided by one or
more sensors.
10. A method for determining a characteristic of a mixed airflow
stream in a air handling unit (AHU), wherein the AHU includes at
least two flow inputs that are mixed to provide the mixed airflow
stream, and a flow output, the method comprising: providing a
measure of a first characteristic and a second characteristic of
air in a first flow input; providing a measure of the first
characteristic and the second characteristic of air in a second
flow input; calculating the first characteristic of the mixed air
stream in the AHU as a function of the first and second
characteristics of the first flow input and the first and second
characteristics of the second flow input.
11. The method of claim 10 wherein the first characteristic is
temperature and the second characteristic is air flow.
12. The method of claim 10 wherein the first characteristic is
moisture content and the second characteristic is air flow.
13. The method of claim 10 wherein the first characteristic is
carbon dioxide concentration and the second characteristic is air
flow.
14. The method of claim 11 wherein the first input is a fresh
outdoor air input.
15. The method of claim 14 wherein the second input is a return air
input.
16. The method of claim 15 wherein the flow output is a supply air
output.
17. The method of claim 10 wherein the AHU includes a heating,
ventilation, and air conditioning (HVAC) system.
18. The method of claim 10 wherein the measure of the first
characteristic and the second characteristic are provided by one or
more sensors.
19. A method for determining the temperature of a mixed airflow
stream in a air handling unit (AHU), wherein the AHU includes an
outdoor fresh air input port, a return air input port, a return air
exhaust port, and a supply air port, wherein the mixed airflow
stream is a mixture of the air passing through the outdoor fresh
air input port and the return air input port, the method
comprising: providing a measure of temperature and air flow for air
passing through the outdoor fresh air input port; providing a
measure of temperature for air passing through the return air input
port; providing a measure of air flow through the supply air port;
and calculating a temperature for the mixed air stream in the AHU
as a function of the temperature of the air flowing through the
outdoor fresh air input port and the return air input port, and the
air flow through the outdoor fresh air input port and the supply
air port.
20. The method of claim 19, wherein the AHU further includes a
damper for controlling the air flow through the outdoor fresh air
input port, the method further comprising the steps of: adjusting
the damper to reduce the air flow through the outdoor fresh air
input port if the temperature of the mixed air stream falls below a
predetermined threshold temperature value.
21. The method of claim 20 wherein the adjusting step only occurs
if the temperature of the mixed air stream falls below the
predetermined threshold temperature value for a predetermined time
period.
22. A method for determining the temperature of a mixed airflow
stream in a air handling unit (AHU), wherein the AHU includes an
outdoor fresh air input port, a return air input port, a return air
exhaust port, and a supply air port, wherein the mixed airflow
stream is a mixture of the air passing through the outdoor fresh
air input port and the return air input port, the method
comprising: providing a measure of temperature for air passing
through the outdoor fresh air input port; providing a measure of
temperature and air flow for air passing through the return air
input port; providing a measure of air flow for air passing through
the return air exhaust port; calculating a temperature for the
mixed air stream in the AHU as a function of the temperature of the
air flowing through the outdoor fresh air input port and the return
air input port, and the air flow flowing through the return air
input port and the return air exhaust port.
23. An air handling unit (AHU) having an outdoor fresh air input
port, a return air input port, and a supply air port, wherein the
AHU mixes the air passing through the outdoor fresh air input port
with the return air input port to produce a mixed air stream, the
AHU comprising: a first temperature sensor positioned in the
outdoor fresh air input port for providing a measure of the
temperature of the air passing through the outdoor fresh air input
port; a first air flow sensor positioned in the outdoor fresh air
input port for providing a measure of the air flow of the air
passing through the outdoor fresh air input port; a second
temperature sensor positioned in the return air input port for
providing a measure of the temperature of the air passing through
the return air input port; a second air flow sensor positioned in
the supply air port for providing a measure of air flow through the
supply air port; and a controller coupled to the first temperature
sensor, the second temperature sensor, the first air flow sensor
and the second air flow sensor, the controller adapted to calculate
a temperature for the mixed air stream as a function of the
temperature of the air flowing through the outdoor fresh air input
port and the return air input port, and the air flow passing
through the outdoor fresh air input port and the supply air
port.
24. The air handling unit (AHU) of claim 23 further comprising a
damper for controlling the air flow through the outdoor fresh air
input port.
25. The air handling unit (AHU) of claim 24 wherein the controller
is adapted to adjust the damper to reduce the air flow through the
outdoor fresh air input port if the temperature of the mixed air
stream falls below a predetermined threshold temperature value.
26. The air handling unit (AHU) of claim 25 wherein the controller
is further adapted to only adjust the damper if the temperature of
the mixed air stream falls below the predetermined threshold
temperature value for a predetermined time period.
27. A method for determining the mixed air temperature in a air
handling unit (AHU), the air handling unit including a fresh air
region, a return air region, and a supply air region, the method
comprising: determining the supply airflow rate in the supply
region; sensing the outdoor air temperature in the fresh air
region; sensing the outdoor airflow rate in the fresh air region;
sensing the temperature of the return airflow in the return air
region; and calculating the mixed airflow temperature as a function
of the supply airflow rate, the outdoor air temperature, the
outdoor airflow rate, and the temperature of the return
airflow.
28. The method of claim 27, wherein the supply airflow is
calculated.
29. The method of claim 27, wherein the supply airflow is
measured.
30. The method of claim 27 further comprising the steps of:
comparing the outdoor air temperature and the mixed air temperature
to a threshold temperature; and adjusting an outdoor air damper to
regulate the outdoor airflow into the AHU if the outdoor air
temperature and/or the mixed air temperature fall below the
threshold temperature.
31. A method for protecting a HVAC system of an air handling unit
from low temperatures wherein the air handling unit includes a
fresh air region, a return air region, a supply air region, and a
damper situated in or adjacent to the fresh air region to regulate
the flow of outdoor air into the HVAC, the method comprising:
determining the flow rate of the supply air in the supply air
region; sensing the outdoor air temperature in the fresh air
region; sensing the outdoor airflow rate in the fresh air region;
sensing the temperature of the return airflow in the return flow
region; calculating the temperature of the mixed air in the air
handling unit as a function of the supply airflow rate, the outdoor
air temperature, the outdoor airflow rate, and the temperature of
the return airflow; comparing the outdoor air temperature and the
mixed air temperature to a threshold temperature; and closing the
position of the damper to decrease the flow of outdoor air into the
air handling unit when the outdoor air temperature and the mixed
air temperature are less than the threshold temperature.
32. The method of claim 31 further comprising issuing a warning
signal, an alarm, and/or a message when the outdoor air temperature
and the mixed air temperature are less than the threshold
temperature.
33. The method of claim 31 further comprising the steps of:
counting the length of time that the mixed air temperature and
outdoor air temperature are below the threshold temperature with a
timer; comparing the length of time that the mixed air temperature
and outdoor air temperature are below the threshold temperature to
a predetermined limit of time allowable for the mixed air
temperature and outdoor air temperature to be below the threshold
temperature; and closing the position of the damper when the length
of time that the mixed air temperature and outdoor air temperature
are below the threshold temperature is greater than the
predetermined limit of time allowable for the mixed air temperature
and outdoor air temperature to be below the threshold temperature.
Description
FIELD
[0001] The present invention relates generally to Heating,
Ventilation, and Air Conditioning (HVAC) systems, and more
particularly to methods and apparatus for regulating the flow of
outdoor air to help protect the HVAC systems from low temperature
air.
BACKGROUND
[0002] Modern buildings have an HVAC system that is used to control
the environment of the building's inside space. In many systems,
air from the building's inside space is drawn into return ducts and
provided back to the HVAC system. To meet desired ventilation
requirements, many HVAC systems include an exhaust port for
exhausting at least some of the return air to the outside
environment, and an intake port for bringing in fresh air to the
HVAC system. In some cases, a damper is provided that selects how
much return air is exhausted and how much outside air is brought
into the building. Thus, and depending on the conditions, the air
entering the rooms is often a mixture of fresh outdoor air and
return air.
[0003] In some systems, an HVAC economizer is provided to act as a
first stage of cooling to increase fuel economy of the HVAC system
during some cooling cycles. The HVAC economizer may mix cooler
outdoor air and warmer return air to provide essentially "free"
cooling during some or all cooling cycles. At times of heating or
when the outdoor air temperature is unsuitably low, the economizer
may automatically enter a "lockout" position. The lockout position
may hold the outdoor air damper at a closed position or minimum
outdoor airflow setting to prevent the undesirably low temperature
air from entering the HVAC system.
[0004] In many HVAC economizer applications, a low temperature
controller is installed in the mixed air stream. The low
temperature controller limits the amount of cooler outdoor airflow
when the mixed air temperature drops below a safe operating low
temperature limit of the HVAC system. If the mixed air temperature
is too low, it may cause condensation or freezing in the HVAC
cooling and/or heating coils, which in many cases, is undesirable.
In the case of modular or packaged air handling unit components, it
is difficult to install low temperature protection sensors and
control. In some cases it may be physically impossible to install
an averaging or other sensor in the mixed air stream plenum of many
HVAC economizers. In other cases, the economizer outdoor air
dampers, airflow station, and controller are fabricated and shipped
as an enclosed module, which makes it difficult or impossible to
install a mixed air low temperature sensor/controller. Thus, a
method and system for determining the mixed air temperature to help
protect the HVAC system from undesirably low temperatures would be
desirable in these and other applications.
SUMMARY
[0005] The following summary of the invention is provided to
facilitate an understanding of some of the innovative features
unique to the present invention and is not intended to be a full
description. A full appreciation of the invention can be gained by
taking the entire specification, claims, drawings, and abstract as
a whole.
[0006] The present invention relates generally to Heating,
Ventilation, and Air Conditioning (HVAC) systems, and more
particularly to methods and apparatus for regulating the flow of
outdoor air to help protect the HVAC systems from undesirably low
temperature air. In one illustrative embodiment, an Air Handling
Unit (AHU) may be provided to determine a characteristic of a mixed
air stream, wherein the AHU includes at least two flow inputs and a
flow output. In order to determine a characteristic of the mixed
air stream such as mixed air temperature, predetermined
characteristics of the first flow input, predetermined
characteristics of the second flow input and predetermined
characteristics of the flow output may be measured or otherwise
determined. The characteristic of the mixed air stream may then be
calculated as a function of these predetermined characteristics, as
desired. In some cases, the predetermined characteristics may
include air temperature, air flow, air humidity, dew point and/or
some other characteristic or characteristics, as desired.
[0007] In one example, the air in the first flow input may be fresh
outdoor air, the air in the second flow input may be return air,
and the air in the flow output may be a supply air. The AHU may
measure or otherwise determine the temperature and air flow of the
fresh outdoor air, the temperature of the return air, and the air
flow of the supply air. From this, the AHU may calculate the air
temperature of the mixed air stream that passes to the HVAC cooling
coils of the HVAC system. It is contemplated that various
combinations of air temperature and air flow of the fresh outdoor
air, return air, supply air may be used to compute the temperature
of the mixed air stream, as desired. In some cases, if the
temperature of the mixed air stream falls below a predetermined
threshold, the AHU may begin to close the damper that controls the
air flow of the fresh outdoor air to help increase the temperature
of the mixed air stream.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic diagram of an illustrative Air
Handling Unit (AHU) having an HVAC system for use with a
building;
[0009] FIG. 2 is a flow diagram of an illustrative mixed air
temperature calculation method;
[0010] FIG. 3 is a flow diagram of another illustrative mixed air
temperature calculation method;
[0011] FIG. 4 is a flow diagram of another illustrative mixed air
temperature calculation method;
[0012] FIG. 5 is a flow diagram of another illustrative mixed air
temperature calculation method;
[0013] FIG. 6 is a flow diagram of an illustrative calculation of
the supply airflow rate;
[0014] FIG. 7 is a flow diagram of an illustrative method for
protecting an HVAC system from undesirably low temperatures;
and
[0015] FIG. 8 is a legend that defines the parameters used in the
flow diagrams of FIGS. 2-7.
DETAILED DESCRIPTION
[0016] The following description should be read with reference to
the drawings wherein like reference numerals indicate like elements
throughout the several views. The detailed description and drawings
show several embodiments which are meant to be illustrative of the
claimed invention.
[0017] FIG. 1 is a schematic diagram of an illustrative air
handling unit (AHU) 16 in a building 20. The building 20 may be a
residential, commercial, or any other suitable building, as
desired. The AHU 16 may include a heating, ventilation, and air
conditioning (HVAC) unit 40, which in some cases, may include one
or more cooling and/or heating coils. In the illustrative
embodiment, the AHU 16 includes at least two inputs and one output.
A first input may correspond to the fresh outdoor air input 32. The
temperature and/or flow rate of the fresh outdoor air input may be
measured or otherwise determined by one or more sensors, by
computational methods, and/or any other method as desired.
Additionally, any other characteristic, such as humidity, dew
point, carbon dioxide level, etc., of the fresh outdoor air input
32 may be measured and/or determined, as desired.
[0018] A second input to the AHU 16 may correspond to the return
air input 30. The return air input 30 may include air that is
pulled from the rooms inside of the building 20. In some cases,
some of the return air may be exhausted as shown at 38 through a
damper 29, and some of the return air may be recirculated back into
the HVAC 40. The temperature and/or flow rate of the return air
input 30 may be measured and/or otherwise determined by one or more
sensors, by computational methods, and/or any other method as
desired. Additionally, any other characteristic of the return air
may be measured and/or determined as desired.
[0019] A mixed air stream 34 may correspond to the AHU output. The
mixed air stream 34 may be a mixture of the fresh outdoor air input
32 and the return air input 30. The temperature and/or flow rate of
the mixed air stream 34 may be measured and/or otherwise determined
by one or more sensors, by computational methods, and/or any other
method as desired. Additionally, any other characteristic of the
mixed air stream 34 may be measured and/or determined as desired.
The HVAC system 40 may include a fan 18 to induce flow of air
through the HVAC 40 and ductwork, as desired, to produce supply air
36 to the building 20. In some cases, the temperature and/or flow
rate of the supply air 36 may be measured and/or otherwise
determined by one or more sensors, by computational methods, and/or
any other method as desired. Additionally, any other characteristic
of the return air may be measured and/or determined as desired.
[0020] In some illustrative embodiments, a damper 28 may be
provided to regulate the flow of fresh outdoor air 32 into the
building 20. Likewise, a damper 29 may be provided to regulate the
amount of return air that is exhausted 38 from the building 20. Yet
another damper 31 may be provided to regulate the flow of return
air 30 to mix with the fresh outdoor air 32. In many cases, the
dampers 28, 29 and 31 may be mechanically coupled together so that
the dampers 28 and 29 open and close together or in sequence, and
damper 31 opens and closes in an opposite manner to dampers 28 and
29. Thus, when damper 28 is opened to allow more fresh outdoor air
into the building, damper 29 also opens to allow a similar amount
of return air to be exhausted from the building. In this example,
the return air damper 31 may close as the dampers 28 and 29 open.
This arrangement may help balance the pressure inside the AHU
16.
[0021] In some cases, the dampers 28 and 29 and associated duct
work may be provided in an economizer, shown generally at 50. Under
some conditions, the economizer 50 may provide a first stage of
"free" cooling by mixing cooler fresh outdoor air 32 with the
sometimes warmer return air 30 to provide a mixed air stream 34 to
the cooling coils of the HVAC system 40. If the temperature of the
mixed air stream 34 is too low, it may cause condensation or
freezing in the HVAC cooling and/or heating coils, which in many
cases, is undesirable. Thus, the present invention may include a
controller 54 that calculates the temperature and possibly other
characteristics of the mixed air stream 34 and, by adjusting the
damper 28 and sometimes dampers 29 and 31, limits the low
temperature of the mixed air stream 34 that is provided to the HVAC
cooling and/or heating coils.
[0022] In some cases, the AHU 16 may also include a heat exchanger
generally shown at 52. The heat exchanger 52 may be adapted to
efficiently transfer heat energy between the incoming fresh outdoor
air 32 and the exhausted air stream 38, which may be useful under
some operating conditions.
[0023] FIG. 2 is a flow diagram of an illustrative mixed air stream
34 temperature calculation method. To perform the illustrative
mixed air stream 34 temperature calculation, sensor data is first
acquired, as shown at block 60. In the illustrative embodiment,
sensor data from an outdoor air temperature sensor (OAT), return
air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and
supply airflow 36 sensor (SplyFlow) is acquired by the controller
54 (see FIG. 1). Note that the outdoor air temperature sensor
(OAT), return air temperature sensor (RAT), outdoor airflow sensor
(OAFlow), and supply airflow 36 sensor (SplyFlow) may be provided
at convenient locations outside of the mixed air flow stream 34, if
desired. Once this data is acquired, and as shown at block 62, the
mixed air stream temperature (MAT) may be determined as a function
of these parameters using the illustrative function: Mixed .times.
.times. Air .times. .times. Temp = ( Outdoor .times. .times. Air
.times. .times. Temp * Outdoor .times. .times. Air .times. .times.
Flow + Return .times. .times. .times. Air .times. .times. Temp * (
Supply .times. .times. Flow - Outdoor .times. .times. Air .times.
.times. Flow ) ) * ( 1 / Supply .times. .times. Flow ) ##EQU1##
[0024] Note that when other characteristics of the various flow
streams are measured, different characteristics of the mixed air
flow stream may be calculated. For example, if an outdoor dew point
sensor (OAD) and a return air dew point sensor (RAD) are provided,
a Mixed Air Dew Point (MAD) value may be determined using a similar
illustrative function: Mixed .times. .times. Air .times. .times.
Dew .times. .times. Point = ( Outdoor .times. .times. Air .times.
.times. Dew .times. .times. Point * Outdoor .times. .times. Air
.times. .times. Flow + Return .times. .times. Air .times. .times.
Dew .times. .times. Point * ( Supply .times. .times. Flow - Outdoo
.times. r .times. .times. Air .times. .times. Flow ) ) * ( 1 /
Supply .times. .times. Flow ) ##EQU2## Instead of using dew point
sensors, humidity and temperature sensors may be used to calculate
the Outdoor Air Dew Point and the Return Air Dew Point, if desired.
Also, Mixed Air Carbon Dioxide levels (MACD), as well as many other
mixed air parameters may be determined in a similar manner, if
desired.
[0025] In any event, the illustrative mixed air temperature (MAT)
calculation may be used to help protect the HVAC system 40 from low
outdoor air 32 temperatures. For example, and in one illustrative
embodiment, the mixed air temperature (MAT) may be compared to a
mixed air temperature threshold temperature, as shown at block 64.
The mixed air temperature threshold may be any temperature in the
range of, for example, 32 degrees Fahrenheit to 50 degrees
Fahrenheit, but other temperatures may also be used as desired. If
the illustrative mixed air temperature (MAT) is less than the mixed
air temperature threshold, the fresh outdoor airflow 32 may be
reduced, as shown at block 66. The fresh outdoor airflow 32 may be
reduced by, for example, adjusting the damper 28 position to reduce
the incoming fresh outdoor airflow 32. In some cases, the damper 29
position may be equally decreased and the return air damper 31 may
be increased to help balance the pressure within the HVAC system
40.
[0026] FIG. 3 is a flow diagram of another illustrative mixed air
temperature calculation method. To perform the illustrative mixed
air stream 34 temperature calculation, sensor data is first
acquired, as shown at block 70. In the illustrative embodiment,
sensor data from an outdoor air temperature sensor (OAT), return
air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and
return air flow sensor (RAFlow) is acquired by the controller 54
(see FIG. 1). Note that the outdoor air temperature sensor (OAT),
return air temperature sensor (RAT), outdoor airflow sensor
(OAFlow), and return air flow (RAFlow) may be provided at
convenient locations outside of the mixed air flow stream 34, if
desired. Once this data is acquired, and as shown at block 72, the
mixed air stream temperature (MAT) may be determined as a function
of these parameters using the illustrative function: Mixed .times.
.times. Air .times. .times. Temp = ( Outdoor .times. .times. Air
.times. .times. Temp * Outdoor .times. .times. Air .times. .times.
Flow + Return .times. .times. .times. Air .times. .times. Temp *
Return .times. .times. Air .times. .times. Flow ) * ( 1 / ( Outdoor
.times. .times. Air .times. .times. Flow + Return .times. .times.
Air .times. .times. Fow ) ) ##EQU3## The illustrative mixed air
temperature (MAT) calculation may then be used to help protect the
HVAC system 40 from low outdoor air 32 temperatures. For example,
and in one illustrative embodiment, the mixed air temperature (MAT)
may be compared to a mixed air temperature threshold temperature,
as shown at block 74. The mixed air temperature threshold may be
any temperature in the range of, for example, 32 degrees Fahrenheit
to 50 degrees Fahrenheit, but other temperatures may also be used
as desired. If the illustrative mixed air temperature (MAT) is less
than the mixed air temperature threshold, the fresh outdoor airflow
32 may be reduced, as shown at block 76. The fresh outdoor airflow
32 may be reduced by, for example, adjusting the damper 28 position
to reduce the incoming fresh outdoor airflow 32. In some cases, the
damper 29 position may be equally decreased and the return air
damper 31 may be increased to help balance the pressure within the
HVAC system 40.
[0027] FIG. 4 is a logic diagram of another illustrative mixed air
temperature calculation method. To perform the illustrative mixed
air stream 34 temperature calculation, sensor data is first
acquired, as shown at block 80. In the illustrative embodiment,
sensor data from an outdoor air temperature sensor (OAT), return
air temperature sensor (RAT), return airflow sensor (RAFlow), and
supply airflow sensor (SplyFlow) is acquired by the controller 54
(see FIG. 1). Note that the outdoor air temperature sensor (OAT),
return air temperature sensor (RAT), return airflow sensor
(RAFlow), and supply airflow sensor (SplyFlow) may be provided at
convenient locations outside of the mixed air flow stream 34, if
desired. Once this data is acquired, and as shown at block 82, the
mixed air stream temperature (MAT) may be determined as a function
of these parameters using the illustrative function: Mixed .times.
.times. Air .times. .times. Temp = ( Outdoor .times. .times. Air
.times. .times. Temp * ( Supply .times. .times. Flow - Return
.times. .times. Air .times. .times. Flow ) + Return .times. .times.
Air .times. .times. Temp * Return .times. .times. Air .times.
.times. Flow ) * ( 1 / Supply .times. .times. Flow ) ##EQU4## The
illustrative mixed air temperature (MAT) calculation may then be
used to help protect the HVAC system 40 from low outdoor air 32
temperatures. For example, and in one illustrative embodiment, the
mixed air temperature (MAT) may be compared to a mixed air
temperature threshold temperature, as shown at block 84. The mixed
air temperature threshold may be any temperature in the range of,
for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but
other temperatures may also be used as desired. If the illustrative
mixed air temperature (MAT) is less than the mixed air temperature
threshold, the fresh outdoor airflow 32 may be reduced, as shown at
block 86. The fresh outdoor airflow 32 may be reduced by, for
example, adjusting the damper 28 position to reduce the incoming
fresh outdoor airflow 32. In some cases, the damper 29 position may
be equally decreased and the return air damper 31 may be increased
to help balance the pressure within the HVAC system 40.
[0028] FIG. 5 is a logic diagram of another illustrative mixed air
temperature calculation method. To perform the illustrative mixed
air stream 34 temperature calculation, sensor data is first
acquired, as shown at block 90. In the illustrative embodiment,
sensor data from an outdoor air temperature sensor (OAT), return
air temperature sensor (RAT), return airflow sensor (RAFlow), and
return air exhaust air flow sensor (RAExhaustFlow) is acquired by
the controller 54 (see FIG. 1). Note that the outdoor air
temperature sensor (OAT), return air temperature sensor (RAT),
return airflow sensor (RAFlow), and return air exhaust air flow
sensor (RAExhaustFlow) may be provided at convenient locations
outside of the mixed air flow stream 34, if desired. Once this data
is acquired, and as shown at block 82, the mixed air stream
temperature (MAT) may be determined as a function of these
parameters using the illustrative function: Mixed .times. .times.
Air .times. .times. Temp = ( Outdoor .times. .times. Air .times.
.times. Temp * Return .times. .times. Air .times. .times. Exhaust
.times. .times. Flow + Return .times. .times. Air .times. .times.
Temp * Return .times. .times. Air .times. .times. Flow ) * ( 1 / (
Return .times. .times. Air .times. .times. Flow + Return .times.
.times. Air .times. .times. Exhaust .times. .times. Flow ) )
##EQU5## This function assumes that that the return air exhaust
airflow (RAExhaustFlow) is approximately equal to the outdoor
airflow (OAFlow), and thus dampers 28 and 29 preferably move
together in this illustrative embodiment. Also, return air damper
31 may open and close in an opposite manner to dampers 28 and
29.
[0029] The illustrative mixed air temperature (MAT) calculation may
be used to help protect the HVAC system 40 from low outdoor air 32
temperatures. For example, and in one illustrative embodiment, the
mixed air temperature (MAT) may be compared to a mixed air
temperature threshold temperature, as shown at block 94. The mixed
air temperature threshold may be any temperature in the range of,
for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but
other temperatures may also be used as desired. If the illustrative
mixed air temperature (MAT) is less than the mixed air temperature
threshold, the fresh outdoor airflow 32 may be reduced, as shown at
block 96. The fresh outdoor airflow 32 may be reduced by, for
example, adjusting the damper 28 position to reduce the incoming
fresh outdoor airflow 32. The damper 29 position may equally or
substantially equally decrease the return air exhaust airflow
(RAExhaustFlow), and the return air damper 31 may increase the
return air flow 30 accordingly.
[0030] FIG. 6 is a flow diagram of an illustrative method for
determining the supply airflow (SplyFlow) 36, as shown at block
100. The illustrative method may be run on a regular basis, such as
every second, minute, hour, day, etc., and may be used to set
and/or update the supply airflow (SuplyFlow) parameter.
[0031] The supply airflow (SplyFlow) 36 may depend on the type of
HVAC system 40 used. Block 110 determines if a constant volume
system is used. If a constant volume system is used, control is
passed to block 112. Block 112 determines if an outdoor airflow
station is present, which provides a measure of the outdoor air
flow (OAFlow). If an outdoor air flow station is present, control
is passed to block 114. Block 114 determines if the damper 28 of
the economizer is at the full open (>=100%) position. If the
damper 28 is at the full open (>=100%) position, control is
passed to block 116, which updates a MAXFLOW parameter with the
outdoor air flow (OAFlow) that is currently measured by the outdoor
airflow station. Control is then passed to block 120, which updates
the supply air flow (SplyFlow) with the updated MAXFLOW
parameter.
[0032] Referring back to block 114, if the damper 28 is not at the
full open (>=100%) position, control is passed to block 120,
which updates the supply air flow (SplyFlow) with the old MAXFLOW
parameter.
[0033] Referring back to block 112, if an outdoor air flow station
is not present in the system, or is otherwise not working, control
is passed to block 118. Block 118 updates the supply air flow
(SplyFlow) with the designed flow rate of the system (DsgFlow)
multiplied by a Dirty Filter Factor (DirtyFltrFctr). The Dirty
Filter Factor (DirtyFltrFctr) may be a value ranging from one
(clean) to zero (very dirty), and may provide a measure of the
reduction in supply air flow caused by the HVAC filter. From blocks
118 and 120, control is passed to block 136, wherein the algorithm
is exited.
[0034] Referring back to block 110, if a constant volume system is
not used, the system must be a Variable Air Volume (VAV) system,
and control is passed to block 122. Block 122 determines if the
system is a Variable Air Volume (VAV) HVAC system that includes a
Supply Flow Station for providing a measure of the supply air flow.
If so, control is passed to block 124. Block 124 receives the
current supply air flow (SplyFlow) from the Supply Flow Station.
Control is then passed to block 136, wherein the algorithm is
exited.
[0035] Referring back to block 122, if a Variable Air Volume (VAV)
HVAC system 40 is used that does not include a Supply Flow Station,
control is passed to block 126. Block 126 determines if the
Variable Air Volume (VAV) HVAC system 40 includes an outdoor air
flow Station that provides a measure of the outdoor air flow
(OAFlow). If so, control is passed to block 128. Block 128
determines if the economizer damper 28 is at the full open
(>=100%) position. If the damper 28 is not at the full open
(>=100%) position, control is passed to block 134. If the damper
28 is at the fill open (>=100%) position, control is passed to
block 130, which updates the MAXFLOW parameter with the outdoor air
flow (OAFlow) currently measured by the outdoor airflow station.
Control is then passed to block 134.
[0036] Block 134 updates the supply air flow (SplyFlow) as a
function of the flow capacity signal of the Variable Air Volume
system, the minimum air flow setting of the Variable Air Volume
system times a dirty filter parameter, and the MAXFLOW parameter.
Control is then passed to block 136, wherein the algorithm is
exited.
[0037] Referring back to block 126, if the Variable Air Volume
(VAV) HVAC system 40 does not includes an outdoor air flow Station
that provides a measure of the outdoor air flow (OAFlow), control
is passed to block 132. Block 132 calculates the supply air flow
(SplyFlow) as a function of the flow capacity signal of the
Variable Air Volume system, the minimum air flow setting of the
Variable Air Volume system, the MAXFLOW parameter, and a dirty
filter parameter. The flow capacity signal Page: 11.sub.[0]typical
ranges from 0 to 100%, depending on the cooling demand of the zone
terminals. Control is then passed to block 136, wherein the
algorithm is exited.
[0038] FIG. 7 is a flow diagram of an illustrative method of
protecting an HVAC system from undesirably low temperatures. It is
contemplated that the illustrative method shown in FIG. 7 may be
run on a regular basis, such as every second, minute, hour, day,
etc.
[0039] To perform the illustrative method, sensor data is first
acquired as shown at block 200. In the illustrative embodiment,
sensor data from an outdoor air temperature sensor (OAT), return
air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and
supply airflow 36 sensor (SplyFlow) is acquired by the controller
54 (see FIG. 1). Note that the outdoor air temperature sensor
(OAT), return air temperature sensor (RAT), outdoor airflow sensor
(OAFlow), and supply airflow 36 sensor (SplyFlow) may be provided
at convenient locations outside of the mixed air flow stream 34, if
desired. Once this data is acquired, and as shown at block 210, the
mixed air stream temperature (MAT) may be determined as a function
of these parameters using the illustrative function: Mixed .times.
.times. Air .times. .times. Temp = ( Outdoor .times. .times. Air
.times. .times. Temp * Outdoor .times. .times. Air .times. .times.
Flow + Return .times. .times. .times. Air .times. .times. Temp * (
Supply .times. .times. Flow - Outdoor .times. .times. Air .times.
.times. Flow ) ) * ( 1 / Supply .times. .times. Flow ) ##EQU6##
Control is then passed to block 220. Block 220 determines if the
Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT)
are less than a Heat Exchanger Low Limit (HtgExchgLow) or a Low
Comfort Limit (LowComfort). If so, control is passed to block 222.
Block 222 limits the outdoor air flow ventilation that is provided.
The fresh outdoor airflow 32 may be reduced by, for example,
adjusting the economizer damper 28 position to reduce the incoming
fresh outdoor airflow 32. In some cases, the damper 29 position may
be equally decreased to help balance the pressure within the HVAC
system 40. Control is then passed to block 224, which issues a
diagnostic warning signal or message.
[0040] Referring back to block 220, if the Outdoor Air Temperature
(OAT) and the Mixed Air Temperature (MAT) are not less than the
Heat Exchanger Low Limit (HtgExchgLow) or the Low Comfort Limit
(LowConfort), control is passed to block 230. Block 230 determines
if the Outdoor Air Temperature (OAT) and the Mixed Air Temperature
(MAT) are less than a Safety Limit. If not, control is passed to
block 240, wherein the algorithm is exited.
[0041] If the Outdoor Air Temperature (OAT) and the Mixed Air
Temperature (MAT) are less than a Safety Limit, then control is
passed to block 232. Block 232 starts a delay timer. Control is
then passed to block 234. Block 234 determines if the Delay Timer
has exceeded a timer limit. If the Delay Timer has not exceeded the
timer limit, control is passed to block 240, wherein the algorithm
is exited. The timer limit allows the Outdoor Air Temperature (OAT)
and the Mixed Air Temperature (MAT) to be less than a Safety Limit
for a predetermined period of time, which may help reduce nuisance
fan stops, as further described below.
[0042] If the Delay Timer has exceeded the timer limit, control is
passed to block 236. Block 236 stops the fan system and closes the
outside air damper 28. Control is then passed to block 238, which
issues an alarm signal or message. Control is then passed to block
240, wherein the algorithm is exited. FIG. 8 is a legend that
defines the parameters used in the flow diagrams of FIGS. 2-7.
[0043] Having thus described the preferred embodiments of the
present invention, those of skill in the art will readily
appreciate that yet other embodiments may be made and used within
the scope of the claims hereto attached. Numerous advantages of the
invention covered by this document have been set forth in the
foregoing description. It will be understood, however, that this
disclosure is, in many respect, only illustrative. Changes may be
made in details, particularly in matters of shape, size, and
arrangement of parts without exceeding the scope of the invention.
The invention's scope is, of course, defined in the language in
which the appended claims are expressed.
* * * * *