Note: Descriptions are shown in the official language in which they were submitted.
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1
AN HVAC SYSTEM HAVING A
DIAGNOSTICS CONTROLLER ASSOCIATED THEREWITH
TECHNICAL FIELD
This application is directed, in general, to heating,
ventilating and air conditioning (HVAC) systems, and more
specifically, to a diagnostics controller that can be used in
those HVAC systems.
BACKGROUND
HVAC systems can be used to regulate the environment
within an enclosed space.
Typically, an air blower is used
to pull air (i.e., return air) from the enclosed space into
the HVAC system through ducts and push the air (i.e., return
air) back into the enclosed space through additional ducts
after conditioning the air (e.g., heating, cooling or
dehumidifying the air). Various types of HVAC systems may be
used to provide conditioned air for enclosed spaces.
For
example, some HVAC units are located on the rooftop of a
commercial building. These so-called rooftop units, or RTUs,
typically include one or more blowers and heat exchangers to
heat and/or cool the building, and baffles to control the
flow of air within the RTU.
Some RTUs also include an air-
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side economizer that allows selectively providing fresh
outside air (i.e., ventilation or ventilating air) to the RTU
or to recirculate exhaust air from the building back through
the RTU to be cooled or heated again. A pressure sensor that
has sensors on opposite sides of the economizer is often
present to provide pressure information within the air-side
of the RTU.
SUMMARY
Certain exemplary embodiments can provide a controller
for a heating, ventilating and cooling (HVAC) system having
an economizer with a damper and pressure sensors on opposing
sides of said economizer, comprising; a diagnostics
controller having a program configured to use pressure
difference sensor data to determine if an error exists in the
HVAC system by determining if a measured pressure difference
across the economizer is outside an expected range for the
pressure difference across the economizer, and when said
pressure difference is outside said expected range, generate
an error signal indicative of an error in the HVAC system,
wherein the expected range for the pressure difference across
the economizer is based at least in part on damper position
data.
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2a
Certain exemplary embodiments can provide a heating,
ventilating and cooling (HVAC) system, comprising:
an
economizer having a damper and an actuator to move blades
thereof; a pressure sensor configured to determine a pressure
difference across said damper; and a diagnostics controller
having a program stored thereon configured to use pressure
difference sensor data to determine if a measured pressure
difference across said economizer is outside an expected
range for the pressure difference across the economizer and
generate an error signal when said pressure difference is
outside said design parameter wherein the expected range for
the pressure difference across the economizer is based at
least in part on damper position data.
Certain exemplary embodiments can provide a computer
program product, comprising a non-transitory computer usable
medium having a computer readable program code embodied
therein, said computer readable program code adapted to be
executed to implement a method of using pressure difference
sensor data to determine if a pressure difference across an
economizer of a heating ventilation air conditioning (HVAC)
system is outside an expected range for the pressure
difference across the economizer, and generate an error
signal when said pressure difference is outside the expected
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range, comprising:
receiving pressure sensor feedback data
from said HVAC system, said feedback data corresponding to a
measured pressure difference across an economizer of said
HVAC system; comparing said feedback data to a diagnostics
data table of said computer readable program; determining if
said measured pressure difference is outside said expected
range for the pressure difference across the economizer; and
sending an error signal when said pressure difference is
outside of said expected range, wherein the expected range
for the pressure difference across the economizer is based at
least in part on damper position data.
In another embodiment, the present disclosure provides a
controller for a HVAC system having an economizer with a
damper and pressure sensors on opposing sides of the
economizer.
The controller comprises a diagnostics
controller having a program configured to use pressure
difference sensor data to determine if a pressure difference
across the economizer is outside an
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operational design parameter of the HVAC system, and
generate an error signal when the pressure difference is
outside the operational design parameter.
In yet another aspect, an HVAC system is disclosed.
In one embodiment, the HVAC system comprises an
economizer having a damper and an actuator to move the
blades of the economizer, a pressure sensor configured to
determine a pressure difference across the damper, and a
diagnostics controller. The diagnostics controller has a
program stored thereon that is configured to use the
pressure difference sensor data to determine if a
pressure difference across the economizer is outside an
operational design parameter of the HVAC system, and
generate an error signal when the pressure difference is
outside the design parameter.
In another aspect, a computer program product,
including a non-transitory computer usable medium having
a computer readable program code embodied therein, the
computer readable program code is adapted to be executed
to implement a method of using pressure difference sensor
data to determine if a pressure difference across an
economizer of a HVAC system is outside an operational
design parameter of the HVAC system, and generate an
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error signal when the pressure difference is outside the
operational design parameter. The method comprises
receiving pressure sensor feedback data from the HVAC
system, wherein the feedback data corresponds to a
pressure difference across an economizer of the HVAC
system at a given damper blade position, comparing the
feedback data to a diagnostics data table of the computer
readable program, determining if the pressure difference
is outside the operational design parameter, and sending
an error signal when the pressure difference is outside
of the operational design parameter.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is now made to the following descriptions
taken in conjunction with the accompanying drawings, in
which:
FIG. 1 illustrates a block diagram of an embodiment
of an HVAC system constructed according to the principles
of the disclosure;
FIG. 2 illustrates a block diagram of one embodiment
of a diagnostics controller, as provided by this
disclosure;
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FIG. 3 illustrates a block diagram of an embodiment
of the diagnostics controller, as provided by this
disclosure;
FIG. 4 illustrates a flow diagram of an embodiment
5 of a method of diagnosing the operation of a HVAC system
as provided by this disclosure; and
FIG. 5 illustrates a chart showing the relationship
between damper position and damper position differences
which the diagnostics controller uses to determine when
the damper pressure difference is outside a preferred
range of damper pressure differences.
DETAILED DESCRIPTION
Knowing the ventilation airflow rate (i.e., airflow
rate through the damper of the economizer) during the
various operating modes of an economizer, such as the
ventilation mode and the free cooling mode, is
advantageous. When in
the ventilation mode, the
ventilation airflow rate provides verification that
ventilation, as required, is being provided. If the
ventilation airflow rate is too high, then energy may be
wasted due to over ventilation. In a free cooling mode,
knowing the ventilation airflow rate provides an
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indication of the energy savings provided by the
economizer. Thus, determining the ventilation airflow of
an HVAC system is often needed to verify that the system
is providing the desired ventilation.
However, such systems may also rely on pressure
sensors to monitor the ventilation airflow. Over
time,
these sensors or other components, such as the dampers in
an economizer, within the ventilation system can
malfunction and give false pressure readings or filters
may become excessively dirty, thereby impending good
airflow. Further,
the actuator that drives the blades'
positions of the economizer can also malfunction. In one
embodiment, the present disclosure uses pressure sensor
data to detect faults within the HVAC system. In another
embodiment, the controller uses pressure sensor data in
conjunction with damper positions of an economizer to
detect faults within the HVAC system. Thus, embodiments
of the present disclosure provide a diagnostic controller
for an airside economizer using a pressure sensor. For
example, the relationship between the damper position and
the pressure sensor reading can be used to determine if
the pressure sensor is operating correctly, or if the
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damper blades of the economizer are moving properly or if
a duct is unduly restricted due to a dirty filter, etc.
FIG. 1 illustrates a block diagram of an embodiment
of an HVAC system 100 constructed according to the
principles of this disclosure. The system
100 includes
an enclosure 101 (e.g., a cabinet) with openings for
exhaust air, ventilation air, return air and supply air.
The enclosure 101 includes exhaust vents 102 and
ventilation vents 103 at the corresponding exhaust air
and ventilation air openings. Within the enclosure 101,
the system 100 includes an optional exhaust fan 105,
economizer 110, a cooling element 120, an indoor fan or
blower 130 and a heating element 140. Additionally, the
system 100 includes a fan controller 150 and a HVAC
controller 160. The fan controller 150 is coupled to the
blower 130 via a cable 155. The cable 155 is a
conventional cable used with HVAC systems. The HVAC
controller 160 can be connected (not illustrated) to
various components of the system 100, including a
thermostat 119 for determining outside air temperature,
via wireless or hardwired connections for communicating
data.
Conventional cabling or wireless communications
systems may be employed. Also
included within the
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enclosure 101 is a partition 104 that supports the blower
130 and provides a separate heating section.
In the embodiment that is illustrated, the HVAC
system 100 is an RTU. One
skilled in the art will
understand that the system 100 can include other
partitions or components that are typically included
within an HVAC system, such as a RTU. While
the
embodiment of the system 100 is discussed in the context
of a RTU, the scope of the disclosure includes other HVAC
applications that are not roof-top mounted.
The blower 130 operates to force an air stream 170
into a structure, such as a building, being conditioned
via an unreferenced supply duct. A return airstream 180
from the building enters the system 100 at an
unreferenced return duct.
A first portion 181 of the air stream 180 re-
circulates through the economizer 110 and joins the air
stream 170 to provide supply air to the building. A
second portion of the air stream 180 is air stream 182
that is removed from the system 100 via the optional
exhaust fan 105.
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The economizer 110 operates to vent a portion of the
return air 180 and replace the vented portion with the
air stream 175. Thus, indoor air quality characteristics,
such as 002 concentration and humidity, may be maintained
within defined limits within the building being
conditioned. The economizer 110 includes damper, such as
an indoor damper 111, an outdoor damper 113 and an
actuator 115 that drives (opens and closes) the indoor
and outdoor dampers 111, 113 (i.e., the blades of the
indoor and outdoor dampers 111, 113). Though the
economizer 110 includes two damper assemblies, one
skilled in the art will understand that the concepts of
the disclosure also apply to those economizers or devices
having just a single damper assembly.
The controller 160 includes an interface 162 and a
ventilation director 166. The
ventilation director 166
may be implemented on a processor and/or a memory of the
controller 160. The interface 162 receives feedback data
from sensors and components of the system 100 and
transmits control signals thereto. As such, the
controller 160 may receive feedback data from, for
example, the exhaust fan 105, the blower 130 and/or the
fan controller 150, the economizer 110 and the thermostat
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119, and transmit control signals thereto if applicable.
One skilled in the art will understand that the location
of the controller 160 can vary with respect to the HVAC
system 100. The HVAC
controller 160 is configured to
5 determine supply airflow according to conventional means.
For example, in one embodiment, the HVAC controller 160
is configured to calculate the supply airflow rate based
on a set of blower curves, fan power and fan speed.
The interface 162 may be a conventional interface
10 that employs a known protocol for communicating (i.e.,
transmitting and receiving) data. The interface 162 may
be configured to receive both analog and digital data.
The data may be received over wired, wireless or both
types of communication mediums. In some
embodiments, a
communications bus may be employed to couple at least
some of the various operating units to the interface 162.
Though not illustrated, the interface 162 includes input
terminals for receiving feedback data.
The feedback data received by the interface 162
includes data that corresponds to a pressure drop across
the outdoor damper 113 and damper position of the
economizer 110. In some
embodiments, the feedback data
also includes the supply airflow rate. Various
sensors
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of the system 100 are used to provide this feedback data
to the HVAC controller 160 via the interface 162. In
some embodiments, a return pressure sensor 190 is
positioned in the return air opening to provide a return
static pressure. The return pressure sensor 190 measures
the static pressure difference between the return duct
and air outside of the HVAC system 100. In one
embodiment, a supply pressure sensor 192 is also provided
in the supply air opening to indicate a supply pressure
to the HVAC controller 160. The supply pressure sensor
192 measures the static pressure difference between the
return duct and the supply duct. Pressure sensor 193 is
used to provide the pressure drop across outdoor damper
113 of the economizer 110. The pressure sensor 193 is a
conventional pressure transducer that determines the
static pressure difference across the outdoor damper 113.
The pressure sensor 193 includes a first input 194 and a
second input 195 for receiving the pressure on each side
of the outdoor damper 113. The pressure sensors discussed
herein can be conventional pressure sensors typically
used in HVAC systems.
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A diagnostics controller 196 is also present in the
HVAC system 100 that is coupled to the pressure sensor
193 and the controller 160 that is configured to monitor
the pressure sensor 193 and the economizer 110 and send
an error signal when an improper pressure difference is
read by the diagnostics controller 196, and in another
embodiment, the controller 196 may also use a damper
position of the economizer 110 in conjunction with the
pressure senor data to detect improper pressure
differences. Though the pressure sensor 193 is shown as
a separate component from the diagnostics controller 196,
it should be understood that they may both be
incorporated into a single unit. In one embodiment, the
diagnostic controller 196 has a diagnostics table stored
in memory. The values in the table are selected based on
the airflow properties of the economizer damper assembly,
which are known at time of manufacture.
It should be noted that the values in the
diagnostics table may vary from unit to unit. For
example, if the outdoor air inlet is relatively large and
unrestricted, the minimum pressure when the damper is
100% open would be near zero. However,
if the outdoor
inlet is relatively small and restrictive, min pressure
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when 100% open could be 0.2" h2o. At the 0% open range
of the table, the range of values are a function of the
return static pressure drop, which is unknown. In
another aspect of this embodiment, the pressure sensor
190 in the return duct could be used to narrow the range
at 0% open. The pressure at the 50% open position is a
function of the flow curves of the damper assembly.
Different values are obtained if the damper blades within
the economizer 110 move opposed to each other vs.
parallel to each other.
Economizer damper position data is provided to the
HVAC controller 160 via the actuator 115 and the
ventilation director 166. The actuator 115 is configured
to rotate or move the indoor and outdoor dampers 111,
113, of the economizer 110 in response to a received
signal, such as control signals from the HVAC controller
160 (i.e., the ventilation director 166). The
actuator
115 is a conventional actuator, such as an electrical-
mechanical device, that provides a signal that
corresponds to the economizer damper position (i.e.,
blade angle of the outdoor damper 113 of the economizer
110). The
signal is an electrical signal that is
received by the ventilation director 166 which is
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configured to determine the relative angle of the outdoor
damper 113 based on the signal from the actuator 115. A
lookup table or chart may be used by the processor 117 to
determine a relative blade angle with respect to an
electrical signal received from the actuator 115. The
angle can be based on (i.e., relative to) the ventilation
opening of the HVAC system 100.
In some embodiments, the economizer damper position
can be determined via other means. For
example, an
accelerometer coupled to a blade (or multiple
accelerometers to multiple blades) of the outdoor damper
113 may be used to determine the economizer damper
position. The
outdoor damper 113 is opened at 100
percent when the blades thereof are positioned to provide
maximum airflow of ventilation air 175 into the system
100 through the ventilation opening. In FIG.
1, the
blades of the outdoor damper 113 would be perpendicular
to the ventilation opening or the frame surrounding the
ventilation opening when opened at 100 percent. In the
illustrated embodiment, the blades of the outdoor damper
113 would be parallel to the ventilation opening when
opened at zero percent.
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The ventilation director 166 is configured to
determine an operating ventilation airflow rate of the
HVAC system based on the static pressure difference
across the outdoor dampers 113, the economizer damper
5 position and economizer ventilation data. In some
embodiments, the ventilation director 166 also employs
the supply airflow rate to calculate the operating
ventilation airflow rate. In one
embodiment, using the
supply airflow rate for the calculation is based on the
10 economizer damper position being above 50 percent. In
one embodiment, the economizer ventilation data is
developed during manufacturing or engineering of the
system 100 or similar type of HVAC systems. During
development, a ventilation airflow rate is measured in,
15 for example, a laboratory, at a variety of operating
conditions. Various
sensors and/or other type of
measuring devices are employed during the development to
obtain the measured data for the various operating
conditions.
Economizer ventilation data is developed
from the measured data and loaded into the HVAC
controller 160, such as a memory thereof. During
operation in the field, the HVAC controller 160 (i.e.,
the ventilation director 166) receives the feedback data
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and calculates the ventilation airflow rate employing the
feedback data and the economizer ventilation data. FIG.
3 provides a more detailed embodiment of a ventilation
director 166.
The ventilation director 166 may further be
configured to adjust a position of the economizer 110
based on the economizer damper position and a desired
ventilation airflow rate and provide damper position data
to the diagnostics controller 196. The
desired
ventilation airflow rate can be preprogrammed into a
memory of the HVAC controller 160 during manufacturing.
In some embodiments, the desired ventilation airflow rate
is entered into the HVAC controller 160 in the field
during, for example, installation, a maintenance visit or
a service visit. The ventilation director 166 generates
a signal that directs the actuator 115 to adjust a
position of the blades of the economizer 110 based on the
desired ventilation airflow rate. In some
embodiments,
this signal represents a difference between the operating
ventilation airflow rate and the desired ventilation
airflow rate.
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FIG. 2 illustrates a block diagram of one embodiment
of the diagnostics controller 200 (196 of FIG. 1). The
controller 200 is configured to perform a diagnostics
routine during system calibration, which occurs after
installation and initial power-up, or during normal
operation of the HVAC system 100, to determine if the
pressure sensor is functioning properly or to determine
if the damper blades of the economizer 110 are working
properly based on pressure readings across the economizer
110. As such,
the controller 200 is configured to
generate diagnostic signals that may be transmitted to
the controller 160. The controller 200 may generate the
diagnostic signals in response to feedback data received
from the pressure sensor 193. The
controller 200
includes an interface 210 that is configured to receive
and transmit the feedback data and diagnostic signals.
The interface 210 may be a conventional interface that is
used to communicate (i.e., receive and transmit, by
either hard wire or wirelessly) data for a controller,
such as a microcontroller.
The interface 210 may include a designated input
terminal or input terminals that are configured to
receive feedback data from the pressure sensor 193. The
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controller 200 also includes a processor 220 and a memory
230. The
memory 230 may be a conventional memory
typically located within a controller, such as a
microcontroller, that is constructed to store data and
computer programs. The memory
230 may store normal
operating damper pressure differences as they relate to
various damper positions of the economizer 110 and may
also store diagnostic routines. The diagnostic routines
may correspond to algorithms that provide the
functionality of the diagnostic schemes disclosed herein.
For example, the diagnostic routines may correspond to
the algorithm or algorithms that implement the methods,
as described below. The
processor 220 may be a
conventional processor, such as a microprocessor. The
controller 200, in certain embodiments, may also include
a display 240 for visually providing information to a
user. The
interface 210, processor 220 memory 230 and
display 240 may be coupled together via conventional
means to communicate information. The controller 200 may
also include additional components typically included
within a controller for a HVAC unit, such as a power
supply or power port.
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The controller 200 is configured to receive feedback
data from the pressure sensor including, for example, a
pressure difference across a damper of the economizer 110
of the HVAC system 100 based on a particular damper
position. The controller 200 compares the pressure data
received with normal pressure data stored in the
controller 200.
If the pressure reading is outside
prescribed operating parameters, the diagnostics
controller 200 will generate an error or alarm signal.
FIG. 3 illustrates a simple block diagram of an
embodiment of a diagnostics controller 300, as provided
herein. In one embodiment, the diagnostics controller 300
may be embodied as a series of operation instructions
that direct the operation of a processor when initiated
thereby. In
one embodiment, the diagnostics controller
300 is implemented in at least a portion of a memory of
an HVAC controller, such as a non-transistory computer
readable medium of the HVAC controller. The diagnostics
controller 300 includes a diagnostics controller 310
having a comparison table stored in memory and an
economizer damper position reader 315.
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The diagnostics controller 310 is configured to
compare the pressure sensor data received from the
pressure sensor with the stored table, based on the
position of the damper positions of the economizer, which
5 is provided by the economizer position read function 315.
As stated above, the values in the table are selected
based on the airflow properties of the economizer damper
assembly, which are known at time of manufacture.
FIG. 4 illustrates a flow diagram of an embodiment
10 of a method 400 of diagnosing an HVAC unit based on
pressure sensor data. The method 400 may be carried out
under the direction of a computer program product. In
one embodiment, a controller of an HVAC system is
employed to carry out the method 400. The
method 400
15 begins in a step 405.
In a step 410, pressure sensor data is received from
the pressure sensor and damper position data from the
damper or ventilation controller of the HVAC system. In
one embodiment, the data corresponds to the pressure
20 difference across an outdoor economizer damper and
economizer damper position of the HVAC system. The data
is real time data obtained during operation of the HVAC
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system, either during calibration or during continuous
operation of the HVAC system.
The data is compared with standardized data within
the memory of the diagnostics controller in a step 420.
The controller then performs calculations to determine
whether the pressure sensor reading is within standard
operating parameters based on comparing the calculated
pressure reading with the standardized pressure in step
430.
In a step 440, an error signal is generated if the
pressure reading is outside the normal operating
parameters. The method 500 ends in a step 450. For
example, the error signal may be one or more of a duct
restriction, an incorrect damper position, or a pressure
sensor malfunction.
The above-described methods may be embodied in or
performed by various conventional digital data
processors, microprocessors or computing devices, wherein
these devices are programmed or store executable programs
of sequences of software instructions to perform one or
more of the steps of the methods, e.g., steps of the
method of FIG. 4. The
software instructions of such
programs may be encoded in machine-executable form on
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conventional digital data storage media that is non-
transitory, e.g., magnetic or optical disks, random-
access memory (RAM), magnetic hard disks, flash memories,
and/or read-only memory (ROM), to enable various types of
digital data processors or computing devices to perform
one, multiple or all of the steps of one or more of the
above-described methods, e.g., one or more of the steps
of the method of FIG. 4.
Additionally, an apparatus,
such as dedicated HVAC controller, may be designed to
include the necessary circuitry to perform each step of
the methods disclosed herein.
As discussed above, embodiments of the diagnostics
controller provided herein uses pressure difference
sensor data across an economizer to detect faults with
duct system, such as airflow restrictions, the
economizer, or the pressure sensor itself. One
embodiment of the diagnostics routine is based on the
principles illustrated in FIG. 5 that shows a plot of the
pressure drop across the damper as a function of damper
position at varying return duct static pressures. Each
line represents the damper pressure difference as a
function of damper position for a given return duct
static design pressure drop. The upper curve represents
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the pressure difference corresponding to a design return
static of 0.5" which is the highest one should expect to
see in the field. The bottom most curve represents the
pressure difference corresponding to a damper static
pressure drop on 0.1" which is the lowest typical return
static. Areas outside the boundaries defined by these
curves are unlikely to occur in a normally operating
system. Thus, there appearance in a diagnostics routine
could indicate a malfunction within the HVAC unit.
A high pressure and low pressure fault region is
defined by high and low limits at closed, 50% open and
full open. The red shaded regions on figure 5 indicate
fault areas defined by the high and low fault limits.
The fault limits 0.05" outside the expected pressure
values to account for potential uncertainty in the
pressure reading.
Table 1: Damper pressure 3 point fault curves
Damper Position Low Limit High Limit
0% 0.05 0.60
50% 0.05 0.25
100% 0 0.20
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The diagnostics algorithm can be used in at least
two ways, during airflow calibration and continuously.
During the airflow calibration procedure, the supply fan
is commanded to run at a torque corresponding to 400 CFM
per ton. The damper
position is moved from closed to
open. Pressure
readings are taken at the closed, 50%
open, and full open positions. They are
then compared
with the corresponding limits listed in Table 1, which is
coded in the memory of the diagnostics controller. If
the pressure is above the high limit and the damper
position is less than 50% open, then a fault 1 condition
is triggered, indicating that the damper pressure is too
high. In such
instances the alarm could instruct the
technician to check the return for a restriction, such as
a dirty filter, etc. If the pressure is above the high
limit and the damper position is 50% or greater, then a
fault 2 condition is triggered, indicating that the
damper pressure is too high. In such
instances, the
alarm could instruct the technician to check the
economizer damper. If the
pressure is below the low
limit, then a fault 3 condition is triggered, indicating
that the damper pressure is too low. In such instances
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the alarm could instruct the technician to check the
pressure sensor.
Anytime the fan is running, the function could be
called to continuously monitor the status of the
5 economizer damper. In one
embodiment, during normal
operation, the function uses a persistence criterion to
ensure that a fault persists before triggering an error.
During the supply fan calibration procedure, the
persistence criteria is disabled to provide the installer
10 immediate feedback regarding the installation. One
embodiment of calibration involves the following steps:
Calculate the High and low limit using the following
procedure to linearly interpolate high Low Limits. If
the damper position is between 0 and 50%, then:
15 LowLimit = MinDPClosed + DamperPosition x MinDP50%-MinDPClosed
HighLimit = MaxDPClosed + DamperPosition x MaxDP50%-MaxDPClosed
20 If the damper position greater than 50%, then:
LowLimit = MinDP50% + (DamperPosition ¨ 50) x MinDPOpen-MinDP50%
HighLimit =
MaxDPOpen-MaxDP50%
MaxDPClosed + (DamperPosition ¨ 50) x
25 Correct the limits for airflow as follows:
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The pressure limits listed in the table are based on
full design airflow, which is typically 400 CFM/ton. At
lower airflow rates the static pressure drops through the
return ducts and damper drops with the square of the
airflow rate. So, the
pressure limits is corrected for
the airflow rate, as follows:
(CurrentAirf lon2
LowLimitc,õ = LowLimit X __________________
NomCap x 400)
(CurrentAirf low2
HighLimitc,õ = Hig hLimit X
NomCap x 400)
Add uncertainty band to account for uncertainty in
the pressure measurement. An addition
error band is
added to the limits to prevent false alarms as follows:
LowLimitfinai = LowLimitcoõ
HighLi
-m-t f inal = HighLimitc,õ
When the fan is on, the damper pressure reading is
compared to the high and low final limit values. If the
pressure is above the high limit and the damper position
is less than 50% open, then a fault 1 condition is
triggered, indicating that the damper pressure is too
high. In such
instances, the alarm could instruct the
technician to check return restriction. If the pressure
is above the high limit and the damper position is 50% or
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greater, then a fault 2 condition is triggered,
indicating that the damper pressure is too high. In such
instances the alarm could instruct the technician to
check the economizer damper for proper operation. If the
pressure is below the low limit, then a fault 2 condition
is triggered, indicating that the damper pressure is too
low. In such
instances the alarm could instruct the
technician to check the pressure sensor.
During calibration, faults are directly reported to
the calibration function, and no persistence is required.
During normal operation, a persistence criteria is
typically met before the fault is reported. A separate
instance of the persistence function is kept for each of
the 3 faults, and a fault ratio of 5 is used for faults 1
and 3. In certain
embodiment, the persistence routine
may be active any time the fan is on and the damper
position is less than 50%. For fault 2, the persistence
routine may be active any time the fan is on and the
damper position is less than 50%.
Those skilled in the art to which this application
relates will appreciate that other and further additions,
deletions, substitutions and modifications may be made to
the described embodiments.
