Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A CONSTANT AIR VOLUME HVAC SYSTEM WITH A
DEHUMIDIFICATION FUNCTION AND DISCHARGE AIR TEMPERATURE
CONTROL, AN HVAC CONTROLLER THEREFOR AND A METHOD OF
OPERATION THEREOF
TECHNICAL FIELD
This application is directed, in general, to heating,
ventilating and air conditioning (HVAC) systems and, more
specifically, to a dehumidification function and a discharge
air temperature control function for HVAC systems.
BACKGROUND
HVAC systems can be used to regulate the environment
within an enclosure. Typically, an air blower is used to pull
air from the enclosure into the HVAC system through ducts and
push the air back into the enclosure through additional ducts
after conditioning the air (e.g., heating, cooling or
dehumidifying the air). Various types of HVAC systems, such
as roof top units, may be used to provide conditioned air for
enclosures. Additionally, different techniques may be
employed for controlling the capacity of the HVAC systems.
Two such techniques are variable air volume (VAV) and constant
air volume (CAV).
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In a VAV system, the temperature of the supply air
for the enclosure is substantially constant and the air
flow rate is varied to meet the thermal changes in the
enclosure. A control
function based on discharge air
temperature may be used to control operation of a VAV
HVAC system. As such, a
sensor may be employed to
determine the discharge air temperature of the HVAC
system. Employing a discharge air temperature control
allows an HVAC unit to typically deliver around 55 F
discharge supply air temperature.
In a CAV system, the air flow rate of the supply air
is substantially constant and the temperature of the
supply air is varied to meet thermal changes. In a CAV
HVAC system, humidity may be more of a problem than with
a VAV HVAC system. Thus, CAV HVAC systems may include a
dehumidification function, such as employing reheat
coils. Humiditrole from Lennox Industries, Incorporated
of Richardson, Texas, is an example of such a
dehumidification function. Humiditrol0 or other hot-gas
reheat systems may reheat conditioned air to about 70 F
or more to provide discharged air that has a lower
relative humidity, but not temperature, than the
enclosure. This can be
a result of the enclosure
requiring more latent than sensible cooling.
SUMMARY
One aspect an HVAC controller. In one
embodiment,
the HVAC controller includes: (1) an interface configured
to receive both a latent cooling demand and a sensible
cooling demand and (2) a processor configured to direct
both a dehumidification function and a discharge air
temperature control function for a constant air volume
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(CAV) HVAC system employing the latent cooling demand and
the sensible cooling demand.
In another aspect, a method of operating a CAV HVAC
unit having compressor and evaporator coils and employing
both a discharge air temperature control function and a
dehumidification function. In one embodiment, the method
includes: (1) receiving at least one of a latent cooling
demand or a sensible cooling demand, (2) determining if
both a latent cooling demand and a sensible cooling
demand are being simultaneously processed and (3)
activating the compressor and evaporator coils of the
HVAC unit having corresponding reheat coils in response
to the latent cooling demand if the latent cooling demand
has been received and the sensible cooling demand has not
been received.
In yet another aspect, a CAV HVAC system is
provided. In one embodiment, the CAV HVAC system
includes: (1) compressor and evaporator coils, (2) reheat
coils corresponding to at least one of the compressor and
evaporator coils, (3) an air blower configured to move
air across the compressor and evaporator coils and the
reheat coils and (4) a controller configured to direct
operation of the compressor and evaporator coils, the
reheat coils and the air blower, the controller having
(4A) an interface configured to receive both a latent
cooling demand and a sensible cooling demand for the CAV
HVAC system and (4B) a processor configured to direct
both a dehumidification function and a discharge air
temperature control function for the CAV HVAC system
employing the latent cooling demand and the sensible
cooling demand.
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In one aspect, there is provided a heating, ventilating
and air conditioning (HVAC) controller, comprising:
an interface configured to receive both a latent cooling
demand and a sensible cooling demand; and
a processor configured to:
simultaneously process said latent cooling demand
and said sensible cooling demand and direct both a
dehumidification function and a discharge air
temperature control function for a constant air volume
(CAV) HVAC system employing said latent cooling demand
and said sensible cooling demand; and
calculate a cooling sum by summing a number of
cooling stages of said CAV HVAC system having a
corresponding reheat coil with a number of cooling
stages required to maintain a discharge air temperature
of said CAV HVAC system.
In one aspect, there is provided a method of operating
a constant air volume (CAV) heating, ventilating and air
conditioning (HVAC) unit having compressor and evaporator
coils and employing both a discharge air temperature control
function and a dehumidification function, said method
comprising:
receiving at least one of a latent cooling demand or a
sensible cooling demand;
determining if both a latent cooling demand and a
sensible cooling demand are being simultaneously processed;
directing both a dehumidification function and a
discharge air temperature control function for said CAV HVAC
system when simultaneously processing both said latent
cooling demand and said sensible cooling demand, wherein said
directing includes calculating a cooling sum by summing a
number of cooling stages of said CAV HVAC system having a
corresponding reheat coil with a number of cooling stages
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required to maintain a discharge air temperature of said CAV
HVAC system; and
activating said compressor and evaporator coils of said
HVAC unit having corresponding reheat coils in response to
said latent cooling demand if said latent cooling demand has
been received and said sensible cooling demand has not been
received.
In one aspect, there is provided a constant air volume
(CAV) heating, ventilating and air conditioning (HVAC)
system, comprising:
compressor and evaporator coils;
reheat coils corresponding to at least one of said
compressor and evaporator coils;
an air blower configured to move air across said
compressor and evaporator coils and said reheat coils; and
a controller configured to direct operation of said
compressor and evaporator coils, said reheat coils and said
air blower, comprising:
an interface configured to receive both a latent
cooling demand and a sensible cooling demand for said
CAV HVAC system; and
a processor configured to:
simultaneously process said latent cooling
demand and said sensible cooling demand and direct
both a dehumidification function and a discharge
air temperature control function for said CAV HVAC
system employing said latent cooling demand and
said sensible cooling demand; and
calculate a cooling sum by adding a number of
cooling stages of said CAV HVAC system having a
corresponding reheat coil with a number of cooling
stages required to maintain a discharge air
temperature of said CAV HVAC system.
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BRIEF DESCRIPTION
Reference is now made to the following descriptions
taken in conjunction with the accompanying drawings, in
which:
FIG. 1 is a block diagram of an embodiment of an
HVAC system constructed according to the principles of
the disclosure;
FIG. 2 is a diagram of an embodiment of a HVAC
controller constructed according to the principles of the
disclosure; and
FIG. 3 is a flow diagram of an embodiment of a
method of operating a CAV HVAC system carried out
according to the principles of the disclosure.
DETAILED DESCRIPTION
In addition to a dehumidification function,
applications may also call for a CAV HVAC system to
employ a discharge air temperature control function.
Operating a CAV HVAC system employing both a
dehumidification function and a discharge air temperature
control function can be difficult. The disclosure
provides an embodiment of a CAV HVAC system that monitors
unit discharge temperature to optimally operate
compressors for conditioning air and reheat coils for
dehumidification. The HVAC system balances the need for
both sensible and latent cooling demands to improve the
effect of the available cooling capacity. As such, CAV
HVAC embodiments are disclosed that provide both
dehumidification control using reheat coils and discharge
air temperature control using a discharge air temperature
sensor. An HVAC
controller is also disclosed that
manages the operation of both the dehumidification
control and the discharge air temperature control to
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allow these two functions to operate together. The
dehumidification control with reheat coils may be
Humiditrol by Honeywell.
FIG. 1 is a block diagram of an embodiment of an
HVAC system 100 constructed according to the principles
of the disclosure. The HVAC system 100 includes a return
duct 102, a return plenum 104, a supply duct 106 and a
supply plenum 108.
Additionally, the HVAC system 100
includes a compressor and evaporator coils 110, an air
blower 120, reheat coils 130, a discharge air temperature
sensor 140 and a HVAC controller 150. The compressor and
evaporator coils 110 include a first, second, third and
fourth unit that is denoted by the numbers 1-4 in FIG. 1.
Each compressor and evaporator coil combination may
represent a cooling stage of the HVAC system 100. The
reheat coils 130 include a first unit and a second unit
that are denoted by the numbers 1 and 2 in FIG. 1. The
first and second units of the reheat coils 130 correspond
to the first and second units of the compressor and
evaporator coils 110.
One skilled in the art will understand that the HVAC
system 100 may include additional components and devices
that are not presently illustrated or discussed but are
typically included in an HVAC system, such as, a power
supply, exhaustion fans, etc. A thermostat
(not shown)
is also typically employed with the HVAC system 100 and
used as a user interface.
The HVAC system 100 is a constant air volume (CAV)
unit wherein the supply air flow rate is constant or
substantially constant and the supply air temperature is
varied to meet cooling demands. The various illustrated
components of the HVAC system 100 may be contained within
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a single enclosure (e.g., a cabinet). In one embodiment,
the HVAC system 100 is a rooftop unit.
The compressor and evaporator coils 110, the air
blower 120, the reheat coils 130 and the discharge air
temperature sensor 140 may be conventional devices that
are typically employed in HVAC systems. The HVAC
controller 150 causes the air blower 120 to move air
across the evaporator coils, the reheat coils 130, the
discharge air temperature sensor 140 and into a
conditioned space. At least some of the operation of the
HVAC system 100 can be controlled by the HVAC controller
150 based on inputs from various sensors of the HVAC
system 100 including the discharge air temperature sensor
140.
The HVAC controller 150 may include a processor,
such as a microprocessor, configured to direct the
operation of the HVAC system 100. Additionally, the HVAC
controller 150 may include a memory section including
instructions that direct the operation of the processor.
The memory section may be a conventional memory. The
memory section may include a series of operating
instructions that direct the operation of the HVAC
controller 150 (e.g., the processor) when initiated
thereby. The series
of operating instructions may
represent algorithms that are used to coordinate the
operation of a dehumidification function and a discharge
air temperature control function at the same time when
processing both a sensible cooling demand and a latent
cooling demand.
The HVAC controller 150 receives and responds to
cooling demands for the HVAC system 100. As illustrated,
the cooling demands may be a latent cooling demand or a
sensible cooling demand. A latent cooling demand is used
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for dehumidification. A latent cooling demand may be the
result of high humidity in the conditioned space or even
in outside air being used by the HVAC system 100. In
response to a latent cooling demand, the HVAC controller
150 may respond by activating all of the compressor and
evaporator coils 110 that have corresponding reheat coils
installed therewith. Thus, in the
HVAC system 100, the
HVAC controller 150 may energize the first and second
units of the compressor and evaporator coils 110 and the
first and second units of the reheat coils 130.
Accordingly, the air that has been cooled by moving
across the first and second units of the compressor and
evaporator coils 110 is reheated by moving across the
first and second units of the reheat coils 130. The
cooled air is reheated to an approximately neutral
temperature by the reheat coils and returned to the
conditioned space with reduced humidity but at
approximately the same dry-bulb temperature as when the
air left the conditioned space.
A sensible cooling demand may be the result of a
high dry-bulb temperature in the conditioned space or
even a high dry-bulb temperature of outside air being
used by the HVAC system 100. In response,
the HVAC
controller 150 responds by activating the compressor
evaporator coils 110 in sequence (e.g., first through
fourth) until the discharge temperature value determined
by the discharge air temperature sensor 140 reaches a
setpoint value. When the
discharge temperature value
falls below the setpoint value, the compressor and
evaporator coils are deactivated in sequence (e.g.,
fourth through first) to maintain the discharge
temperature value within a permitted range of the
setpoint value.
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In addition to having both a dehumidification
function and a discharge air temperature control
function, the HVAC system 100 may also process both of
these functions at the same time. In the HVAC
system
100, the HVAC controller 150 is configured to control the
operation of the HVAC system 100 when latent and sensible
cooling demands are processed at the same time
When both latent and sensible cooling demands are
present at the same time, the HVAC controller 150
calculates a cooling sum by adding all of the cooling
stages installed with a reheat coil to the number of
cooling stages required for sensible cooling to maintain
the discharge temperature value that corresponds to the
sensible cooling demand. If the value of the cooling sum
is less than or equal to the number of compressors
installed in the HVAC system 100, then the value of the
cooling sum is the number of compressors that are
activated. Additionally, all of the reheat coils are
activated.
For example, for the HVAC system 100, if a sensible
cooling demand requires one cooling stage, the HVAC
controller 150 would add one and two (the number of
cooling stages with corresponding reheat coils) to obtain
a cooling sum of three. Since three
is less than four
which is the number of compressors that the HVAC system
100 includes, then three of the compressor and evaporator
coils 110 are activated and all of the reheat coils 130
are activated.
If the value of the cooling sum is greater than the
number of compressors installed in the HVAC system, then
all of the installed compressors are activated.
Additionally, the reheat coils 130 are turned-off until
the number of compressors running without reheat is equal
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to the number required for the sensible cooling demand.
In some embodiments, the reheat coils 130 may be kept
off.
For example, if a sensible cooling demand requires
three cooling stages, the HVAC controller 150 would add
three and two (the number of cooling stages with
corresponding reheat coils) to obtain a cooling sum of
five. Since five
is greater than four which is the
number of compressors that the HVAC system 100 includes,
then all four of the installed compressor and evaporator
coils 110 are activated. The reheat
coils 130 remain
deactivated until the number of compressors running
without reheat is equal to three.
As illustrated in FIG. 1, the HVAC controller 150 is
coupled to the various components of the HVAC system 100.
In some embodiments, the connections therebetween are
through a wired-connection. A
conventional cable and
contacts may be used to couple the HVAC controller 150 to
the various components of the HVAC system 100. In other
embodiments, a wireless connection may also be employed
to provide at least some of the connections.
FIG. 2 is a block diagram of an embodiment of an
HVAC controller 200 constructed according to the
principles of the disclosure. The HVAC controller 200 is
configured to control operations of an HVAC system. The
HVAC controller 200 includes a processor 210 and a memory
section 220. Additionally, the HVAC controller 200
includes additional components such as a signal interface
having an input port 230 and an output port 240. The
HVAC controller 200 may also include other components
typically included within a controller for a HVAC system,
such as a power supply or power port. The processor 210
is configured to direct both a dehumidification function
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and a discharge air temperature control function for a
CAV HVAC system employing a latent cooling demand and a
sensible cooling demand. Latent and
sensible cooling
demands may be received at the input port 230 of the
signal interface. Control
instructions may then be
transmitted via the output port 240 of the signal
interface.
The memory 220 may be a conventional memory. The
memory 220 may include a series of operating instructions
that direct the operation of the processor 210 when
initiated thereby. The series of operating instructions
may represent algorithms that are used to control the
dehumidification function and the discharge air
temperature control function. The
algorithm may be
represented by the flow diagram illustrated in FIG. 3.
FIG. 3 is a flow diagram of an embodiment of a
method 300 of operating a CAV HVAC unit carried out
according to the principles of the disclosure. The CAV
HVAC unit includes compressor and evaporator coils and
employs both a discharge air temperature control function
and a dehumidification function. An HVAC
controller
such as described with respect to FIG. 1 or FIG. 2 may be
used to perform the method 300. The method
300 may
represent an algorithm that is stored on a computer
readable medium, such as a memory of an HVAC controller
(e.g., the memory 220 of FIG. 2) as a series of operating
instructions. The method 300 begins in a step 305.
In a step 310 at least one type of cooling demand is
received. The cooling demand or demands are received at
a controller for the HVAC unit via an interface. The
interface may be an input port. The cooling demand may
be a latent cooling demand or a sensible cooling demand.
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Both cooling demands may be received at approximately the
same time.
A determination is then made in a first decisional
step 320 if both a latent cooling demand and a sensible
cooling demand are being simultaneously processed. A
latent cooling demand and a sensible cooling demand may
be simultaneously processed if received by the controller
within a designated period of time. The
designated
period of time may be due to the operating
characteristics of the HVAC controller or associated
circuitry. In some embodiments, the designated period of
time may be selected by a manufacturer of the HVAC
controller or HVAC system. The designated period of time
may be programmed into the HVAC controller.
If both of the cooling demands are not being
simultaneously processed, the method 300 continues to a
second decisional step 322 where a determination is made
if received cooling demand is a latent cooling demand.
If a latent cooling demand has been received, the method
300 continues to a step 324 where the compressor and
evaporator coils of the HVAC unit having corresponding
reheat coils are activated. The method
300 continues
until a step 360 and ends.
Returning now to the second decisional step 322, if
the received cooling demand is not a latent cooling
demand, then the method 300 continues to a step 326. If
the received cooling demand is not a latent cooling
demand, then the received cooling demand is a sensible
cooling demand. As such, at step 326, the compressor and
evaporator coils of the HVAC system are sequentially
activated based on a discharged air temperature of the
HVAC system. The method
300 continues until the step
360.
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Returning to first decisional step 320, if both a
latent cooling demand and a sensible cooling demand are
being simultaneously processed, a cooling sum is
calculated in a step 330. The cooling
sum may be
determined by adding a number of cooling stages of the
HVAC system having a corresponding reheat coil with a
number of cooling stages required to maintain a discharge
air temperature of the HVAC system.
In a third decisional step 340, a determination is
made if the cooling sum is less than or equal to a number
of compressors installed in the HVAC system. If the
cooling sum is less than or equal to the number of
compressors installed in the HVAC system, each reheat
coil of the HVAC system is activated and the number of
compressors of the HVAC system equivalent to a value of
the cooling sum are activated in a step 342. The method
300 continues until the step 360.
Returning now to the third decisional step 330, each
reheat coil of the HVAC system is deactivated and each of
the compressors are activated when determining the
cooling sum is greater than the number of compressors in
a step 340. The
deactivation of the reheat coils is
maintained until a number of compressors activated
without reheat is equal to a number of compressors
required for the sensible cooling demand. The method 300
ends at the step 360.
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. 3. The software
instructions of such
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programs may be encoded in machine-executable form on
conventional digital data storage media, 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. 3.
Additionally, an apparatus, such as dedicated HVAC
controller, may be designed to include the necessary
circuitry to perform each step of the methods of FIG. 3.
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.
