U.S. patent application number 11/799068 was filed with the patent office on 2007-12-20 for air handler unit fan installation and control method.
Invention is credited to Thomas J. Mathews.
Application Number | 20070289322 11/799068 |
Document ID | / |
Family ID | 38860258 |
Filed Date | 2007-12-20 |
United States Patent
Application |
20070289322 |
Kind Code |
A1 |
Mathews; Thomas J. |
December 20, 2007 |
Air handler unit fan installation and control method
Abstract
A method is provided including installing a variable frequency
drive unit to drive a previously non-variable air handler fan of an
HVAC system. The method includes setting a control strategy for the
variable frequency drive. The control strategy includes a drive
speed for each mode of the HVAC system. The method includes
operating the HVAC system in each mode and monitoring the HVAC
system. The method includes adjusting the HVAC system or the
control strategy of the variable frequency drive to increase drive
speed based on monitoring the HVAC system.
Inventors: |
Mathews; Thomas J.;
(Fayette, ME) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
38860258 |
Appl. No.: |
11/799068 |
Filed: |
April 30, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60796347 |
Apr 28, 2006 |
|
|
|
Current U.S.
Class: |
62/225 |
Current CPC
Class: |
F24F 11/85 20180101;
F24F 11/77 20180101; Y02B 30/70 20130101 |
Class at
Publication: |
062/225 |
International
Class: |
F25B 41/04 20060101
F25B041/04 |
Claims
1 . A method comprising: installing a variable frequency drive unit
to drive a previously non-variable air handler fan of an HVAC
system, said HVAC system having a cooling unit for operating in at
least one cooling mode; setting a control strategy for said
variable frequency drive, said control strategy including a drive
speed for each cooling mode of said at least one cooling mode;
operating said HVAC system in each cooling mode of said at least
one cooling mode; monitoring a superheat of an evaporator of said
cooling unit and comparing said superheat with a predetermined
superheat threshold, for operation of said HVAC system in each
cooling mode of said at least one cooling mode; adjusting at least
one of an expansion valve of said evaporator and said control
strategy of said variable frequency drive when said monitored
superheat is less than said superheat threshold; wherein said
adjusting said expansion valve includes decreasing a flow of
refrigerant in said evaporator and said adjusting said control
strategy includes increasing said drive speed for at least one
cooling mode.
2. The method of claim 1, further comprising: adjusting additional
HVAC system components to increase air flow within said HVAC system
when said monitored superheat is less than said superheat
threshold.
3. The method of claim 1, further comprising: modifying program
interlocks for said HVAC system when said monitored superheat is
less than said superheat threshold; wherein said modifying said
program interlocks includes setting an HVAC system component to
cycle based on a current mode of said HVAC system.
4. The method of claim 1 wherein said setting said control strategy
includes setting a preset speed control strategy.
5. The method of claim 4 wherein said control strategy includes a
maximum mode and a minimum mode, and wherein said setting said
preset speed control strategy includes setting said maximum mode at
about ninety percent capacity speed and said minimum mode at about
sixty percent capacity.
6. The method of claim 5 wherein said control strategy includes an
intermediate mode, and wherein said setting said present speed
control strategy includes setting said intermediate mode at about
seventy five percent capacity.
7. The method of claim 1 wherein said setting said control strategy
includes setting a temperature based control strategy.
8. The method of claim 7 wherein said setting said temperature
based control strategy includes setting a base capacity and a
maximum capacity for said variable frequency drive and setting a
start ramp-up temperature and an end ramp-up temperature.
9. The method of claim 8 wherein said base capacity is about sixty
percent, said maximum capacity is about one hundred percent, said
start ramp-up temperature is about fifty five degrees Fahrenheit
and said end ramp-up temperature is about forty five degrees
Fahrenheit.
10. A method comprising: installing a variable frequency drive unit
to drive a previously non-variable air handler fan of an HVAC
system, said HVAC system having a heating unit for operating in at
least one heating mode; setting a control strategy for said
variable frequency drive, said control strategy including a drive
speed for each heating mode of said at least one heating mode;
operating said HVAC system in each heating mode of said at least
one heating mode; monitoring a temperature of said heating unit and
comparing said temperature with a predetermined temperature
threshold, for operation of said HVAC system in each heating mode
of said at least one heating mode; adjusting at least one of a
setpoint of said heating unit and said control strategy of said
variable frequency drive when said monitored temperature is greater
than said temperature threshold; wherein said adjusting said
setpoint includes decreasing a heating capacity of said heating
unit and said adjusting said control strategy includes increasing
said drive speed for at least one heating mode.
11. The method of claim 10, further comprising: adjusting
additional HVAC system components to increase air flow within said
HVAC system when said monitored temperature is greater than said
temperature threshold.
12. The method of claim 10, further comprising: modifying program
interlocks for said HVAC system when said monitored temperature is
greater than said temperature threshold; wherein said modifying
said program interlocks includes setting an HVAC system component
to cycle based on a current mode of said HVAC system.
13. The method of claim 10 wherein said setting said control
strategy includes setting a preset speed control strategy.
14. The method of claim 13 wherein said control strategy includes a
maximum mode and a minimum mode, and wherein said setting said
preset speed control strategy includes setting said maximum mode at
about sixty percent capacity speed and said minimum mode at about
forty percent capacity.
15. The method of claim 14 wherein said control strategy includes
an intermediate mode, and wherein said setting said present speed
control strategy includes setting said intermediate mode at about
fifty percent capacity.
16. The method of claim 10 wherein said setting said control
strategy includes setting a temperature based control strategy.
17. The method of claim 16 wherein said setting said temperature
based control strategy includes setting a base capacity and a
maximum capacity for said variable frequency drive and setting a
start ramp-up temperature and an end ramp-up temperature.
18. The method of claim 17 wherein said base capacity is about
forty percent, said maximum capacity is about one ninety percent,
said start ramp-up temperature is about eighty five degrees
Fahrenheit and said end ramp-up temperature is about one hundred
and five degrees Fahrenheit.
19. A method comprising: installing a variable frequency drive unit
to drive a previously non-variable air handler fan of an HVAC
system, said HVAC system having a cooling unit for operating in at
least one cooling mode and a heating unit for operating in at least
one heating mode; setting a control strategy for said variable
frequency drive, said control strategy including a drive speed for
each cooling mode of said at least one cooling mode and each
heating mode of said at least one heating mode; operating said HVAC
system in each cooling mode of said at least one cooling mode;
monitoring a superheat of an evaporator of said cooling unit and
comparing said superheat with a predetermined superheat threshold,
for operation of said HVAC system in each cooling mode of said at
least one cooling mode; adjusting at least one of an expansion
valve of said evaporator and said control strategy of said variable
frequency drive when said monitored superheat is less than said
superheat threshold; operating said HVAC system in each heating
mode of said at least one heating mode; monitoring a temperature of
said heating unit and comparing said temperature with a
predetermined temperature threshold, for operation of said HVAC
system in each heating mode of said at least one heating mode;
adjusting at least one of a setpoint of said heating unit and said
control strategy of said variable frequency drive when said
monitored temperature is greater than said temperature threshold,
wherein said adjusting said expansion valve includes decreasing a
flow of refrigerant in said evaporator, said adjusting said control
strategy includes increasing said drive speed for at least one
heating or cooling mode, and said adjusting said setpoint includes
decreasing a heating capacity of said heating unit.
20. The method of claim 19 wherein setting said control strategy
for said variable frequency drive includes setting at least one of
a temperature based control strategy and a preset speed control
strategy.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/796,347, filed on Apr. 28, 2006, the disclosure
of which is incorporated herein by reference.
FIELD
[0002] The present disclosure relates to an air handler unit fan
installation and control method.
BACKGROUND
[0003] The statements in this section merely provide background
information related to the present disclosure and may not
constitute prior art.
[0004] Heating, ventilating and air conditioning (HVAC) systems may
generally be equipped with an air handler unit that may control the
flow of air. Air handler units may traditionally be non-variable,
in that the unit is equipped with a single or two speed fan that
may operate at certain fixed or incremental speeds. Because such
fans may only operate at either one or two fixed speeds, the fans
may not be efficiently controlled to match HVAC system load.
Consequently, use of such fans may result in excessive electrical
energy consumption, excessive cycling of system components, and
excessive system component wear.
[0005] Variable frequency drives (VFD) may be installed in the HVAC
system air handler. Traditional retrofitting of a fixed speed air
handler unit with a VFD may be accomplished by simply installing
the VFD drive unit in line with the air handler unit power supply.
Operating the air handler fans at variable speeds may result in
increased energy efficiency, as well as decreased air flow in the
HVAC system. In a cooling mode, when the air handler fans are
operated at slower speeds, less air flow contacts evaporator coils
resulting in low evaporator superheat temperatures and a risk of a
refrigerant flood-back condition. Likewise, in a heating mode, less
air flow contacting a heating element may result in a high heating
element temperature and a risk of a heating element overheating
condition.
SUMMARY
[0006] A method is provided and includes installing a variable
frequency drive unit to drive a previously non-variable air handler
fan of an HVAC system. The HVAC system has a cooling unit for
operating in at least one cooling mode. The method further includes
setting a control strategy for the variable frequency drive. The
control strategy includes a drive speed for each cooling mode of
the at least one cooling mode. The method further includes
operating the HVAC system in each cooling mode of the at least one
cooling mode, monitoring a superheat of an evaporator of the
cooling unit and comparing the superheat with a predetermined
superheat threshold. The method further includes adjusting an
expansion valve of the evaporator the control strategy of the
variable frequency drive when the monitored superheat is less than
the superheat threshold. The adjusting the expansion valve includes
decreasing a flow of refrigerant in the evaporator and the
adjusting the control strategy includes increasing the drive speed
for at least one cooling mode.
[0007] Another method is provided and includes installing a
variable frequency drive unit to drive a previously non-variable
air handler fan of an HVAC system. The HVAC system has a heating
unit for operating in at least one heating mode. The method further
includes setting a control strategy for the variable frequency
drive. The control strategy includes a drive speed for each heating
mode of the at least one heating mode. The method includes
operating the HVAC system in each heating mode, monitoring a
temperature of the heating unit, and comparing the temperature with
a predetermined temperature threshold. The method further includes
adjusting at least one of a setpoint of the heating unit and the
control strategy of the variable frequency drive when the monitored
temperature is greater than the temperature threshold. The
adjusting the setpoint includes decreasing a heating capacity of
the heating unit and the adjusting the control strategy includes
increasing the drive speed for at least one heating mode.
[0008] Further areas of applicability will become apparent from the
description provided herein. It should be understood that the
description and specific examples are intended for purposes of
illustration only and are not intended to limit the scope of the
present disclosure.
DRAWINGS
[0009] The drawings described herein are for illustration purposes
only and are not intended to limit the scope of the present
disclosure in any way.
[0010] FIG. 1 is a graph showing percent air flow versus percent
fan speed;
[0011] FIG. 2 is a graph showing percent input power versant
percent air flow and fan speed;
[0012] FIG. 3 is a schematic of an HVAC system;
[0013] FIG. 4 is a schematic of an air handler fan and a variable
frequency drive;
[0014] FIG. 5 is a flow chart of a system check and initial
setup;
[0015] FIG. 6 is a flow chart of a system operation adjustment and
verification for cooling modes;
[0016] FIG. 7 is a flow chart of system operation adjustment and
verification for heating modes;
[0017] FIG. 8 is a chart showing variable frequency drive control
settings for a preset speed control strategy; and
[0018] FIG. 9 is a chart showing variable frequency drive control
settings for a temperature based control strategy.
DETAILED DESCRIPTION
[0019] The following description is merely exemplary in nature and
is not intended to limit the present disclosure, application, or
uses. It should be understood that throughout the drawings,
corresponding reference numerals indicate like or corresponding
parts and features.
[0020] A method for installing a variable frequency drive (VFD)
unit in an air handler unit of an HVAC system may include
installing a VFD in line with a power supply to an air handler unit
fan. In this way, a previously single or two speed fan may be
converted to operate as a variable speed fan to allow for more
efficient fan operation. By monitoring and comparing system
conditions after VFD installation, a VFD control strategy may be
implemented and adjusted to minimize a probability of a refrigerant
flood-back condition, in a cooling mode, or an overheating
condition, in a heating mode, while maximizing energy
efficiency.
[0021] As shown in FIG. 1, fan air flow varies linearly with
rotational fan speed. As shown in FIG. 2, fan input power
consumption varies with the cube of rotational fan speed. Reducing
fan speed may decrease power consumption. However, because fan air
flow does not decrease as rapidly as power consumption, reducing
fan speed may result in decreased power consumption and increased
efficiency, while continuing to meet HVAC system load.
[0022] Generally, the method may include verifying that the VFD has
been properly installed, verifying that associated HVAC equipment
and system operation are not adversely affected by VFD control of
the air handler fan speeds and air volumes, and utilizing control
strategies and set-points available in the existing control system,
or in new controls installed as part of the VFD installation, to
create the optimal balance of energy savings and reliable system
operation. In a cooling mode, to verify that the HVAC system and
associated HVAC equipment are not adversely affected by
installation of the VFD, a refrigerant temperature or pressure may
be used to calculate a superheat temperature. The superheat
temperature may be compared with a component manufacturer
recommended operating superheat temperature, or threshold, to
determine whether a refrigerant flood-back condition is probable.
In a heating mode, a heating element temperature in the area of a
high temperature limit safety switch sensor may be monitored to
determine whether an overheat condition is probable.
[0023] With reference to FIG. 3, an HVAC system 10 may include a
VFD 12 and an HVAC controller 14. VFD 12 may be connected to an air
handler unit supply fan 16 which may control the flow of air in
HVAC system 10. One or more additional fans 18 may also be used as
additional supply fans or additional return fans. The additional
fans 18 may also be configured with a corresponding VFD 20. VFD's
12, 20 may be connected to a single or three phase power supply 22.
In FIG. 3, power supply 22 is shown as a three phase power
supply.
[0024] With reference to FIG. 4, VFD 12, 20 may include a frequency
modulator 24 connected to power supply 22 and controlled by a VFD
controller 26. Frequency modulator 24 may include solid state
electronics to modulate frequency of electrical power delivered by
power supply 22. Generally, frequency modulator 24 may convert each
phase of electrical power from AC to DC. Frequency modulator 24 may
then convert each phase of electrical power from DC back to AC at a
desired frequency. For example, frequency modulator 24 may directly
rectify electrical power delivered by power supply 22 with a
full-wave rectifier bridge. Frequency modulator 24 may then chop
electrical power delivered by power supply 22 using insulated gate
bipolar transistors (IGBT's) or thyristors to achieve the desired
frequency. Other suitable electronic components may be used to
modulate the frequency of electrical power delivered by power
supply 22. VFD 12, 20 may also include a bypass module to allow
inputted electrical power to be outputted directly without
modulation by frequency modulator 24.
[0025] Rotational speed of fans 16, 18 may be controlled by a
rotational speed of electric motors that drive fans 16, 18.
Rotational speed of electric motors may be controlled by the
frequency of electrical power received from the frequency modulator
24 of VFD 12, 20. For example, if power supply 22 operates at sixty
hertz, fan speed may be varied by reducing frequency to a frequency
less than sixty hertz. At thirty hertz, fan 16, 18 may operate at
half speed or half capacity. In this way, VFD 12, 20 may vary speed
and capacity of fans 16, 18 of HVAC system 10.
[0026] VFD controller 26 may control the output frequency of the
frequency modulator 24. VFD controller 26 may monitor electrical
current, voltage, and/or power and may communicate electrical data
to HVAC controller 14. In this way, HVAC controller 14 may monitor
electrical current, voltage, and/or power of VFD.
[0027] With reference again to FIG. 3, HVAC system 10 may include a
heating unit 30 and a cooling unit 32. Heating unit 30 may include
a boiler, gas pressure burner, atmospheric gas furnace, heat pump,
or other elements to heat air flow of HVAC system 10. Cooling unit
32 may include a compressor, condenser, evaporator, or other
elements to cool air flow of HVAC system 10. For example, cooling
unit 32 may use chilled water, direct expansion (DX) or the like to
cool air flow of HVAC system 10. HVAC system may include various
dampers. For example, HVAC system may include a return air damper
34, a fresh air damper 36, an exhaust damper 38, and various
heated/cooled space dampers. The components of HVAC system 10
operate to heat, cool, dehumidify or ventilate the heated/cooled
spaces 40. While two spaces 40 are shown, a building may include
many different spaces 40 in various HVAC system zones.
[0028] Other HVAC system components may selectively be installed.
For example, HVAC system 10 may include duct mounted heating coils
42, terminal cooling coils 44, recovery wheels 46, desiccant wheels
48, and/or other HVAC devices. Variable air volume (VAV) boxes 49
and/or economizers 50 may also be installed. HVAC system components
may generally be affected by variable volume air flow resulting
from operation of VFD 12, 20 may require adjustment, as described
in more detail below.
[0029] Duct mounted heating coils 42 or terminal cooling coils 44
may provide additional heating or cooling to the heated/cooled
space 40. VAV box 49 may control air volume delivered to
heated/cooled space 40. Economizer 50 may be used to allow cooling
with outside air. Recovery wheel 46 may be used to exchange heat
between exhaust air and the fresh air makeup. For example, if
exhaust air is warmer than fresh air makeup, exhaust air may be
used to warm fresh air makeup. If exhaust air is cooler, exhaust
air may be used to cool fresh air makeup. Desiccant wheel 48 may be
used to remove moisture from return air stream.
[0030] HVAC system 10 may be operated in a number of different
modes. In a heating mode, return air may be heated with heating
unit 30. In a cooling mode, return air may be cooled with cooling
unit 32. In an economizer mode, cooling may be effectuated by use
of fresh outside air. In economizer mode, return air may be
completely exhausted to the outside, while fresh air may be taken
in. In a dehumidification mode, return air may be cooled to remove
moisture with cooling unit 32, but may also be heated with heating
unit 30. In ventilation mode, air may be circulated with a mix of
fresh outside air.
[0031] HVAC system 10 may be configured with a number of sensors.
HVAC system 10 may include a supply/discharge air temperature
sensor 52, a heating air-off temperature sensor 54, a cooling
air-off temperature sensor 56, a mixed air temperature sensor 58, a
return air temperature sensor 60, and heated/cooled space
temperature sensors 62. In addition, static pressure sensors may be
installed in various locations of the HVAC system. Signals
generated by the various sensors may be received and monitored by
HVAC controller 14 and used to control operation of HVAC
system.
[0032] Return air temperature sensor 60 may measure the temperature
of return air before any exhaust or fresh air dampers. Mixed air
temperature sensor 58 may measure the temperature of the return air
stream before it enters cooling unit 32 or heating unit 30, and
after any exhaust dampers 38 or fresh air dampers 36. Cooling
air-off temperature sensor 56 may measure the temperature of air
leaving the cooling unit 32. Heating air-off temperature sensor 54
may measure the temperature of air leaving heating unit 30.
Supply/discharge air temperature sensor 52 may measure the
temperature of air after all heating and cooling units.
[0033] The method may include a system check and initial setup
routine, as shown in FIG. 5, a system operation adjustment and
verification routine for cooling modes, as shown in FIG. 6, and a
system operation adjustment and verification routine for heating
modes, as shown in FIG. 7.
[0034] With reference to FIG. 5, the system check and initial setup
method 500 may start in step 501. In step 502, HVAC system sensors
may be checked and calibrated where needed. In step 502, the
location and accuracy of return air temperature sensor 60, mixed
air temperature sensor 58, cooling air-off temperature sensor 56,
heating air-off temperature sensor 54, and discharge/supply air
temperature sensor 52 may be confirmed where present.
[0035] In step 502, the accuracy of these sensors during full and
part speed fan operation, for all available HVAC operating modes
may be checked. When present, and to be used as part of the VFD
control strategy, the location and accuracy of any static pressure
sensors may be checked. Further, the accuracy of these sensors
during full and part speed fan operation, for all unit operating
modes available may be verified.
[0036] Other sensors may additionally be used as part of the HVAC
system control strategy and may also be checked and/or calibrated
in step 502. For example, electrical current transducers, or other
electrical sensors may be used. Further, VFD 12, 20 may include
electrical sensors that may be checked and/or calibrated in step
502.
[0037] In step 504, existing air handler operation may be checked
and current HVAC system design and equipment may be reviewed. Any
downstream heating or cooling devices, for example duct mounted
heating coils 42, additional furnaces, terminal cooling coils 44,
VAV boxes 49, economizers 50, recovery wheels 46, desiccant wheels
48, and other devices likely to be affected by VFD operation, may
be identified and checked.
[0038] Further in step 504, the source of heating and cooling,
i.e., (gas pressure burner, atmospheric gas furnace, chilled water,
direct expansion (DX), etc.) where present may be identified. If DX
cooling is used, a coil type may be determined. For example, the DX
coil type may be split or full face. Additionally, the number of
coil rows may be identified. Whether the rows are separate or
intertwined may also be determined. Configuration of the coil type
may affect VFD operation. For example, a split face coil type may
include divided coil areas, such as a low coil and a high coil. If
a split face configuration is used, during HVAC system operation
all of the return air does not contact a cooling coil as it passes
cooling unit 32. When VFD 12, 20 is operated at a lower capacity
speed, and when the split face configuration is such that the
return air load does not entirely contact the coil face, a risk of
a refrigerant flood-back condition may be more likely. The control
strategy may be adjusted, as discussed in more detail below, to
address such a situation and reduce the risk of a refrigerant
flood-back condition.
[0039] Further in step 504, operation of all existing HVAC system
equipment may be verified for each HVAC system operating mode. HVAC
system 10 may be constant run or run on demand only during building
unoccupied periods. Operating mode information may be collected
primarily as documentation of conditions prior to VFD installation,
for use as a basis of comparison to evaluate or diagnose any
operation problems. Where available, the existing building control
system data, such as elapsed run hours for each fan, each stage of
heat, each stage of cooling, and each stage of dehumidification may
be checked and recorded.
[0040] Further in step 504, steady state full speed fan motor
volts, amps, and kW power may be measured and recorded.
Additionally, supply air volume, return air volume, fresh air
makeup volume, store pressurization, and outside temperature may be
checked and recorded.
[0041] In step 506, VFD 12, 20 may be installed in the air handler
and VFD operation may be verified. Alternatively, VFD 12, 20 may be
installed prior to step 502 above and operated in a bypass mode or
full speed mode during steps 502 and 504. Steady state full speed
fan motor volts, amps and kW power may be measured and recorded.
Additionally, supply air volume, return air volume, fresh air
makeup volume, store pressurization, and outside temperature may be
measured and recorded.
[0042] Further in step 506, with VFD 12, 20 controlling the fan 16,
18 at full speed, a voltage waveform may be checked with an
oscilloscope at the electric motor connections. If voltage
excursions above a voltage limit, for example seven hundred and
fifty volts, are detected, all connections between the fan motor
and the VFD outputs may be checked to determine whether loose or
dirty connections or pitted disconnect or contactor points of
electrical contact surfaces are present. In some cases,
installation of an output filter on the VFD may be required.
[0043] In step 508, a VFD control system or strategy may be set
and/or initialized. VFD control may be based on a preset-based
speed control or a temperature based speed control, and/or
associated set points. Referring now to FIG. 8, in a preset-based
speed control system, VFD 12, 20 may be programmed, or HVAC
controller 14 may be programmed to control VFD 12, 20, to operate
fan 16, 18 at preset speeds indicated in the table. VFD control may
depend on the number of stages present. For example, one, two,
three, four, or more, stages of cooling may be present. If, for
example, three stages of cooling are present, VFD 12, 20 may be
operated at sixty percent during stage one, seventy-five percent
during stage two, and ninety percent during stage three. As a
further example, if two stages of heating are present, VFD may be
operated at forty percent in stage one and sixty percent in stage
two.
[0044] With reference to FIG. 9, VFD control may also be set
according to a temperature based speed control system corresponding
to supply air temperature. VFD 12, 20 may be programmed, or HVAC
controller 14 may be programmed to control VFD 12, 20, to operate
fan 16, 18 at the initial setting indicated in the table of FIG. 9,
using a supply air temperature-based speed control strategy and
set-points. For example, when on call for AC or dehumidification,
the drive speed may start at the cooling base speed setting, such
as sixty percent, and may ramp the speed and air flow up linearly
if and when supply air temperature declines below the `start
ramp-up` temp set-point. The ramp-up may be completed to the
maximum speed, for example one hundred percent, by the `end of
ramp-up` temp set-point.
[0045] When on call for heat, the drive speed may start at the
heating base speed setting indicated in the table of FIG. 9, for
example forty percent, and may ramp speed up linearly if and when
the supply air temperature rises above `start ramp-up` temp
set-point. The ramp-up may be completed to maximum speed, for
example ninety percent, by the `end of ramp-up` temp set-point.
When on call for economizer, the drive speed may be set at the
economizer base speed setting, for example sixty percent. When
there is no call for AC, dehumidification, heating or economizer,
the drive speed may be set at ventilation-only base speed, for
example thirty-five percent.
[0046] With reference again to FIG. 5, after VFD control is set in
step 508, the system check and initial setup routine may end in
step 708.
[0047] With reference now to FIGS. 6 and 7, VFD operation may be
checked in each cooling and heating mode. Problems with respect to
low superheat temperatures or high heating element temperatures, if
any, may be identified for trouble-shooting and system adjustment,
as necessary.
[0048] In FIG. 6, a system operation adjustment and verification
routine for cooling modes 600 is shown, and begins in step 601. in
step 602, HVAC system 10 is set to an initial cooling mode. For
example, an initial mode may be stage one of four stages. In step
604, HVAC system 10 is allowed to reach equilibrium before the
routine proceeds. In step 606, fan motor volts, amps, motor kW, air
flow, supply air temperature, fresh air makeup volume, and building
pressurization may be measured and recorded.
[0049] In step 608, evaporator coil outlet superheat may be
measured and recorded. In step 610, the measured superheat is
compared with a desired superheat to determine whether measured
superheat is too low. For example, desired superheat may be
determined based on manufacturer recommendations for a particular
evaporator used with cooling unit 32. When measured superheat is
less than desired, a refrigerant flood-back condition may be
likely.
[0050] When measured superheat is too low in step 610, HVAC system
adjustments are made in step 612 to increase superheat and reduce
the likelihood of a refrigerant flood-back condition. Four HVAC
system adjustment options are generally shown in step 612. First,
an expansion valve of an evaporator of cooling unit 32 may be
adjusted to increase superheat in the current cooling mode. An
evaporator may have an mechanical or electronic expansion valve
that may be adjustable to control a flow of refrigerant in the
evaporator. Proper adjustment of the expansion valve may increase
superheat. For example, adjusting the expansion valve to decrease
refrigerant flow may increase superheat.
[0051] Second, a VFD control strategy may be modified to increase
superheat. For example, VFD speed may be incremented in the current
mode to increase air flow through cooling unit 32, thereby
increasing the volume of air contacting evaporator coils and
increasing superheat. For example, the speed percent setting, as
shown in FIG. 8, or the base VFD percent, as shown in FIG. 9, may
be increased for the current cooling mode.
[0052] Third, operation of other HVAC components may be checked or
modified in the current mode. For example, air flow may be measured
to see if the air flow is lower than anticipated at any given
preset speed set-point. Additionally, HVAC system may be checked
for blocked filters, blocked or collapsed ductwork, fire doors
partially or fully closed, and other air flow obstruction.
Additionally, HVAC system may be checked for slipping fan belts or
damaged fan sheaves.
[0053] Fourth, program interlocks may be modified. For example, a
program interlock may be set such that operation of duct mounted
terminal cooling coils 44 may be required in the current cooling
mode. Program interlocks are established such that specified HVAC
system components are forced to cycle when the HVAC system enters a
particular mode.
[0054] After adjusting HVAC system operation in step 612, HVAC
system is allowed again to reach equilibrium in step 604. Steps
606, 608, 610, and 612 are repeated until a desirable superheat is
attained in step 610. When superheat is not too low in step 610,
the routine checks for additional cooling modes in step 614. When
additional cooling modes remain, the routine cycles to the next
cooling mode in step 616, and waits for HVAC system 10 to reach
equilibrium again in step 604. Steps 604 to 614 are repeated until
all cooling modes have been checked. In step 614, when no
additional modes are available, the routine proceeds to step 618.
In step 618, the routine determines whether all cooling modes have
been checked without any adjustment. Adjustments made in subsequent
modes may affect operation in a previous mode. Therefore, in step
618, when adjustments were made in any cooling mode in the last
round of checks, the routine returns to step 602 and starts with
the initial cooling mode again. When in step 618, all cooling modes
have been checked without adjustment, the routine ends in step
620.
[0055] In FIG. 7, a system operation adjustment and verification
routine for heating modes 700 is shown, and begins in step 701. in
step 702, HVAC system 10 is set to an initial heating mode. For
example, an initial mode may be stage one of four stages. In step
704, HVAC system 10 is allowed to reach equilibrium before the
routine proceeds. In step 706, fan motor volts, amps, motor kW, air
flow, supply air temperature, fresh air makeup volume, and building
pressurization may be measured and recorded.
[0056] In step 708, heating unit temperature is measured and
recorded in the area of any high temperature limit safety switch
sensor. Heating unit 30 may be equipped with a high temperature
limit safety switch that deactivates heating unit 30 when the
heating unit is too hot such that there is a risk of damage to
heating unit. In step 710, the measured temperature is compared
with a temperature limit, such as a manufacturer recommended
temperature limit. When measured heating element temperature is too
high, an overheat condition may be may be likely.
[0057] When measured temperature is too high in step 710, HVAC
system adjustments are made in step 712 to decrease heating element
temperature and reduce the likelihood of an overheating. Four HVAC
system adjustment options are generally shown in step 712. First, a
heating unit 30 setpoint may be adjusted such that heating unit 30
does not provide as much heat. When the heating unit temperatures
are too high, operation of the set-points for the heating unit and
high limit switches may be checked. Comparison may be made between
the set-points and the manufacturer's recommended set-points for
actual application. Comparison may be made between the set-points
and user defined set-points.
[0058] Second, a VFD control strategy may be modified to increase
airflow and decrease heating unit temperature. For example, VFD
speed may be incremented in the current mode to increase air flow
through heating unit 30, thereby increasing the volume of air
contacting heating unit 30 and decreasing temperature. For example,
the speed percent setting, as shown in FIG. 8, or the base VFD
percent, as shown in FIG. 9, may be increased for the current
heating mode.
[0059] Third, operation of other HVAC components may be checked or
modified in the current mode. For example, air flow may be measured
to see if the air flow is lower than anticipated at any given
preset speed set-point. Additionally, HVAC system may be checked
for blocked filters, blocked or collapsed ductwork, fire doors
partially or fully closed, and other air flow obstruction.
Additionally, HVAC system may be checked for slipping fan belts or
damaged fan sheaves.
[0060] Fourth, program interlocks may be modified. For example, a
program interlock may be set such that operation of duct mounted
terminal heating coils 44 may be required in the current heating
mode.
[0061] After adjusting HVAC system operation in step 712, HVAC
system is allowed again to reach equilibrium in step 704. Steps
706, 708, 710, and 712 are repeated until a desirable temperature
is attained in step 710. When temperature is not too high in step
710, the routine checks for additional heating modes in step 714.
When additional heating modes remain, the routine cycles to the
next heating mode in step 716, and waits for HVAC system 10 to
reach equilibrium again in step 704. Steps 704 to 714 are repeated
until all heating modes have been checked. In step 714, when no
additional modes are available, the routine proceeds to step 718.
In step 718, the routine determines whether all cooling modes have
been checked without any adjustment. Adjustments made in subsequent
modes may affect operation in a previous mode. Therefore, in step
718, when adjustments were made in any heating mode in the last
round of checks, the routine returns to step 702 and starts with
the initial heating mode again. When in step 718, all heating modes
have been checked without adjustment, the routine ends in step
720.
[0062] Further, HVAC system operation may be reviewed for smooth
and reliable operation. Additional adjustments to strategies or
set-points may be made as required to secure efficient system
operation. These may include adjustments to allow lower speed
set-points in one or more operating modes, operation of chillers at
evaporator suction and chilled water temperatures, and operation of
heating appliances at lower firing rates.
[0063] As shown in FIGS. 6 and 7, the process is iteratively
repeated as necessary as adjustments are made and the affects of
those adjustments are reviewed.
[0064] Adjustments continue until the lowest total energy usage in
each operating mode is obtained while maintaining a reduced risk of
refrigerant flood-back. For example, as shown in FIGS. 6 and 7,
HVAC system 10 is checked in each mode for excessive heating unit
30 temperatures and for low superheat temperatures.
[0065] Additionally, HVAC system 10 may be checked to determine
whether additional efficiencies may be gained by slowing fan
operation even further. Fan speed may be initially set high, at or
near full speed. Then, fan speed may be gradually lowered, while
allowing HVAC system to reach equilibrium. At equilibrium,
evaporator superheat for cooling modes and heating element
temperatures for heating modes may be checked and compared with
corresponding thresholds to determine whether fan speed may be
lowered further without adverse effects. In this way, efficient
HVAC system operation is achieved.
* * * * *