U.S. patent application number 12/741276 was filed with the patent office on 2011-04-28 for variable air volume economizer minimum position reset.
This patent application is currently assigned to CARRIER CORPORATION. Invention is credited to Richard G. Lord.
Application Number | 20110097988 12/741276 |
Document ID | / |
Family ID | 40626016 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110097988 |
Kind Code |
A1 |
Lord; Richard G. |
April 28, 2011 |
VARIABLE AIR VOLUME ECONOMIZER MINIMUM POSITION RESET
Abstract
A rooftop air conditioner having a variable speed fan and an
economizer also includes a control feature for adjusting the
minimum damper position in response to the speed of the fan during
predetermined conditions in order to maintain a desired volume of
fresh air from the economizer.
Inventors: |
Lord; Richard G.;
(Murfreesboro, TN) |
Assignee: |
CARRIER CORPORATION
Farmington
CT
|
Family ID: |
40626016 |
Appl. No.: |
12/741276 |
Filed: |
November 6, 2007 |
PCT Filed: |
November 6, 2007 |
PCT NO: |
PCT/US07/23365 |
371 Date: |
May 4, 2010 |
Current U.S.
Class: |
454/256 ;
454/329; 454/333 |
Current CPC
Class: |
F24F 2011/0002 20130101;
F24F 2011/0006 20130101; F24F 11/77 20180101; F24F 13/14 20130101;
Y02B 30/70 20130101 |
Class at
Publication: |
454/256 ;
454/333; 454/329 |
International
Class: |
F24F 7/007 20060101
F24F007/007 |
Claims
1. A system for controlling air quality in a building having an air
temperature conditioning unit with a variable speed flow inducing
apparatus, and an economizer comprising: a damper for controlling
the volume of outside air flow through the economizer, said damper
having a variable minimum damper position; and a control for
varying the minimum damper position in response to the speed of
said flow inducing apparatus during predetermined operating
conditions.
2. A system as set forth in claim 1 wherein said flow inducing
apparatus is a variable speed fan.
3. A system as set forth in claim 2 wherein said fan receives its
power from an inverter.
4. A system as set forth in claim 2 wherein the characteristics of
the fan are such that, for a given damper position, the amount of
outside air delivered will be substantially proportional to the
speed of the fan.
5. A system as set forth in claim 2 wherein the characteristics of
the fan are such that the volume of the air delivered by the fan
will be substantially proportional to the speed of the fan.
6. A system as set forth in claim 1 wherein said control also
includes software which determines the relationship between the
percent of outside air and the damper position for the
economizer.
7. A method for improving air quality in a building having a
climate control system that includes in serial flow relationship,
an economizer with a minimum damper position, a variable speed flow
inducing apparatus and a supply air duct, comprising the steps of:
as the demand for air in the supply air duct is reduced, reducing
the speed of the flow inducing apparatus and, when that speed is
within a predetermined range, adjusting the minimum damper position
in response to the speed of the flow inducing apparatus.
8. A method as set forth in claim 7 and including the step of
determining the relationship between the percent of outside air and
the damper position for the economizer.
9. A method as set forth in claim 7 wherein said minimum damper
position is adjusted non-linearly with respect to the speed of the
flow inducing apparatus.
10. A method as set forth in claim 7 wherein said flow inducing
apparatus is a variable speed fan.
11. A method as set forth in claim 10 and including the step of
providing variable power to said flow inducing device.
Description
TECHNICAL FIELD
[0001] This invention relates generally to variable air volume air
conditioning systems and, more particularly, to a system for
operating the economizer damper therein to ensure sufficient
outside air is introduced in order to meet minimum ventilation code
requirements.
BACKGROUND OF THE INVENTION
[0002] In a typical climate control system of a building, a central
thermostat controls the temperature of air in the building. A
building central heating/cooling unit forces a constant amount of
heated or cooled air to various points in the building through duct
work comprising a supply air duct, a plurality of zone ducts and a
return air duct.
[0003] In variable air volume (VAV) systems the space temperature
is controlled by varying the airflow delivered to the space. This
is done by means of a variable speed fan controlled by a variable
frequency inverter, the position of inlet dampers to the fan or
other airflow control devices. For larger or by varying VAV
systems, individual zone dampers are used to control the zone cfm
and the supply fan speed is controlled to provide a constant supply
duct static pressure. On smaller VAV systems used in single zone
applications the supply cfm is directly controlled on the basis of
a space temperature sensor.
[0004] When outdoor temperatures are lower than the indoor
temperature, many units are equipped with outside and return air
dampers that are modulated to control the use of outside air to
provide free cooling to the space. These damper systems are
typically called economizers. The economizer controls the flow of
outside air through the inlet air duct and return air through the
return air duct and using the inlet air damper and return air
damper with the respective positions being controlled such that air
temperature to the space is maintained.
[0005] Because of air quality control needs in a building, the
industry has established standards (i.e. ASHRAE 62.1) to ensure
that a certain amount of fresh air (i.e. 15 cfm per person) is
brought in through the economizer. For that purpose, a minimum open
position is established such that the economizer damper is never
fully closed but can only be closed to a predetermined minimum
position. It has been recognized that with a variable indoor flow
rate as with a VAV system and other zoning systems, the fixed
minimum damper position would result in insufficient ventilation
air when the indoor air flow rate is decreased below the design
full load airflow. To solve this problem the industry sometimes
uses expensive airflow sensors that measure the actual amount of
outside air that is coming through the outside air inlet and then
uses the value to readjust the economizer damper minimum position
accordingly to ensure that the minimum ventilation air is delivered
even through the supply airflow is varying to meet the cooling
load. For example for a 20 ton unit the required full load cfm
might be 8,000 cfm. For 20% outside air the ventilating air would
be 1,600. As per the previous discussions this might be the amount
of air that is obtained with a damper position of 16%. As the
building load decreases the supply fan will be slowed to provide
the amount of air to cool the space. If the supply air flow reduces
to 4,000 cfm then at the 16% damper position the outside air would
also reduce in proportion to the supply air and the amount of
outside air would reduce to around 1,000 cfm and the proper supply
air would not be delivered. For the units equipped with the outside
air cfm sensor the unit controls would readjust the economizer
minimum damper position to a larger position where the required
2,000 cfm of air would be delivered. Depending on the
characteristics of the dampers and the building ductwork this might
be at a damper position of around 45%.
[0006] What is needed is an inexpensive and effective approach to
ensure that sufficient outside air will be brought in, particularly
during periods of time when the variable air volume fan is slowed
because of reduced building cooling requirements.
DISCLOSURE OF THE INVENTION
[0007] Briefly, in accordance with one aspect of the invention, the
minimum damper position does not remain fixed but is varied to
ensure that sufficient outside air is brought in even during
periods in which the VAV fan speed is reduced.
[0008] By another aspect of the invention, the minimum damper
position is varied as a function of the fan speed such that,
generally, as the fan speed is reduced, the minimum damper position
is adjusted to a further opened position.
[0009] By another aspect of the invention, the minimum damper
position is varied in a non-linear relationship with the variable
fan speed to compensate for the typical non-linearity of the
economizer dampers.
[0010] In the drawings as hereinafter described, a preferred
embodiment is depicted; however, various other modifications and
alternate constructions can be made thereto without departing from
the spirit and scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a schematic illustration of a rooftop air
conditioning system with the present invention incorporated
therein.
[0012] FIG. 2 is a graphic illustration of the percentage of
outside air in relationship to the damper position of the
economizer.
[0013] FIG. 3 is a graphic illustration of the economizer damper
position in relationship to the fan speed of the system.
DETAILED DESCRIPTION OF THE INVENTION
[0014] Shown in FIG. 1 is a typical packaged rooftop air
conditioner having a condenser section 11, an evaporator section 12
and an economizer section 13. The condenser section 11 includes a
compressor 14 for receiving refrigerant vapor from the evaporator
section 12 and compressing the vapor before it is condensed. Also
included in the condenser section 11 is a condenser coil 16 and a
condenser fan 17 for passing ambient air through the condenser coil
16.
[0015] The evaporator section 12 includes a supply fan 18 which is
driven by a fan motor 19. The fan motor is adapted to operate at
variable speeds to meet the cooling/heating requirements of the
system. One manner of providing the variable speed is by the use of
an inverter for providing variable frequency power to the fan motor
19.
[0016] Leading into the evaporator section 12 from the economizer
section 13 is a cooling coil 21 and its associated filters 22. A
heater 23 is placed in a downstream position from the supply fan
18. In operation, the supply fan 18 draws air in through the filter
22 and the cooling coil 21 where it is cooled by refrigerant
passing through the cooling coil 21. The cooled air then passes to
the supply air duct 24 from which it is distributed within the
building. Alternatively, in the heating mode, the air is passed
from the supply fan 18 through the heater 23 where it is heated
prior to being passed into the supply air duct 24.
[0017] Included within the economizer section 13 is an outside air
intake vent 26, an exhaust air vent 27 and its associated exhaust
fan 28, and an economizer damper 29. The economizer damper 29
includes an inlet air damper 30 and a linked return air damper 35,
which are adjustable to selectively mix an amount of outside air
coming in the outside air intake vent 26 with the portion of the
return air that is flowing into the economizer section 13 from the
return air duct 31. Another portion of the return air is caused to
pass out the exhaust air vent by the exhaust fan 28.
[0018] It will be understood that, although the economizer damper
may, to a degree, be placed in further opened positions, as the
speed of the fan motor 19 is reduced, less air, and therefore less
outside air, is drawn into the evaporator section 12. Provision is
therefore made to ensure that sufficient outside air is available
to meet the requirements of the system. This is accomplished by way
of a control 32 which senses fan speed 33 and controls the damper
position 34 in response thereto in a manner to be described
hereinafter.
[0019] The first step is to define the characteristic curve for the
damper position vs. the % outside air by a series of points.
Typically curves defining the damper position vs. % outside are not
linear. Depending on how non-linear the curve is, a number of
points are required to define the curve. A typical curve is shown
in FIG. 2. In this example, the curve is defined by four (4) points
as shown. Point A represents a damper position of 68% open,
resulting in 80% outside air. Similarly, point B, a 47% position
with 60% outside air, point C, a 39% position with 40% outside air,
and point D, a 14% position with 20% outside air. It is also known
that the dampers will provide 0% cfm at the 0% position and 100%
cfm at the 100% position. Between these points one can then use
straight line interpolation to determine the damper position
required to maintain the percentage of outside air.
[0020] With the characteristic curve now defined in the unit
control software one can then use the curve to adjust the outside
air percentage required as a function of the indoor fan speed.
[0021] We know that for a given damper position the amount of
outside air delivered will vary just about directly with the fan
speed on inverter driven fans due to the characteristics of the
fan. Therefore we can take the fan speed and use it to adjust the
percent outside air that we want to deliver to the space. For
example if the VAV controls only require the indoor fan to run at
50% speed, then we know we would have to provide 40% outsider air
to maintain the same absolute amount of outside air that would have
been deliver at 100% fan speed with 20% outside air. So using the
curve we would adjust the dampers open from the 100% fan speed
point of 14% to the 40% point damper position which is 29%. In
between the defined points, a straight line interpolation can be
used to solve for the damper position to deliver the required
percentage of outside air and cfm.
[0022] Using the characteristic curve for the damper position vs.
outside air percentage, one can then relate the indoor fan speed to
the damper setting as shown in Table I as follows:
TABLE-US-00001 Fan Supply Outside % Outside Damper Speed CFM Air
CFM Air Position 100% 10000 2000 20% 14% 75% 7500 2000 27% 20% 50%
5000 2000 40% 39% 25% 2500 2000 80% 68% 20% 2000 2000 100% 100%
[0023] One knows that the supply cfm is going to vary linearly with
the fan speed so in order to maintain constant outside air cfm, one
can use the control 32 to maintain the proper damper position 34 on
the basis of fan speed as set forth in FIG. 3 so as to maintain
constant outside air. The control 32 thus includes look-up tables
relating the damper position to percent of outside air and the
damper position to the fan speed as shown in FIGS. 2 and 3,
respectively.
[0024] While the present invention has been particularly shown and
described with reference to the preferred mode as illustrated in
the drawings, it will be understood by one skilled in the art that
various changes in detail may be effected therein without departing
from the spirit and scope of the invention as defined by the
claims.
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