Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
Passive Constant Pressure Hatch for Fresh Air Direct Fired Gas
Heated Ventilation Systems
BACKGROUND
[0001] The present disclosure relates to direct gas-fired heated
ventilation systems for industrial and commercial buildings, and particularly
to
systems which use variable amounts of fresh air.
[0002] More specifically, the present disclosure relates to a constant
pressure self adjusting burner bypass for such direct gas-fired heated
ventilation
systems.
[0003] Direct gas-fired heated ventilating systems provide heated
make-up air to buildings by drawing fresh air into them then exhausting it
into a
building. Such make-up air is necessitated by the loss of heated inside air
through various exhaust fans and chimney flues. The quantity of heated
replacement air is usually substantially equal to the exhausted heated air. To
prevent infiltration of cold outside air, it is also common practice to
provide
slightly more make-up air than is lost so that a slight positive pressure is
maintained inside the building. The' operation of the heated ventilation
system is
controlled to maintain a selected inside air temperature and a desired
building
inside/outside air pressure differential. This type of heated ventilation
system is
commonly called a make-up air unit. Generally, it is not permitted to use a
direct
gas-fired heated ventilating systems to heat a building because the combustion
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gases are introduced into the building. It can only be used to heat
replacement
air.
[0004] There are two basic types of direct gas-fired make-up air units
(DGF make-up air unit). A first basic type includes a fresh air intake which
communicates with a chamber containing a direct gas-fired burner. The burner
heats the air as it is drawn or blown over the flame by a blower fan. The
heated
fresh air and the combustion gases are exhausted into the building. In some of
such systems, the amount of air being exhaust out of the building may vary
requiring that the make-up air also vary. In some systems, the rotational
speed of
the blower fan is controlled to vary the amount of heated make-up air
generated,
and in other systems the blower fan is turned on and off as needed..
[0005] An inherent characteristic of known DGF make-up air units is
that the airflow velocity at the burner must remain constant to maintain
optimum
combustion chemistry. To allow variable quantities of heated air to be
introduced
into the building, the quantity of air heated by the burner is made constant
and a
variable quantity of replacement air is then shunted passed the burner
unheated
and then mixed with the fixed quantity of heated air such that the resulting
mixture will be at the desired temperature. Example systems of this type are
disclosed in United States Patent nos. 3,591,150, issued on July 6, 1971 to
Weatherston and titled "Gas Furnace" and 4,325,352 issued on April 20, 1982 to
Dirkes and titled "Internal Recirculation Device".
[0006] The second basic type of system recirculates inside air and
mixes it with the heated fresh air before delivering it to the inside space.
These
systems include a recirculating air intake which draws air from inside the
building
and mixes it with the heated fresh air. With this system type, the airflow
through
the replacement air blower fan remains constant. Dampers control the relative
amounts of each air stream. Examples of this type of direct gas-fired air make-
up
units are disclosed in the following references:
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[0007] - United States Patent No. 3,417,977, issued on December 24,
1968 to Nelson and titled "Air Control System for Heating Unit";
[0008] - United States Patent No. 4,429,679, issued on
February 7, 1984 to Dirkes and titled "Modular Air Heater";
[0009] - United States Patent No. 4,573,912, issued on March 4, 1986
to Albritton et al. and titled "Space Heater";
[0010] - United States Patent No. 4,674,475, issued on June 23, 1987
to Powis and titled "Gas Fired Furnace"; and
[0011] - United States Patent No. 4,917,074, issued on April 17, 1990
to Brekke and titled "Direct gas-Fired Heating and Ventilation Method and
Apparatus".
[0012] As mentioned above, the air stream over the direct gas-fired
burner is maintained at a constant velocity for optimum combustion. This
implies
that the pressure drop across the burner be maintained at a substantially
constant level. In the above-cited patents this is achieved by controlling
dampers
that supply air to the burner or dampers that shunt air around the burner. In
all
cases, the dampers are either controlled by complex interlocking mechanisms,
linkages with springs or counterweights or are separately controlled by
electronic
controllers. Such control methods are disclosed in the following documents:
[0013] - United States Patent No. 4,829,447, issued on May 9, 1989 to
Parker et I. and titled "Bypass Controller and Bypass System";
[0014] - United States Patent No. 5,257,958, issued on November 2,
1993 to Jagers and titled "Pressure Override Control for Air Treatment Unit";
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[0015] - United States Patent No. 5,597,354, issued on
January 28, 1997 to Janu et at. and titled "Indoor Air Quality Control for
Constant
Volume Heating, Ventilating and Air Conditioning Units";
[0016] - A known manufacturer of make-up air units, CaptiveAire of
Raleigh, North Carolina, USA, employs vertical spring biased blades on both
side
of the burner.
SUMMARY
[0017] Embodiments of a passive constant pressure hatch (PCP
hatch) for regulating the pressure drop over a direct gas-fired burner are
provided
in a DGF make-up air unit that includes three chambers:
[0018] a discharge chamber containing a blower fan;
[0019] a burner chamber placed within the discharge chamber,
wherein a common opening is made between the discharge chamber and the
burner chamber to allow air to pass from the burner chamber to the discharge
chamber, and wherein a direct gas-fired burner is mounted in this opening to
heat the air passing therethrough; and
[0020] an intake chamber is mounted within the discharge chamber
upstream and adjacent to the burner chamber such that, in operation, air
introduced into the intake chamber through an outside air intake opening may
flow from the intake chamber to the burner chamber through a common opening.
[0021] Heater systems using direct gas-fired burner are often used in
air make up units. To maintain a constant airflow at the burner and thus
maintain
good combustion chemistry, the pressure drop over a direct gas-fired burner
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must remain substantially constant. A system for air replacement/recirculation
as
described above can be used to vary the total quantity of air through the DGF
air
make-up unit but the inclusion of a direct gas-fired burner requires
additional
components.
[0022] A bypass opening is cut in the top of the intake chamber such
that air can be shunted as required from the intake chamber through this
opening
directly into the discharge chamber. An embodiment of a PCP hatch cover is
then mounted over the bypass opening to regulate the shunted portion of the
intake air.
[0023] A passive constant pressure hatch according to illustrative
embodiments described herein improves on the constant pressure floating door
taught in my United States Patent No. 5,365,975 issued on November 22, 1994
and titled "Constant Air Pressure Unit for Air Handling System" (hereinafter
referred to as 'my '975 patent).
[0024] As will be explained in more detail below, as the floating door
from my '975 patent begins to rise off the bypass opening at the desired pre-
set
pressure, air begins to escape at its edges. The air velocity close to its
edge
creates an aerodynamic lift on the perimeter surface of the floating door.
This
aerodynamic lift tends to draw the floating door back towards the bypass
opening
and reduces to varying degrees the flow of air. The greater the operating
pressure, the greater the reduction in airflow. The net effect of this
phenomenon
is that the apparent weight of the floating door is increasing as the escaping
air
flow increases and the aerodynamic lift extends its effect away from the edge
of
the floating door and over a greater surface of the floating door. At low
operating
pressures, such as below 0.15" water gauge (w.g.), which was the design
criteria
for the floating door of my '975 patent, this phenomenon is negligible. The
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operating pressure required for make-up air units is much higher, above 0.45"
w.g.. The aerodynamic lift at theses pressures negates the desired constant
pressure characteristic of the floating door of my '975 patent.
[0025] An object of illustrative embodiments is to provide a PCP hatch
for DGF make-up air units that minimize the variation of pressure required to
lift
the PCP hatch cover.
[0026] The problem of the variation of the pressure required to lift a
PCP hatch cover is solved by distancing the lifting surface of the hatch cover
from the zone of high velocity airflow.
[0027] In accordance with an illustrative embodiment, there is
provided a direct gas-fired fresh air heating system comprising:
[0028] an air intake chamber;
[0029] a discharge chamber;
[0030] a burner chamber in fluid communication with both the air
intake and discharge chambers therebetween;
[0031] a direct-gas fired (DGF) heater mounted in the burner chamber
to heat air flowing therethrough from the air intake chamber to the discharge
chamber; and
[0032] a passive constant pressure (PCP) hatch including:
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[0033] a bypass
opening between the air intake and
discharge chambers to allow air to be shunted passed the burner chamber; and
[0034] a PCP hatch
cover having a top panel covering the bypass
opening; the PCP hatch cover being movable between a first position, wherein
the PCP hatch cover closes the bypass opening and a second position wherein
the PCP hatch cover is lifted away from the bypass opening so as to
substantially
maintain a fixed pressure differential across the burner chamber; the PCP
hatch
cover being configured to lift further away from the bypass opening when there
is
an attempt to increase the pressure differential between the air intake and
discharge chambers;
[0035] the improvement comprising:
[0036] the PCP
hatch cover further including a peripheral wall
extending from the top panel to distance the top panel from the bypass opening
when the PCP hatch cover is in the first position;
[0037] whereby
negative effects of aerodynamic lift on the top panel
and an inherent reduction of static pressure associated thereto, are
substantially
reduced throughout.
[0038] According
to another illustrative embodiment, there is provided
a passive control hatch cover for selectively closing a bypass opening between
an air intake chamber and a discharge chamber, the passive control hatch cover
comprising a lifting surface that is distanced from the bypass opening when
the
passive control hatch closes the bypass opening.
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[0039] According
to still another embodiment, there is provided a
constant air pressure unit for an air handling system, said unit comprising a
partition therein having an air passage opening to allow pressurized air to
fluidly
circulate therethrough, a door having a panel displaceably retained over the
air
passage opening such that, in a closed position of the door, substantially no
air
can pass through the air passage opening; the door panel having loading means
whereby the door is caused to move away from the air passage opening when a
pressure differential across the partition attains a desired level; the door
panel
further allowing the controlled passage of air through the air passage opening
while maintaining a substantially constant air pressure differential across
the
partition, the improvement wherein the door further includes a peripheral wall
which comes in contact with the partition when the door fully covers the air
passage opening.
[0040] A PCP hatch
according to embodiments of the present
invention is so designed that the air arriving under it lifts it at a pre-set
static
pressure differential making it float on the air flow. Any attempt to increase
the
pre-set static pressure differential further raises the PCP hatch cover,
letting
more air escape. The pre-set static pressure is substantially proportional to
its
weight divided by the area of the horizontal projection of its top panel.
[0041] Generally,
the burner opening is sized to allow twenty (20)
percent of the total flow rate to pass over the burner at a specified pressure
drop
(usually between 0.4" and 0.6" w.g.). This flow rate is the minimum airflow
capacity of the DGF make-up air unit. The remaining airflow capacity may vary
between 0 and 80% of the maximum flow rate as it is shunted passed the burner
through the bypass opening.
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[0042] Constructed as taught in my '975 patent , the operation of a
constant pressure floating door works well at relatively low pressure
differentials
(below 0.15" w.g.). The passive constant pressure hatch according to
embodiments of the present invention successfully operates at pressures well
above 0.15" w.g.
[0043] A passive constant pressure hatch according to an illustrative
embodiment maintains a constant air velocity across the direct gas-fired
burner
by opening and closing as is necessary to shunt air in excess of that required
to
provide the optimal burner performance. Such a passive constant pressure hatch
thereby "regulates" the pressure drop and, thus, the air velocity across the
burner.
[0044] Such a regulation of the pressure drop across the burner is
achieved passively, i.e. that no control system or interconnecting linkages
are
required to operate the constant pressure hatch. The biasing force supplied by
the weight of the floating PCP hatch cover is sufficient to maintain the
pressure
level required by the direct gas-fired burner.
[0045] A quick response time to changes in flow conditions is
achieved , compared with conventional pressure regulation means that use
control actuators with dampers, resulting in a slow response to changing flow
conditions. Firstly, commercially available control actuators have fixed and
relatively slow operating speeds. Depending on the rapidity with which the
flow
conditions change, the damper may be continually attempting to attain the
desired pressure differential by playing catch-up. Secondly, commercially
available controls signal their damper actuators to open the damper or close
the
damper but also have a small "no signal" range or dead band where the damper
actuator is allowed to stop before changing directions. Thus, the actual
pressure
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=
differential will remain within plus or minus five percent (5%) of the set
pressure
differential. These hardware limitations create a lag between the required
pressure differential and the actual pressure differential generated by the
damper. With a PCP hatch according to an embodiment of the present invention,
this inherent time lag of the control actuator is overcome since none is
required.
[0046] A PCP hatch according to an embodiment of the present
invention yields a reliable and inexpensive pressure drop regulator across the
burner. No dedicated control systems are required and no complex mechanical
linkages, springs, pivot pins or rotating blades are needed to operate the PCP
hatch. A PCP hatch according to an embodiment of the present invention
operates autonomously to perform the regulation function with only one moving
part: the PCP hatch itself, as it floats on a cushion of air in a
substantially
frictionless fashion.
[0047] Other objects, advantages, and features will become more
apparent upon reading of the following non-restrictive description of
illustrative
embodiments thereof, given by way of example only with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] In the appended drawings:
[0049] Figure 1a and lb are cross-sections of a direct gas-fired (DGF)
make-up air unit including a passive constant pressure (PCP) hatch according
to
a first illustrated embodiment; the hatch cover being illustrated respectively
in
close and open positions;
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[0050] Figure 2 is
a bottom perspective view of the PCP hatch cover
shown in Figures la and lb;
[0051] Figures 3a
and 3b are close up side elevations of the PCP
hatch cover in the position shown respectively in Figures la and 1 b, showing
the
pivot means of the PCP hatch cover;
[0052] Figure 4a,
which is labeled "prior art" is a schematic cross-
section of an opened floating door from my '975 patent, showing velocity lines
and pressure gradients on the door;
[0053] Figure 4b
is a schematic cross-section of a PCP hatch
according to the first illustrative embodiment of the present invention,
showing
velocity lines and pressure gradients thereon;
[0054] Figure 5 is
a graph showing the pressure response of the hatch
according to the first illustrative embodiment;
[0055] Figure 6 is
a cross-section of a DGF make-up air unit ,
including a PCP hatch according to a second illustrative embodiment; the PCP
hatch being illustrated in an open position;
[0056] Figure 7 is
a cross-section of a DGF make-up air unit according
to a third illustrative embodiment, including the PCP hatch illustrated in
Figure 6
with a recirculation damper;
[0057] Figures 8a
and 8b are perspective views of a PCP hatch
according to a fourth illustrative embodiment; the pressure hatch being shown
in
respectively open and closed configurations;
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[0058] Figure 9a, which is labeled "Prior Art', and Figure 9b are
schematic views with corresponding pressure graphs of an heating, ventilating
and air conditioning (HVAC) system with a return fan according to the ASHREA
(American Society of Heating, Refrigeration and Air-Conditioning Engineers)
Guideline 16-2003, respectively with and without a hatch cover from Figure 2;
[0059] Figure 10a, which is labeled "Prior Art', and Figure 10b are
schematic views with corresponding pressure graphs of an HVAC system with a
return fan according to the ASHREA Guideline 16-2003, respectively with and
without a hatch cover from Figure 2; and
[0060] Figure 11a, which is labeled "Prior Art', and Figure 11b are
schematic views with corresponding pressure graphs of an HVAC system with a
gravity or motorized relief damper according to the ASHREA Guideline 16-2003,
respectively with and without a hatch cover from Figure 2.
DETAILED DESCRIPTION
[0061] In the following description, similar features in the drawings
have been given similar reference numerals, and in order not to weigh down the
figures, some elements are not referred to in some figures if they were
already
identified in a previous figure.
[0062] The use of the word "a" or "an" when used in conjunction with
the term "comprising" in the claims and/or the specification may mean "one",
but
it is also consistent with the meaning of "one or more", "at least one", and
"one or
more than one". Similarly, the word "another" may mean at least a second or
more.
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[0063] As used in this specification and claim(s), the words
"comprising" (and any form of comprising, such as "comprise" and "comprises"),
"having" (and any form of having, such as "have" and "has"), "including" (and
any
form of including, such as "include" and "includes") or "containing" (and any
form
of containing, such as "contain" and "contains"), are inclusive or open-ended
and
do not exclude additional, unrecited elements.
[0064] With reference first to Figures 1a and lb, a heated ventilation
system 10 according to a first illustrative embodiment will be described. The
system 10 is shown mounted on a roof curb 12 of a building 13.
[0065] The system 10 comprises a centrifugal blower fan 20 that
draws air from an enclosed discharge chamber 22 and delivers it to the
building
13 through the air duct 14. The blower fan 20 is driven by a motor 21 and
operated in a well known manner by a control system (not shown) to provide the
required amount of air to the building 13.
[0066] Fresh outside air (see arrows 15) is drawn in by the blower
fan 20 through an intake opening 30 into an intake chamber 31 that is within
the
discharge chamber 22 (see arrows 17). The intake chamber 31 is defined by side
and top dividing walls 40 and 41 within the discharge chamber 22. A bank of
air
filters 32 are mounted in the intake opening 30 of the intake chamber 31 to
filter
the fresh air as it enters intake chamber 31.
[0067] The system 10 further comprises a burner chamber 38
between the inlet and discharge chambers 31 and 22 so as to be in fluid
communication with both chambers 22 and 31. According to the first
illustrative
embodiment, the burner chamber 38 is defined by a wall 37 within the intake
chamber 31 and by the dividing walls 40 and 41.
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[0068] The burner chamber 38 includes an air inlet opening 36
provided in the wall 37 to allow passage for the fresh air 17 from the intake
chamber 31 to enter the burner chamber 38. An inlet damper 34 is mounted in
the air inlet opening 36 to shut off the flow of fresh air when the heating
and
ventilation system 10 is not in use. A control system (not shown) is further
provided to operate both the inlet damper 34 and the blower fan 20.
[0069] The dividing wall 40 between the burner chamber 38 and
discharge chamber 22 has a burner opening 28 through which the heated fresh
air (see arrows 39) is drawn into the discharge chamber 22 from the burner
chamber 38. An air heater in the form of a direct gas-fired burner 42 is
mounted
in the burner opening 28. During the heating season, the burner 42 heats that
portion of the fresh air drawn through openings 36 and 28, into the discharge
chamber 22.
[0070] Also, it is to be noted that the relative dimensions of the
intake
and discharge chambers 31 and 22, of the burner chamber 38 and intake
opening 30 are not limitative and the overall configuration of the system 10
is
provided for illustrative purposes only. The chambers 22, 31 and 38, and the
opening 30 are adapted for example to the dimension of the building 13 and to
the air conditioning requirements.
[0071] A hatch opening 29 is provided in the dividing wall 41 between
the air intake and discharge chambers 22 and 31 such that fresh air may be
shunted directly from the intake chamber 31 into the discharge chamber 22
without passing through the burner chamber 38 and around burner 42. A hatch
cover 44 is mounted over the hatch opening 29 to control the airflow between
intake chamber 31 and discharge chamber 22. As will be described in more
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detail below, the hatch cover 44 is so designed to maintain a substantially
constant air pressure differential across burner opening 28.
[0072] With reference to Figure 2, the hatch cover 44 includes a top
panel 44a, a pair of spaced sidewalls 44b, an upstream wall 44c and a
downstream wall 44d. Returning to Figure 1a, the hatch cover 44 pivotally
rests
on the dividing wall 41 such that walls 44a, 44c, 44d and 44b fully cover the
hatch opening 29 and stop the passage of air between the intake chamber 37
and the discharge chamber 22 when
[0073] a) the heated ventilation system 10 is not in operation or
[0074] b) the vacuum generated by the blower fan 20 as it draws in
fresh air is not sufficient to lift the hatch cover 44.
[0075] Referring to Figure 3a, the hatch cover 44 is pivotally mounted
to the horizontal dividing wall 41 at the upstream wall 44c by two pivot tabs
45
(only one shown) and two pivot clips 46 (only one shown). As shown in Figure
2,
the pivot tabs 45 can be formed as lateral extensions of the upstream wall
44c.
According to another embodiment (not shown), the tabs 45 are fastened to the
upstream wall 44c.
[0076] Referring back to Figure 3a and 3b, the two pivot clips 46 are
fixedly attached to the dividing wall 41. A "V' shaped notch 47, having sides
47a
and 47b, is cut in each pivot clip 46 to loosely receive the pivot tabs 45.
Sides
47a and 47b meet at a truncated apex 47c having a width substantially equal to
the thickness of the pivot tabs 45. Side 47a is substantially parallel to
upstream
wall 44c. Side 47b is angled with side 47a such that tab 45 can pivot about
truncated apex 47c allowing hatch cover 44 to freely pivot to its maximum open
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position. The angle between sides 47a and 47b defines the maximum pivoting
angle of the hatch cover 44 with regards to the wall 41.
[0077] In the operation of the PCP hatch, whenever the pressure
differential between intake and discharge chambers 31 and 22 is greater than
the
weight of the hatch cover 44 divided by the area of the horizontal projection
of
the top panel 44a, the hatch cover 44 begins to float on a cushion of air and
is
therefore no longer in contact with dividing wall 41. As the hatch cover 44
begins
to rise, pivot tabs 45 rest against pivot clips 46 at truncated apexes 47c and
the
hatch cover 44 begins to pivot upward. The pivoting angle of the hatch cover
44
increases with any attempt to increase the pressure differential up to when
the
pivot tabs 45 abut the sides 47b of notches 47.
[0078] As can be seen in Figure 3b, the height of notch 47 at apex 47c
is sufficiently greater than the height of the pivot tab 45 to allow free
pivoting of
pivot tabs 45 when it comes in contact with truncated apex 47c. The two pivot
clips 46 also prevent lateral movement of the hatch cover 44.
[0079] The system 10 and hatch cover 44 are not limited to the
illustrated pivotal attachment of the hatch cover 44 to the dividing wall 41.
A
heated ventilation system according to another embodiment includes a hatch
cover that is pivotally secured to the dividing wall using another friction-
free
pivoting mechanism that the one illustrated in Figures 3a and 3b.
[0080] As mentioned above and referring to Figure 1 b, an inherent
characteristic of known DGF make-up air units is that the airflow velocity at
burner 42 must remain substantially constant to maintain optimum combustion
chemistry. This implies that to maintain a constant air velocity through
burner
opening 28, the air pressure differential across it must also remain constant.
As
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will now be explained, this is achieved by the hatch cover 44 without any
actuators, springs, or additional linkage.
[0081] The hatch cover 44 regulates the air pressure differential
across the burner opening 28 and thus the air velocity therein. This is
achieved
by shunting fresh air from the intake chamber 31 directly into the discharge
chamber 22 in sufficient quantity so as to neutralize any change in the
pressure
differential between the burner chamber 38 and discharge chamber 22.
[0082] In order to better explain the operation of the hatch cover 44,
the limitation found in a single panel floating door 60 from my '975 patent
will first
be briefly described.
[0083] Considering Who, the weight of the floating door 60, and Ahc,
the area of its horizontal projection, the pressure required to lift hatch
cover 60 is
equal to
PlIft = VVhc = constant
Ahc
[0084] With the application of pressure Pim under floating door 60, it
begins to float on a cushion of air; its displacement being frictionless, only
an
extremely low force is required to move it. Since the floating door 60 is
pivotably
mounted over the opening 29, it pivots as the airflow increases consequently
to
any attempts to increase the pressure Rift under it. However, as the floating
door
60 pivots upward, the horizontal distance between its center of gravity and
the
pivot axis decreases: this causes the floating door 60 to seem slightly
lighter as
the angle of rotation Afift increases, reducing the pressure Pim required to
hold it
open. Such a pressure reduction is equal to
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[0085] API,ft = Put X (1- cos(Aiift)), where Aft is the lift angle in
degrees.
[0086] It has been found that, for pivoting angles of eight (8)
degrees,
the reduction of the lifting pressure Put is one (1) percent; for eleven (11)
degrees, the reduction is two (2) percent; and for fourteen (14) degrees, the
reduction is three (3) percent. A reduction of two (2) percent can be
considered
negligible for a typical direct fired gas burner. However, for any greater
pivoting
angle, the horizontal translation of the center of gravity with regards to the
pivot
axis should be considered.
[0087] It has been found that AP Hft is reduced for a given /Nut by
having
the pivoting of the floating door 60 begin at ¨1/2Aut and by rotating it by
Aft to +1/2
Aift. Thus, for a Alift of eleven (11) degrees, theAP lift is reduced from two
(2)
percent to 0.5 percent, allowing Aim to be increased to 22 degrees without
causing theAP lift to exceed the threshold of negligibility that has been set
hereinabove to two (2) percent.
[0088] As mentioned hereinabove, the rotation of the floating door 60
may have a detrimental side effect on it constant pressure characteristic.. In
practice, other phenomena come into play, which limit the use of a single top
panel floating door 60.
[0089] Firstly and with reference to Figure 4a, the geometry of the
hatch cover greatly determines the manner in which it operates at pressure
differentials above 0.15" w.g. The addition of the side walls 44b, 44c and 44d
and
their relative size influence its effective operation. When used to control
relatively
high pressures (at or above 0.45" w.g. such as in typical of commercially
available direct fired gas burners), the velocity of the air escaping around
the
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floating door 60 is relatively high (above 2500 feet per minute) and generates
an
associated "velocity pressure" or dynamic pressure as per the formula:
[0090] Pd = (Vair / 4005 )2 , where
[0091] Pd = dynamic pressure (inches w.g.) and
[0092] Vair = air velocity (feet per minute).
[0093] With no substantial pressure losses due to turbulence just
upstream from the throat of the opening 29, the total pressure in the
airstream at
this location remains substantially constant. In accordance with Bernoulli's
law of
airflow:
[0094] Pt Pd + Ps = constant,
[0095] where Pt = total pressure, Pd = dynamic pressure and Ps =
static pressure.
[0096] Based on this relationship, as the air accelerates approaching
the opening to a maximum velocity at the throat of the opening, the static
pressure decreases as the velocity of the airflow increases. The floating door
60
is reactive to this decrease in static pressure which tends to pull the
floating door
60 towards the high velocity air flow (called also aerodynamic lift as with an
airplane wing). Since the surface of the underside of the floating door 60
over
which the drop in static pressure occurs increases with the quantity of air
being
shunted, so will the static pressure required to lift the floating door 60
further
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open. This is unacceptable since the required constant pressure for optimal
burner combustion conditions is not met.
[0097] To overcome this adverse effect, side walls 44b, 44c and 44d
are added to distance the lifting surface of the top panel 44a away from the
zone
of high velocity airflow at the perimeter of the hatch cover 44.
Experimentally, an
acceptable distance can been found such that the reduction in static pressure
at
high flow rates close to the top panel 44a becomes negligible (approximately
five
(5) inches (12,7 cm) for air velocities of three thousands (3000) feet per
minutes
(15.2 m/sec )).
[0098] Also, as can be seen in Figure 1a, the side walls 44b, 44c and
44d can be made of unequal heights, for example when the dividing wall 41 is
slanted. Upstream wall 44c being the pivot edge, it is reduced in height since
substantially no air is shunted along its edge. Downstream wall 44d is made
larger, positioning the top panel 44a further from the high velocity airflow
at the
edge of the downstream wall 44d as taught above.
[0099] A second phenomenon is the response of the hatch cover 44 to
back pressure caused by less than ideal flow conditions downstream, i.e.,
changes in flow direction or obstacles in flow stream. As back pressure
increases
with an increase in airflow, it adds to the hatch cover opening pressure
making
the pressure at the burner opening 28 increase. This effect is detrimental to
the
desire pressure equilibrium.
[00100] Again, the proper selection of the height of the side walls can
be used to correct this increase. As the height of the side walls is
increased, the
center of gravity rises further above the edge pivot axis. Thus, when the
hatch
begins to rotate upward, the horizontal distance between the center of gravity
CA 02818376 2013-06-17
21
and the pivot axis decreases. This makes the hatch cover seem lighter and
lighter as it rotates open. Less and less pressure is requires to lift it. The
phenomena of reduced static pressure is now used to counteract the increase in
back pressure. Thus the side wall heights are selected so that both the back
pressure and static pressure variations cancel each other. This allows the
pressure response of the hatch cover 44 to be tuned to a specific design of
the
heated ventilation system 10.
[00101] It is to be noted that the heights of the side walls of the
hatch
cover 44 are obtained through a calibration process when the hatch is
installed,
along with adding calibration weight as required onto the hatch cover. This
may
be necessary since sheet material used to fabricate the hatch cover is
available
in standard thicknesses and weights per square foot. Thus, the available
material
may not give the desired final weight and extra calibration weight may be
required..
[00102] A graphical representation of the "pressure versus airflow"
characteristic through the hatch opening 29 is shown in Figure 5 and will be
used
to illustrate the operation of the hatch cover 44.
[00103] As the pressure differential across the burner opening 28
begins to increase, substantially zero leakage occurs between the side walls
44b,
44c and 44d and the dividing wall 41 as the hatch cover 44 fully covers the
hatch
opening 29 (shown as a vertical line along the Y axis at zero airflow). It is
to be
noted that no sealing means is required between the hatch cover 44 and the
dividing wall 41 if dividing wall 41 is made flat and matting edges of side
walls
44b, 44c and 44d are made straight.
CA 02818376 2013-06-17
22
[00104] While the pressure differential AP s remains below the threshold
pressure level at point 100, the hatch cover 44 remains closed. As the
pressure
level attempts to increase above the pressure level at point 100, the hatch
cover
44 begins to rise and shunt air. The airflow curve then becomes a horizontal
line
shown as segment 102. Any attempt to increase the pressure differential above
point 100 results into an airflow increase (Cubic Foot per Minute (CFM)) at
constant pressure.
[00105] When the hatch cover 44 reaches the designed maximum
airflow (see intersection 104), the airflow may still be increased (see
segment
106) with no impact on the pressure differential, until the hatch cover 44 is
physically prevented from further opening, i.e. when a mechanical means, such
as the side 47h of the groove 47, is used to restrain the pivoting of the
hatch
cover 44.
[00106] There is no hysteresis when the airflow decreases, since the
operation of the hatch cover 44 is substantially frictionless (in fact the
friction is
100% but no slippage is happening thus no frictional loses). Thus, the
"return" or
decreasing airflow curve is identical to the increasing pressure curve 102.
[00107] Thus, without any mechanical linkage, springs, electronic
controls or measurements, the hatch cover 44 responds instantaneously to
changes in the rotational speed of the blower fan 20 to regulate the air flow
through the burner 42 at the optimal level.
[00108] The hatch cover 44 also responds to airflow variations caused
by external factors. Such variations may be due to:
CA 02818376 2013-06-17
23
[00109] - changes within the heated ventilation system 10, including the
filters 32 becoming dirty or an accumulation of dirt on the fan wheel (not
shown)
of the blower fan 20;
[00110] - changes in flow conditions within the building 13 such as
adding or removing air diffuser, or building pressurization;
[00111] - changes in atmospheric conditions such as wind gusts, frost
build-up or temperature changes.
[00112] Regardless of the cause, the hatch cover 44 automatically
responds to maintain a constant pressure drop across the burner opening 28 by
shunting the required amount of air around the burner 42 and thereby regulate
the airflow therethrough at the optimal amount.
[00113] The hatch cover is not limited to the first illustrative
embodiment, nor to the configuration of the illustrated heated ventilation
system
10.
[00114] As discussed hereinabove, the configuration of the hatch cover
44, including the weight and dimension of the side walls 44a, 44b, 44c and
44d,
and the initial and final angular position thereof is adapted to the operating
parameters of the heated ventilation system 10 and to the geometry of the
bypass opening 29.
[00115] It should be apparent to those skilled in the art that many
variations are possible from the first illustrative embodiment. Further
characteristics and features of the PCP hatch will become apparent upon
reading
the following descriptions of further illustrative embodiments thereof.
CA 02818376 2013-06-17
24
[00116] Figure 6 shows an existing make-up air unit 10 retrofitted with
a
PCP hatch according to a second illustrative embodiment. Since the second
embodiment is similar to the first embodiment, only the differences
therebetween
will be described herein for conciseness.
[00117] The configuration of a retrofit system 52 (shown in dashed
lines) is particularly well adapted to convert conventional air make-up units
from
fixed flow units to variable flow units. According to this embodiment, the
horizontal roof 54 of the existing make-up air unit 10 becomes the dividing
wall
41 of the preferred embodiment. The side wails 50b, 50c and 50d of the hatch
cover 50 are equal in height and extend downwardly from a substantially
horizontal top panel 50a. Two openings are provided in the horizontal roof 54:
opening 56 over the intake chamber 31 to receive the hatch cover 50 and a
larger second opening 58 above the discharge chamber 22. A plenum chamber
60 is formed by the interior space within retrofit system 52 such that it
becomes
an extension of the discharge chamber 22 for shunting air directed from the
hatch opening 56 towards the blower fan 20, and bypassing the burner chamber
38. Existing adjustable baffles 62 (also known as "profile plates") are then
repositioned to yield the minimum allowable burner opening 28 around the
burner
42. When the make-up air unit 10 is not in use, the damper 34 is closed and
the
hatch cover 50 closes opening 56, stopping air from escaping or entering the
building.
[00118] With reference to Figure 7, a third illustrative embodiment of
an
air make-up unit 64, incorporating the hatch cover 50, will now be described.
Since the third embodiment is similar to the second embodiment, only the
differences therebetween will be described herein for conciseness.
CA 02818376 2013-06-17
[00119] Downstream from the burner chamber 38 and before the
supply fan 20, a damper opening 66 is cut within the floor wall 68 of the air
make-
up unit 64 and an adjustable recirculating damper 70 is provided therein to
allow
building air (see arrow 72) to be drawn into the discharge chamber 22 by
blower
fan 20.
[00120] The control of this damper 70 allows a variable amount of
building air 72 to be admitted into the air make-up unit 64 so as to maintain
the
desired building inside / outside air pressure differential. Since adding air
into the
fan discharge chamber would tend to drop the pressure therein, the hatch cover
50 reacts by reducing by an equivalent amount the quantity of outside air
coming
through the hatch opening 56. At the blower fan 20, the flow of air is
constant
since the static pressure just upstream from the blower fan 20, as set by the
hatch cover 50, remains unchanged.
[00121] The fresh outside air 15 can thus be varied from minimum flow
set by the burner opening 28 to one hundred percent (100%) of the airflow
range
of the make-up air unit 64. This is achieved by adjusting the recirculating
damper
70. With the required pressure drop at the hatch cover 50 being set by the
burner
combustion conditions, the maximum pressure drop through the recirculating
damper 70 and its associated ductwork is adjusted to be equal to the required
pressure drop at the burner 42. In this way, when the recirculating damper 70
is
at its maximum opening, the air make-up unit is in eighty percent (80%)
recirculation mode: the hatch cover 50 is substantially closed since the
pressure
downstream from the recirculating damper 70 has reached the pressure required
to start lifting it off the opening . This combo damper system operates in
unison
without any mechanical interconnection and with a single control actuator (not
shown) driving the recirculating damper 70. Since the pressure upstream from
the blower fan is constant, the blower fan will deliver a constant airflow
rate
CA 02818376 2013-06-17
26
regardless of where the air is coming from: the recirculated air, heated
outside
air, cold outside air, or any mixture thereof.
[00122] With reference now to Figures 8a and 8b, a passive pressure
hatch cover 80 according to a fourth illustrative embodiment will now be
described. Since the hatch cover 80 is installed in a heated ventilation
system
that is similar to the system 10, only the differences between these two make-
up
units will be described herein for conciseness.
[00123] According to this fourth illustrative embodiment, the fixed
burner chamber 38 is removed and replaced by a pair of opposing modified
hatch covers: a top hatch cover 82 above burner 42 and a bottom hatch cover 84
below thereof. The top and bottom hatch covers 82 and 84 defines the hatch
cover assembly 80 and a variable burner chamber 86 therein.
[00124] The top hatch cover 82 includes a top panel 82a, a pair of
spaced sidewalls 82b, a downstream wall 82d and pivot pins 88. The bottom
hatch cover 84 includes a top panel 84a, a pair of spaced sidewalls 84b, a
downstream wall 84d and pivot pins 88. No upstream walls are required. The
pivot pins 88 are pivotally mounted in brackets 89 which are fixedly mounted
to
the back wall 37. Back wall 37 has the same functions as in previous
embodiments in that it allows the fresh outside air to enter the burner
chamber 86
through an inlet opening 36 in which an inlet damper 34 is mounted.
[00125] The closed position of the hatch cover assembly 80
corresponds to the minimum air flow. In this position, the adjacent side walls
82b
and 84b rest against each other and against the back wall 37 as shown in
Figure
8a.
CA 02818376 2013-06-17
27
[00126] The hatch cover assembly 80 is made movable between the
close position of Figure 8a and the open position illustrated in Figure 8b.
The two
adjacent side walls 82b and 84b are linked by U-shaped link rods 90 such that
the hatch covers 82 and 84 pivot in unison and in opposite directions away
from
burner 42 and from the back wall 37. One end of each link rod 90 is attached
to
the side wall 82b near the corner thereof that is close to the back wall 37
and
adjacent to the side wall 84h; the opposite end of the link rod 90 is attached
to
the side wall 84b near the corner thereof that is closest to both the side
wall 84d
and the side wall 82b. Both side walls 82d and 84d have complementary U
shaped openings 92a and 92b that together yields an adjustable burner opening
94.
[00127] According to this embodiment, the weight of the top hatch
cover 82 is counterbalanced by the weight of the bottom hatch cover 84.
Calibration weights (not shown) are added onto the top cover hatch 82 to
obtain
the biasing pressure for optimum combustion conditions at the burner 42 as
discussed hereinabove. The weights are attached to the side walls 82b and are
positioned to fine tune the response of the hatch covers 82 and 84 as they
pivot
about their pivot pins 88. The positioning of the weights allows corrections:
[00128] = for the varying area of the horizontal projection of both
hatch
covers 82 and 84 which increases as they open;
[00129] = while the center of gravity moves horizontally away from the
pivot points at pivot pins 88.
[00130] With the increase in projected area, the pressure required to
move the hatch covers 82 and 84 into their open position decreases.
Oppositely,
with the center of gravity moving horizontally away from the pivot points at
pivot
CA 02818376 2013-06-17
28
pins 88, the pressure required to move the hatch cover 82 and 84 into their
open
position increases. It can be made to increase further by positioning
calibration
weight on side walls 82b near the back wall 37 distal from the pivot pins 88.
[00131] No additional hatch, controls, actuators or dampers are
required in other than the hatch defined by the cover assembly 80.
[00132] The linkage between the top and bottom hatch covers 82 and
84 is not limited to the link rod 90 and other mechanism can be provided to
force
the movement thereof in unison. For example, fork-shaped elements can be
mounted on top or bottom covers 82 and 84 and guide rods be fixedly mounted
to the other cover such that the rods can slide within the fork-shaped
elements.
To reduce the friction, a frictionless bearing can be included on the guide
rod to
substantially eliminate the friction as the guide rods move up and down within
the
fork-shaped elements, making the hatch covers 82 and 84 operate in unison and
in opposite directions. According to a second embodiment (not shown) the link
rods 90 are replaced by a multi-strand cable wire. The attachment points are
as
described with reference to the previous embodiment, but the multi-strand
cable
wire being flexible, no pivot points would be required as with the ends of the
link
rods 90.
[00133] It is believed to be within the reach of a person skilled in
the art
to use the present teaching to adapt the heated ventilation systems 10, 52 or
64
to receive the hatch cover 80.1t is to be understood that embodiments of the
PCP
hatch cover and of air make-up systems incorporating such PCP hatch covers
are not limited in their application to the details of construction and parts
illustrated in the accompanying drawings and described hereinabove. A PCP
hatch cover and air make-up systems incorporating such hatch cover are
capable of other embodiments and of being practiced in various ways.
CA 02818376 2013-06-17
29
[00134] Further applications of the PCP hatch will now be described.
[00135] An illustrative embodiment of a PCP hatch as described
hereinabove is incorporated into "Air-Side Economizer Systems" as shown in
Figures 9a to 11 b. Air side economizer systems are designed to take advantage
of outside condition to afford free cooling when cooling is required. This is
usually
done by a modulating damper or set of dampers used to control the outdoor
airflow drawn into the heating, ventilating and air conditioning (HVAC) system
in
excess of minimum ventilation outdoor air to allow free cooling. To do so,
varying
quantities of cool outside fresh air is mixed with warm inside air such that
the
mixture has the appropriate temperature to meet the air-conditioning needs of
a
building and thus reduce or eliminate the use of the air-conditioning system
compressor. In some cases, only outside air may be used.
[00136] As defined by ASHREA (American Society of Heating,
Refrigeration and Air-Conditioning Engineers) Guideline 16-2003 several
arrangements are suggested to achieve the free cooling needs. The most
common variations are:
[00137] - arrangement 1 (Figure 9a ¨ Prior art): HVAC system with a
return fan;
[00138] - arrangement 2 (Figure 10a ¨ Prior art): HVAC system with a
relief fan;
[00139] - arrangement 3 (Figure lla ¨ Prior art): HVAC system with a
gravity or motorized relief damper.
[00140] Refer to "ASHREA Guideline 16-2003" for more details.
CA 02818376 2013-06-17
[00141] By the incorporation of PCP hatches, the HVAC systems are
greatly simplified requiring in most cases only one control damper, fewer
controls
components such as actuators, multi-blade dampers, pressure sensors or
differential pressure controllers. The operation of PCP hatches has much
better
pressure and flow control as compared to the use of motorized dampers
especially when two (2) motorized dampers are used as a constant flow mixing
box. In such a configuration, one damper opens while the second damper closes
and at the same time, maintain a constant total quantity of air through the
mixing
box as pressures upstream of both dampers vary.
[00142] Referring to Figure 9a ¨ Prior art, the known arrangement for
the "HVAC system with return air fan (with minimum outside air control)" is
shown. Referring now to Figure 9b, the proposed arrangement of the "HVAC
system with return air fan (with minimum outside air control)" using PCP
hatches
is shown. As can be seen, the assembly using the PCP hatches is simpler,
requiring:
1. two (2) fewer actuators (at D-1 & D-3),
2. one (1) less fan (injection fan),
3. one (1) less pressure controller (P-3),
4. one (1) less airflow sensor (at injection fan) and
5. one (1) less speed control (at injection fan).
[00143] In Figures 9a to 11 b, the graphs show the pressure variations
in the ducts.
CA 02818376 2013-06-17
31
[00144] With the PCP hatch in air plenum PL-1, pressure in the plenum
is now constant. With the PCP hatch in air plenum PL-2, pressure in this
plenum
is also constant. This entails that the pressure drop across damper D-2 is
fixed.
The pressure in plenum PL-1 is selected to be equal the minimum pressure
required to lift the PCP hatch D-1. . The pressure in plenum PL-2 is designed
to
be equal to the pressure in plenum PL-1 minus the pressure drop through the
return air damper when full open at maximum design airflow. The pressure drop
through the minimum outside air (OA) damper is fixed (pressure in PL-2) so
once
adjusted, a modulating actuator is no longer required. A two-position actuator
(open/closed) is used to allow closure of the minimum outside air damper when
the HVAC system in not in operation. Both hatches are calibrated as taught
above to account for back-pressure associated with the ductwork: downstream
from the D-1 relief PCP hatch and upstream from the D-3 OA PCP hatch
economizer.
[00145] Referring now to Figure 10a ¨ Prior art, the known arrangement
for the "HVAC system with relief fan (with minimum outside air control)" is
shown.
Referring now to Figure 10b, the proposed arrangement of the "HVAC system
with relief fan (with minimum outside air control)" using PCP hatches is
shown.
As can be seen, the assembly using the PCP hatches is simpler, requiring:
1. two (2) fewer actuators (at D-1 & D-3),
2. one (1) less pressure controller (P-3) and,
3. one (1) less airflow sensor (at D-4).
[00146] With the PCP hatch in air plenum PL-1, pressure in the plenum
is now constant. With the PCP hatch in air plenum PL-2, pressure in this
plenum
CA 02818376 2013-06-17
32
is also constant. This entails that the pressure drop across damper D-2 is
fixed.
The negative pressure in plenum PL-1 is selected to be equal the minimum
pressure required to lift the PCP hatch D-1. The pressure in plenum PL-2 is
designed to be equal to the pressure in plenum PL-1 minus the pressure drop
through the return air damper when full open at maximum design airflow. The
pressure drop through the minimum outside air (OA) damper is fixed (pressure
in
PL-2) so once adjusted, a modulating actuator is no longer required. A two-
position actuator (open/closed) is used to allow closure of the minimum
outside
air damper when the HVAC system in not in operation. Both hatches are
calibrated as taught above to account for back-pressure associated with the
ductwork: upstream from the D-1 relief PCP hatch and upstream from the D-3
OA PCP hatch economizer.
[00147] Referring now to Figure 11 a ¨ Prior art, the known arrangement
for the "HVAC system with a gravity or motorized relief damper fan (with
minimum outside air control)" is shown. Referring now to Figure 11b, the
proposed arrangement of the "HVAC system with a gravity or motorized relief
damper fan (with minimum outside air control)" using PCP hatches is shown.
This configuration yields a pressurized building space as might be required
for
clean rooms or operating rooms where introduction of unfiltered air is not
allowed. The response time of the system when using a PCP hatch is quicker
with instant response to changes in building pressure. As can be seen, the
assembly using the PCP hatches is simpler, requiring:
1. one (1) fewer actuator (at 0-1) and
2. one (1) less airflow sensor (at D-3).
CA 02818376 2013-06-17
33
[00148] With the PCP hatch in air plenum PL-1, pressure in the plenum
is now constant. With the use of a counterweight of the PCP hatch D-1, the PCP
hatch can be adjusted to a very low opening pressure regardless of the total
weight of the hatch cover. In air plenum PL-2, pressure is also constant. This
entails that the pressure drop across damper D-2 is fixed. The positive
building
pressure is sensed in plenum PL-1 and is equal to the minimum pressure
required to lift the PCP hatch. The pressure in plenum PL-2 is designed to be
equal to the pressure in plenum PL-1 minus the pressure drop through the
return
air damper when full open at maximum design airflow. The pressure drop
through the minimum outside air (OA) damper is fixed (pressure in PL-2) so
once
adjusted, a modulating actuator is no longer required. A two-position actuator
(open/closed) is used to allow closure of the minimum outside air damper when
the HVAC system in not in operation. Both hatches are calibrated as taught
above to account for back-pressure associated with the ductwork: upstream from
the D-1 relief PCP hatch and upstream from the D-3 OA PCP hatch economizer
[00149] It is also to be understood that the phraseology or terminology
used herein is for the purpose of description and not limitation.
