Draft limiting device

A draft limiting device for controlling the draft in a flue gas passage, includes a flap which is supported on a swing shaft in a secondary air inlet for swiveling movement between a closed position, at which the flap extends in a substantially vertical plane to block entry of air from a room into the secondary air inlet, and a fully open position at which the flap is displaced a certain angle about the axis of the swing shaft to admit air through the air inlet to the flue gas passage. A solid, semi-cylindrical member is fixed to the flap so that the center of radius of the member is parallel to the swing shaft axis, and the member extends on diametrically opposite sides of the swing shaft axis. The member thereby provides a restoring moment to the flap as the flap is moved in response to a differential pressure from the closed to the open position. The magnitude of the restoring moment is either constant or decreases as the flap moves in the direction from the closed to the open position. This arrangement serves to maintain the draft in the flue gas passage at a desired value.

BACKGROUND OF THE INVENTION 
The present invention relates to a draft limiting device for a flue gas 
passage of a fireplace. The device includes a flap arranged to swivel 
about the axis of a swing shaft, and the device is located in a secondary 
air inlet of the flue gas passage. In response to a differential pressure 
which acts on the flap, the flap moves against a restoring moment from a 
closed position to a fully open position to admit air from a room to the 
flue gas passage. 
Draft limiting devices are known which adjust the draft in the flue gas 
passage to an optimum value for a combustion process, taking into 
consideration that too great a draft in the flue gas passage leads to 
excessive combustion air. Since fresh air enters through a secondary air 
inlet, the draft in the flue gas passage can be reduced and thus an 
optimum amount of combustion air can be fed to the combustion process. 
Since the draft in the flue gas passage depends on operating variables, 
e.g., the ambient temperature or the chimney temperature, the fresh air 
supply through the secondary air inlet cannot be set permanently. Rather, 
some control is necessary in order to keep the draft nearly constant. 
Test devices are known which attempt to keep the draft in the flue gas 
passage nearly constant, independent of the above-mentioned variables. 
However, these draft limiting devices were insufficient because the 
desired value set for the draft, i.e., the reduced pressure which prevails 
at the fireplace, did not remain substantially constant, especially when 
the chimney draft was greater. 
It was discovered that the unsatisfactory performance of the known draft 
limiting devices is due to the fact that the restoring moment on the flap 
becomes increasingly larger as the flap approaches an open position. 
However, when the flap is in the open position, the force which acts on 
the flap and which tends to deflect the flap toward the open position is 
at a minimum. This is true for both of the force components which 
originate from the static pressure differential conditions, as well as 
from the flow conditions. Accordingly, the known draft limiting devices 
open only partially because, as they move from the closed to the open 
position, the restoring force on the flap becomes greater while the force 
urging the flap toward the open position becomes increasingly smaller. The 
draft in the flue gas passage at the fireplace therefore does not remain 
constant, but increses in an undesirable manner with increasing chimney 
draft. 
An object of the invention is to construct a draft limiting device such 
that an at least substantially constant draft and, consequently, a reduced 
pressure in the region of the fireplace can be maintained over a large 
range of variable operating parameters. 
According to the invention, restoring means on the flap of a draft limiting 
device provides a restoring moment to the flap which either remains 
substantially constant or becomes smaller over at least a portion of the 
path of movement of the flap as it moves between a closed and a fully open 
position. 
In one embodiment of a draft limiting device according to the invention, 
the restoring moment does not increase as the flap moves from the closed 
toward the open position, but remains constant or becomes smaller, so that 
the static and dynamic force components acting on the flap serve to move 
the flap further toward the open position than has been possible up to 
now. 
It has been demonstrated that a constant effective draft at the fireplace 
can be achieved when the restoring moment is substantially constant for a 
first part of the path of movement of the flap which first part includes 
the closed position, and when the restoring moment becomes smaller in a 
second part of the path of movement which second part includes the open 
position. 
The arrangement of the invention is not only advantageous because of the 
constant draft at the fireplace, but also because the restoring moment is 
at its highest value in the closed position of the flap. The high 
restoring moment permits the flap reliably to remain closed up to a 
desired value of the effective draft at the fireplace. Therefore, when the 
draft is low, i.e., below the desired value, the flap does not start to 
swing as in the known draft limiting devices. While in the known devices, 
the flap always swings open even when the chimney draft is below the value 
set for the draft at the fireplace, the device of the present invention is 
arranged to prevent the flap from swinging open under such conditions. 
Only when the value of the chimney draft corresponds substantially to the 
value set at the flap does the flap begin to swing open. 
According to a preferred embodiment of the invention, the restoring moment 
which is effective in the closed position and the restoring moment which 
is effective in the fully open position can be adjusted to a great extent 
independently of one another. This feature has the advantage that a 
pressure threshold at which the flap starts to open can be set when the 
flap is in the closed position, and that a restoring moment still exists 
in the open position which, depending on the desired control function, can 
be set to a value which is as small as possible. This makes it possible to 
compensate for tolerances such as are required when the restoring moment 
is produced by gravitational forces and, therefore, depends on the 
position of the device when installed, as well as dimensional variations 
from production of the device. 
The draft limiting device of the invention can also be constructed with 
return springs. Return springs with a substantially flat force 
characteristic are available in the form of, e.g., gas springs. Spring 
arrangements with a decreasing force characteristic can be realized with 
gear means, e.g., by the selection of the system points of the spring 
arrangement. 
When arranging the flap as a pendulum which is supported by a horizontal 
swing shaft for swiveling movement between the closed and the open 
position, a restoring torque or moment developed by gravitational force 
can be used. The concept of the invention then can be easily realized by 
providing that the center of gravity of the flap arrangement which 
includes all parts fixed to the flap, lies substantially on the same level 
as the axis of the swing shaft in the closed position. Then, when the flap 
approaches the open position, the center of gravity follows a circular 
path, and the horizontal distance or moment arm from the center of gravity 
to the swing shaft axis decreases in a co-sinusoidal relation. This 
results in a decreasing value of the restoring moment as the flap moves 
toward the open position. 
In the above embodiment, the value of the restoring moment which is 
provided in the closed position can be set in a simple manner by making 
the center of gravity of the flap arrangement adjustable in the horizontal 
direction. The restoring moment which is provided in the open position can 
be set by making the center of gravity of the flap arrangement adjustable 
in the vertical direction when the flap arrangement is viewed in the 
closed position. 
The above two settings for the center of gravity can be implemented in a 
simple manner by providing an elongated arm extending transversely of and 
horizontally from the swing shaft (when the flap arrangement is viewed in 
the closed position), and a weight on the arm which can be set at a 
desired position along the longitudinal direction of the arm. Further, the 
arm is secured to the swing shaft so that it can be adjustably rotated 
about the axis of the swing shaft and remain fixed at a desired position 
relative to the swing shaft. Although the longitudinal axis of the arm 
need not intersect the axis of the swing shaft, it is preferable that both 
axes intersect one another for reasons of structural simplicity. It is 
also preferable that the arm face toward the room, thereby facilitating 
its operation and allowing the arm weight to be easily accessible. 
The flap preferably is constructed so that during the change from the 
closed to the open position, a secondary air passage is defined between 
only one side of the flap and the secondary air inlet. This is contrary to 
the known draft limiting devices in which a secondary air passage is 
formed on both sides of the flap when the flap is opened. The disadvantage 
of the known arrangement is that an airfoil condition results from the 
flow of the secondary air, which adds to the restoring force as the flap 
opening increases. This situation does not occur with the present device 
which acts to maintain a constant draft at the fireplace. 
The formation of a single air passage on only one side of the flap is 
achieved by placing the swing shaft in the region of an inside edge of the 
secondary air inlet. While such an arrangement might appear to make it 
difficult to locate the center of gravity of the closed flap arrangement 
on the same level as the swing shaft axis, a preferred embodiment of the 
present device overcomes such a problem by providing a raised shield 
member fixed to the flap. Therefore, when the swing shaft is arranged 
parallel to an inside edge of the secondary air inlet, the entry of the 
secondary air inlet located between the swing shaft and the inside edge 
always remains closed. The shield member may be in the form of, e.g., a 
semi-cylinder having a center of radius which coincides with the axis of 
the swing shaft, and having a radius which corresponds to the distance 
between the swing shaft axis and the adjacent inside edge of the secondary 
air inlet. The semi-cylinder may be arranged so that its flat outer 
surface which is parallel to the center of radius abuts the surface of the 
flap. By suitably locating the semi-cylinder, the center of gravity of the 
entire flap arrangement can be brought to the desired location. The shield 
member can be arranged, for example, so that it extends at least partly 
over the smaller portion of the flap which is bounded by the swing shaft, 
so that the shield member acts as a counterbalance weight for the 
remaining larger portion of the flap at the desired setting of the center 
of gravity. 
In order to provide for fine adjustment of the center of gravity, it is 
preferred that the arm weight be substantially lighter than the 
counterbalance weight of the shield member. Accordingly, fine adjustment 
of the center of gravity is achieved easily by moving the arm weight over 
relatively large distances along the arm. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the drawing and descriptive matter in which there are 
illustrated and described preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a frame 10 of a draft limiting device according to the 
invention. Frame 10 includes a bottom inside edge 16 and a top inside edge 
18. The frame 10 is placed into a secondary air inlet (not shown) which 
can be provided in, e.g., a chimney side wall, or a flue gas pipe between 
a fireplace and a chimney. The side of the device which faces the flue gas 
is identified by A, and the side facing the room, i.e., the room in which 
the fireplace is located, is identified by B. 
A horizontal swing shaft 14 is arranged at the frame 10 and is spaced 
further apart from the bottom inside edge 16 of the frame 10 than from the 
top inside edge 18. The swing shaft 14 is supported for rotation in 
bearings 20 which are fixed to the frame 10 by means not shown in the 
drawing. The shaft 14 is fixed to a flap 22 at two locations 24. The 
perimeter of the flap 22 corresponds to the inner boundary of the frame 
10. 
As shown in FIG. 2, the frame 10 forms a stop for the flap 22 at the bottom 
edge 16, so that the flap 22 can be rotated only clockwise from the closed 
position as viewed in FIG. 2. 
A generally semi-cylindrical shield member 26 is arranged on the flap 22 so 
that the axis of the shield member 26 is substantially parallel to that of 
the swing shaft 14, and the shield member 26 extends on diametrically 
opposite sides of the axis of the swing shaft 14. Shield member 26 
preferably is solidly formed of metal. The mass of the semi-cylindrical 
shield member 26 causes the center of gravity S of the entire swinging 
flap arrangement including the flap 22 and all parts fixed to the flap 22 
to lie substantially at the same level as the swing shaft 14, as shown in 
FIGS. 1 and 2. Therefore, the shield member 26 serves as a counterbalance 
weight in addition to other functions, discussed below. 
A bearing bush 28 having a cross bore 30 is arranged on the swing shaft 14. 
The swing shaft 14 is held in the cross bore 30 with a friction fit, so 
that the bearing bush 28 can be forcibly rotated about the axis of the 
swing shaft 14, but will remain in a fixed position relative to the swing 
shaft once the bearing bush 28 has been set in such position. A threaded 
spindle 32 is fixed at one end to the bearing bush 28. Two setting weights 
34,36 threadably engage the spindle 32 and are arranged for movement along 
the spindle 32 in the direction of the double arrow 38. The weights 34,36 
can be effectively locked together at a desired position along the spindle 
32. 
When a reduced pressure condition exists on the flue gas side A due to a 
chimney draft, then a differential in pressure develops on both sides of 
the flap 22 between the reduced pressure on side A and the room pressure 
on side B. This differential pressure causes the flap 22 to open by 
rotating clockwise as viewed in FIG. 2. During opening movement of the 
flap 22, the center of gravity S of the flap arrangement moves along a 
circular path 40 which is identified with dash lines in FIG. 2. 
Starting with the position of the center of gravity S shown in FIG. 2, it 
can be seen that when the flap 22 is moved, the horizontal distance 
between the center of gravity S and the axis of the swing shaft 14 
increases only very slightly until the center of gravity S is exactly 
level with the axis of the swing shaft 14. From this point on, the 
horizontal distance between the center of gravity S and the axis of swing 
shaft 14 decreases. This means that the restoring moment which acts on the 
swinging flap arrangement and stems from the center of gravity S becomes 
smaller with increasing deflection of the flap 22 toward the open 
position. 
The restoring moment which urges the flap 22 against the bottom edge 16 at 
the closed position of FIG. 2 depends on the position of the setting 
weights 34,36. The further the setting weights 34,36 are moved to the left 
in FIG. 2, the further the center of gravity S is moved to the left and 
the greater the restoring moment on the flap 22 at the closed position of 
FIG. 2. In this way, a threshold pressure can be set at which the flap 22 
begins to open. 
FIG. 3 shows the flap 22 in a partially open position. The center of 
gravity S is moved upwardly and to the right relative to its position in 
FIG. 2, so that the restoring moment which tends to swing the flap 22 back 
toward the bottom edge 16 of frame 10 becomes smaller. 
The restoring moment can be changed by swiveling the bearing bush 28 about 
the axis of the swing shaft 14 either upwardly or downwardly. Accordingly, 
when the flap 22 approaches a horizontal position during continued opening 
movement, it will be subjected to a restoring moment which is as small as 
possible. By swiveling the bearing bush 28 about the axis of the swing 
shaft 14, balancing can be achieved even when the frame 10 is installed in 
a position such that the frame 10 lies in a plane which deviates from the 
vertical direction. 
In the draft limiting device according to the invention, less than the 
entire open cross-section of the frame 10 is used to admit secondary air 
to the flue gas passage, since a part of the air passage is always closed 
by the shield member 26 (compare FIGS. 1 and 3). However, the flap 22 can 
swing open over a much greater angle about the swing shaft axis than in 
the known devices, so that the present draft limiting device occupies less 
space in which it can be easily installed. Further, material costs are 
reduced. 
It may be desirable to move the swing shaft 14 further to the right as 
viewed in FIG. 2, and to reshape the outer periphery of the shield member 
26 accordingly, while leaving the present device otherwise unchanged. If 
the horizontal distance between the swing shaft 14 and the plane of the 
frame 10 is equal to the vertical distance between the swing shaft 14 and 
the top inside edge 18, then the total frame cross-section is available as 
a flow passage when the flap 22 is swung 90.degree. from the position of 
FIG. 2. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the inventive principles, it 
will be understood that the invention may be embodied otherwise without 
departing from such principles. 
As can be seen from FIG. 2 the center of gravity S is in vertical 
direction, slightly below the axis of the shaft 14 in the closed 
condition. The horizontal distance of the center of gravity S from the 
axis of the shaft 14 defines a predetermined initial closing torque 
exerted onto the flap arrangement. When the flap arrangement is rotated 
towards the position of FIG. 3, the horizontal distance of the center of 
gravity S for the axis of the shaft 14 is increased only slightly until a 
maximum horizontal distance is achieved, when the center of gravity S is 
exactly on the same height level than the axis of the shaft 14. So also 
the closing torque is only increased slightly. On further rotation in the 
clockwise sense, the horizontal distance of the center of gravity S from 
the axis of the shaft 14 is decreased again and correspondingly the 
closing torque also decreases again. It is essential for this invention 
that the increase of the closing torque from the initial value existing in 
the closed position of FIG. 2 to the maximum value existing when the 
center of gravity S is on the level of the axis of the shaft 14 is less 
than 40% of the initial value of the closing torque, preferably less than 
20% and still more preferably less than 10%. 
The center of the shield member 26 substantially coincides with the axis of 
the shaft 14.