US2242667A - Automatic air control for forced draft firing - Google Patents

Description

May 20, 1941. N. CUNNINGHAM AUTOMATIC AIR CONTROL FOR FORCED DRAFT FIRING Filed May 22, 1940 DIREC r/ou 0F 41/? To W/A/DBOX\] Znve1utor:

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Patented May 20, 1941 2,242,667 AUTOMATIC AIR CONTROL FOB, FORCED DRAFT FIRING Noel Cunningham, Aurora, 111., assignor to Durabilt Steel Locker 00., Aurora, 111.

Application May 22, 1940, Serial No. 336,549

8 Claims.

My invention is directed to apparatus for automatically controlling the air supply in forced draft firing of solid fuels. More specifically, it is directed to apparatus that will facilitate the proper proportioning to fuel bed requirements of the amount of air delivered by the blower or fan used in such forced draft firing. Also, it assures a minimum of useless combustion during the periods when the fan or blower producing the forced draft is not in operation.

In the firing of solid fuels it is impossible to maintain at all times uniform fuel bed depths and textures and, as the depths or textures of fuel beds vary, so does the air permeability of such fuel beds. Where forced draft is used, it is customary to maintain uniform the rate of rotation of fans used in producing the forced draft and, at any particular rate of rotation, volume of air varies in inverse relation to the static back pressure against which such fans are required to When air permeability of any fuel bed increases, the static back pressure against the fan decreases and, unless fan output is controlled by manual or automatic means, the volume of air from the fan increases and the fuel bed is consumed at an excessive rate. Conversely, when a fuel bed becomes less air permeable, static pressure increases and the fan delivery, if uncontrolled, decreases thereby causing rate of combustion within the fuel bed to be reduced. Either of these conditions is undesirable.

It is not practicable to control closely the proper rates of air delivery from fans used in forced draft firing by manual regulation and a variety of automatic means is utilized for the purpose, consisting of devices which respond, either directly or indirectly, to changing static back pressures occuring within the windbox below the fuel bed against which pressure fans used for producing forced draft operate. In one type of such controls, impulses from varying static pressures within the duct leading from the fan to the windbox below the fuel bed provide the means for regulation. These pressure variations act upon pressure responsive mechanisms which in turn operate dampers control.

The regulating member of an automatic air control is customarily a butterfly damper, that is, one which opposes to the flow of air equal surfaces on either side of an axis about which it may rotate. Such butterfly dampers may be either single blades effective over entire duct cross-sections or multiple blades, each centrally balanced and inter-connected to act in unison. TOP D- to effect the desired erly position such dampers there must be provided actuating means responsive to changes in static back pressures which occur in the windbox below the fuel bed.

In forced draft firing, the periods during which the fan is inoperative are herein referred to as off periods while the periods during which the fan is supplying forced draft are referred to as on periods. During on periods in any forced draft system, the true static back pressure of a fuel bed which is receiving adequate air for combustion is the resultant of fan pressure and fuel bed air permeability. When an on period occurs after an off period there should be an immediate build-up of static pressure below' the fire and this necessitates a flow of air. from the fan to the windbox. Without such flow 'of air, static back pressure will not develop and the actuating member will not receive sufficient impulse to cause proper response and there will not occur a correct setting to fuel bed requirements of the regulating member of an automatic control.

The proper function of an automatic'air control is two-fold; it should assure minimum fuel bed combustion during off periods and also correctly proportion air delivery during on periods. the regulating member of an automatic air control was effective in minimizing air flow to the fuel bed during "off periods it failed to allow the passage of sufficient air to develop against the actuating medium the true static backpressure represented bythe'depth and air permeability of the fuel bed. It has been considered impossible to provide an air regulating member to be both effective in minimizing air flow during off periods and also to allow air passage in initial on period operation sufficient to cause a proper immediate build-up of static back pressure.

Since optimum conditions both during on and off periods have been difficult to secure and, as

correct air proportionment during on periods is'the more important of the two, it has been customary to design the regulating members of such automatic air controls to fit loosely within the air duct even though in so doing much of their effectiveness during off periods is sacrificed.

Coal is the normal fuel used where forced draft firing is practiced. Air permeability of fuel beds is chiefly determined by coal caking and coking characteristics. Some coals form coke which is hard and relatively air impermeable whereas the 'coke which forms when another type of coal is used is soft and readily air permeable. Still other In the past it has been found that, if-

- steadying coals form a surface cake during off periods and, until this cake is broken, it is resistant to passage of air. Due to variations in fuel bed characteristics of coals from various producing areas, it is necessary that air control devices have means for adjustment and regulation in order to adapt them to operate within proper zones suitable to all characters of fuel beds so as to properly meter air in forced draft firing.

My invention is an automatic air control having one compartment which houses a damper for quantitative regulation of flow of air from a fan to the windbox below a fuel bed, and a second, adjoining compartment which houses a pressure responsive device for translating variations of static back pressure within the windbox into impulses which cause the damper to assume positions which accurately apportion the volume of regulating member and the compartment housing the impulse responsive device will be called the power member. I have devised and incorporated new and novel features appertaining to'both the regulating member and the power member of my automatic control and it is these new features, in conjunction with a novel combination of elements, which constitute my invention. Thus:

My invention provides a damper element as part of the regulating member which is close fitting and which allows the passage of a minimum of air during off periods of forced draft firing My invention also provides simple, novel and effective means for adjusting the amount of impulsive force which the power member will transmit to the regulating member in response to any static back pressure which may occur within the windbox. As an instance, where a coalis being used in forced draft firing which produces a fuel bed always easily air permeable, my invention provides a means for causing accurate positioning the damper element of the regulating member so as to pass adequate air for combustion in spite of the relatively small static back pressure which will exist; on the other hand, the adjusting means can be set to cause the regulating member to correctly apportion the air for forced draft firing of coals characterized by fuel beds of extreme air impermeability or for coals characterized by fuel beds of intermediate air permeability. Novelty is not claimed for the fact that regulating means is incorporated as a feature of my device but for the manner in which plished.

Another feature of my invention is the incorporation of an adjustment which limits the extent to which the damper element of the regulating member may open irrespective of the amount of static back pressure which may develop. Thereby, when normal fuel bed conditions are reached during an on period of forced draft firing, the

said regulation is accomaction of my air control is to assure that the damper element of the regulating member may reach but not exceed a maximum opening adequate to provide an amount of air which will pro- Vide for combustion of fuel sufficient to produce a release of heat units at a desirable rate.

My invention also incorporates means for the movement of the control elements against the surge of static pressure which characteristically occurs in windboxes when regulating dampers open beyond a critical point. These pressure surges result in a common tendency in all forms of automatic air controls to open slowly at first and, upon reaching approximately a midopening position, immediately to fly wide open. Usually counterweightsare provided to exert increasing effect as the regulating dampers approach and pass midposition. I have eliminated the necessity for counterweighting and obtain a similar effect by a means which avoids the many disadvantages which counterweighting introduces,

Still another novel feature of my invention is p the'placement of the power member relative to the regulating member whereby a minimum number of operating parts are involved and the greatest possible protection is made possible from dust and dirt which would impair proper functioning of the air control. In any automatic air control which utilizes static back pressure to regulate air flows from fan to windbox, both a regulating member and a power member are necessary features. However, in prior art, it has been customary to either superpose the power member compartment upon the regulating member compartment or to separate the one from the other. In my device, I so place the power member compartment that it horizontally parallels the regulating member compartment, the two being separated by a common wall through which passes the shaft upon which are mounted both the damper element of the regulating member and the pressure responsive element of the power member. Owing to the fact that these members are neither superposed nor spacially separated, the device is placeable as is no other comparable device, in positions where maximum protection is afforded from all forms of contamination or physical injury.

The full nature of my invention will be-apparent from the accompanying drawing and the following description and claims.

The accompanying drawing shows the general arrangement of my invention.

In the drawing:

I is, an air passage which, when positioned in the air duct, becomes part of the air communieating passage from the fan to the fuel bed.

2 is a damper having a larger exposure of surface above its axis than below its axis.

3 is a chamber separated from air passageway I by a partition 4.

5 is a swinging plate within chamber 3 and is mounted on shaft 6 which is common to both the swing'member 5 and the damper 2.

6 and 6" are bearings supporting the ends of shaft 6.

1 is a stationary curved backing plate conforming with the arc of swing of the free end of plate 5.

8 is a stationary filler in the lower portion of passageway lagainst which the lower portion of damper 2 seats itself when in closed position as shown.

9 is an aperture in the wall 4.

I i) is a movable filler which in its closed position as shown prevents communication above the n ing plate 5 but which provides an adjustable by-lpassage when partially opened by movement by an arm ll.

I2 is an opening to the atmosphere from chamber 3.

I3 is a disc fastened to the swinging plate 5, and M is a projection in the periphery of disc I3.

[5 is an adjustable stop which at its maximum setting as shown will allow swinging plate 5 a full 90 swing, but, when moved counter-clockwise, limits the swing of 5 to any lesser swing which may be desirable.

I5 is a projecting segment beyond the periphery of disc l3 which progressively covers the opening I2 as swinging plate 5 moves clockwise.

During off period of forced draft firing, the control will be as shown in drawing. Damper 2 is closed and only such air reaches the fuel bed as leaks through the small spaces between the walls of passage l and the edges of damper 2 which is normally as close fitting as good mechanical construction will allow.

When an on period occurs, action within the automatic air control is in the following sequences:

Immediately the fan which provides forced draft is in full rotation, damper 2 will tend to rotate 'about'axis' 6 in clockwise direction. This is due to the provision of a greater surface exposure in the portion of damper 2 which is above axis 6 than in its portion below the axis. Damper 2 thus opens initially as soon as an on period" occurs, the amount of opening being the result of the uniform pressure of the fan against the unbalanced damper and the amount of counterforce which is exerted against rotation of damper 2 by the greater dependent weight below the axis due to the swinging member 5 being also mounted on axis 6. Swinging member 5 being out of the air flow from the fan, has, in this stage. no effect other than to limit the initial swing of the damper 2. The extent to which damper 2 opens initially when an on period of forced draft firing occurs is controllable in design of my apparatus and I provide that this will be about 5% from the closed position.

The initial opening of damper 2 when an on period of forced draft firing occurs allows passage of air which is inadequate for production of a required rate of combustion within the fuel bed but which will, in conjunction with whatever resistance to air passage exists due to depth and condition of the fuel bed, establish a static back pressure in the windbox below the fuel bed.

As soon as static back pressure in the windbox builds up, it is communicated to the swinging plate 5 in chamber 3 due to the provision through wall 4 of the aperture 9 on the windbox side of damper 2. Thus, on one side of swinging plate 5 exists static windbox back pressure while on its reverse side, a less pressure exists due to the relief to the atmosphere which is provided by opening [2 in casing 3. Due to the pressure differential on the two sides of swinging plate 5 its free end will move over the curved backing plate I, the amount of such movement being conditioned upon the amount of pressure differential which in turn is in direct relation to the amount of static back pressure existing within the windbox. This static back pressure in turn varies with the depth and air permeability of the fuel bed. As this varies so will the positioning of swinging plate 5 and the extent to which damper 2 will be open. Greater or less opening of damper 2 causes a greater or less air flow from fan to fuel bed, thus regulating rate of combustion to comparative uniformity.

As the damper 2 opens, a rush of air to the windbox tends to produce a pressure surge and this being communicated to swinging plate 5 'would cause it to fly up to its limit of travel segment I6 progressively covers opening l2 so as to allow an equalization of pressure on the two sides of plate 5 and. thus to counteract the effect of pressure surges and to slow the clockwise travel of 5 when surges occur.

As has been stated, some coals form fuel beds of relatively low air permeability, whereas other coals form fuel beds which allow freer air passage. On this account, a regulation is needed which will control the extent to which swinging plate 5 is affected by static pressures within the windbox. For a fuel bed of low air permeability, there will be a correspondingly high back pressure, whereas the burning of a coal which forms fuel beds which allow freer air passage will be characterized by low static back pressures. However, both these coals will take approximately the same amount of air for a given combustion rate. This regulation is provided by the movable filler It. When this filler is closed, as shown in drawing, maximum pressure differential will exist on the two sides of swinging plate 5 and the greatest possible opening of damper 2 will result fromany windbox back pressure. This will be correct adjustment of ID for a coal which produces fuel beds of low air resistance. By opening H] the amount of pressure differential which will exist on the two sides of swing plate 5 for any given windbox back pressure will be decreased by a partial equalization of the pressure differential. This partial equalization will affect the amount of swing of 5 and the extent of opening of 2. By varying the movement of 10, corresponding variations in placements of 5 and 2 will result.

Depending upon the rate at which it is desirable to release heat units from a fuel bed, there are corresponding limits to which it is desirable that damper 2 may open. This limiting effect is'provided by adjustable stop l5 and the projection M on the wheel [3 which is rigidly attached to swinging plate 5.

Having described my invention, I claim:

1. Automatic air control for use in forced draft firing comprising an air conduit, a damper in said conduit, a horizontal axial support rigidly attached to said damper and positioned to allow greater damper surface above than below said axial support, a chamber adjoining said conduit and separated therefrom by a wall common to said conduit and said chamber, an extension of said axial damper support within said chamber, a swing plate dependently and rigidly attached to said extension, a curved backing conforming to the free end of said swing plate, an aperture through the wall common to said conduit and said chamber positioned on the firebox side of said damper, a movable filler within said chamher and above said axial extension and an opening from said chamber to the atmosphere positioned oppositely with reference to the swing plate to said aperture from conduit to chamber.

2. Automatic air control for use in forced draft firing comprising an air conduit, a damper in said conduit, a horizontal axial support rigidly attached to said damper and positioned to allow greater damper surface above than below said axial support, a chamber adjoining said conduit and separated therefrom by a wall common to said conduit and said chamber, an extension of said axial damper support within said chamber, a swing plate d p dently and rigidly attached to said extension, a curved backing conforming to the free end of said swing plate, an aperture through the Wall common to said conduit and said chamber positioned on the firebox side of said damper, a movable filler within said chamber and above said axial extension, an opening from said chamber to the atmosphere positioned oppositely with reference to the swing plate to said aperture from conduit to chamber and an adjustable stop controlling maximum allowable movement of said swing plate and said damper. 3. Automatic air control for use in forced draft firing comprising an air conduit, a damper in 4. Automatic air control for use in forced draft firing comprising an air conduit, a damper in the conduit exposing greater surface above than below a horizontal axial support, a chamber adjoining the air conduit, a plate within said chamber swingingly dependent from and rigidly attached to an extension within the chamber of said axial support, means for exposing one surface of said plate to static pressure existing below a fuel bed and manually adjustable means for. providing a lesser pressure against the reverse surface of said plate in a direction to assist the damper to open and to pass a greater volume of air to the fuel bed and an adjustable stop controlling maximum allowable movement of said damperand said plate.

5. Automatic air control for use in forced draft firing comprising an unbalanced damper within an air duct leading from a fan to a Windbox below a fuel bed, a chamber adjoining said air conduit, an extension within the chamber of the axial support of said damper, a plate within said chamber swingingly and rigidly attached to said extension, meansfor exposing one surface of said plate to static pressure within the windbox, an,

opening to the atmosphere on'the opposite side of said plate, an adjustable by-pass above said plate to allow any desired equalization of the pressure differential on the two sides of said plate.

6. Automatic air control for use in forced draft firing comprising an unbalanced damper within an air duct leading from a fan to a windbox below a fuel bed, a chamber adjoining said air conduit, an extension within the chamber of the axial support of said damper, a plate within said chamber swingingly and rigidly attached to said extension, means for exposing one surface of said plate to static pressure within the windbox, an opening to the atmosphere on the opposite side of said plate, an adjustable by-pass above said plate to allow any desired equalization of the pressure differential on the two sides of said plate and an adjustable stop controlling maximum allowable movement of said plate and said damper.

7. Automatic air control for use in forced draft firing comprising an air conduit, an unbalanced damper within the conduit, a chamber adjoining the conduit, an extension Within the chamber of the axial support of said damper, a plate swingingly and rigidly attached to said extension, means for exposing one surface of said plate to static windbox pressure and the opposite surface to atmospheric pressure so as to cause the plate to exert a turning effect upon the damper in the direction of its opening and adjustable means for partially equalizing the pressure differential on opposed surfaces of the plate.

8. Automatic air control for use in forced draft firing comprising an air conduit, an unbalanced damper within the conduit, a chamber adjoining the conduit, an extension Within the chamber of the axial support of said damper, a plate swingingly attached to said extension, means for exposing one surface of said plate to static windbox pressure and the opposite surface to atmospheric pressure so as to cause the plate to exert a turning effect upon the damper in the direction of its opening, adjustable means for partially equalizing the pressure differential on opposed surfaces of the plate and an adjustable stop controlling maximum allowable movement of said plate and said damper.

NOEL CUNNINGHAM.

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