Combustion – Timer – programmer – retarder or condition responsive control – By condition of burner feed or feed means
Reexamination Certificate
1997-01-24
2001-03-27
Price, Carl D. (Department: 3743)
Combustion
Timer, programmer, retarder or condition responsive control
By condition of burner feed or feed means
C431S089000, C431S012000, C431S187000, C431S266000, C431S062000, C431S265000, C137S625500, C137S625480, C251S206000
Reexamination Certificate
active
06206687
ABSTRACT:
TECHNICAL FIELD
The present invention is a gas burner. More particularly, the present invention is a gas burner with a high turndown capability, permitting the burner to operate between less than 5% and 100% of the maximum firing rate.
BACKGROUND OF THE INVENTION
A gas burner is the fire producing device used in a warm air furnace, a heat exchanger, a boiler, an oven, and the like. Typically, the gas burner controls the flow rate and mixing of air and gas and includes the controls that do the ignition and safety monitoring of the flame. For many applications of a gas burner, the amount of heat required is not constant. The amount of heat required may vary according to the weather, the process load, and other conditions. To deal with varying loads, banks of multiple burners have been used. The banks of multiple burners may be sequenced to produce the required amount of heat. Alternatively, a burner with a variable firing rate may also be used. A burner with a variable firing rate can be a staged burner, capable of operating either at a low fire or high fire, or it can be a modulating burner. A modulating burner is capable of being controlled to operate at any firing rate within a range between its minimum and maximum firing rates. That range is typically 50%-100%, with some of the better burners being capable of 33%-100%. That means that when the heat requirement is less than the minimum firing rate of the burner, 33% in the case of the better burners, the only alternative is to periodically cycle the burner on and off at the minimum rate in order to produce a lesser amount of heat than is produced at the minimum rate. Unfortunately, this results in fluctuating temperatures and therefore less than ideal control when operating in this mode.
Accurate and consistent temperature control is improved if a burner is capable of operating at a very low minimum firing rate. A great deal of effort in the industry has been expended toward achieving the goal of having a very low minimum firing rate. Typically, efforts at providing such capability have concentrated on control of the gas flow and control of the secondary air.
With respect to control of the gas flow, the maximum fire rate of a gas burner is typically controlled by the sizing of the main gas orifice. The size is typically set when the burner is manufactured and is invariable thereafter. The maximum firing rate occurs when a specified gas pressure is present at the fixed orifice. To effect the minimum firing rate on a modulating gas burner, it is common practice to control a butterfly gas valve or other similar device to cause a reduction in the gas pressure to the fixed orifice. Reducing the gas pressure causes a reduction in the gas flow rate through the fixed orifice, thereby reducing the firing rate of the burner. Typically, a control actuator is mechanically linked to the butterfly gas valve to also control a combustion air damper, such that both the gas and the combustion air are simultaneously reduced to achieve the minimum firing rate. Alternatively, the combustion air damper only is controlled. Such control reduces the air pressure within the burner. A suitable pressure regulator is then used to sense the reduced air pressure and to control the gas pressure proportionately.
Because the flow rate to a fixed orifice varies as the square of the pressure across it, there are practical limits as to how low the flow can be reduced using either of the foregoing techniques. As an example, if the burner utilizes 4.0 inches water column orifice pressure at the maximum firing rate, the pressure would have to be reduced to unmanageably low levels to operate in the region below approximately 20% of the maximum firing rate. Such levels are indicated in Table 1 below.
100%
4.00 In. W.C.
50%
1.00 In. W.C.
33%
.44 In. W.C.
20%
.16 In. W.C.
10%
.04 In. W.C.
5%
.01 In. W.C.
As indicated above, secondary air may be also controlled to achieve a minimum firing rate. Secondary air is that air which is introduced directly into the combustion zone. Typically the combustion air to a modulating gas burner is controlled by a pivoting damper blade. A pivoting damper blade is inadequate for a burner that is going to be modulated down to a minimum firing rate that is less than 25% of the maximum firing rate. A pivoting damper blade simply does not allow precise enough control near and at the desired minimum firing rate.
On gas burners that control secondary air to proportion combustion air, primary air is not presently varied in any fashion in order to affect the minimum and maximum burning rates. Primary air is that air that is mixed directly with the gas stream before it enters the combustion zone. Having a source of primary air is common practice with many types of gas burners.
As previously indicated, there is a need in the industry for a gas burner that is capable of operating efficiently at very low minimum firing rate. Such firing rate should be in the range of less than 25% of the maximum firing rate. In order to achieve such a low minimum firing rate, a new means of accurate and consistent temperature control is required.
SUMMARY OF THE INVENTION
The present invention substantially meets the aforementioned needs of the industry. The apparatus of the present invention maintains a relatively constant pressure on the gas flow orifice but varies the area of the orifice. This is accomplished by having a square orifice and controlling the open area of the orifice by positioning a tapered plug at various positions within the orifice. Generally, the valve will have a specific stroke length for the tapered plug and the taper of the tapered plug will be defined for a particular capacity profile along that stroke. Accordingly, valves sized for lower capacity will have less taper and therefore there will be less open area at the maximum capacity position. Although a square orifice has been described, the present invention may also utilize round or other shaped orifices with an appropriate shaped plug. Additionally, the profile of the tapered plug can be characterized so that a specific flow rate will occur at specific stroke positions. In this manner, the plug can have a linear rate of change or with a compound face of the taper the plug can have a slow rate of increase at the minimum firing rate end of the stroke and a fast rate of increase toward the maximum firing rate end the stroke.
The gas burner of the present invention meters secondary air using a sliding blade under a plate that had characterized openings responsive to the need of the burner from the minimum firing rate to the maximum firing rate. Accordingly, the apertures admitting the secondary air can be precisely determined along the stroke of the blade.
The aforementioned sliding blade also controls air to a port that supplies the primary air to the burner. Preferably, at the minimum firing rate, a specific amount of primary air is mixed with the gas. As the amount of gas increases when a higher firing rate is commanded, the amount of primary air is also increased. When the firing rate increases beyond a certain point, the primary air is cut off. At this point, the primary air is not needed for good combustion and the addition of the primary air needlessly adds to the gas port pressure drop in the burner gun.
For the gas burner of the present invention, a new source of air is utilized to enhance the combustion of the gas. At very low firing rates, good combustion requires that the combustion air be greatly reduced and that the flame receives that air at the correct location relative to the gas. Toward this end, a source of base air is supplied directly into the burner gun assembly. The base air and the gas are mixed proximate the point at which the gas emerges from the burner gun.
A further advantage of the present invention is that both the sliding blade of the air valve and the wedge of the gas valve are linearly actuated. Accordingly, they can be directly connected to a single linearly actuated rod, thus eliminating the need for crank arms, adjustable linkage, and the like
AAF-McQuay Inc.
Patterson Thuente Skaar & Christensen P.A.
Price Carl D.
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