Combustion – Process of combustion or burner operation – Controlling or proportioning feed
Patent
1997-11-20
1999-12-07
Lazarus, Ira S.
Combustion
Process of combustion or burner operation
Controlling or proportioning feed
431 90, 431 76, 431 77, 236 15BD, 236 15E, F23N 1144, F23N 500
Patent
active
059972785
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to apparatus for providing an air/fuel mixture for example to fuel cell, but particularly an air/fuel gas mixture to a fully premixed burner.
BACKGROUND OF THE INVENTION
In such a burner fuel gas is mixed, before combustion in the burner, with air in a plenum chamber.
The fuel gas is usually supplied from a main while the air is supplied by a fan.
To prevent incomplete combustion of the gas and the production of poisonous carbon monoxide gas, the volume flow rate of air is usually intended to be maintained in excess of the rate theoretically necessary for full combustion of the gas. Typically this excess amounts to 30%, and the burner is then said to be operating with 130% of the stoichiometric air requirement or, for brevity, "at 130% aeration".
SUMMARY OF THE INVENTION
According to the present invention apparatus for providing an air/fuel mixture to a fully premixed burner comprises means for providing fuel at a variable flow rate to the burner, means for supplying air at a variable flow rate for mixing with the fuel in a plenum chamber within the burner, means for sensing aeration by measuring the composition of the combustion products and control means for controlling the fuel flow rate in dependence upon a heat output demand and the air flow rate in dependence upon both the fuel flow rate and the sensed aeration in such a way that the air flow rate is sufficient to maintain the aeration at a predetermined value, the values of fuel flow rate and air flow rate being particular values within a respective range of predetermined values which form a geometric series with a constant ratio between successive terms.
The means for providing fuel at a variable rate may comprise a modulating valve having a variable opening to vary the fuel gas flow rate, while the means for supplying air at a variable rate may comprise either a variable speed fan or alternatively a variable throttle valve in association with a fan operating at a nominally constant speed.
The means for sensing aeration may comprise a sensor for sensing the oxygen content of the fuel combustion products and for providing a signal representative of the oxygen content.
Conveniently the geometric series contains a predetermined number N.sub.max of terms, each term being in accordance with the following relationship:
Q.sub.N is the respective fuel flow rate or air flow rate at the N.sub.th step in the predetermined series of steps,
Q.sub.1 is the respective fuel flow rate or air flow rate at step one in the series and therefore for both the fuel flow rate and the air flow rate constitutes the respective lowest of the permitted rates of flow,
R is a constant term equal to the common ratio of the geometric series, the value of R being chosen according to the resolution desired between successive steps in flow rate and being furthermore the same for the respective series defining the permitted rates of flow of fuel gas and of air, and
N is a number uniquely identifying any individual step and having a lowermost value of unity and an uppermost value of N.sub.max, the latter being determined jointly by the chosen value of the constant R and the ratio of magnitude between the highest and lowest rates of flow to be provided.
Suitably the constant R is allocated a value of 1.025.
This apparatus can satisfy a variable heat demand largely without on-off cycling of the burner yet with accurate control of the burner aeration.
The advantage of making changes on the basis of a geometric series of flowrate values is that it becomes possible to make adjustments to the parameters which control the process of combustion, as percentage changes in the existing values of the parameter(s).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a domestic combustion system in a gas-fired domestic heating appliance, together with control apparatus therefor, and
FIG. 2 is a schematic circuit diagram illustrating how the heat demand signal is produced.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, there is illustra
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patent: 5401162 (1995-03-01), Bonne
patent: 5630408 (1997-05-01), Versluis
BG plc
Cocks Josiah C.
Holt William H.
Lazarus Ira S.
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