BURNER WITH PRIMARY MR FLOW CONTROL GATE
The present invention relates to natural aspiration burners and, in particular, to natural aspiration venturi burners for kilns.
An important operating parameter of kilns, particularly industrial kilns, is the extent to which it is possible to control carefully the temperature profile of the kilns.
A particularly important operating parameter of kilns is the extent to which it is possible to selectively control ramping of the temperature of the kilns at start-up from ambient to an operating temperature
The variables for achieving temperature control are the combustible gas flow rate through the burners and the air flow rate to the burners. In practice, temperature control is achieved by a combination of combustible gas flow rate control and air flow rate control.
Natural aspiration venturi burners with unrestricted primary air flow to the burners have a minimum combustible gas input with which a stable flame can be maintained. Natural aspiration venturi burners with unrestricted primary air flow to the burners also have a maximum combustible gas input that is compatible with efficient combustion. The ratio of maximum/minimum combustible gas inputs can be termed, and is referred to hereinafter as, the natural "turn down ratio" (T.D.R.3 of the burner. It can readily be appreciated that the natural turn down ratio is an indication of the effective operating range of a burner.
The maximum energy input for a natural aspiration venturi burner is fixed by the overall design parameters of the burner.
The minimum energy input for a burner can be reduced from the natural value (i.e. the minimum combustible gas input possible with unrestricted primary air flow to the burner) by the use of a primary air flow control means to limit the flow of primary air to the burner, albeit at some loss of combustion efficiency.
The standard primary air flow control means are manually-operated and usually comprise a slide or wormthread-drive plate. The main disadvantages of such devices are that:
i. precise control of flame condition is not
possible and in large part the accuracy of
the control is dependent on operator
judgement; and
ii. where automatic ramping control systems are
employed, a manual procedure is required
thus preventing the firing procedure from
being fully automatic.
However, the automation of such devices, which would improve the level of flame control and subsequent temperature control, is difficult and requires complex and finely-balanced mechanical and electronic systems associated with a sophisticated firing control system.
An object of the present invention is to provide a natural aspiration burner, in particular a venturi gas burner, with a primary air flow control means which is not subject to the disadvantages of the standard air flow control means described above.
According to the present invention there is provided a burner comprising:
(a) a burner nozzle;
(b) a combustible gas inlet assembly;
(c) a primary air inlet assembly; and
(d) a means for automatically controlling the
primary air flow rate when operating the
burner at relatively low combustible gas
inputs which would place the burner outside
the operating limits determined by the
natural turn down ratio of the burner, the
air flow rate control means being
responsive directly or indirectly to the
flow rate of combustible gas into the
burner to adjust automatically the flow
rate of primary air into the burner with
changes in the flow rate of combustible gas
to maintain a predetermined flame
characteristic for the burner.
The term "predetermined flame characteristic for the burner" is understood herein to mean a flame that is required for a particular burner operation. For example, in many instances the predetermined flame characteristic would be a stable, smoke-free, flame which, at low combustion gas inputs, can be maintained without "burn back".
It is preferred that the primary air flow rate control means comprise a gate means to selectively close the air inlet means in response to changes in the combustion gas pressure in the burner, such changes being directly related to the flow rate of the combustible gas into the burner.
It is preferred that the burner further comprise a combustible gas flow rate control means for selectively controlling the combustible gas flow rate of the burner.
It can readily be appreciated that, with such an arrangement, by selectively controlling the combustible gas flow rate to the burner there will also result an automatic control of the primary air flow rate to the burner whilst maintaining the predetermined flame characteristic for the burner.
The gate means may be of any suitable configuration.
In a preferred embodiment of the present invention the gate means comprises a flap which is suspended for swinging movement between:
i. a 11closed" position at which the flap
substantially covers the primary air inlet
to allow the inspiration of sufficient air
into the burner to maintain a small but
stable flame without burn-back; and
ii. a range of "open" positions which allow the
inspiration of varying amounts of air into
the burner.
It is preferred particularly that the primary air flow rate control means comprises a means to bias the flap into the "closed1, position.
It is preferred that the burner be a venturi burner
According to the present invention there is also provided a kiln comprising the burner described in the preceding paragraphs.
The present invention is described hereinafter by way of example with reference to the accompanying drawings, in which:
Figure 1 is a front view of the back plate of a preferred embodiment of a natural aspiration venturi burner in accordance with the present invention; and
Figure 2 is a vertical cross-section through the burner.
The figures illustrate a natural aspiration venturi burner, generally identified by the numeral 3.
The burner 3 comprises:
i. a burner nozzle 6 for igniting a mixture of
combustible gas and primary (and secondary)
air and for projecting the flame generated
by combustion into a kiln or other form of
furnace (not shown); and
ii. a chamber, generally identified by the
numeral 5, for mixing combustible gas and
primary air and for supplying the mixed
combustible gas and primary air to the
burner nozzle 6.
With reference to Figure 2, the mixing chamber 5 is defined by a venturi throat 7, a cylindrical side wall 9, and a back plate 11.
The burner 3 further comprises a combustible gas injector 13 extending centrally through the back plate 11 for injecting combustible gas into the mixing chamber 5.
The burner 3 further comprises two generally rectangular primary air inlets 15 vertically spaced apart above and below the combustible gas injector 13 for allowing the aspiration of primary air into the mixing chamber 5.
The burner 3 further comprises a primary air flow rate control means in the form of a flap 17 for selectively closing/opening each primary air inlet 15. The upper section of each flap 17 is suspended by a support system which comprises pins (not shown) extending between posts 19 that are mounted to the back-plate 11.
It can readily be appreciated that, with such an arrangement, the flaps 17 are free to swing from:
i. the substantially "closed" position shown
in the figures at which there is a small
gap between the flaps 17 and the primary
air inlets 15 which allows the inspiration
of a small flow rate of primary air into
the mixing chamber 5; and
ii. a range of "open" positions in which the
effective inlet area of the primary air
inlets 15 increases to allow the
inspiration of increasing flow rates of air
into the mixing chamber 5
The "closed" configuration is selected as required to allow a sufficient flow rate of primary air into the mixing chamber 5 to maintain a stable, rich, smoke-free, flame at minimum combustible gas flow rates, without burn-back.
It can readily be appreciated that the position of the flaps 17 is directly responsive to the internal pressure within the mixing chamber 5 which, in turn, is directly responsive to the combustible gas flow rate into the mixing chamber 5. Thus, the air flow rate into the mixing chamber 5 varies directly, in use, with the combustible gas flow rate into the mixing chamber 5.
The above described burner is a significant improvement over prior art burners which incorporate manual air flow control means, because the flaps 17 enable adjustment of the air flow rate automatically with changes in combustible gas flow rate and enable the burner 3 to operate efficiently and effectively at combustible gas inputs which are below the minimum combustible gas input with unrestricted primary air for the burner 3, i.e. at combustible gas inputs outside the operating limits for the burner 3 established by the natural turn down ratio of the burner 3.
Many modifications may be made to the preferred embodiments described above without departing from the spirit and scope of the present invention.
For example, whilst the preferred embodiment comprises a natural aspiration venturi burner, it can readily be appreciated that the present invention is not so limited and extends to any form of combustible gas burner.
Furthermore, whilst the preferred embodiment comprises flaps 17 suspended for swinging movement between closed and open positions relative to the air inlets 15, it can readily be appreciated that the present invention is not so limited and extends to any suitable air flow rate control means.
In this context, it is also noted that the present invention is not limited to the particular configuration and location of air inlets 15 and extends to any suitable air inlets.
Furthermore, it is also noted that the flaps 17 are finely-balanced and the operation of the flaps 17 may be influenced by external factors such as air flow in the region of the rear section of a burner. In situations where this is a factor, it is within the scope of the present invention to provide a shield means to shield the rear section of the burner.