Air/gas ratio

Flame treatment is predominantly used with articles of relatively thick section, such as blow moulded bottles, although it has been applied to polyolefin films as well. The most important variables in the process are the air-gas ratio and their rate of flow, the nature of the gas, the separation between burner and surface, and the exposure time. 

Figure 19.1 indicates the flue losses to be expected for different temperatures and excess air. It is seen that considerable savings can be made, particularly at higher temperatures, by reducing excess air levels to a practical minimum. It is also evident that a reduction in air/gas ratio to below stoichiometric will cause a rapid deterioration in efficiency caused by the energy remaining in the incomplete combustion of fuel. 

The ideal air/gas ratio is that which is marginally higher than stoichiometric. It is not possible to run a burner with no excess air for various reasons (e.g. changing ambient temperature, a slight change in calorific value, variation in barometric pressure, wear of control equipment, etc.). All of these and other factors dictate that the burner is operated with sufficient excess air to avoid the production of carbon monoxide in any quantity. 

Reference to Figure 19.1 shows how efficiency can be adversely affected by deviation from the optimum air/gas ratio. By maintaining combustion close to stoichiometric, efficiency will be improved, but the practical limitations of burners discussed above must be noted. 

The quality of combustion can be measured with suitable instrumentation, on either a periodic or a continual basis. If continuous analysis is practiced then there may be feedback to continuously adjust the air/gas ratio and/or ratio and/or record the data derived. 

Most oxygen trim systems interpose an additional link in the air/gas ratio controller. Others use an additional valve. Most types are based on the zirconia cell installed in the flue, while others use paramagnetic or electrolytic cell methods. The zirconia type has the advantage that there is no time lag in sampling, nor is there a risk of contamination of the sample. 

If the flue is operating under negative pressure, which is often the case, care should be taken that no tramp air is allowed to enter the flue upstream of the sampling point, as this will give erroneous measurements. The trim system should be set to follow the practical firing curve and, in the event of a malfunction, be disabled so that it ceases to have any influence on the air/gas ratio, which reverts to the normal load control only. 

Types of air/gas ratio control There are various types of air/gas ratio device commonly used, including ... 

Linked characterized valves. The relationship between angular rotation of the valve spindle and open area of the valve can be adjusted over different portions of the flow range using a series of screws. In this way, the air/gas ratio can be characterized to any desired profile over the whole firing range. These valves are suited to fully modulating systems and are commonly used on steam boilers. 

In general, a fully modulating burner will be thermally more efficient, particularly if it maintains air/gas ratio accurately at intermediate rates. For natural-draft burners this may not be the case, as, in general, the air is not modulated, leading to progressively more excess air at lower rates. 

A test procedure which has proved very useful was first described by Hatfield. The samples are cylinders 32 x 12-5 mm in diameter with a standard abraded finish which are supported on open-ended refractory boats in a tubular furnace. In the original test the atmosphere, which was produced by burning towns gas with a 50% excess of air, was passed over the specimens at a standard velocity after first preheating to test temperature over refractory packing in a separate furnace chamber. More latterly, natural gas has been used with suitable modification of air gas ratio to give... 

Fig. 4.13. Integrated ratio actuator controls air/gas ratio by comparing pressure drops across air and gas orifices. It automatically compensates for varying air temperature, thus providing mass flow control. An adjustment allows use of low-fire excess air for thermal turndown. Courtesy of North American Mfg. Co.

The most important variables in the process are the air gas ratio, the air/gas flow rate, the namre of the gas, the burner-surface separation and the exposure time. The gas used is either mains (mainly methane) or bottled (propane or butane). For optimum treatment, an oxidizing flame should be used, that is, with an excess of oxygen over that required for complete combustion. For a given air gas ratio, treatment level in a given exposure time increases as the volume of mixture burned increases. It is clearly important that the exposure time is not sufficient for the surface polymer to melt (the flame temperature is 2000 °C) in the case of film treatment, the film is treated as it passes over a cooled roll. 

Air gas ratio Suiphur in gas Oxidation gain in seven 6-h lest cycles (g/m )... 

Burner output must be increased as the speed of the process line is increased in order to achieve the same level of treatment. This power density control requirement is served efficiently by increasing the mass flows of both the hydrocarbon gas and air electronically so the preset stoichiometric air/gas ratio is maintained. 

Figure 6.10 Adhesion as a function of value ranges of air/gas ratio, burner vs. substrate distance, gas rate, and line speed [2]...

Closed loop air/gas ratio control, to provide a constant flame discharge across the entire adjustment range. 

The reaction is exothermic, that is, it produces heat. The ideal air/fuel ratio, i.e., the exact amount of oxygen present to burn the fuel completely. There is no excess oxygen or fuel. This is called the stoichiometric air/fuel ratio. A lower air/gas ratio is called sub-stoichiometric, or rich , containing more fuel than there is oxygen a higher ratio is lean , containing excess air. 

The effect of air/gas ratio on treatment level is shown in Figures 3 4 for natural and propane gases. This illustrates that a lean air/gas ratio provides the highest treatment level. At sub-stoichiometric air/fuel ratio, the treatment is not as high as the ideal ratio which while lean, depends on the substrate, and line speed. If the air/gas mixture is too lean, the treatment drops off. This shows how critical the air/gas ratio is in regard to optimizing flame surface treatment. 

AIR/GAS RATIO 3/0" FROM INNER CONE FUEL- METHANE... 

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