By Timmie McElwain Gas/Propane Basics President, Gas Appliance Service Riverside, RI gastc@cox.net Understanding Combustion Atmospheric gas equipment gets its combustion air from within the room in which it is installed. There are three air requirements for proper burner operation: primary air, secondary air and excess air. As the gas stream enters the throat of the mixing tube, it tends to spread and induce air in through the opening of the adjustable shutter. This is primary air, which mixes the gas before the air-gas mixture is forced through the burner ports. The energy in the gas stream forces the mixture through the mixing tube into the burner manifold casting, from which it issues through ports where additional air must be added to the flame to complete combustion. This secondary air supply flows into the heat exchanger and around the burner. Its function is to mix with unburned gas in the heat exchanger in order to complete the combustion process. The primary air is admitted at about a 10:1 ratio for natural gas and 24:1 for propane. These ratios are generally used as theoretical values of air for purposes of complete combustion. Most burners used in gas fired furnaces or boilers operate efficiently on 40–60% of the theoretical value. Primary air is regulated by means of an adjustable air shutter. When burning natural gas, the air adjustment generally aims to secure as blue a flame as possible. Primary air does not, however, affect the combustion analysis. Burner manufacturers provide a number of different ways to field adjust primary air. For example, shutters for primary air control are part of the main burners. Factory adjusted, angled orifices are also used on some burners. The secondary air is drawn into the burner by natural draft. No provision is made by manufacturers to field adjust secondary air. Secondary air is controlled primarily by the design of the flue restrictors and draft diverters (draft hood). The general configuration of the heat exchanger or boiler sections and burners are also important factors in secondary air control. Excess secondary air constitutes a loss and should be reduced to a proper and safe minimum (usually not less than 25–35%). A typical gas flame, as shown above, consists of a well-defined inner cone surrounded by an outer envelope. Combustion of the primary air and gas mixture occurs along the outer surface of the inner cone, the hottest part of the flame. Any unburned gas combines with the secondary air to complete the combustion process in the outer envelope of the flame. The height of the inner cone can be reduced by increasing the amount of primary air contained in the air-gas mixture. The extra oxygen increases the burning rate, which causes the inner cone to decrease in size. This helps to prevent impingement of the flame on cooler furnace or boiler surfaces which will cause carbon monoxide and aldehyde. A burner is generally considered to be properly adjusted when the height of the inner cone is approximately 70% of the maximum visible cone height. The gas should flow out of the burner ports fast enough so that the flame cannot travel or flash back into the burner head. The velocity must not be so high that it blows the flame away from the port(s). We will consider some of these adjustments when we go over burner problems in a later article. Primary air shutters are provided on most furnaces and boilers to enable adjustment of the primary air supply. The purpose of this adjustment is to obtain the most suitable flame characteristics. Experience has shown that the most efficient burner flame for natural gas has a soft blue cone. Many in the propane industry say that a propane burner flame should have a little yellow showing in the tips, but it Continued on p. 24 20 ICM/March/April 2015
ICM September-October 2014
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