Primary air flow

Air Flow - The capacity of a multijet flare to induce air flow must be calculated, to make sure that it is adequate to meet the maximum air flow requirement for smokeless combustion. (W, of Equation 4 below must be > W, of Equation 5). The term air flow capacity refers to the primary air flow rate which will be induced around each jet, and may be estimated from the following equation ... 

The primary air flow rate per jet necessary for smokeless combustion depends on the molecular weight and degree of unsaturation of the flare gas. Experience indicates that it varies linearly with percent unsaturates, from a minimum of 20 % excess air for a flare gas containing 0 % unsaturates to 35 % excess air for a gas containing 67 mol % unsaturates. Based on this relationship and a gas flow rate of 72.2 mVh per jet, the required primary air flow rate can be computed directly from the gas composition, or approximated conservatively from the following equation ... 

The underfired combustion mode corresponded to the operation of an updraft gasifier. The tests carried out on wet wood chips and peat lumps (35-60%) showed that the combustion rates are many times higher than the same conditions for the overfired mode. Koistinen et al were able to gasify wood chips with a moisture content of 58% d.b. However, the off-gas was so humid that it was not ignitable until the end of each run when the drying had ceased. Koistinen et al concluded that the combustion rate increased linearly with an increased primary air flow rate. 

Heat fluxes (grate loadings) on 1500 kW/m can be obtained by cocurrent combustion, but for wet biofuels the heat fluxes are only 500 kW/m. Wood chips with a moisture content of 40% are very hard to bum in a cocurrent configuration. On the other hand, if a countercurrent configuration is used between fuel feed and primary air flow it is possible to bum very wet fuels upto 55-60%. Heat fluxes on 1 500 kW/m can be attained for wood chips with a moisture content of 40%, if the air flow velocity is 0.1 m/s. 

The primary air flow rate and moisture content of the fuel was given before each combustion run. Flue gas samples were taken continuously and the bed temperature was monitored on-line. The bed weight and mass loss rate were also continuously measured. 

Both the primary air flow and the fuel feed can have at least three possible directions through the fuel-bed system. The primary air directions are here referred to as updraft (upwards), downdraft (downwards) and crossdraft (sidewards) [19,35], whereas the fuel feed is referred to as overfeed, underfeed and crossfeed [9,10], Figure 24 illustrates different continuous conversion systems with varying direction of both primary air flow and fuel feed. 

Friberg R. and Blasiak W., Preliminary Measurements of the Conversion Gas Rate for Two Types of Wood-Fuels as Function of Primary Air Temperature and Primary Air Flow , in the proceedings of the conference Survey of Combustion Research in Sweden, Chalmers University of Technology, Gothenburgh, 21-22 October, 111-119(1998). 

Figure 20.13 shows the temperature contours for the three different cases 1, 2, and 3 indicated in the figure. The cases correspond to three stable flow conditions of the secondary flow rate Q, entering through the offset gaps and primary air flow rate Qp entering from the base at the bottom (1) Qs = 8.8 scfm, Qp = 0.0 scfm (2) Qs = 5.0 scfm, Qp = 3.8 scfm and (3) Qs = 8.8 scfm Qp = 3.8 sfcm. 

The general arrangement of the multiplexed diode-laser sensors for measurements in the forced combustor at Stanford University is shown in Fig. 24.2. The primary air flow (65 1/min) through the central jet (d = 2.1 cm) was acoustically forced (up to 30% RMS of the flow rate) to create coherent vortices at... 

Temperature BS 7990 Primary ISOTS 19700 Primary Air Flow IEC 60695-7-50 Primary Air Flow... 

Primary air flow, supplied at the bottom of the bed, is insufficient for complete combustion of the fuel. The zone below the elevation of secondary-air jets is endothermic, cracking oil to yield carbon and fuel gas species. These burned in the upper, exothermic zone. Alumina product is withdrawn from a standpipe receiving solid from the upper zone, and burn-off of carbon in this zone is sufficient to yield a product that is acceptably white. Fluidizing-gas velocity being lower in the primary combustion zone than in the secondary, density is higher. Provision of the two zones accomplishes two purposes (1) affording a sufficient solid residence time in the primary zone and (2) reducing horsepower needed for air compression. 

Due to the Langrangian formulation applied to the solid phase, the use of an effective thermal conductivity as usually applied to porous media is not necessary. In a packed bed heat is transported between solid particles by radiation and conduction. For materials with low thermal conductivity, such as wood, conduction contributes only to a minor extent to the overall heat transport. Furthermore, heat transfer due to convection between the primary air flow through the porous bed and the solid has to be taken into account. Heal transfer due to radiation and conduction between the particles is modelled by the exchange of heat between a particle and its neighbours. The definition of the neighbours depends on the assembly of the particles on the flow field mesh. 

Use multiple steel flexible hoses installed in only one primary air channel for flame shaping to bring the whole primary air flow into a swirl of desired intensity without any reduction of primary air momentum at different flame shapes. Essential benefits of M.A.S. claims by the manufacturer are as follows ... 

The burner is equipped with a flexible swirl setting device to adjust the flame shape. Quasi whole amounts of primary air flow through the flexible hoses. The swirl intensity of primary air stream is determined by the angle of deflection of the hose nozzles, which are easily adjustable from cold end of the burner. The total primary air is internally divided in the air circuit to the flexible hoses and the central air circuit. It is particularly important that the fuels, the burner primary air, and the kiln secondary air mix properly to ensure... 

Utilization of the multichannel burners able to control separately the primary air flow stream on each channel (axial, tangential-swirl, transport, and core). 

The adjustment of fhe primary air flow rate and velocities at burner hp were done in two different modes ... 

The secondary air flow rate has been calculated by balancing the kiln air flow rates (see Figure 31.24). The primary air flow rate has been measured (all the measured flow rates are resumed in Table 31.40). The exhausted air flow rate (112,910 Nm /h) is lower than the exhausted air flow rate measured at the cooler stack (130,700 Nm /h). The difference is given by ... 

Experimental and theoretical studies of mist cooling were conducted over three decades ago [2, 3]. These studies showed that the heat transfer performance of a single-phase flow can be significantly improved by adding mist into the primary air flow. A number of two-phase flow studies have been made for a variety of configurations [4, S]. Results show that latent heat of evaporation within the boundary layer is important and e formation of a thin liquid film over the surface will greatly enhance heat transfer rates. A constraint of the current study was to avoid the deposition of drops on the heated plate, therefore only evaporation of the liquid in the bound layer will be considered. 

Semi-indirect firing denotes a system whereby the primary air flow can be reduced to such an extent as is compatible with adequate removal of the moisture from the coal grinding/drying mill, while the rest of the mill exhaust air is returned to the mill. 

Flow meters on field sampling instruments are calibrated using one of several methods. Primary standards are tliose which measure volume directly and are, therefore, preferred. Primary air flow standards include bubble meters, spirometers and Mariotti botdes. The time required to draw a measured volume of air through these systems is measured and the resulting flow rate is calculated. These methods are typically accurate to within one percent. 

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