Sizing Example | 4950 Flame Grid
Example
CONSTRUCTION
Given: 9900 scfm fume-laden air at 400 °F. Available pressure drop 1.2"w.c.
Metal parts are heat-resistant cast iron, steel or stainless steel. Burner mounting are often lined with insulation to reduce its outer surface temperature. Standard flame grids include an ignition/flame supervision boss with: — spark-ignited pilot assembly or direct spark igniter — observation port — provision for flame detection device. Grid plates are 309 stainless steel and fuel tubes are 304. Up- graded materials are available in special engineered designs for very harsh applications. Optional hard refractory internal insulation (adds 6" to “A” dimension) is available for use with refractory-lined ducts and 900-1100 °F inlet temperatures.
Selection procedure: 1. Convert acfm to scfm, using ratio of absolute temperatures:
9900 acfm x 460 + 60 = 5990 scfm 160 + 400 ( )
2. Temperature correction Multiply scfm by Table 1 air capacity factor:
5990 x 1.29 (for 400 °F) = 7727
3. Pressure correction Mulitply corrected air capacity by Table 2 factor:
7727 x 0.65 (for 1.2"w.c.) = 5020 (equiv. scfm air at 60 ° inlet and 0.5"w.c. pressure drop)*
4. Select Flame Grid fro Table 3: 5020 scfm falls between wide open and minimum settings of 4950-3624 Burner, which is the proper selection. 5. Fuel input requirement from Table 1: 5990 scfm x 1390 Btu/HVV per scfm (at 400 °F inlet) = 8,330,000 Btu (approx. 8330 cfh natural gas).
SELECTION
Because the fume stream is the burner’s air source, it must con- tain enough oxygen for complete combustion of all contaminants and auxiliary fuel (gas). Minimum oxygen requirements vary with fume stream temperature, see Figure 1. Minimum oxygen content is shown on a wet basis; i.e., as a percentage of total gases, including water vapor. Most oxygen analyzers report oxygen on a dry basis, so their readings must be corrected for water vapor content. If a potential application dœs not satisfy the requirements listed above, an alternate method of incineration may be necessary using furnace burners with an external source of combustion air. Check with your North American field engineer. If fume stream contains sufficient oxygen, a Flame Grid can be selected by the following procedure, which assumes combustible contaminants make a negligible contribution to heat required to raise fume stream to incineration temperature. It also assumes 1400 °F is adequate for incineration of all combustible contami- nants.
GAS PRESSURE REQUIREMENT
Available pressure to Flame Grid must overcome pressure drops through the burner and its gas train. Table 4 shows natural gas capacities at 1"w.c. pressure drop (with standard gas port drilling).
To determine gas pressure drop across the 4950-3624 Burner selected above, apply the flow square root law:
8330 cfh 2380 cfh ( ) 2
= 12.3"w.c.
1"w.c. x
Supply pressure must equal this plus duct pressure at the burner plus pressure drops through the gas train.
Warning: Minimum gas pressure drop across the burner is 0.7"w.c. (1.8 mbar). Lower drops may cause uneven gas distribu- tion in the burner. Maximum drop with good flame stability is 3 psi (200 mbar)
Basic selection information required: 1. Maximum flow rate of fumes, acfm 2. Fume temperature at burner inlet 3. Fume stream pressure drop available across burner.
Bulletin 4950 Page 3
*See "Fluid Flow" chapter in North American COMBUSTION HANDBOOK for explanation of correction procedure.
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