Fraction of heat radiated

D = minimum distance from flame center, m X = fraction of heat intensity transmitted F = fraction of heat radiated Q = heat release, kW K = allowable radiation, kW/m  

This can be estimated by the procedure outlined by Brzustowski and Sommer [20]. However, for a conservative analysis, the value oft is assumed as 1. 

This depends on the composition of gas and the burner diameter. An approximate value of F can be applied based on Table 4.15. The values presented in Table 4.15 are applicable to radiation from a gas. If liquid droplets of the 

Process engineering and design using Visual Basic 

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F = Fraction of heat radiated. Refer to the last section of this chapter for some appropriate values of F. 

WTien steam is injected at a rate of approximately 0.3 pounds of steam per pound of flare gas, the fraction of heat radiated is decreased by 20%. T is based on hydrocarbon flame at 2240°F, 80 °F dry buib air, relative humidity > 10%, distance from flame between 100 and 500 feet, and is acceptable to estimate under wide conditions. 

F = environment factor for Table 7-8 F gs = relief valve factor for non-insulated vessels in gas service exposed to open fires F = Fj, = fraction of heat radiated... 

Steam-assist flares use high pressure steam to entrain surrounding air and inject it into the core of the flare gas stream. The rapid mixing of the steam and air with the flare gas helps reduce soot formation that tends to lower the flame radiant fraction. Figure 30.14 shows a steam-assisted flare operating under identical flare gas flow conditions with and without steam-assist. Notice without steam-assist, the flame is more luminous and contains more soot this results in higher radiant fractions. The fraction of heat radiated from a flame can also be greatly increased by the presence of liquid droplets in the gas. Droplets within a hot flame can easily be converted to soot [21]. 

Height of flame center above flare tip, m h = Height of flare tip above grade, m F = Fraction of heat release radiated from the flame m = Mass flaring rate, kg/s H = Lower heating value of the flare gas, MJ/kg r = Relative humidity, percent The following are the calculation steps ... 

Another problem related to the validity of equation 9.9 is that equation 9.6 applies only to heat conduction. If T — 12 is large, some significant fraction of heat will be transferred by convection and radiation and thus will not be monitored by the thermopile. Consequently, the use of partial compensating Peltier or Joule effects was essential in the experiments involving Calvet s calorimeter, whose thermopiles had a fairly low thermal conductivity. 

In all cases, the fraction of heat energy radiated, F, lies between 0.2 and 0.4. For more detailed modeling, a correlation giving based on initial vapor pressure of the fluid at ambient temperature (Roberts, 1982) can be used ... 

The radiative fraction is the fraction of heat that is radiated from the fire plume (SFPE, 1999) it can be estimated using ... 

The higher crystalline fraction of the radiation cross-linked polyethylene even after a melt-freeze cycle has great technological merit for the heat-shrink packaging and electrical connector products. ... 

A vertical 1.5-m-bigh and 3.0-m-wide enclosure con-.sists of two surfaces separated by a 0.4-m air gap at atmospheric pressure. If the surface temperatures across the air gap are measured to be 280 K and 336 K and the surface emissivi-ties to be 0.15 and 0.90, determine tire fraction of heat transferred through the enclosure by radiation. Aosivcr.- 0.30... 

A 4-m-long section of a 5 cm diameter horizontal pipe in wliich a refrigerant flows passes tluough a room at 20°C. Tlie pipe is not well insulated and the outer surface temperature of the pipe is observed lo be - 10°C. The emissivity of the pipe surface is 0.85 and the surrounding surfaces arc at 15"C. The fraction of heat transferred to the pipe by radiation is... 

Eor the EPA evaluations, it is to be assumed that the hazardous intensity of thermal radiation q would be 5 kilowatts per square meter (0.44 BTU/sec/fE) and that the duration of exposure would be 40 seconds. Thus, if the exposed person(s) retreated from the pool fire or took refuge in the shadow of a structure, the hazard of thermal-radiation exposure could be reduced significantly. It was also assumed that the fraction of heat e that would be radiated from a pool fire would be 0.4 and that the atmospheric transmittivity x would be 1.0. Values for e, x, and q are available in the literature [26, 27, 28]. 

Before a test is started, the coordinates of the flare and the radiometers (see Chapter 6) used to measure radiation are determined by utilizing a laser range finder to measure distances to three fixed objects with known coordinates and a technique called "triangulation." Multiple radiometers are used to measure various radiant fluxes simultaneously. A photo of the radiation measurement system is shown in Figure 28.12. The measured radiant fluxes, through sophisticated mathematical analysis, are used to determine the coordinates of the effective "epicenter(s)" of the flame, and the radiant fraction, which is defined as the fraction of heat release from combustion that is emitted as thermal radiation [43]. Solar radiation is subtracted from the radiation measurements as appropriate. 

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