Radiant absorption rate

The radiant absorption rate has been expressed in different ways by different investigators. 

A higher radiant heat absorption rate can be permitted with colder fluids and higher velocities. In steam boilers, rates of 60,000 Btu/hr. sq. ft. are not uncommon. Some data report up to four times T iis rate with exceptionally pure, silica-free water. Companies that design heaters once recommended radiant absorption rates of 12,500 to 15,000 Btu/hr. sq. ft. of circumferential tube surface. In recent years, rates up to 17,500 Btu/hr. sq. ft. have been offered. In practice, rates up to 50% higher than these are not uncommon. 

Maximum Permissible Radiant Absorption Rate Rate Of Heat Release, Btu/Hr.Ft. ... 

Choose a center-to-center spacing for the radiant tubes which is compatible with the selected tube size. Wide tube-spacing permits high radiant absorption rates with relatively low firebox temperatures and gives good circumferential heat... 

Radiant-absorption Rate. Radiation between solid surfaces is dependent upon the fourth power of the temperature difference and upon a constant, the value of which is dependent on the kind of material and the condition of the surface. Radiation from a flame, as in pipestill or boiler furnaces, is governed by the same laws except that the size of the flame... 

According to Fig. 23-6, a stack temperature of 600 F wiU be economical. This corresponds to a stack loss of about 15 to 16 per cent (Fig. 14-3). Wall losses for a steaight-up-type still (Fig. 18-ld) will be only about 4 per cent (2 per cent in the radiant and 2 per cent in the convection section). In the topping of crude oil, a radiant-absorption rate or q of 35,000 Btu p sq ft of projected area (Table 18-5) is safe, and a velocity of 3 ft per sec will not give a prohibitive pressure drop. According to Tig 18-4, the percentage of the heat liberation that is absorbed in the radiant section is about 39.4, but since the gas fuel used here is different from the one of Fig. 18-4, an R of 41 will be used. Note that this applies only to radiant sections in which two rows of tubes are used, and for center-to-center spacings that are twice the tube diameter. The heat balance is... 

Considering the distribution of tubes between the radiant and convection sections, it would probably cost little more if more tubes were put in the radiant section, and this would be highly advisable because it would reduce the radiant-absorption rate to such low values that no danger of overheating would exist. In addition, the radiant section is so small that only one row of radiant tubes (rather than two) would allow a better arrangement of surface. 

The radiant absorption rate must be larger (sometimes double) in the earlier reaction zones in order to maintain a constantly rising temperature. 

The heating operation is not a difficult one, and hence the radiant section can be designed for a radiant-absorption rate of about 35,000 Btu per sq ft of projected area per hr (Table 18-5). 

The term Qsh is the net solar radiant energy absorption rate on the basin bottom. It is equivalent to total radiation incident on the basin cover minus reflection from the cover, the water surface, and the basin bottom, and minus loss due to structural shadowing. Its determination from Weather Bureau records of total daily radiation on a horizontal surface is complicated by many factors such as variation in angle of incidence, and resulting transmissivity of cover, hourly and seasonally, intensity change due to cloudiness, and different properties of direct and diffuse radiations. Detailed explanation of these meteorological and optical calculations is beyond the scope of this paper, but may be found in the literature (6). 

This section is devoted to the results of some theoretical studies on solar radiation, absorption rates, and primary photochemical processes in smog, undertaken by Leighton (18). In work sponsored by the Air Pollution Foundation, Leighton made a critical analysis of the chemical effects that sunlight and sky radiation may have on smog formation in urban atmospheres. The radiant energy available for photochemical... 

The radiant surface — including its supporting casting, refractory walls and a portion of the supporting frame and casing — costs more per square foot than does the more compact convection bank. The cheapest overall cost will be obtained if the radiant heat absorption rate is made as high as practicable. 

For each square foot of radiant section plus the convection section, the heat absorbed per hour is Qr + Qc. The surface thus found is the equivalent plane surface having the same radiant absorption as the most exposed element of the tubes. This can be converted into other forms by applying the factors from Table 1-1. After finding the overall efficiency, the fuel rate and the weight of the combustion gases can be computed. The convection bank can then be designed by conventional methods. 

It is now necessary to check the computed radiant absorption to be sure it meets the design limitations. First, divide the heat absorption by the total exposed tube area to get the average heat flux. If it is higher than the allowable maximum, a new furnace with more tube surface must be selected and the rating repeated. If the actual heat flux is considerably below the allowable value, a smaller furnace should be considered. 

On the other hand, the specific local photon absorption rate (in [imolhi/ kg s ) (x) = fpyiKkinetic rates and radiant energy absorption rates can be easily formulated from the definition, as for any photo-reactive process (Cassano et al., 1995 Comet and Dussap, 2009 Comet et al., 2003 Pmvost and Comet, 2012), of the overall quantum yield as follows ... 

Contrary to the complete expression estabhshed previously by the authors for photobioreactors (Comet et al., 1994), the specific radiant fight absorption rate here is ignored in the dissipation fimction because the radiant fight energy in the photochemical process was taken into account directly in the affinity definitions (see Eqs. (131) to (134)). 

The first situation that can be envisaged concerns functioning close to the compensation point for photosynthesis (very low specific photon absorption rates 0) corresponding to the strongest physical limitation by radiant light energy transfer. In this condition, the specific rate is the... 

Preheating the air that is used in combustion has an effect that is the reverse of flue-gas recirculation. It tends to increase the absorption rate in the radiant section and to increase the flame temperature. To be most effective, the air should be heated by only the gases that are passing to the stack. 

The ratio of air to fuel is important mainly because it affects the flame temperature. The larger the quantity of air (or products of combustion) that must be heated in the flame the lower will be the resultant flame temperature and the lower the rate of radiation. Thus large quantities of excess air reduce the radiant absorption. For the same reason the recirculation of flue gas reduces the flame temperature, but air preheat increases it. The arrangement of the cooling surfaces also affects the rate of radiant absorption. The closer the surface is tb the flame the greater the amount of radiation that falls upon it. Thus corners and ends of furnaces receive less radiation than other surfaces (pages 609 and 611). 

Fio. 18-4. Rate of radiant absorption (Btu per square foot of projected area) for a center-to-center tube spacing that is double the tube diameter, and for two rows of radiant tubes [also see Eq. (18-6)]. 

A study of this equation for the effect of the type of fuel is useful. Tabl 18-3 is based on the fuel analyses and data given in Chap. 14. The radiant-absorption factors and rates of absorption per square foot of projected area are computed for 30 p>er cent excess air when half of the heat liberation is absorbed in the radiant section. 

Example 18-2. Rate of Absorption by Lobo-Evans. What is the total heat input required and the radiant absorption for the following still and operating conditions ... 

The rate of radiant absorption varies at different parts of the radiant section. In ordinary stills the exact distribution of radiation is not important but for (1) heating sensitive stocks such as treated lubricating oils, (2) heating to very high temperatures as in cracking stills, and (3) heating two different stocks in separate coils in the same radiant section, the rate of heat absorption in the different parts of the furnace box becomes very important. Data on several stills have been reported that indicate radian tion rates that varied by 400 per cent in different parts of the radiant section. Combustion space is not of direct importance in pipestills. 

In the unimolecular reactions which are also of the first order, only one molecule takes part in the reaction. The process of activation in unimolecular reactions, if caused by collisions should ordinarily lead to second order reactions. How then the observed rate of reaction could be of first order. If however, the activation is by absorption of the radiant energy, this problem can be avoided. But many unimolecular reactions take place under conditions where there is no absorption of radiant energy. For example... 

In a purely photochemical reaction the absorption of radiant energy is plainly responsible for the activation. This suggested the possibility that thermal reactions are also due to activation by the thermal radiation which is present at every temperature. The argument was very forcibly presented by Perrin who showed that if the specific rate of a imimolecular gas reaction remains constant, with indefinite diminution in pressure, activation must be by radiation since the number of opportunities for activation by collision also diminishes without limit. In fact, the decomposition of nitrogen pentoxide, the first gas reaction shown to be unquestionably unimolecular, was found to have a specific reaction rate constant over a wide range of pressure, and apparently increasing at very low pressures. ... 

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