Radiation heat transmission

Thickness. The traditional definition of thermal conductivity as an intrinsic property of a material where conduction is the only mode of heat transmission is not appHcable to low density materials. Although radiation between parallel surfaces is independent of distance, the measurement of X where radiation is significant requires the introduction of an additional variable, thickness. The thickness effect is observed in materials of low density at ambient temperatures and in materials of higher density at elevated temperatures. It depends on the radiation permeance of the materials, which in turn is influenced by the absorption coefficient and the density. For a cellular plastic material having a density on the order of 10 kg/m, the difference between a 25 and 100 mm thick specimen ranges from 12—15%. This reduces to less than 4% for a density of 48 kg/m. References 23—27 discuss the issue of thickness in more detail. 

Specialized heat transfer nomenclature used for radiative heat transfer is defined in the subsection Heat Transmission hy Radiation. Nomenclature for mass transfer is defined in the subsection Mass Transfer. ... 

Example 5 Radiation in a Furnace Chamber A furnace chamber of rectangular paraUelepipedal form is heated hy the combustion of gas inside vertical radiant tubes hningthe sidewalls. The tubes are of 0.127-m (5-in) outside diameter on 0.305-m (12-in) centers. The stock forms a continuous plane on the hearth. Roof and end walls are refractory. Dimensions are shown in Fig. 5-20. The radiant tubes and stock are gray bodies having emissivities of 0.8 and 0.9 respectively. What is the net rate of heat transmission to the stock by radiation when the mean temperature of the tube surface is SIG C (1500 F) and that of the stock is 649 C (1200 F) ... 

Design Methods for Calciners In indirect-heated calciners, heat transfer is primarily by radiation from the cyhnder wall to the solids bed. The thermal efficiency ranges from 30 to 65 percent. By utilization of the furnace exhaust gases for preheated combustion air, steam produc tion, or heat for other process steps, the thermal efficiency can be increased considerably. The limiting factors in heat transmission he in the conductivity and radiation constants of the shell metal and solids bed. If the characteristics of these are known, equipment may be accurately sized by employing the Stefan-Boltzmann radiation equation. Apparent heat-transfer coefficients will range from 17 J/(m s K) in low-temperature operations to 8.5 J/(m s K) in high-temperature processes. 

Radiation Heat The transmission of heat through the medium of heat rays. 

Heat transfer (heat transmission) is an important unit operation in chemical and bioprocess plants. In general, heat is transferred by one of the three mechanisms, namely, conduction, convection, and radiation, or by their combinations. However, we need not consider radiation in bioprocess plants, which usually operate at relatively low temperatures. The heating and cooling of solids rarely become problematic in bioprocess plants. 

The most important property for insulation is thermal conductivity. The following transport types participate in the transmission of heat heat conduction in PS, heat conduction in the filling gas (air), radiation heat transfer and heat convection by convection flows in the closed cells. The thermal conductivity of the air in the cells contributes the most to the total heat transport. The radiation fraction depends on the diameter of the cells formed. The thermal conductivity depends on the density of the foamed PS material. Thermal conductivity decreases with increasing bulk density, reaches a minimum and then rises again (Figure 9.15). The following processes are responsible for this characteristic. 

Many familiar heal transfer application. involve natural convection as the primary mechanism of heat transfer. Some examples are cooling of electronic equipment such as power transistors, TVs, and DVDs heat transfer from electric baseboard heaters or steam radiators heat transfer from the refrigeration coils and power transmission lines and heat transfer from the bodies of animals and human beings. Natural convection in gases is usually accompanied by radiation of comparable magnitude except for low-emissivity surfaces. 

H. C. Hottel. "Heat Transmission by Radiation from Non-tuminous Gases," Transaction of the AlChE 921), pp. 173-205. 

The additional heat flow is transferred by liquid — vapor circulation and the contribution of this flow to the total coefficient of thermal conductivity is unrelated to the four well-known mechanisms of heat transfer in plastic foams heat transmission of gas heat transmission of the poljmieric matrk, heat radiation and convection. 

All bodies with a temperature above absolute zero radiate heat in the form of electromagnetic waves. The radiation may be transmitted, reflected, or absorbed by matter, the fraction absorbed being transformed into heat. Radiation is of great importance at very high or very low temperatures and under circumstances in which the other modes of heat transmission are suppressed. Although the heat losses can, in some cases, equal the losses by natural convection, the mechanism is, from the standpoint of pharmaceutical processing, least important and needs only brief consideration. 

In the case of solid crystalline oxides, thermal conductivity decreases with increasing temperature but begins to rise above 1500— 1600 °C because transmission of heat by radiation (photons) begins to take a significant part besides the conduction of heat (phonon mechanism). In completely transparent materials (the coefficient of absorption a = O), no interaction with the radiation occurs in an opaque body (a = oo) the heat is transferred by conduction alone. With translucent materials, each element of the substance absorbs some of the incident radiation, and emits simultaneously,This internal radiation mechanism of heat transmission is characteristic for glasses. At high temperatures, a considerable proportion of heat is therefore transmitted by radiation the so-called apparent thermal conductivity is a sum of true conductivity with radiation conductivity ... 

Hottel, H.C. Mangelsdorf, H.G. Heat transmission by radiation from nonluminous gases. Trans. Amer. Inst. Chem. Eng. 31 (1935) 517-549... 

Figure 15-17. Block diagram of the thermal flow in a lubricated gear and bearing system. Heat sources.—A oil film at tooth contact B churning of bulk oil C oil film in bearings and bulk churning D oil film at Seals E external sources. Heat transmission.—F m G c H c, m I c J f K m L f M f N m P f Q c, f, r R c S n, f, r T c. c = conduction f = forced convection m = mass transport n = natural convection r = radiation.

Absorptivities of various solids vs, source temperature and peak wavelength of incident radiation. (By permission of author and publisher, from H. C. Hottel, p. 62 in W. H. McAdams, Heat Transmission. Copyright by author, 1954, McGraw-Hill Book Company.)... 

Heat Transmission Coefficient - Any coefficient used to calculate heat transmission by conduction, convection, or radiation through materials or structures. 

As far as the heat transmission coefficient from the containment outside surface to the atmosphere in the absence of external spray is concerned, it is worthwhile remembering that the contribution of radiation is important. The coefficient values usually range from 5 to 20 Cal m hr°C according to the building layout adopted. If the external spray is supposed to operate, the transmission coefficient between paint and spray water is of the order of 500-5000 Cl m2hr°C. ... 

---