Heat exchangers surface emissivity

As alluded to in Chapter 8, the ideal biomass feedstock for thermal conversion, whether it be combustion, gasification, or a combination of both, is one that contains low or zero levels of elements such as nitrogen, sulfur, or chlorine, which can form undesirable pollutants and acids that cause corrosion, and no mineral elements that can form inorganic ash and particulates. Ash formation, especially from alkali metals such as potassium and sodium, can lead to fouling of heat exchange surfaces and erosion of turbine blades, in the case of power production systems that use gas turbines, and cause efficiency losses and plant upsets. In addition to undesirable emissions that form acids (SOx), sulfur can... 

This paper presents some of the opportunities CFD offers when applied to analyse different combustion systems. Practical examples presented are ash deposition predictions on heat exchanger surfaces and walls in a bubbling fluidised bed furnace and detailed nitrogen oxide emission predictions for the same furnace type. Furthermore, the extension of a standard model using process specific data is presented for the fuel conversion process in a black liquor recovery furnace. 

Form view factor A factor which describes the effects of the relative area of two surfaces, the geometry of the surfaces in relation to each other, and the two emissivities on radiation heat exchange between the surfaces. 

Two large parallel plates with grey surfaces are situated 75 mm apart one has an emissivity of 0.8 and is at a temperature of 350 K and the other has an emissivity of 0.4 and is at a temperature of 300 K. Calculate the net rate of heat exchange by radiation per square metre taking the Stefan-Boltzmann constant as 5.67 x 10-8 W/m2 K4. Any formula (other than Stefan s law) which you use must be proved. 

For the given radii and temperatures of the sphere and the disk, the following four possible fiiodificalioas could increase the net rate of radiation heat exchange paint each of the two surfaces to alter their emissivities, adjust the distance between them, and provide an (refractory) endosu. Calculate the net rate of radiation heat ex-chang belween the two bodies if the best values are selected for each of the above inudificalions. 

The second issue is the improvement of the low-temperature performance of combustion catalysts, i.e., the activity at combustor inlet conditions. All the proposed catalytic combustor designs available today need a pilot flame, or a heat exchanger in the case of recuperative gas turbines, to heat the compressed combustion air to a temperature sufficient for ignition of the catalyst. The possibility of avoiding this pilot flame is considered very important, since it would further reduce NO emissions. The catalyst surface area and washcoat loading are very important for the low-tempcraturc activity. 

The term e is called the emissivity of the surface and has a value between 0 and 1. When two blackbodies exchange heat by radiation, the net heat exchange is then proportional to the difference in T4. If the first body sees only body 2, then the net heat exchange from body 1 to body 2 is given by... 

Open-weave liner (Murphy/Jahn Architects, 2006 Holst, 2006). The low-emissivity coating has the effect of blocking the radiative heat exchange between the external membrane surface and internal surfaces whilst reflecting the coolness of the chilled floor (Holst and Schuler, 2003). Without this... 

Another NG-fired OTM-based oxy-fuel plant is the zero emission ion transport membrane oxygen power (ZEITMOP) cycle (Fig. 10.11) proposed by Yantovski and co-authors. The cycle is based on a supercritical CO2 cycle, where CO2 is compressed in an intercooled compressor to over 200 bar, heated in a recuperative heat exchanger and expanded in a high pressure turbine to 15 bar. It is then used as sweep gas in an OTM, where it is enriched with O2 separated from a stream of compressed air. The CO2/O2 flow is used as oxidant in a NG combustor which produces high temperature oxidized gas to be expanded to nearly ambient temperature in a low pressure turbine. Efficiencies of 50.4-52.0% with virtually zero CO2 emissions are reported. " A reactive membrane configuration can also be adopted to reduce the required membrane surface area. In this case, cycle efficiency... 

Hence, knowledge of the emissivity completely characterizes the surface. Of course, non-gray and non-diffuse surfaces bring several degrees of complication into the calculations. By employing the concept of radiosity and irradiation illustrated in Figure 7.10, the net thermal radiation heat exchange between surfaces / can be computed by... 

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