Heat absorption

Compared to natural gas and oil, complete combustion of coal requires higher levels of excess air, about 15% as measured at the furnace outlet at high loads, and this also serves to avoid slagging and foifling of the heat absorption equipment. 

Furnaces for Oil and Natural Gas Firing. Natural gas furnaces are relatively small in size because of the ease of mixing the fuel and the air, hence the relatively rapid combustion of gas. Oil also bums rapidly with a luminous flame. To prevent excessive metal wall temperatures resulting from high radiation rates, oil-fired furnaces are designed slightly larger in size than gas-fired units in order to reduce the heat absorption rates. 

Tanks with contents at lower than atmospheric temperature may require insulation to minimize heat absorption. The insulation must have a vapor barrier at the outside to prevent condensation of atmospheric moisture from reducing its effectiveness. An insulation not damaged by moisture is preferable. The insulation techniques presently used for refrigerated systems can be apphed (see subsection Fow-Temperature and Cryogenic Storage ). 

Low-temperature tanks may be installed underground to take advantage of the insulating value of the earth. Frozen-earth storage is also used. The frozen earth forms the tank. Some installations using this technique have been unsuccessful because of excessive heat absorption. 

FIG. 27-37 Heat absorption distribution for various types of boilers. (Adapted ft vm Singer, Combustion—Fossil Power, 4th ed., Combustion Kngineeting, Inc., Windsor, Conn., 1991.)... 

The thicker plate for operation at higher pressures increases the cost of the boiler. As a result, it is normally not economical to use a boiler bank for heat absorption at pressures above 10.7 MPa (1550 psia). 

TABLE 27-22 Heat Absorption Distribution in Bubbling AFBCs... 

During heat dissipation by radiation the colour and condition of the surface plays a similar role. Dark-coloured bodies dissipate more heat than the light-coloured ones. The amount of heat absorption and emission for the same body may therefore be assumed to be almost the same. Accordingly, Table 31.1, for selected colours, may be considered for the coefficients of absorption and emission of heat due to solar radiation and natural radiation respectively. 

Due to the great variation in pressures, flux rates, materials of construction, heat recovery, burner configuration, etc., correlation of process heaters is difficult even with large amounts of data. For similar furnaces, heat absorption vs. cost gives the best correlation. It is again recommended that vendor help be obtained for estimating process furnaces, unless data on similar furnaces is available. Data can be found in References 24 and 25. 

All heat absorption from fire exposure is considered as latent heat and no credit is taken for the sensible heat capacity of the fluid in the vessel. The vapor release rate W is calculated from ... 

Superimposing Figures 6-19 and 6-20, respeetively, possible points of equality in heat generation and heat absorption ean be observed as in Figure 6-21. 

From the slope and intereept of the heat absorption line, it is possible to manipulate Equation 6-117 by ehanging operating variables sueh as u, Tq, and T or design variables sueh as the dimensionless heat transfer group UA/puCp. It is also possible to alter the magnitude of the reaetion exotherm by ehanging the inlet reaetant eoneentrations. Any of tliese manipulations ean be used to vary tlie number of loeations of the possible steady states. 

Absorption coefficient Measure of the amount of sound or heat absorption provided by a material. 

Q = total heat absorption (input) to the wetted surface, in BTU per hour. 

Note Above 2800 square feel of weiied surface area, the total heat absorption is considered to remain constant for nonrefrigerated tanks below I pound per squa. e inch gage. For nonrefrigerated tanks above I pound per square inch gage and for all refrigerated tanks, the total heat absorption continues to increase with wetted surface area. This is the reason why the curve splits above 2800 square feel. 

The total furnace heat absorption may be estimated by using the calculated furnace exit gas temperature and analysis to determine the enthalpy (excluding the latent heat of water vapor) and thus deducting the heat rejection rate from the net heat input rate. 

We see that the sign of AH is sensible. It is positive when heat content is rising (by heat absorption) and it is negative when heat content is dropping (by heat evolution). This is shown diagrammatically in Figure 7-1. 

Qa = required flow, cu ft/min at actual flowing temperature and pressure, ACFM Qt = heat released by flame, Btu/hr Qr = heat release, lower heating valve, Btu/hr Qs = required flow, cu ft/min at standard conditions of 14.7 psia and 60°F, SCFM q = Average unit heat absorption, Btu/hr/sq ft of wetted surface... 

It is not essential, however, that the unit of heat should be defined in terms of the rise of temperature produced when heat is absorbed by a standard body, say unit mass of water. Any effect of heat absorption which is capable of measurement and numerical expression might be used, and the method of measurement would in all cases be consistent with the axiom that if two identical systems are acted upon by heat in the same way so as to produce two other identical systems, the quantities of heat supplied to the systems are equal. Lavoisier and Laplace (1780-84) took as unit that quantity of heat which must be absorbed by unit mass of ice in order to convert it completely into water. This unit is of course different from the one we adopted, but if a quantity of heat A has been found to raise from lo ° to 16 ° twice as much water as another quantity of heat B, then A will also melt twice as much ice as B. 

Our problem is to determine how the changes of total and free energy, AU and A P, or, what are the same, the heat absorption at constant configuration and the maximum work, Qx and At, of an isothermal and reversible process, alter with the temperature of execution of the process. 

In the development of the theory of freezing-points for very dilute solutions (Chap. XI.) it was assumed that both the change of total volume and the heat absorption on further dilution are zero. With solutions of moderate concentration (say up to 5N), neither assumption is true, but we know that the change of volume is always very small, and possibly negligible, whilst the heat absorption is not usually of such small magnitude. It... 

Designs include the once-through boiler, which can operate at sub-critical, critical, or supercritical pressures and at all commercial temperatures, and the radiant boiler, where heat absorption is largely by radiant energy transfer. 

Equipment located inside the boiler furnace area to direct the path of hot combustion gases and thus gain maximum heat absorption. 

FIGURE 14.16 An evaporation x>ncl The blue color is due to a dye added to the brine to increase heat absorption and hence speed up evaporation. 

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