Area for heat transfer

Fill Packing Specially designed baffling used to provide a large surface area for heat transfer. Two classes of materials are used splash bars of wood, metal transite or plastic and film pack (cellular fill). The splash type cools the water as the droplets bounce down a series of bars in the air stream film packing converts droplets into a thin film. 

Before eonsidering the effects of water injection in an EGT type plant, it is worthwhile to refer to the earlier studies on the performanee of some dry recuperative cycles. Fig. 6.6 shows the T..s diagram of a [CBT i X r cyele, with a heat exchanger effectiveness of unity. It is implied that the surface area for heat transfer is very large, so that the outlet temperature on the cold side is the same as the inlet temperature on the hot side. However, due to the higher specific heat of the hot gas, its outlet temperature is higher than the inlet temperature of the cold air. 

The exposed area for heat transfer (A g) is then replaced on the premise that, for a set of similar gas turbines, there is a reasonably constant ratio between A g and the cross-sectional area of the main hot gas flow A g. Thus, writing A g = AA g = XWg/pgVg in Eq. (A3) gives... 

In some cases, particularly for the radial flow of heat through a thick pipe wall or cylinder, the area for heat transfer is a function of position. Thus the area for transfer applicable to each of the three media could differ and may be A, A2 and A3. Equation 9.3 then becomes ... 

In model equations, Uf denotes the linear velocity in the positive direction of z, z is the distance in flow direction with total length zr, C is concentration of fuel, s represents the void volume per unit volume of canister, and t is time. In addition to that, A, is the overall mass transfer coefficient, a, denotes the interfacial area for mass transfer ifom the fluid to the solid phase, ah denotes the interfacial area for heat transfer, p is density of each phase, Cp is heat capacity for a unit mass, hs is heat transfer coefficient, T is temperature, P is pressure, and AHi represents heat of adsorption. The subscript d refers bulk phase, s is solid phase of adsorbent, i is the component index. The superscript represents the equilibrium concentration. 

Fig. 3.2 shows the case of a jacketed, stirred-tank reactor, in which either heating by steam or cooling medium can be applied to the jacket. Here V is volume, Cp is specific heat capacity, p is density, Q is the rate of heat transfer, U is the overall heat transfer coefficient, A is the area for heat transfer, T is temperature, H is enthalpy of vapour, h is liquid enthalpy, F is volumetric flow... 

The rate of heat transfer is most conveniently expressed in terms of an overall heat transfer coefficient, the effective area for heat transfer and an overall temperature difference, or driving force, where... 

Here, Um is the film heat transfer coefficient between the reactor and the reactor wall, Uj is the film heat transfer coefficient between the reactor wall and the jacket. Am is the area for heat transfer between the reactor and the wall and Aj is the area for heat transfer between the wall and the jacket. 

When the areas for heat transfer are not the same, it can be shown that... 

This mode is seldom used except for extremely slow processes such as fermentation or for very small reactors where the surface area for heat transfer is large enough to maintain the reactor thermostatted at the temperature of the surroundings. In fact, we seldom want to operate a reactor isothermally, because we want to optimize the temperature and temperature profile in the reactor to optimize the rate and selectivity, and this is most efficiently achieved... 

A = area for heat transfer t = specific heat capacity of medium H = heat content of steam relative to initial medium temperature = mass flow rate of steam At = initial mass of medium q= rate of heat transfer t = time T = temperature = initial temperature of medium = temperature of heat source and U = overall heat transfer coefficient. 

Surface area for heat transfer per unit hydraulic radius... 

In principle, however, the effective surface area for heat transfer from the single monomer is decreased by the contact area between the particles. The fractal dimension of the aggregates varies in different systems and depends on the evolution process. For dense aggregates, heat transfer rates are decreased due to the reduced heat exchange area compared to primary particles only connected by point contact in a chain like structure (Liu et al., 2006a-c). This leads to an overestimation of... 

The area for heat transfer varies with radius therefore, we define the overall heat transfer coefficient based upon the outer area of the cylinder, A0=2nr0L, where L is the length of the cylinder, giving... 

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