Surface wall resistance

This section is concerned with the UA xtiT — Text) term in the energy balance for a stirred tank. The usual and simplest case is heat transfer from a jacket. Then A xt refers to the inside surface area of the tank that is jacketed on the outside and in contact with the fluid on the inside. The temperature difference, T - Text, is between the bulk fluid in the tank and the heat transfer medium in the jacket. The overall heat transfer coefficient includes the usual contributions from wall resistance and jacket-side coefficient, but the inside coefficient is normally limiting. A correlation applicable to turbine, paddle, and propeller agitators is... 

In general, as an adsorbate is transported in the internal matrix of adsorbent, there is tendency of adsorbate-adsorbate interaction in the pores and hopping, from site to site, of adsorbed species along the wall of the adsorbent. These phenomena give rise to pore and surface diffusion resistances to intraparticle... 

If the recommended factor of 0.7 on the refractory area is used, the effective area of the tubes is [22.0-1- (0.7)(3.18)]/22.0= 1.10 mVm of actual area. The exact evaluation of the outside tube temperature from the known oil temperature would involve a knowledge of the oil-film coefficient, tube-wall resistance, and rate of heat flow into the tube, the evaluation usually involving trial and error. However, for the present purpose the temperature drop through the tube wall and oil film will be assumed to be 41.7°C ( 5°F), making the tube surface temperatures 357°C (675°F) and 468°C (875°F) the average is 412°C (775°F). The radiating gas temperature is... 

The scale or fouling resistances represent a necessary safety factor that increases the surface of the heat exchanger. This enables the full process duty requirements to be attained between cleaning periods. When an exchanger is first placed in operation there is no dirt or scale on the tubes consequently, the overall resistance consists of the two film and the tube wall resistances. During operation, dirt or scale accumulates on the surface of the tubes and the overall heat transfer rate decreases as the dirt buildup increases. The rate of this scale or dirt depends on the cleanliness or fouling tendencies of the process fluids. 

The second step in the mechanism is diffusion into the pores leading to the reacting surface sites. Resistance to this diffusion is through collisions either with other molecules (bulk diffusion) or with the walls of the pore (Knudsen diffusion). Satterfield has described methods for calculating bulk and Knudsen diflusivities, and respectively. It is important to remember that... 

In these equations, and h are the inside and outside lihn heat-transfer coefficients. A, and A are the inside and outside surface areas of the clean tube, and Rf are the inside and outside surface fouling resistances, is the interface temperature between the fluid and the surface of the fouling deposit inside the tube, tf is the interface temperature between the fluid and fouling surface on the outside, and are the inside and outside tube wall temperatures, and is the thermal conductivity of the tube material. Each of the terms in the denominator is a resistance to the heat flow, and their sum is the total resistance to heat transfer between the two fluids. These equations can be combined to eliminate the intermediate temperatures and find the heat-transfer rate using only the overall temperature difference (T- t) ... 

These resistances are illustrated in Figure 9.51. The subscript i in Equation (9.83) refers to the coefficient at the inside wall of the mixing vessel the subscript j refers to the jacket side. The other terms are the wall resistance and the fouling resistances for either side. A similar equation can be written for an internal coil or other device. In situations where both a jacket and an internal device are used, the overall coefficients for each type of surface should be calculated separately, and the two g s should be added to obtain the overall heat-transfer capability. 

The adsorbers should be protected with dust filters to prevent them from becoming filled with particulate material which would add resistance and restrict the flow of air. It is also desirable to make provision for the continuous removal of aerosols because these are not appreciably adsorbed by carbon. Smoke (a typical aerosol) is a colloidal dispersion of tars in air and, unless it is removed as soon as formed, the colloidal tars will deposit on all surfaces—walls, floors, ceilings—from which they will slowly evaporate to cause a persistent odorous environment. A typical example is a smoke-filled room on the morning after. All such after-effects can be avoided by placing... 

EXAMPLE 4S-1. Heating of Air in Turbulent Flow Air at 206.8 kPa and an average of 477.6 K is being heated as it flows through a tube of 25.4 mm inside cfiameter at a velocity of 7.62 m/s. The heating medium is 488.7 K steam condensing on the outside of the tube. Since the heat-transfer coefficient of condensing steam is several thousand W/m K and the resistance of the metal wall is very small, it will be assumed that the surface wall temperature of the metal in contact with the air is 488.7 K. Calculate the heat-transfer coefficient for an L/D > 60 and also the heat-transfer flux q/A. 

Soil—geotextile interaction in pull-out mode The geotextile length for slope and wall reinforcement is designed to provide anchorage beyond the potential failure surface. Such resistance can be modeled in the laboratory using a puU-out test. The pull-out resistance of the geotextile obtained in the test can be expressed as (Eq. [15.6]) ... 

The heat-transfer coefficient of most interest is that between the bed and a wall or tube. This heat-transfer coefficient, is made up of three components. To obtain the overall dense bed-to-boiling water heat-transfer coefficient, the additional resistances of the tube wall and inside-tube-waH-to-boiling-water must be added. Generally, the conductive heat transfer from particles to the surface, the convective heat transfer... 

For primary insulation or cable jackets, high production rates are achieved by extmding a tube of resin with a larger internal diameter than the base wke and a thicker wall than the final insulation. The tube is then drawn down to the desked size. An operating temperature of 315—400°C is preferred, depending on holdup time. The surface roughness caused by melt fracture determines the upper limit of production rates under specific extmsion conditions (76). Corrosion-resistant metals should be used for all parts of the extmsion equipment that come in contact with the molten polymer (77). 

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