Surface coefficient of heat transfer

Btu/ (hr) (ft ) (°F) or, heat transfer film coefficient between the insulated or bare pipe and air see Figure 10-171 assume 6 = 0.90 and ambient air = 70°F = surface coefficient of heat transfer,... 

Would it be feasible to use a magnesia insulation which will not stand temperatures above 615 K and has a thermal conductivity 0.09 W/m K for an additional layer thick enough to reduce the outer surface temperature to 370 K in surroundings at 280 K Take the surface coefficient of heat transfer by radiation and convection as 10 W/m- IC... 

For constant surface coefficient of heat transfer a, the heat flux is given by... 

A copper fin L-ft long is triangular in cross-section [6]. It is w-ft thick at the base and tapers off to a line (see Figure 3.10). The base of this wedge-shaped piece of metal is maintained at a constant temperature Ta, and the fin loses heat by convection to the surrounding air, which is at a temperature T. The surface coefficient of heat transfer is h Btu/h ft °F. Derive the relationship between the temperature, T, of the metal fin and the distance from the base. L x. (Hint assume J is a function of X only.)... 

On the left-hand side is the heat flux Cht, liie surface coefficient of heat transfer, is an empirical constant that depends very much on the nature of the heat transport as well as on the surface structure, in case of two solid bodies on the contact pressure, and on the presence and nature of a fluid medium (gas or liquid) between the two bodies. This is of importance in calorimeters in which the tested substance is put into special containers (crucibles) that are then placed inside the calorimeter. If no measures are taken to ensure well-defined, reproducible heat transfer, the temperature difference involved in the heat exchange between the measuring system and the sample (or respectively, the crucible) may differ from one measurement to another, so variations will occur in the temperature as shown by the sensor relative to the actual temperature of the sample. As a consequence, the measured heat quantity may also differ, leading to an uncertainty of the result. 

U Overall coefficient of heat transfer for outside surface basis U for overall coefficient between liquid-vapor interface and coolant J/(s-m -K) Btu/(h-fF- F)... 

T = absolute temperature. Subscript 1 (or G), radiating surface (or gas) temperature subscript E, exit-gas subscript o, base temperature subscript E, pseudoadiabatic flame temperature based on C averaged from to Te-U = overall coefficient of heat transfer, gas convec tion to refractory wall to ambient air. 

Equivalent-Area Concept The preceding equations for batch operations, particularly Eq. 11-35 can be appliedforthe calculation of heat loss from tanks which are allowed to cool over an extended period of time. However, different surfaces of a tank, snch as the top (which would not be in contact with the tank contents) and the bottom, may have coefficients of heat transfer which are different from those of the vertical tank walls. The simplest way to resolve this difficulty is to nse an equivalent area A in the appropriate equations where... 

The value of the coefficient of heat transfer from steam to sheet is determined by the conditions prevailiug on the inside and on the surface of the dryers. Low coefficients may be caused by (1) poor removal of air or other noncoudeusables from the steam in the cyhn-ders, (2) poor removal of condensate, (3) accumulation of oil or rust on the interior of the drums, and (4) accumulation of a fiber lint on the outer surface of the drums. In a test reported by Lewis et al. [Pulp Pap. Mag. Can., 22 (Februaiy 1927)] on a sulfite-paper diyer, in which the actual sheet temperatures were measured, a value of 187 W/(m °C) [33 Btu/(h ft" °F)j was obtained for the coefficient of heat flow between the steam and the paper sheet. 

Double-Pipe Scrapea-Surface Crystallizer This type of equipment consists of a double-pipe heat exchanger with an internal agitator fitted with spring-loaded scrapers that wipe the wall of the inner pipe. The cooling hquid passes between the pipes, this annulus being dimensioned to permit reasonable shell-side velocities. The scrapers prevent the buildup of solids and maintain a good film coefficient of heat transfer. The equipment can be operated in a continuous or in a recirculating batch manner. 

Figure 5-24B. Forced convection past heat transfer surfaces improves the overall coefficient of heat transfer. By permission, Weber, A. R, Chem. Engr., Oct. 1953, p. 183 [23].

A heat exchanger is required to cool 20 kg/s of water from 360 K to 340 K by means of 25 kg/s water entering at 300 K. If the overall coefficient of heat transfer is constant at 2 kW/m2K, calculate the surface area required in < a) a countercurrent concentric tube exchanger, and (b) a co-current flow concentric tube exchanger. 

Radiation arrives at a grey surface of emissivity 0.75 al a constant temperature of 400 K, at the rate of 3 kW/m2. What is the radiosity and the net rate of radiation transfer to the surface What coefficient of heat transfer is required to maintain the surface temperature at 300 K if the rear of the surface is perfectly insulated and the front surface is cooled by convective heat transfer to air at 295 K ... 

A vessel contains 1 tonm (I Mg) of a liquid of specific heat capacity 4.0 kj/kg K. The vessel is heated by steam at 393 K which is fed to a cod immersed in the agitated liquid and heat is lost to the surroundings at 293 K from the outside o." the vessel. How long dots it take to heat the liquid from 293 to 353 K and what is the maximum temperature to which the liquid can be heated When the liquid temperature has reached 353 K, the steam supply is tinned off for 2 hours (7.2 ks and the vessel cools. How long will it take to reheat the material to 353 K The surface area of the coil is 0 5 m2 and the overall coefficient of heat transfer to the liquid may be taken as 600 W/m2 K. The outside area of Lie vessel is 6 m2 and the coefficient of heat transfer to the surroundings may be taken as 10 W/m2 K. 

A process requires a flow of 4 kg/s of purified water at 340 K to be heated from 320 K by 8 kg/s of untreated water which can be available at 380, 370, 360 or 350 K. Estimate the heat transfer surfaces of one shell pass, two tube pass heat exchangers suitable for these duties. In all cases, the mean heat capacity of the water streams is 4.18 kJ/kg K and the overall coefficient of heat transfer is 1.5 kW/m2 K. 

In describing chillers for the production of wax distillates, Nelson1 L 2> points out that the rate of cooling depends very much on the effectiveness of the scrapers, and quotes overall coefficients of heat transfer ranging from 15 W/m2 K with a poorly fitting scraper to 90 W/m2 K where close fitting scrapers remove the wax effectively from the chilled surface. 

Eagle , A. and Ferguson. R.M. Proc. Roy. Soc. 127 (1930) 540. On the coefficient of heat transfer from the internal surfaces of tube walls. 

If a layer of insulating material 25 mm thick, of thermal conductivity 0.3 W/m K, is added, what temperatures will its surfaces attain assuming the inner surface of the furnace to remain at 1400 K The coefficient of heat transfer from the outer surface of the insulation to the surroundings, which are at 290 K, may be taken as 4.2. 5.0, 6.1, and 7.1 W/m K, for surface temperatures of 370, 420, 470, and 520 K respectively. What will he the reduction in heat loss ... 

The coefficients of heat transfer from the inner metal surface to the boiling liquid depend upon the temperature difference are ... 

A longitudinal tin on the outside of a circular pipe is 75 mm deep and 3 mm thick. If tire pipe surface is at 400 K. calculate the heat dissipated per metre length from the fin to the atmosphere at 290 K if the coefficient of heat transfer from its surface may be assumed constant at 5 W/m2 K, The thermal conductivity of the material of the fin is 50 W/m K and the heat loss from the extreme edge of the fin may be neglected. It should be assumed that the temperature is uniformly 400 K at the base of the fin. 

A pipe, 50 mm outside diameter, is carrying steam at 413 K and the coefficient of heat transfer from its outer surface to the surroundings at 288 K is 10 W/m2 K. What is the heat loss per unit length ... 

A single-effect evaporator is used to concentrate 7 kg/s of a solution from 10 to 50 per cent solids. Steam is available at 205 kN/m2 and evaporation takes place at 13.5 kN/m2. If the overall coefficient of heat transfer is 3 kW/m2 deg K, estimate the heating surface required and the amount of steam used if the feed to the evaporator is at 294 K and the condensate leaves the heating space at 352.7 K. The specific heats of 10 and 50 per cent solutions are 3.76 and 3.14 kJ/kg deg K respectively. 

In the case where a fluid flows across a bank of tubes, the film coefficient of heat transfer at the tube outside surface can be estimated using the following equation [1] ... 

The steam-side coefficient of heat transfer is converted to based on the inside tube surface. 

For reactors of a larger diameter (e.g. more than 600 mm) it is more advisable to use a bundle of small tubes as heat exchangers. Such a distribution of heat exchange surfaces virtually does not inhibit fluidising and ensures that the heat is efficiently withdrawn from the whole surface of the apparatus. It should be also kept in mind that tube bundles allow one to select the reaction space and place rotating gas distribution devices between sections this considerably increases the coefficient of heat transfer and ensures a more uniform gas distribution in the reaction zone. 

The following symbols are used in the definitions of the dimensionless quantities mass (m), time (t), volume (V area (A density (p), speed (u), length (/), viscosity (rj), pressure (p), acceleration of free fall (p), cubic expansion coefficient (a), temperature (T surface tension (y), speed of sound (c), mean free path (X), frequency (/), thermal diffusivity (a), coefficient of heat transfer (/i), thermal conductivity (/c), specific heat capacity at constant pressure (cp), diffusion coefficient (D), mole fraction (x), mass transfer coefficient (fcd), permeability (p), electric conductivity (k and magnetic flux density ( B) ... 

Derive an expression for the optimum economic thickness of insulation to put on a flat surface if the annual fixed charges per square foot of insulation are directly proportional to the thickness, (a) neglecting the air film, (b) including the air film. The air-film coefficient of heat transfer may be assumed as constant for all insulation thicknesses. 

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