Overall coefficient of heat transfer

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)... 

Overall Coefficient of Heat Transfer In testing commercial heat-transfer equipment, it is not convenient to measure tube temperatures (t,3 or t4 in Fig. 5-6), and hence the overall performance is expressed as an overall coefficient of heat transfer U based on a convenient area dA, which may be dAi, oi an average of dAi and dA whence, by definition,... 

U is called the overall coefficient of heat transfer, or merelv the overall coefficient. The rate of conduction through the tube wall and scale deposit is given by... 

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. 

In the above example, 1 lb of initial steam should evaporate approximately 1 lb of water in each of the effects A, B and C. In practice however, the evaporation per pound of initial steam, even for a fixed number of effects operated in series, varies widely with conditions, and is best predicted by means of a heat balance.This brings us to the term heat economy. The heat economy of such a system must not be confused with the evaporative capacity of one of the effects. If operated with steam at 220 "F in the heating space and 26 in. vacuum in its vapor space, effect A will evaporate as much water (nearly) as all three effects costing nearly three times its much but it will require approximately three times as much steam and cooling water. The capacity of one or more effects in series is directly proportional to the difference between the condensing temperature of the steam supplied, and the temperature of the boiling solution in the last effect, but also to the overall coefficient of heat transfer from steam to solution. If these factors remain constant, the capacity of one effect is the same as a combination of three effects. 

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].

An exchanger has heen examined, and the following individual coefficients and resistances determined. What is the overall coefficient of heat transfer referenced to outside coefficients (Methods for determining these film coefficients are given later). 

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. 

Toluene is continuously nitrated to mononitrotoluene in a cast-iron vessel, 1 m diameter, fitted with a propeller agitator 0.3 m diameter rotating at 2.5 Hz. The temperature is maintained at 310 K by circulating 0.5 kg/s cooling water through a stainless steel coil 25 mm o.d. and 22 mm i.d. wound in the form of a helix, 0.80 m in diameter. The conditions are such that the reacting material may be considered to have the same physical properties as 75 per cent sulphuric acid. If the mean water temperatute is 290 K, what is the overall coefficient of heat transfer ... 

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. 

Estimate the heat transfer area required for the system considered in Examples 9.1 and 9.36. assuming that no data on the overall coefficient of heat transfer are available. 

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. 

A reaction mixture is heated in a vessel fitted with an agitator and a steam coil of area 10 m2 fed with steam at 393 K. The heat capacity of the system is equal to that of 500 kg of water. The overall coefficient of heat transfer from the vessel of area 5 m2 is 10 W/m2 K. It takes 1800 s to heat the contents from ambient temperature of 293 to 333 K. How long will it take to heat the system to 363 K and what is the maximum temperature which can be reached ... 

U overall coefficient of heat transfer between reaction mixture and coolant W m K 500-1500... 

U0 Overall coefficient of heat transfer, based on outside tube area, Btu/(h)(ft2)(°F)... 

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. 

It has long been established 18 that, with scale formation, the overall coefficient of heat transfer may be expressed as a function of the boiling time by an equation of the form ... 

A solution containing 23 per cent by mass of sodium phosphate is cooled from 313 to 298 K in a Swenson-Walker crystalliser to form crystals of Na3P04.12H20. The solubility of Na3P04 at 298 K is 15.5 kg/100 kg water, and the required product rate of crystals is 0.063 kg/s. The mean heat capacity of the solution is 3.2 kJ/kg deg K and the heat of crystallisation is 146.5 kJ/kg. If cooling water enters and leaves at 288 and 293 K, respectively, and the overall coefficient of heat transfer is 140 W/m2 deg K, what length of crystalliser is required ... 

Values of overall coefficients of heat transfer are collected in Tables 17.10-17.12. Two sets of formulas for tank-side film coefficients are in Tables 17.13 and 17.14. They relate the Nusselt number to the Reynolds and Prandtl numbers and several other factors. In the equation for jacketed tanks, for example,... 

In a condenser, the rate of heat flow (Q) is proportional to the condenser area (A), the mean temperature difference (A7"m) between the coolant and vapour streams and the overall coefficient of heat transfer ( ) ... 

Take the coefficient of heat transfer on the water side as 2.5, and on the vapour side as 0.8 kW/m2 K and assume that the overall coefficient of heat transfer from vapour to water, based upon these figures, is reduced 20% by the effects of the pipe walls, dirt and scale. The latent heat of the hydrocarbon vapour at 345 K is 315 kJ/kg. 

A continuous evaporator is being used to concentrate a scale-forming solution of sodium sulfate in water. The overall coefficient of heat transfer decreases according to the following expression ... 

Many of the important cases of heat transfer involve the flow of heat from one fluid through a solid retaining wall into another fluid. This heat must flow through several resistances in series. The net rate of heat transfer can be related to the total temperature-difference driving force by employing an overall coefficient of heat transfer [/ thus, for steady-state conditions,... 

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