Evaporation heat transfer area

Sindlady, heating surface area needs are not direcdy proportional to the number of effects used. For some types of evaporator, heat-transfer coefficients decline with temperature difference as effects are added the surface needed in each effect increases. On the other hand, heat-transfer coefficients increase with temperature level. In a single effect, all evaporation takes place at a temperature near that of the heat sink, whereas in a double effect half the evaporation takes place at this temperature and the other half at a higher temperature, thereby improving the mean evaporating temperature. Other factors to be considered are the BPR, which is additive in a multiple-effect evaporator and therefore reduces the net AT available for heat transfer as the number of effects is increased, and the reduced demand for steam and cooling water and hence the capital costs of these auxiUaries as the number of effects is increased. 

Heat transfer equipment has a great variation in heat transfer area per unit of material volume. Table 4.4 compares the surface compactness of a variety of heat exchanger types. Falling film evaporators and wiped film heat exchangers also reduce the inventory of material on the tube side. Process inventory can be minimized by using heat exchangers with the minimum volume of hazardous process fluid for the heat transfer area required. 

If a more complex mathematical model is employed to represent the evaporation process, you must shift from analytic to numerical methods. The material and enthalpy balances become complicated functions of temperature (and pressure). Usually all of the system parameters are specified except for the heat transfer areas in each effect (n unknown variables) and the vapor temperatures in each effect excluding the last one (n — 1 unknown variables). The model introduces n independent equations that serve as constraints, many of which are nonlinear, plus nonlinear relations among the temperatures, concentrations, and physical properties such as the enthalpy and the heat transfer coefficient. 

A liquor containing 15 per cent solids is concentrated to 55 per cent solids in a double-effect evaporator, operating at a pressure in the second effect of 18 kN/m2. No crystals are formed. The flowrate of feed is 2.5 kg/s at 375 K with a specific heat capacity of 3.75 kJ/kg K. The boiling-point rise of the concentrated liquor is 6 deg K and the steam fed to the first effect is at 240 kN/m2. The overall heat transfer coefficients in the first and second effects are 1.8 and 0.63 kW/m2 K, respectively. If the heat transfer area is to be the same in each effect, what areas should be specified ... 

The two key temperatures of the liquefaction process (inversion and liquid product temperatures) are controlled by TC-25 and TC-26. TC-25 modulates the level set point of LC-24 of the LN2 accumulator, and TC-26 adjusts the level set point of FC-27 of the LH2 accumulator. The LH2 (or nitrogen) supplies to the evaporators are controlled by cascade loops that adjust the levels in the accumulators, which in turn vary the heat transfer area, and therefore, the rate of evaporation. 

A shell-and-tube heat exchanger is used for preheating the feed to an evaporator. The liquid of specific heat 4.0 kJ/kg K and density 1100 kg/m3 passes through the inside of tubes and is heated by steam condensing at 395 K on the outside. The exchanger heats liquid at 295 K to an outlet temperature of 375 K when the flowrate is 1.75 x 10 4 m3/s and to 370 K when the flowrate is 3.25 x 10 4 m3/s. What is the heat transfer area and the value of the overall heat transfer coefficient when the flow rate is 1.75 x 10 4 m3/s ... 

Heat Transfer Area, A 2 m Available Energy Dissipation,(T a) kcal Pressure in the Evaporator, P atm Fraction of the Generated Steam Reused,... 

The only factor which affects the overall coefficient is the scale formation. The liquid enters the evaporator at the boiling point, and the temperature and heat of vaporization are constant. At the operating conditions, 990 Btu are required to vaporize 1 lb of water, the heat-transfer area is 400 ft2, and the temperature-difference driving force is 70°F. The time required to shut down, clean, and get back on stream is 4 h for each shutdown, and the total cost for this cleaning operation is 100 per cycle. The labor costs during operation of the evaporator are 20 per hour. Determine the total time per cycle for minimum total cost under the following conditions ... 

In this chapter we have presented an overview of scale-up considerations involved as one moves from bench-scale reaction calorimetry to larger scale pilot plant and production reactors. Our focus has been on heat transfer and single-phase processes, addressing primarily the problem that the heat transfer area per unit reactor volume decreases with scale. Clearly, there are many challenging problems associated with multiphase vessels, with evaporation/distillation and crystallization as obvious examples, but these topics are beyond the scope of this chapter. 

The temperature of the chilled process stream is controlled by adjusting the setpoint of the evaporator level controller (the heat transfer area is varied to change the heat transfer rate). 

Distilled water at 34 °C is cooled to 30 °C by a raw-water feed at 23 °C flowing to an evaporator. Estimate the heat-transfer area required to cool 79,500 kg/h (8.16x10 Ib/h) of distilled water using a 1-2 heat exchanger. 

SOLUTION The chilling room of a meat plant witi) a capacity of 450 beef carcasses is considered. The cooling load, the airflow rate, and the heat transfer area of the evaporator are to be determined. 

In this work, microscale evaporation heat transfer and capillary phenomena for ultra thin liquid film area are presented. The interface shapes of curved liquid film in rectangular minichannel and in vicinity of liquid-vapor-solid contact line are determined by a numerical solution of simplified models as derived from Navier-Stokes equations. The local heat transfer is analyzed in term of conduction through liquid layer. The data of numerical calculation of local heat transfer in rectangular channel and for rivulet evaporation are presented. The experimental techniques are described which were used to measure the local heat transfer coefficients in rectangular minichannel and thermal contact angle for rivulet evaporation. A satisfactory agreement between the theory and experiments is obtained. 

Heat transfer can be supplied from within a vessel by a heating coil, but again, the available heat transfer area is not large however, such coils can be designed in ways that make their removal for cleaning relatively easy. The alternative is to have the heat exchange external to the main chamber of the evaporator. 

For viscous liquids, one way to increase the heat transferred is to improve the heat transfer coefficient by scraping or stirring the fluid adjacent to the wall, as in agitated film or wiped film evaporators. Accommodation of the mechanical devices used to mix the fluid close to the wall requires a fairly large diameter tube, so these devices tend to consist of only a single tube thus, heat transfer area is relatively small. The introduction of moving mechanical parts may lead to maintenance problems. 

When liquids are to be evaporated on a small scale, the operation is often accomplished in some form of jacketed tank or kettle. This may be a batch or continuous operation The rate ofheat transfer is generally lower than for other types of evaporators and only a limited heat transfer area is available. The kettles may or may not be agitated. 

Thin-film evaporators are frequently used for extremely viscous fluids, those in the range of 1,000 to 50,000 centipoise, and for concentrating streams with more than 25% suspended solids. Heat transfer coefficients for these types of materials in a thin-film evaporator are typically much greater than coefficients in any other type of evaporator for the same conditions. Very high temperature difference (e.g., 100 to 200°F) can be maintained to better utilize the heat transfer area by increasing the heat flux, Q/A. 

These evaporators are necessarily precision machines and therefore are more expensive than other types, particularly so if compared strictly on equivalent heat transfer area. When the performance for a specific evaporation duty is the basis of comparison, the thin-film evaporator is often the more economical choice because the larger heat transfer coefficient and higher driving force mean much less surface is required than for other evaporators (A = Q/U AT). Thin-film evaporator cost per unit area decreases significantly with unit size, and the largest available unit has 430 square feet of active heat transfer surface. 

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