Surface condensers heat-transfer coefficients

The results for the TEA--water mixtures at atmospheric pressure are shown in Figure 6. These are for TEA mole fractions of x 0.05 and 0.59. The LOST is 18.2 at x - 0.09. We also obtained a very similar data set at the latter mole fraction, but we omitted it for clarity. For contrast and comparison, a data set for pure water is shown. These mixture results again show a sharp rise in heat transfer coefficient as condensate first appeared. In fact, the appearance was remarkably similar to the n-decane--C02 results for x - 0.973 discussed above, but the visibility of the phase separation was enhanced by the presence of a fine emulsion at the phase interface and the absence of strong refractive index gradients characteristic of the supercritical systems. This permitted the structure of the interface to be seen more clearly. In Figure 7 we show photographs that typify the appearance of the two phases. In all cases observed here, both in supercritical vapor--liquid and in liquid--liquid systems, the dense phase appears to wet the cylinder surface regardless of composition. 

Enhanced surfaces can often significandy increase the effective heat-transfer coefficient in condensation, especially if the condensing heat-transfer coefficient is the limiting factor in the overall heat-transfer-coefficient equation. Such enhancements include low fins on horizontal tubes, which increase the heat-transfer area, and fluting on vertical tubes and plane surfaces, which thins the condensate film over part of the surface by surface-tension effects. However, these improvements are limited by condensate retention between the fins and flooding of the drainage paths [7, 34],... 

The equation for thickness of a falling laminar film was first presented by Nusselt, who used the result to predict heat-transfer coefficients for condensing vapors. Measurements of film thickness on a vertical surface (cos = 1) show that Eq. (5.77) is approximately correct for x 1000, but the thickness actually varies with about 0.45 power of the Reynolds number, and the layers are thinner than predicted at low Nr and thicker than predicted above = 1000. The deviations may be due to ripples or waves in the films, which are apparent even at quite low Reynolds numbers. 

Heat transfer coefficients for condensation processes depend on the condensation models involved, condensation rate, flow pattern, heat transfer surface geometry, and surface orientation. The behavior of condensate is controlled by inertia, gravity, vapor-liquid film interfacial shear, and surface tension forces. Two major condensation mechanisms in film condensation are gravity-controlled and shear-controlled (forced convective) condensation in passages where the surface tension effect is negligible. At high vapor shear, the condensate film may became turbulent. 

It is assumed that the thermal resistance on the steam side is negli ble. for this particular example the assumption is justified since the heat transfer coefficient for condensing steam is high and the heat transfer surface is generally clean. The tube wall temperature is close to the steam temperature. 

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. 

For subcooling, a liquid inventory may be maintained in the bottom end of the shell by means of a weir or a hquid-level-controUer. The subcoohng heat-transfer coefficient is given by the correlations for natural convection on a vertical surface [Eqs. (5-33 ), (5-33Z )], with the pool assumed to be well mixed (isothermal) at the subcooled condensate exit temperature. Pressure drop may be estimated by the shell-side procedure. 

Of these special surfaces, only the double-fluted tube has seen extended services. Most of the gain in heat-transfer coefficient is due to the condensing side the flutes tend to collect the condensate and leave the lauds bare [Caruavos, Proc. First Int. Symp. Water Desalination, 2, 205 (1965)]. The coudeusiug-film coefficient (based on the actual outside area, which is 28 percent greater than the nominal area) may be approximated from the equation... 

Pressure can also be controlled by variable heat transfer coefficient in the condenser. In this type of control, the condenser must have excess surface. This excess surface becomes part of the control system. One example of this is a total condenser with the accumulator running full and the level up in the condenser. If the pressure is too high, the level is lowered to provide additional cooling, and vice versa. This works on the principle of a slow moving liquid film having poorer heat transfer than a condensing vapor film. Sometimes it is necessary to put a partially flooded condenser at a steep angle rather than horizontal for proper control response. 

For those condensing duties where permissible pressure loss is less than 0.07kpf/cm there is no doubt but that the tubular unit is most efficient. Under such pressure-drop conditions only a portion of the length of a plate heat exchanger plate would be used and a substantial surface area would be wasted. However, when less restrictive pressure drops are available the plate heat exchanger becomes an excellent condenser, since very high heat transfer coefficients are obtained and the condensation can be carried out in a single pass across the plate. 

Reciprocal of heat transfer coefficient multiplied by 1,000. A term employed when designing surface condensers. 

HEAT TRANSFER IN THE CONDENSATION OF VAPOURS 9.6.1. Film coefficients for vertical and inclined surfaces... 

These expressions give point values of h and Nux at x = x. It is seen that the coefficient decreases from a theoretical value of infinity at the top as the condensate film thickens. The mean value of the heat transfer coefficient over the whole surface, between r = 0 and jc = x is given by ... 

Benzene vapour, at atmospheric pressure, condenses on a plane surface 2 m long and I m wide, maintained at 300 K and inclined at an angle of 45° to the horizontal. Plot the thickness of the condensate film and the point heat transfer coefficient against distance from the top of the surface. 

An organic liquid is boiling at 340 K on the inside of a metal surface of thermal conductivity 42 W/m K and thickness 3 mm. The outside of the surface is heated by condensing steam. Assuming that the heat transfer coefficient from steam to the outer metal surface is constant at 11 kW/m2 K, irrespective of the steam temperature, find the value of the steam temperature to give a maximum rate of evaporation. 

A single-effect evaporator is used to concentrate 7 kg/s of a solution from 10 to 50 per cent of solids. Steam is available at 205 kN/m2 and evaporation takes place at 13.5 kN/m2. If the overall heat transfer coefficient is 3 kW/m2 K, calculate 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 heat capacity of a 10 per cent solution is 3.76 kJ/kgK, the specific heat capacity of a 50 per cent solution is 3.14 kJ/kgK. 

A triple-effect evaporator is fed with 5 kg/s of a liquor containing 15 per cent solids. The concentration in the last effect, which operates at 13.5 kN/m2, is 60 per cent solids. If the overall heat transfer coefficients in the three effects are 2.5, 2.0, and 1.1 kW/m2K, respectively, and the steam is fed at 388 K to the first effect, determine the temperature distribution and the area of heating surface required in each effect The calandrias are identical. What is the economy and what is the heat load on the condenser ... 

A double-effect forward-feed evaporator is required to give a product which contains 50 per cent by mass of solids. Each effect has 10 m2 of heating surface and the heat transfer coefficients are 2.8 and 1.7 kW/m2 K in the first and second effects respectively. Dry and saturated steam is available at 375 kN/m2 and the condenser operates at 13.5 kN/m2. The concentrated solution exhibits a boiling-point rise of 3 deg K. What is the maximum permissible feed rate if the feed contains 10 per cent solids and is at 310 K The latent heat is 2330 kJ/kg and the specific heat capacity is 4.18 kJ/kg under all the above conditions. 

An evaporator, working at atmospheric pressure, is to concentrate a solution from 5 per cent to 20 per cent solids at the rate of 1.25 kg/s. The solution, which has a specific heat capacity of 4.18 kJ/kg K, is fed to the evaporator at 295 K and boils at 380 K. Dry saturated steam at 240 kN/m2 is fed to the calandria, and the condensate leaves at the temperature of the condensing stream. If the heat transfer coefficient is 2.3 kW/m2 K, what is the required area of heat transfer surface and how much steam is required The latent heat of vaporisation of the solution may be taken as being equal to that of water. 

Estimate the overall heat transfer coefficient U, based on the inside tube surface area of a shell-and-tube type vapor condenser, in which coohng water at 25 °C flows through stainless steel tubes, 25 mm i.d. and 30 mm o.d. at a velocity... 

Heat removed by condensation is easy. The heat-transfer coefficient U for condensation of pure, clean, vapors may be 400 to 1000 Btu per hour per ft2 of heat exchanger surface area, per °F of temperature-driving force. The U value for subcooling stagnant liquid may be only 10 to 30. Condensate backup is the major cause of lost heat transfer for heat exchangers, in condensing service. 

---