Heat exchangers forced convection

Maximum shell-side heat-transfer rates in forced convection are apparently obtained by cross-flow of the flmd at right angles to the tubes. In order to maximize this type of flow some heat exchangers are built with segmental-cut baffles and with no tubes in the window (or the baffle cutout). Maximum baffle spacing may thus equal maximum unsupported-tube span, while conventional baffle spacing is hmited to one-h f of this span. 

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries. 

Convection The mechanism of heat transfer due to different temperatures, and hence different densities in fluids. It may be natural, dependent only on thermal forces, or forced, when use is made of a rotodynamic device to improve the rate of heat exchange. 

Convection is heat transfer between portions of a fluid existing under a thermal gradient. The rate of convection heat transfer is often slow for natural or free convection to rapid for forced convection when artificial means are used to mix or agitate the fluid. The basic equation for designing heat exchangers is... 

Design Procedure for Forced Convection Heat Transfer in Exchanger Design... 

Design for Heat Transfer Coefficients by Forced Convection Using Radial Low-Fin tubes in Heat Exchanger Bundles... 

This type of exchanger is used to reject heat from a fluid inside the tubes (and associated headers) directly to ambient air. To be effective, the air must flow in forced convection to develop acceptable transfer coefficients. Figures 10-174, 10-175, and 10-176 illustrate the two types, designated by the type of air movement, induced draft or forced draft. 

The radiant section of an industrial boiler may typically contain only 10 per cent of the total heating surface, yet, because of the large temperature difference, it can absorb 30-50 per cent of the total heat exchange. The mean temperature difference available for heat transfer in the convective section is much smaller. To achieve a thermally efficient yet commercially viable design it is necessary to make full use of forced convection within the constraint of acceptable pressure drop. 

Above this size, the flow of air over the condenser surface will be by forced convection, i.e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 1 and 10 1. 

Shah RK, London AL (1978) Laminar flow forced convection in ducts a source book for compact heat exchanger analytical data. Advances in Heat Transfer, suppl 1. Academic, New York Sher 1, Hetsroni G (2002) An analytical model for nucleate pool boiling with surfactant additives. Int J Multiphase Flow 28 699-706... 

Convective heat exchange, natural or forced Radiation heat transfer, e.g. in furnaces Evaporation Condensation... 

Convective heat exchange, natural or forced Radiant heat transfer, e.g. furnaces Evaporation, e.g. in evaporators Condensation, e.g. in shell and tube heat exchanges Heat transfer to boiling liquids, e.g. in vaporizers, boilers, re-boilers ... 

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

The equation for forced-convective heat transfer in conduits can be used for plate heat exchangers equation 12.10. 

ESDU 93018 (2001) Forced convection heat transfer in straight tubes. Part 2 laminar and transitional flow. ESDU 98003-98007 (1998) Design and performance evaluation of heat exchangers the effectiveness-NTU method. 

There are two types of snow cooling system. One is direct heat exchanging system and the other is indirect heat exchanging system. Direct heat exchanging system is the same as the forced convection type. Room air is circulated between a room and a stored snow room by fans and cooled directly. The advantages of this system are... 

Kast, W., 1964, Significance of Nucleating and Non-stationary Heat Transfer in the Heat Exchanger during Bubble Vaporization and Droplet Condensation, Chem. Eng. Tech. 36(9) 933-940. (2) Katto, Y., 1981, General Features of CHF of Forced Convection Boiling in Uniformly Heated Rectangular Channels, Ini. J. Heat Mass Transfer 24.14131419. (5)... 

Factors involved in heat transfer, such as surface-to-volume ratio, agitation characteristics, mixing efficiency, fouling of heat transfer surfaces, scale of operations, and the resulting heat exchanged depend on the system under consideration (e.g., liquid-liquid transfer, liquid-gas transfer, free convection, or forced convection). Standard chemical engineering texts and reference books contain detailed discussions on heat transfer in process equipment. Only a brief summary follows ... 

During the flight of droplets in the spray, the forced convective and radiative heat exchanges with the atomization gas lead to a rapid heat extraction from the droplets. A droplet undergoing cooling and phase change may experience three states (a) fully liquid, (b) semisolid, and (c) fully solid. If the Biot number of a droplet in all three states is smaller than 0.1, the lumped parameter model 1561 can be used for the calculation of droplet temperature. Otherwise, the distributed parameter model 1541 should be used. 

In process operations, simultaneous transfer of momentum, heat, and mass occur within the walls of the equipment vessels and exchangers. Transfer processes usually take place with turbulent flow, under forced convection, with or without radiation heat transfer. One of the purposes of engineering science is to provide measurements, interpretations and theories which are useful in the design of equipment and processes, in terms of the residence time required in a given process apparatus. This is why we are concerned here with the coefficients of the governing rate laws that permit such design calculations. 

The transfer of heat by convection is also an important component of the indirect cooling of the process. Natural convection currents result from localized heating/cooling effects and the tendency of hot fluids to rise above colder fluids, while forced convection, utilizing a pump, enables higher rates of heat transfer to occur (within the limits of the heat-exchanger design). 

There are several possible mechanisms for the heat exchange between a reacting medium and a heat carrier radiation, conduction and forced or natural convection. Here we shall consider convection only. Other mechanisms are considered in the chapter on heat accumulation. The heat exchanged with a heat carrier (q ) across the reactor wall by forced convection is proportional to the heat exchange area (A) and to the driving force, that is, the temperature difference between the reaction medium and the heat carrier. The proportionality coefficient is the overall heat transfer coefficient (U) ... 

The heat exchanged with a heat carrier across the reactor wall by forced convection is expressed in the classical way by... 

When a liquid warms up, its density decreases, which results in buoyancy and an ascendant flow is induced. Thus, a reactive liquid will flow upwards in the center of a container and flow downwards at the walls, where it cools this flow is called natural convection. Thus, at the wall, heat exchange may occur to a certain degree. This situation may correspond to a stirred tank reactor after loss of agitation. The exact mathematical description requires the simultaneous solution of heat and impulse transfer equations. Nevertheless, it is possible to use a simplified approach based on physical similitude. The mode of heat transfer within a fluid can be characterized by a dimensionless criterion, the Rayleigh number (Ra). As the Reynolds number does for forced convection, the Rayleigh number characterizes the flow regime in natural convection ... 

Such combined convective flows are normally associated with low forced velocities. They can occur, for example, in some electronic cooling situations and in some heat exchangers. 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

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