Hot fluid heating

If the cold and hot fluids heat capacity rates are equal, then R = 1. Equation (9.22) gives an indefinite value, and this equation cannot be used directly. Using I Hopital s rule as R —> 1 gives d... 

The above swii can be eliminated by controlling the preheater heat duty or, better still, the preheater heat duty per unit feed flow (362), instead of the preheater outlet temperature. For steam (or condensing vapor) preheaters, the duly per unit feed flow equals the ratio of the measured steam (or vapor) flow to the measured feed flow times a constant, the constant being the steam latent heat. For a sensible-heated preheater, the above ratio is multiplied by the measured hot-side temperature difference, and the constant is the average hot-fluid heat capacity. For two or more feed preheaters, it is best to compute their total heat duty on-line and ratio it to the feed (68, 259). The computation can be readily performed using conventional analog instrumentation. Similar techniques cured the above-cited swing problems (239,259). 

When fluids heat unevenly, the hot part of the fluid tends to rise with respect to the cooler part of the fluid because of differences in density. The flow is driven by gravity, and distorts resolution in electrophoretic separations. 

Exchanger Performs a double function (1) heats a cold fluid by (2) using a hot fluid which it cools. None of the transferred heat is lost. 

FIG. 11-49 Plate-and frame heat exchanger. Hot fluid flows down between alternate plates, and cold fluid flows up between alternate plates. Theimal Division, Alfa-Laval, Inc.)... 

When a hot fluid stream and a cold fluid stream, separated by a conducting wall, exchange heat, the heat that is transferred across a differential element can be represented by the following expression (refer to Figure 1) ... 

In this manner, an average value of U can be applied to the whole exchanger. Ideally, the heat lost by the hot fluid stream is transferred totally to the cold stream, and hence, integrating results in the following expression ... 

Parameters Cj and Cj are specific heats of the cold and hot fluids in Btu respectively, and the log-mean temperature difference is defined as follows ... 

The heat is transferred by convection and conduction from the hot to the colder fluid through an infinitesimal surface area dA. The temperature of the hot fluid reduces by an amount dTf, and the temperature of the cold fluid increases bv an amount dT.. 

The transfer of heat from one molecule to an adjaeent molecule while the particles remain in fixed positions relative to each other is conduction. For example, if a piece of pipe has a hot fluid on the inside and a eold fluid on the outside, heat is transferred through the wall of the pipe by conduc tion. This is illustrated in Figure 2-1. The molecules stay intact, relative to each other, but the heat is transferred from molecule to molecule by the process of conduetion. This type of heat transfer occurs in solids or, to a much lesser extent, within fluids that are relatively stagnant. 

Most heat transfer processes used in production facilities involve combinations of conduction and convection ti ansfer processes. For example, in heat exchangers the transfer of heat energy from the hot fluid to the coLl fluid involves tliree steps. First, the heat energy is transferred from the luH fluid to the exchanger tube, then through the exchanger tube wall, ctud finally from the tube wall to the cold fluid. The first and third steps are convection transfer processes, while the second step is conduction process. 

Indirect fired heaters (sometimes called line heaters) heat the gas stream before and/or after the choke so that the gas is maintained above the hydrate temperature. Indirect fired heaters can also be used to heat crude oil for treating, heat a hot fluid circulating medium (heat medium) that is used to provide process heat, etc. 

Special operating conditions such as start up of a shut-in well must be considered in sizing the heater. The temperature and pressure conditions found in a shut-in well may require additional heater capacity over the steady state requirements. It may be necessary to temporarily install a heater until the flowing wellhead temperature increases as the hot resei voir fluids heat up the tubing, casing, and surrounding material. 

Find the minimum temperature that a hot fluid at 410°F can be cooled if the cold fluid is heated from an inlet temperature of 167°F to 257°F. Also And the theoretical temperature cross and theoretical minimum hot fluid shell-side outlet temperature, Tg. 

W — Flow rate of hot fluid c= Specific heat of cold fluid. 

In condensers where heat loss is desired, insulation often is omitted from piping carrying hot fluids to take advantage of the heat loss to the atmosphere. In any heat exchange equipment the heat released or lost by one fluid must be accounted for in an equivalent gain by a second fluid, provided that heat losses are negligible or otherwise considered. 

A = total exchanger bare tube heat transfer surface, fF Cp = specific heat, Btu/ (lb) (°F) t = air temperature, °F T = hot fluid temperature, °F U = overall heat transfer coefficient (rate),... 

In a nuclear power plant, heat must be transferred from the core to the turbines without any transfer of matter. This is because fission and neutron capture generate lethal radioactive products that cannot be allowed to escape from the core. A heat-transfer fluid such as liquid sodium metal flows around the core, absorbing the heat produced by nuclear fission. This hot fluid then flows through a steam generator, where its heat energy is used to vaporize... 

Here a steady-state formulation of heat transfer is considered (Pollard, 1978). A hot fluid flows with linear velocity v, through a tube of length L, and diameter D, such that heat is lost via the tube wall to the surrounding atmosphere. It is required to find the steady-state temperature profile along the tube length. 

Fig. 4.25 represents a steady-state, single-pass, shell-and-tube heat exchanger. For this problem W is the mass flow rate (kg/s), T is the temperature (K), Cp is the specific heat capacity (kJ/m s), A (= 7i D Z) is the heat transfer surface area (m ), and U is the overall heat transfer coefficient (kJ/m s K). Subscripts c and h refer to the cold and hot fluids, respectively. 

Steady-State Heat Exchanger Constant U=1.5 Heat transfer coefficient [kJ/s m2 K] Constant CP=4.18 Specific heat capacity [kJ/kg K] Constant WH=8.5 Mass flow rate of hot fluid [kg/s] Constant WC=4.17 Mass flow rate,cold fluid [kg/s] Constant TCIN = 323 Inlet cold water temp.[K] Constant TCOUT = 343 Outlet cold water temp. [K]... 

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