Heat exchangers enthalpy balances

The analysis of the heat exchanger network first identifies sources of heat (termed hot streams) and sinks (termed cold streams) from the material and energy balance. Consider first a very simple problem with just one hot stream (heat source) and one cold stream (heat sink). The initial temperature (termed supply temperature), final temperature (termed target temperature), and enthalpy change of both streams are given in Table 6.1. 

When the dryer is seen as a heat exchanger, the obvious perspective is to cut down on the enthalpy of the air purged with the evaporated water. Minimum enthalpy is achieved by using the minimum amount of air and cooling as low as possible. A simple heat balance shows that for a given heat input, minimum air means a high inlet temperature. However, this often presents problems with heat-sensitive material and sometimes with materials of constmction, heat source, or other process needs. AH can be countered somewhat by exhaust-air recirculation. 

Heat balance equations on the element of heat exchanger length AZ, according to enthalpy balance relationship. 

Adiabatic calorimetry uses the temperature change as the measurand at nearly adiabatic conditions. When a reaction occurs in the sample chamber, or energy is supplied electrically to the sample (i.e. in heat capacity calorimetry), the temperature rise of the sample chamber is balanced by an identical temperature rise of the adiabatic shield. The heat capacity or enthalpy of a reaction can be determined directly without calibration, but corrections for heat exchange between the calorimeter and the surroundings must be applied. For a large number of isoperibol... 

The system equations for the counter-current heat exchanger are derived from the enthalpy balance and the heat transfer rate. 

Next, select a heat exchanger and calculate the heat transfer area. First, calculate the required heat transfer, Q, from an energy balance. Obtain the enthalpy of reaction from Equation 7.4.19 and the standard enthalpies of reaction hsted in Table 7.1.1. 

In chemical process units such as reactors, distillation columns, evaporators, and heat exchangers, shaft work and kinetic and potential energy changes tend to be negligible compared with heat flows and internal energy and enthalpy changes. Energy balances on such units therefore usually omit the former terms and so take the simple form Q = U (closed system) or Q = AH (open system),... 

The above equations relate the heat-transfer rate to the local temperatnre difference (J - t) and the heat-transfer area A throngh the overall heat-transfer coefficient U. In almost all practical sitnations, one or both temperatnres will vary along the length of the heat exchanger. The change in temperatnre of each stream is calcnlated from the enthalpy balance on that stream. To apply Equation (6.98) to the design of a heat exchanger in which the temperature difference is not constant, the equation needs to be written in differential form ... 

The set of bit 2s so obtained are used to make the next trial calculation on column 1. After a solution to the problem has been obtained, the required value of the cooler duty Qcl is readily determined by an enthalpy balance enclosing the trim heat exchanger. 

ENTHALPY BALANCES IN HEAT EXCHANGERS. In heat exchangers there is no shaft work, and mechanical, potential, and kinetic energies are small in comparison with the other terms in the energy-balance equation. Thus, for one stream through the exchanger... 

If only sensible heat is transferred and constant specific heats are assumed, the overall enthalpy balance for a heat exchanger becomes... 

Construct an exchanger operating diagram. The plot provides the local shellside fluid equilibrium temperature Ts as a function of the corresponding fluid specific enthalpy (see Fig. 17.59). A correlation between the shellside and tubeside fluid enthalpies is provided by the enthalpy balance, therefore the tubeside temperature dependence T, can be presented as well. The local equilibrium temperature is assumed to be the temperature of the stream well mixed at the point in question. Note that this step does not involve an estimation of the overall heat transfer coefficient. 

Balanced composite curves are similar with those discussed above, with the difference that now the utilities are considered as streams. Since the utilities covers any imbalance between the streams selected for integration, the enthalpy balance is closed. Moreover, the design of the heat exchangers is done in the balanced grid diagram. 

This formulation assumes that the process is performed under a constant ambient pressure. Strictly considered Equ. 4-43 should therefore be called an enthalpy balance. But in daily routine the established terminology is heat balance. The vector stands for the heat flux density. In setting up a heat balance many more sources and sinks have to be accounted for than in the case of the mass balance. This is indicated by the box for additional sources and sinks enclosed by the dotted line which is enumerated in addition to the main heat fluxes, such as the heat flow exchanged through the heat transfer area. An example for such other heat sources may be the power input of the agitator in a highly viscous system. 

If eqns. (5) and (6) are combined the feuniliar balance equation for enthalpy is obtained. It allows the calculation of the heat exchanged with the environment from the following equation ... 

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