Heat balances exchangers

The output includes material and heat balances, exchanger heat curves with various physical properties, compressor performance calculations and any additional output desired through the use of fortran subroutines. These models were used to predict the plant sensitivity. 

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. 

The air exchange rate, G , required for temperature control in the occupied zone can be calculated from the room heat balance equation ... 

The air temperature of a room at any given time is given hy a heat-balance equation which includes the heat flux exchanged by convection at each wall element (A q ), the heat flow exchanged by ventilation (4>j,), the convective part of heat flow due to internal heat gains the convective part of heat flow due to the HVAC system (< >[,(.), and the variation of energy in the room air(r M ... 

It is assumed that there are no heat losses from the column and that there is zero heat exchange between the gas and liquid phases. Consequently the gas phase temperature will remain constant throughout the column. A liquid phase heat balance, for element of volume dV is given by... 

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

The heat balance equation can now be applied to segment n, of the heat exchanger. The heat transfer rate equations are given by the following terms... 

The heat balances with heat exchange and under adiabatic conditions are summarized in Table 2.5. 

One important component of any evaporator installation is the equipment for condensing the vapour leaving the last effect of a multiple-effect unit, achieved either by direct contact with a jet of water, or in a normal tubular exchanger. If M is the mass of cooling water used per unit mass of vapour in a jet condenser, and H is the enthalpy per unit mass of vapour, then a heat balance gives ... 

The decrease in the heat duty of the pumparound heat exchanger would equal the increase in the heat duty of the overhead condenser. Thus, we say that the heat balance of the tower is preserved. Some of the heat that was being recovered to the cold fluid, shown in Fig. 12.2, is now lost to cooling water, in the overhead condenser. This shows the most important function of pumparounds recovering heat to a process stream that would otherwise be lost to the cooling tower. 

Sufficient data are given for finding the heat balance and the liquor circulation rate, and for sizing the auxiliaries such as lines, pump, heat exchanger and vacuum system, but those calculations will not be made. 

The above overall heat balances state that the heat load of each process stream equals the sum of heat exchanged of that process stream with other process and utility streams. Note that the first subindex denotes the hot stream, the second the cold stream, and the third the stage. 

The plots in Figures 7.8 and 7.9 make both Qer and Qeg infinite and therefore the dense phase and bubble phase conditions are identical and are equal to the output conditions of the reactor and the regenerator in this example. In case of finite exchange rates between the bubble and dense phases in reactor and regenerator, the output conditions from the reactor and the regenerator can be obtained by mass and heat balances for the concentration and the temperature of both phases and these expressions use the same symbols as before, but without the subscript D (used to signify the dense phase before). 

The overall heat balance of a semi-batch reactor can be written by using the three terms mentioned above. If the heat exchange does not compensate exactly, the other terms (heat production, effect of the feed, temperature) will vary as... 

The heat balance of an isothermal semi-batch reaction is represented graphically in Figure 7.2. The maximum heat exchange rate (qeXiialx) calculated for a constant temperature of the heat carrier is also represented in the diagram. It increases linearly with time until the upper limit of the jacket is reached. In this example, the upper limit of the jacket is not reached during the feed time of four hours. 

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