Heat transfer stage

Once the worker has a display which enables the whole furnace unit to be monitored at a glance, it is then necessary to supply more detailed information on each individual furnace. This display should represent the process in a way which facilitates comprehension of the heat transfer stages and provides all relevant information required to fulfill the objecHves of safe and efficient firing. Figure 7.17 presents a recommended graphic display at a more detailed level of system description. 

Figure 10 shows the differential heat exchange between a gas and a solids stream flowing countercurrently and cocurrently. The efficiency of the heat transfer equipment is to represented by the number of heat transfer stages... 

Thus a single stage, NH= 1, corresponds to a solids temperature rise (or drop) equal to the average gas-solids temperature difference, as shown in the lower left-hand side of Fig. 10. On this basis, it is convenient to define equipment efficiency in terms of a unitary heat transfer stage ... 

Thus the number of heat transfer stages can be expressed in terms of the acceleration integral... 

L/pj-A)(S/psA), liquid-solids velocity ratio, dimensionless Number of heat-transfer stages, dimensionless = hdp/kg, Nusselt number, dimensionless Pressure drop, gm-wt/cm2 = Cpu kg, Prandtl number, dimensionless = dpiipj U, Reynolds number, dimensionless S Mass velocity of solids, gm/cirf sec... 

Fig. 43. Number of heat-transfer stages and fractional heat recovery in dilute-phase heat transfer for particles under accelerative motion.

Modern steam turbines are designed to use steam in the temperature range of 1000° to 1100°F. In a nuclear plant, heat must first be transferred from the interior of the fuel element to the fuel element surface, from the fuel element surface to the primary coolant, and from the primary coolant to the heat exchanger to create steam. In order to generate steam at a temperature of, say, 1000°F, it is necessary to have fuel elements that can operate at temperatures sufficiently above 1000°F to allow efficient heat transfer from the fuel element interior through the various heat-transfer stages to the final generation of steam. 

The temperature difference between stages can be manipulated by changing the heat transfer area. Figure 3.136 shows the effect of a decrease in heat transfer area. 

Examination of this equation will show that the required heat transfer area for the heat recovery section per unit mass of distillate produced, increases significantly when PR is increased, and decreases slightly as the number of heat recovery stage, is increased. 

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