negligible changes in the particle internal heat transfer resistance
compared to the external resistance.
4.3. Effect of solid material
Three different kinds of the materials (Stainless steel (S.S.),
Aluminum and Copper) have been investigated in this section to
examine their effects on the heat transfer and to select the best
material to be used in the bed. Physical properties of these materials
are summarized in Table 2.
Fig. 12 A, B and C show t90, Qmax and Qmax/t90 for above
mentioned materials. Stainless steel has the maximum values of
Qmax, t90 and also Qmax/t90.
The results indicate that S.S. is the best choice for maximum
heat saving and regeneration in fluidized bed heat exchangers. For
aluminum although the heat absorption capacity is the lowest, but
it is the fastest heat delivery material.
Due to the importance of t90 value in cold period, the t90 values
for this period are discussed here. As is shown in Fig. 13 the trend of
t90 value for cold period is the same as hot period and S.S. also has a
maximum value of t90.
From operational point of view it is important to know the ratio
of hot/cold time period. Because the time switching between cold
and hot steam depends on the volumetric flow rates of each one,
the ratio of t90,hot to t90,cold is introduced and calculated for different
particle type and is shown in Fig. 14.
As can be seen from the figure all of the values are less than 1.
This shows that the hot period time is less than cold one. This ratio
for Aluminum is about 0.75, which is closer to 1.0 with compared to
the others. This means that with Aluminum particles the operational
switching time ratio between hot and cold streams should be
0.75, applying equal flow rates.