3. Heat transfer between dense bed and horizontal and vertical submerged tubes

In general, in the case of heat transfer between a bed and tubes immersed in the bed (indirect transfer), exchange coefficients are better between bed and immersed tubes than between bed and vertical bed wall.

In this case, the exchange coefficient depends on the geometry and positioning of the tubes in the bed. As before, a reduction in solid particle size and an increase in bed pressure improve exchange coefficients.

The average transfer coefficient for a tube bundle is around 70-80% of that calculated, under the same conditions, for an isolated tube.

In the case of a smooth horizontal tube with diameter D _T , the Vreedenberg correlations can be used:

• for $R{e}_{D_{\text{T}}}=\frac{U{D}_{\text{T}}{\rho }_{\text{p}}}{{\mu }_{\text{g}}}<2000$...