Biot, Reynolds, Nusselt & Eckert Numbers with Boiler Types

Biot, Reynolds, Nusselt and Eckert Numbers

The physical significance of the Biot number is clearer when written as a formula:

Bi = h L_s / k_s

Thus, the Biot number is the ratio between the convective heat transfer coefficient and the conductive conductance of the solid over a characteristic length.

Reynolds, Colburn and Stanton Relations

Reynolds and Colburn parameters are related to the local drag coefficient and the local Stanton number in laminar flow over a flat plate:

The Reynolds and Colburn numbers are connected to the local drag coefficient c_x and the local Stanton number St_x in laminar flow over a flat plate.

The Reynolds number is used as a criterion for the transition from laminar to turbulent flow.

Nusselt and Eckert Interpretations

The Nusselt number can be interpreted as the ratio between convective heat transfer and conduction through a fluid layer of thickness L. Based on this interpretation, a Nusselt number equal to zero implies that there is no convection.

The Eckert number is used to decide whether the effects of viscous dissipation of energy should be considered in the analysis of heat transfer.

Blackbody Radiation

A blackbody absorbs all incident radiation from all directions and at all wavelengths; it does not reflect, transmit, or scatter. At a given temperature and wavelength, no other body at the same temperature emits more radiation than a blackbody.

Boilers

Boilers

Heat source

• Direct combustion or recovery of exhaust from an internal combustion engine

Position of the combustion chamber

• Interior or exterior

Form and position of the tubes inside the generator

• Square, angular, serpentine, straight, inclined, vertical

Building materials

• Steel, copper, iron, special alloys, refractories, mineral wool, aluminum

Location

• Fixed or mobile

Passage of Combustion Products Through Tubes

Passage of the products of combustion through the tubes:

• Cylindrical shape with flat tops
• Thermal fluid: Avolta tubes and fire-tube
• Reverse chamber with mirror
• Large volume of thermal fluid

Advantages

• Compact design
• Compact format — reduced processing time
• Easy cleaning
• Easy replacement of pipes
• Less sensitive to load fluctuations — ideal for peak periods
• Cheap production lines at 18 ton/h

Disadvantages

• Slow start
• Operation limited to low and medium pressure
• Low to medium production of thermal fluid
• Need for 2 or 3 burners for higher yields and pressures