Few Thermal Design Considerations

All considerations for thermal design should consider the four principle facets of thermal design.

Four principle facets of thermal design

1. Heat transfer analysis

2. Materials performance

3. Heating & cooling technology

4. Instrumentation & control

If important considerations are met, then the majority of thermal design problems do not occur. A review of the following few thermal design considerations, before or after a thermal design effort is encouraged.

* Thermal specifications should be realistic. Thermal design specifications must be realistic, necessary, and achievable. If unrealistic specifications are established, then they should be probed to arrive at realistic specifications.

* Physical property variations with temperature. The thermal conductivity and thermal expansion (linear, area, volumetric) of the materials should be considered. Due to varying temperature, the physical properties may change. The viscosity of water and many other liquids is sensitive to temperature changes as is the effective thermal conductivity of insulation. During operating transients, thermal expansion effects have to be taken note of to avoid mechanical distortion and failure.

* Physical property variations with age. Due to aging or performing at higher temperatures, many materials witness loss of mechanical strength and changes in surface properties.

* Materials should be compatible. At a given temperature, many materials may be compatible that may prove to be chemically incompatible at another temperature.

* Limits of temperature. The temperature limits of all materials should be considered and should be adequate.

* The properties of materials should be evaluated. Sometimes the actual physical properties of materials may differ from the values given in the handbook. Thus, design margins should be included or measurement of the properties of the materials should be considered to ensure desired performance.

* Heat gains or losses from supporting systems or components. The heat balance may be dominated by such parasitic heat losses or gains. Thermal “shorts” or transferring heat via piping systems may be considered.

* Application of the first law of thermodynamics. To maintain overall thermal balance, a control volume should be considered and defined.

* Ultimate heat sink/source. The capacity should be considered to evaluate whether over time, due to thermal load, the temperature of the heat sink/source will change or not.

* Heat effects due to chemical reactions. Thermal requirements may be significantly affected due to small amounts of water being evaporated or condensed.

* Heat generated due to structure or friction. Instrument and power measurement circuits generate heat and the effects of such heat gains should be considered. Compressors, bearings, fans contribute towards heat gain.

* Startup and shutdown. To achieve the required startup and shutdown times, whether heating or cooling, the rate of thermal energy exchange may exceed the normal operating requirements. Sizing the thermal power supply accordingly should be considered.

* Radiation, conduction, convection. The three modes of heat transfer should be considered. At near ambient temperatures, radiation should be considered. Techniques for efficient and improved convective heat transfers as well as two-dimensional conduction effects also deserve consideration.

* Surface fouling. This is a potential problem of degrading heat-transfer performance due to corrosion, deposition, or precipitation.

* Oversized systems. If the load is less or in part-load situations, will the oversized system provide efficient control?

* Susceptibility to environmental changes. Internal and external surfaces that are exposed to air should be considered at operating temperatures below ambient for effects of humidity, sunlight, condensation, and frost.

* Safety. Operator and equipment safety should be considered in the event of the failure of the thermal control system along with the burn potential of surfaces.

References:

Eric C. Guyer, David L. Brownell, “Handbook of Applied Thermal Design”, pages 18-19, published by Taylor & Francis, 1999, ISBN 1560328118, 9781560328117.

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