An Answer To A High-Temperature Air Application
The customer manufactures plastic products. The application involves heating dry air to approximately 1150°F (620°C) with a flow rate of 140 SCFM (3.9 SCMM). There are other temperatures and flow rates achieved in stages, but the 1150°F (620°C) outlet temperatures is the worst condition. Since the application does require that such a high temperature be reached, we must use a low-watt density unit.
In this instance, using a very low watt density will work only if baffles are added to the unit. If baffles were left off this unit the sheath temperatures would become too great, and causing the unit to fail almost instantly. As in this case, baffles are used in circulation heaters for the most part.
The theory behind using baffles is easier that it looks. When baffles are inserted into a circulation unit, the normal process flow is diverted. Instead of flowing parallel with the elements, the flow is perpendicular to the elements. This causes the process material to be heated by much more cross-sectional area of heater, which in turn takes more of the heat away from the elements. This keeps the sheath temperature within acceptable limits, while it provides the necessary outlet temperature.
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