Common practice in chemical processing plants is to clad pipe insulation with a

Question

Common practice in chemical processing plants is to clad pipe insulation with a durable thick aluminum

foil. The function of the foil are to confine the batt insulation and to reduce heat transfer by radiation to

the surroundings. Because of the presence of chlorine (at chlorine or seaside plants), the aluminum foil

surface, which is initially bright, becomes etched with in service time. Typically, the emissivity might

change from 0.12 at installation to 0.36 with extended service. For a 300-mm-diamter foil-covered pipe

whose surface temperature is 90 oC, will this increase in emissivity due to degradation of the foil finish

have a significant effect on heat loss from the pipe? Consider the condition: quiescent air, surroundings

and ambient air at 25 oC.

Explanation / Answer

Emissivity : Emissivity is defined as the amount which shows the ability of the substance to emit radiation .

Emitted radiation gives the idea about the temperature of that body.

Shiny object have zero emissivity and black body has 1.

For Aluminium foil emissivity value is 0.03 at normal conditions.

Very thin material emit very less heat than thicker.

Emissivity changes clarely shows that the degradation of the shinyness of the silver foil because as chlorine in the water which reacted with the aluminium and producess the aluminium chloride or corrodes the Aluminium foil due to this reason heat loss is increases and we can say that aluminium is no longer able to insulate the heat .

When Quiescent air means still air has so heat absorption is more and in ambient air at 250C heat absorption is less .

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