Critical thickness of lagging

As the thickness of the lagging is increased, resistance to heat transfer by thermal conduction increases. Although the outside area from which heat is lost to the surroundings also increases, giving rise to the possibility of increased heat loss. It is perhaps easiest to think of the lagging as acting as a fin of very low thermal conductivity. For a cylindrical 

When the relation between heat loss and lagging thickness exhibits a maximum for the unlagged pipe (x 0), then  

When hr/k 1, the addition of lagging always reduces the heat loss. 

When hr/k 1, thin layers of lagging increase the heat loss and it is necessary to exceed the critical thickness given by equation 9.266 before any benefit is obtained from the 

Substituting in equation 9.265 gives the maximum heat loss as  

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By varying input parameters it is found that the critical thickness of the lagging decreases with increasing temperature of the pipe, i.e. the lagging must be thinner in order to avoid self-ignition. This is a requirement counteracting the quality of heat insulation. 

Experimental requirements or assumptions concerning transport not fulfilled, for example, the mixing behavior of fluids and gases, deviation from the ideal residence time distribution, critical dependence on particle size, the thickness of layers of cells, the response lag of the Po electrode. 

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