Critical Thickness of Insulation and It's Effect on Boiler Efficiency

Welcome to the THERMODYNE BLOG, today in this article we are going to explore the thickness of insulation and its effect on boiler efficiency and other parameters.

Critical thickness of insulation and its significance

            The information in this article gives a significant collection of data you need to know about the boiler and its performance, the parameter that affects the boiler efficiency.

       Table of content

• INTRODUCTION
• THICKNESS OF INSULATION CRITICAL RADIUS
• EFFECT ON BOILER EFFIIENCY
• INSULATING MATERIALS
• CONCLUSION

INTRODUCTION-

The thickness up to which heat flow increases and after which heat flow decreases is termed critical thickness. In the case of cylinders and spheres, it is called the critical radius. The critical radius of insulation can be derived depending on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h.

THICKNESS OF INSULATION- CRITICAL RADIUS-

 In boiler the shell and tube are cylindrical and the critical radius is determined by

                                                r = k/h 

K= thermal conductivity of insulation

H = external medium heat transfer coefficient

The insulation in boiler is provided to prevent the heat loss to the surrounding. A layer of insulating material is used over the boiler shell to prevent the heat loss. Here the matter of concern is that what should be the thickness of this layer. At critical radius of insulation there will be maximum heat transfer as it is shown in graph, initially heat transfer rate increases on increasing the insulation thickness and at the critical value it is maximum and after this it decreases. So the insulation thickness should be more than that of the critical value to prevent the heat loss.

            EFFECT ON BOILER EFFICIENCY

The boiler efficiency can be calculated by two methods, one is Direct Method and other is Indirect Method. Indirect method gives the clear idea about where the loss is occurring.

            The heat loss method is used to calculate individual losses, totalize them and determine boiler efficiency by subtracting the total losses from 100%. The individual losses are identified.

                                   

                        Efficiency = 100  - SUM OF ALL LOSSES

 Considering the loss at the surface of boiler due to convection and variation of thickness of insulation a graph is plotted, which shows that as initially on increasing the insulation thickness the efficiency decreases because the heat transfer increases and at the critical value of thickness where the maximum heat transfer to surrounding occurs leads to the lowest efficiency. After increasing the thickness beyond critical value efficiency starts increasing.

            For insulation a material of minimum thermal conductivity is used and the thickness is chosen in such a way that it should be cost effective also. We cannot increase the thickness monotonically because after a certain limit it doesn’t affect the efficiency as much but increases the cost more than the cost saved by increasing the efficiency. So the optimum value of thickness ensures the  high efficiency which means less fuel consumption.

 INSULATING MATERIALS:

            The materials which preserve the heat from transferring and having low thermal conductivity. when deciding which insulation material you should use, you should be sure to be aware of which material would work the best in your situation. We have considered differences like R-value, environmental impact, flammability, sound insulation and other factors below. Here are the 5 most common types of insulation materials:

           

Insulating material	R-value	Eco-friendly	Flammable	Remark
Polyurethane Foam	R-6.3	No	Yes	Makes a great sound insulator
Mineral wool	R-3.1	Yes	No	Does not melt or support combustion
Cellulose	R-3.7	Yes	Yes	Contains the recycled amount
Fiberglass	R-3.1	Yes	No	Does not Absorb water
Polystyrene (EPS)	R-4	No	Yes	Difficult to use imperfection

 

CONCLUSION-

            The thickness of insulation plays a vital role in determining the efficiency of boiler which directly affects the fuel consumption and running cost of boiler. The selection of insulating material and its thickness cannot be avoided. 

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