What is critical thickness of insulation?
Critical Thickness of Insulation – Critical Radius. The thickness upto which heat flow increases and after which heat flow decreases is termed as critical thickness. In the case of cylinders and spheres it is called critical radius.
How does the thickness of insulation affect heat transfer?
Critical Thickness of Insulation – Critical Radius In a plane wall the area perpendicular to the direction of heat flow adding more insulation to a wall always decreases heat transfer. The thicker the insulation, the lower the heat transfer rate. This is due to the fact the outer surface have always the same area.
What is the critical radius of insulation for optimum heat transfer?
The critical radius of insulation for optimum heat transfer from pipe is given by: For insulation to be properly effective in restricting heat transmission, the pipe radius r 0 must be greater than or equal to the critical radius r c. Here r 0 < r c and as such there is no point in using asbestos as the insulating material.
What is the thickness of an insulation wall?
thickness = 305.7 - 203.2 = 102.5 mm Some margin should be taken on the insulation thickness because if the conductive heat transfer rate happens to be higher than the convective heat transfer rate outside the insulation wall, the outer insulation wall temperature will shoot up to higher values than 50 0 C.
What is critical insulation?
The critical radius of insulation is a counterintuitive concept within the study of heat transfer. The theory states that adding insulation to a cylindrical or spherical object will increase the rate of heat loss rather than decrease it, if the radius (thickness) of the insulation is at its “critical” value.
What is critical insulation thickness and its implication?
The critical radius of insulation is the thickness of insulation at which the rate of heat transfer through the body is maximum. It means that the rate of heat transfer increases with an increase in the thickness of insulation up to the critical radius of insulation.
What is insulation thickness?
The economic insulation thickness is the thickness which is corresponding with the lowest point on the total cost curve (y-axis). See graph. In industry, preference is given to the minimum total cost method. This method is particularly suitable if the insulation systems is expected to consist of more than one layer.
How do you find the thickness of critical insulation?
2:122:51Critical Thickness of Insulation l Heat Transfer l GATE 2020 MechanicalYouTubeStart of suggested clipEnd of suggested clipBy solving we get our to equal to K by H equal to rc. We do not require the derivation. We requireMoreBy solving we get our to equal to K by H equal to rc. We do not require the derivation. We require only this formula K upon H. So this is the critical thickness of insulation for the cylinder.
What is the concept of critical thickness?
The thickness up to which heat flow increases and after which heat flow decreases is termed critical thickness.
What is emissivity in heat transfer?
Emissivity is defined as the ratio of the energy radiated from a material's surface to that radiated from a perfect emitter, known as a blackbody, at the same temperature and wavelength and under the same viewing conditions.
What is economic thickness of insulation on heat transfer?
The simplest method of analysing whether you should use 1” or 2” or 3” insulation is by comparing the cost of energy losses with the cost of insulating the pipe. The insulation thickness for which the total cost is minimum is termed as economic thickness.
What are the 3 types of insulation?
Types of InsulationFiberglass: Fiberglass is the most common insulation material. ... Cellulose: Cellulose insulation is made from recycled paper products. ... Foam: Foam insulation may be made from polystyrene, polyisocyanurate or polyurethane, which are all types of plastic.More items...
What is the critical radius of insulation and economic thickness of insulation?
Explanation: Critical radius of insulation = k/h = 0.0825/8 = 0.01031 m = 10.31 mm.
Why are fins used in heat transfer?
Fins are extensions on exterior surfaces of objects that increase the rate of heat transfer to or from the object by increasing convection. This is achieved by increasing the surface area of the body, which in turn increases the heat transfer rate by a sufficient degree.
What is the phenomenon of increase in heat transmission with addition of insulation?
The phenomenon of increase in heat transmission with addition of insulation is most likely to occur when insulating materials of poor quality are applied to pipes and wires of small radius. Such a situation is used to advantage in the insulation of electrical wires and cables. ADVERTISEMENTS:
What is the purpose of heat insulation?
Sheathing of insulation then acts as lagging and that obstructs the flow of heat. Heat insulation is the main objective in steam and refrigeration pipes. For insulation to be properly effective in restricting heat transmission, the outer radius r o must be greater than or equal to the critical radius.
Why do electrical wires need insulation?
The electrical wires are given a coating of insulation with the prime objective to provide protection from electrical hazards. However an increase in the rate of heat dissipation can be made feasible and the conductors maintained within safe temperature limits by a proper choice of the insulation thickness.
Does adding insulation to a surface reduce heat transfer?
ADVERTISEMENTS: Contrary to the common belief that addition of insulating material on a surface always brings about a decrease in the heat transfer rate, there are instances when the addition of insulation to the outside surfaces of cylindrical or spherical walls (geometries which have non-constant cross-sectional areas) does not reduce ...
Does asbestos insulation increase heat transfer?
Addition of asbestos insulation will increase the heat transfer rate and that is not desirable . An insulating material with smaller thermal conductivity need to be employed. For insulation to be effective, the pipe radius should be greater than the critical radius, i.e.,
Example – Critical Thickness of Insulation
Assume a steel pipe of r 1 = 10 mm, which is exposed to natural convection at h = 50 W/m 2 .K. This pipe is insulated by material of thermal conductivity k = 0.5 W/m.K. Determine the critical thickness of this combination:
Critical Thickness of Insulation – Spherical Coordinates
It can be shown in a similar manner that the critical radius of insulation for a spherical shell is:
What is critical radius of insulation?
The critical radius of insulation is the thickness of insulation at which the rate of heat transfer through the body is maximum.
Critical radius of insulation graph
This plot indicates the change in heat transfer with the increase in thickness of insulation.
Critical radius of insulation significance
The critical radius of insulation has different significance based on its purpose.
Critical radius of insulation for cylinder:-
The critical radius of insulation for the cylindrical surface is given by,
Why rate of heat transfer increases till the critical radius of insulation?
As we increase the layer of insulation on a cylindrical or spherical surface, the area exposed to the surrounding also increases.
Why to study Critical thickness or radius of insulation ?
I am preassuming that you have basic knowledge of Heat transfer governing laws. If not, don’t worry, just comment below I’ll be happy to explain that also in detail.
Example for Critical radius of insulation
Let’s consider a steam pipe (steam inside the pipe) having a negligible wall thickness.
Sample Problem Statement
Calculate insulation thickness (minimum value) required for a pipe carrying steam at 180 0 C. The pipe size is 8" and the maximum allowable temperature of outer wall of insulation is 50 0 C. Thermal conductivity of the insulation material for the temperature range of the pipe can be taken as 0.04 W/m·K.
Solution
Solution to this sample problem is quite straightforward as demonstrated below.
