What is critical thickness?
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. It can be derived the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h.
What is the critical radius of insulation?
It can be derived the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h. As can be seen, if r1 < rcr, as it is in this case, the total resistance decreases and the heat rate therefore increases with the addition of insulation.
What is the critical thickness of insulation in heat exchanger?
So Mr. Hakeem need not worry about critical insulation thickness unless his heat exchanger is, say, a 5 mm x 5 mm x 5 mm cube! Unfortunately, the critical thickness of an insulation exist only in spherical and cylindrical heat barrier - the thermal resistance reaches a minimal value for that walls.
What is the critical thickness of insulation in flat plane wall?
Apart from the cost of the insulation, there is no critical thickness of insulation in flat plane wall. But after a certain thickness the redution in heat transfer by ussing more insulation is insignificant. Yes Kharseh you are correct - the "certain thickness" mentioned by you needs a proper term other than critical thickness...
What is meant by critical thickness?
The thickness up to which heat flow increases and after which heat flow decreases is termed critical thickness.
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.
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 critical thickness of insulation of a small diameter pipe?
A steam pipe of 5 cm inside diameter and 6.5 cm outside diameter is insulated with a 2.75 cm radial thickness of high temperature insulation (k = 1.1 W/mK). The surface heat transfer coefficient for inside and outside surfaces are 4650 W/m2K and 11.5 W/m2K respectively.
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.
What is the effect of thickness of insulation?
The numerical calculation results show that structure temperature decreases as the insulation thickness increases as well as temperature difference increases, however, this effect weakens as the insulation thickness increased.
What is meant by critical radius?
In a collection of supported particles of different sizes, the critical radius is the size of the particle that is in equilibrium with the surrounding adatom concentration. Such a particle neither grows nor shrinks due to Ostwald ripening, whereas larger particles grow and smaller particles shrink.
What is the critical thickness of insulation for hollow cylinder Mcq?
Clarification: Critical radius of insulation = k/h0 = 0.1/5 = 0.02 m = 20 mm.
What is meant by critical radius?
In a collection of supported particles of different sizes, the critical radius is the size of the particle that is in equilibrium with the surrounding adatom concentration. Such a particle neither grows nor shrinks due to Ostwald ripening, whereas larger particles grow and smaller particles shrink.
Why is cork a good insulator?
In fact, cork is a very good thermal insulator. The reason has to do with the composition of cork. Cork is made up of countless tiny cells that are filled with air. The air serves to effectively block most heat from getting through a layer of cork.
What is critical radius nucleation?
In other words, it is the lowest radius formed by atoms or molecules clustering together (in a gas, liquid or solid matrix) before a new phase inclusion (a bubble, a droplet or a solid particle) is viable and begins to grow. Formation of such stable nuclei is called nucleation.
What is the critical radius of insulation for a sphere?
The insulation radius at which resistance to heat flow is minimum and consequently heat flow rate is maximum is called “critical radius”. Note that the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h.
What determines the radius of thermal insulation?
Regarding deciding the optimum radius of insulation is based on the economy: Heat insulation. The thickness of thermal insulation can be determined based on two different requirements. One is the economic insulation thickness, the other is the personal protection (sheeting temperature).
What is a limit of insulation?
A limit of insulation where heat flow rate is maximum. if the adition of insulation to the outside s/f of cylinder or spherical walls does not reduce the heat loss.under certain circumstances,it actually increases the heat loss upto a certain thickness of insulation.after this thickness with the addition of insulation,the heat transfer again decrease…….
What happens when you increase the radius of insulation?
If we increase the radius of insulation upto the critical radius, it will dissipate more heat or the heat transfer will increase so the insulation would not be effective . The difference between the radius of insulation and radius of the body on which it is used is called the thickness of insulation. At critical radi.
What happens when insulation is warped?
If insulation is loosely warped then it allows air ingress into the hot surface as hot air is lighter it moves out creating suction of fresh cold air under the insulation causing continuous heat loss. Same insulation thickness with same insulation material will give different result.
What is the difference between the radius of insulation and the radius of the body on which it is used?
The difference between the radius of insulation and radius of the body on which it is used is called the thickness of insulation. At critical radius, the thickness of insulation is called as critical thickness.
Why do we need insulation?
This is mainly due to an increase in exposed area for convective heat transfer from its surface .
What is the maximum temperature for steam condensate line?
If we consider personnel safety factor (usually considered for steam condensate line or steam trap outlet) allowed maximum surface temperature is 65 deg C. This could be wrong insulation thickness in some cases if we compare it with surface heat loss factor 125W/Sq mtr-hr or Ambient + 20 deg C.
What is the thickness of a cylinder called?
In the case of cylinders and spheres it is called critical radius. It can be derived the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h.
What is critical radius?
In the case of cylinders and spheres it is called critical radius. It can be derived the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h. In a plane wall the area perpendicular to the direction of heat flow adding more insulation to a wall always decreases ...
How does R1 affect heat loss?
As can be seen, if r1 < rcr, as it is in this case, the total resistance decreases and the heat rate therefore increases with the addition of insulation. This trend continues until the outer radius of the insulation corresponds to the critical radius, where the heat rate reaches its maximum. This trend is desirable for cooling of an electrical wire, since the addition of electrical insulation would aid in transferring heat dissipated in the wire to the surroundings. On the other hand, any further addition of material (beyond r cr) would increase the total resistance and therefore decrease the heat loss. This behavior would be desirable for insulation of pipes, where insulation is added to reduce heat loss to the surroundings.
What is critical thickness of insulation?
Critical thickness of insulation means maximum heat loss with min thermal resistance. This concept is useful in the situation when surface area of the base object increases with the insulation, i.e., cylindrical and spherical systems.
Can you use insulation on faces?
we cannot use the insulation on faces treating them as 6 isolated walls.
Does insulation reduce heat loss?
In case of plane wall or cuboid, the addition of insulation layer over the base surface will not make any effect on surface area and hence reduce the heat loss from the system irrespective of the thickness of the insulating material.
Does flat insulation show diminishing returns?
Or put another way, if one does not overlap the flat insulation at the edges and the corners, then simply adding 1 sq m sheets of thicker and thicker insulation will show rapidly diminishing returns. Much more rapidly diminishing returns than if optimal thickness were only a matter of added cost of insulation.
Is there a critical thickness of insulation for flat plates?
For flat plates there is not a critical thickness of insulation.
Is there insulation in a flat plane wall?
Apart from the cost of the insulation, there is no critical thickness of insulation in flat plane wall. But after a certain thickness the redution in heat transfer by ussing more insulation is insignificant.
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.
What happens to the rate of heat transfer up to the critical radius of insulation?
Therefore up to the critical radius of insulation, the rate of heat transfer increases.
What happens to the rate of heat transfer when the radius of insulation increases further?
If the radius of insulation increases further of the rcr r c r, the conduction resistance also increases therefore the rate of heat transfer also decreases .
What happens when you increase the insulation on a cylindrical surface?
The increase in a layer of insulation on a cylindrical surface up to the radius rcr r c r results in an increase in the rate of heat transfer through the insulating body.
What happens to the rate of heat transfer when the thickness increases?
If we increase the thickness more than the value of critical thickness then the rate of heat transfer starts to decrease.
Is insulation a good conductor of electricity?
The insulations used for the electric conductors are good insulators for electricity and good conductor for heat.