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 thickness of insulation?
At critical radius, the thickness of insulation is called as critical thickness. The thickness at which maximum heat transfer takes place is critical thickness.
How does the thickness of insulation affect the rate of heat transfer?
The increase in a layer of insulation on a cylindrical surface up to the radius `r_ {cr}` results in an increase in the rate of heat transfer through the insulating body. Therefore for the increase in insulation thickness from `r_ {1}` to `r_ {cr}`, the rate of heat transfer also increases.
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.
How do you find the critical radius of insulation?
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. 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?
What is critical thickness?
How does R1 affect heat loss?
Who wrote Fundamentals of Heat and Mass Transfer?
Does insulation increase convection resistance?
See 2 more
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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 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 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.
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 critical thickness of insulation explain its physical significance?
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 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 critical thickness of cylinder?
critical thickness is 6 - 4 = 2 cm. The maximum heat dissipation is given by Eq. (5). only after 9.59 - 4 = 5.59 em of insulation has been added that q will be lowered below qbare' This might be considered a high value for the insulation thickness, and its addition may not be economically feasible.
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.
Why up to critical thickness of insulation the rate of heat transfer increases and then decreases?
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, and this is because the outer surface always has the same area.
What happens when the thickness of insulation on a pipe exceeds the critical?
The rate of heat transfer from the cylinder increases with the addition of insulation for r2 < rcr, reaches a maximum when r2 = rcr, and starts to decrease for r2 > rcr. Thus, insulating the pipe may increase the rate of heat transfer from the pipe instead of decreasing it when r2 < rcr.
How do you calculate thermal insulation?
How to calculate insulation values?R-value.R-value = thickness of the insulation / ƛ-value. An example: 10 cm of insulating material with a ƛ of 0.05 W/mK results in a R-value of 2 m2K/W.U-value.U = 1/R. A R-value of 2 corresponds to a U-value of 1/2 = 0.5.U-value = ƛ-value / thickness of the insulation.K-value.
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 happens when the thickness of insulation on a pipe exceed the critical value?
The rate of heat transfer from the cylinder increases with the addition of insulation for r2 < rcr, reaches a maximum when r2 = rcr, and starts to decrease for r2 > rcr. Thus, insulating the pipe may increase the rate of heat transfer from the pipe instead of decreasing it when r2 < rcr.
What is the critical thickness of insulation for a square plane wall ...
I have a heat exchanger of cubical shape (say 1m * 1m * 1m) containing a material heated by means of a heat transfer fluid passing through the pipes in it.
Critical thickness of insulation for spheres is given by - Testbook
Concept: The thickness up to which heat flow increases and after which heat flow decreases is termed as Critical thickness. The insulation radius at which resistance to heat flow is minimum and consequently heat flow rate is maximum is called “critical radius”.
Critical Thickness of Insulation | Thermal Engineering
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 the heat loss. In fact, […]
Critical radius of insulation of a hollow cylinder is - Testbook
Explanation:. The critical radius of insulation:. The critical radius of insulation can be defined in many ways. Let's see them all together. The value of R outer for which the heat transfer rate is maximum is called the critical radius of insulation.; The thickness up to which heat flow increases and after which heat flow decreases is termed as c ritical thickness.
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.
What is the thermal conductivity of asbestos?
A pipe of outside diameter 20 mm is to be insulated with asbestos which has a mean thermal conductivity of 0.1 W/m-deg. The local coefficient of convective heat to the surroundings is 5 W/m 2 -deg. Comment upon the utility of asbestos as the insulating material.
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 heat flow rate increase or decrease with insulation?
The fact that heat flow rate attains a maximum at r opposing effects; increasing r increases the thermal resistance of the insulation layer but decreases the thermal resistance of the surface area. At r = r c the total resistance reaches a minimum. Apparently, a pipe carrying a high temperature fluid will lose more heat (compared to a bare pipe) if the conductivity and thickness of insulation are improperly chosen. Dependence of heat loss on the thickness of insulation has been shown in Fig. 3.31.
Does thermal resistance increase with thickness of insulation?
An examination of equation 3.32 would reveal that with increase in r (i.e., thickness of insulation), the thermal resistance of insulating increases but the resistance due to convection coefficient at the outer surface drops. The thermal resistance due to inside film coefficient remains unaffected with change in radius r. Obviously, addition of insulation material can either increase or decrease the rate of heat transmission depending upon a change in the total resistance with outer radius r.
Does insulation increase heat flow?
In fact, under certain circumstances, it actually increases the heat flow upto a certain thickness of insulation. To establish this fact, consider a thin-walled metallic cylinder of length I, radius r i and transporting a fluid at temperature t i which is higher than the ambient temperature t 0. Surrounding this cylinder is an annular section of insulating sheathing of thickness (r – r i) and thermal conductivity k.
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.
Who wrote Fundamentals of Heat and Mass Transfer?
Fundamentals of Heat and Mass Transfer. C. P. Kothandaraman. New Age International, 2006, ISBN: 9788122417722.
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.
How to Calculate the Critical Insulation Radius?
Insulation is a very important thing in heat transfer to reduce the total energy that is expended to make the system hot or cold. Also, staying hot or cold in that system is very important. Because additional energy consumption is required to hold the system hot or cold as intended. In this case, optimum insulation must be designed.
How to Calculate It?
So, the calculation of the critical radius of insulation is very simple. But the calculations of the critical radius values for sphere and cylinder are quite different.
Conclusion
Finally, the general aspects of the critical radius of insulation in heat transfer can be explained as above.
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 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 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.
What is the critical thickness of insulation?
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 critical thickness?
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. Thermal Engineering.
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.
Who wrote Fundamentals of Heat and Mass Transfer?
Fundamentals of Heat and Mass Transfer. C. P. Kothandaraman. New Age International, 2006, ISBN: 9788122417722.
Does insulation increase convection resistance?
But in cylindrical and spherical coordinates, the addition of insulation also increases the outer surface, which decreases the convection resistance at the outer surface.