Critical Radius in Insulation
The critical radius of insulation means the radius that has the highest heat transfer. So now I will try to explain the logic of physic behind that below read it carefully.
Insulation is provided to reduce the rate of heat transfer from an object such as a cylinder or globe. Insulation sometimes contributes to heat transfer based on insulation properties and external settings (h = convective heat transfer coefficient)
Let us consider we are starting the insulation in a pipe, then it will increase the transfer of heat since the convective strength begins to decrease (Outside area A more means that the convective effect is 1 / (hA) low)
If we continue to add insulation, it will reach maximum heat transfer because convective heat transfer is the upper hand compared to convection.
After this, if we increase the insulation this will counteract in behavior. So the effects of movement begin to gain the upper hand. This means that it does not carry more heat outside even the surface and is larger due to increased tensile strength.
So we need to find the best radius of insulation called the radius of insulation judgment. To increase the radius of heat transfer insulation should be less than the required radius and if we want to reduce heat transfer it should be greater than the required radius.
Rc = k / h
If we increase the reduction h, the radius is necessary and therefore we need less insulation to reduce heat transfer. Low Rc means we need less material for insulation so our cost is reduced. Our goal is to reduce heat transfer at a lower insulation thickness so the only way to reduce the required radius is by increasing h and decreasing k. I hope it has some clean how
What is the critical thickness of insulation?
The term “Critical” itself is not complete in itself, it is a thematic term.
Adequate insulation is not acceptable in one case and applications in the other. There are many factors to consider when deciding on the thickness of insulation required.
I explain that providing thermal insulation has some relevant issues for electrical insulation as well.
ISSUE 1: As IS 14164
The recommended surface temperature is Ambient +20 deg C.
Then the Recommended surface loss of heat will be 125 W / Sq mt-hr
The recommended surface temperature for human safety is 65 deg C.
Most of the above factors need to be considered when deciding on the thickness of emergency insulation.
Let me explain this with an example.
If we consider surface temperature as a factor (usually considered for a steam line or boiler wall) the maximum surface temperature is Ambient + 20 deg C. It may be because of thick insulation in some of their cases if we are going to compare it with a surface loss of heat factor of 125W / Sq mt -hr.
If we assume that a manpower safety feature (usually considered for a steam condensate line or steam trap exit) allowed the maximum surface temperature at 65 deg C. This could be thick insulation wrong in some cases if we compare it with a surface heat loss factor of 125W / Sq mt- hr or Ambient + 20 deg C.
ISSUE 2: Insulation Method (Application Usage)
Insulation is heated quickly which allows air to enter the hot surface as hot air is lighter it will move out creating fresh cold air juice under the insulation causing continuous heat loss. The same thickness of insulation with the same insulation material will give a different result.
ISSUE 3: Insulation Density Duty (The same applies if there is also electrical insulation)
Heat taken = M x Cp x DT
M- Mass
Cp - Specific heat or thermal transfer
dT - temperature difference
If the density is higher, the mass is the largest. the more the heat is absorbed.
For example in the case of thermal insulation according to IS 8183 (bonded mineral wool specification)
It is divided into groups based on a range of application density and temperature.
Bulk density Group 1 Between 12- 50 Kg / m3 the recommended hot face temperature is 250 deg C.
Bulk Density Group 2 Between 51-80 Kg / m3 the recommended hot face temperature is 400 deg C.
Bulk density Group 3 Between 81- 120 Kg / m3 the recommended hot face temperature is 550 deg C.
Bulk density Group 4 Between 121- 160 Kg / m3 the recommended hot face temperature is 750 deg C
Average temperature (deg C) = (Th + Tc) / 2
For Group 2, 3 & 4 at 50 deg C Average temperature (Example, Th = 80 Tc = 20) Thermal Conduction 0.43 mW / cm deg C
For Group 2, 3 & 4 at 100 deg C Average temperature (Example, Th = 180 Tc = 20) Thermal Conduction 0.52 mW / cm deg C
For Group 3 & 4 at 150 deg C Average temperature (Example, Th = 280 Tc = 20) Thermal Conduction 0.62 mW / cm deg C
Let's say
If for 250 deg C Higher hot front temperature is considered Group 4 insulation (150 Kg / m3 Commercially Available Density) instead of Group 3 (80 Kg / m3 Commercially Available Density). The thermal conductivity is the same for the same insulation thickness. (Higher the density, Higher the mass)
System heat loss will be higher if the transfer/travel time is longer.
ISSUE 4: Reflective effect (combination factor)
Passing two or more closed lines nearby will create a mixing effect.
A hot steam line approaching the other hot steam line will create a reflective effect on all surfaces resulting in increased surface heat loss.
A hot steam line approaching the cold water or another cold water line will create a reflective effect on all surfaces resulting in increased surface heat loss from a hot surface and heat gain from a cold line.
ISSUE 5: Cost of property
Most of the time I also think about the commercial effects of insulation. The cost impact of heat loss over the lifetime of the cost-effective insulation is also a key factor in making decisions.
Cost-effective lifetime insulation includes the cost of supply, cost of application, the life of effective insulation, maintenance criticism (plant must be planted or not to cause loss of production), and cost of maintenance.
If the cost effectiveness of allowable heat loss over the additional thickness of effective insulation is lower, the organization does not consider the additional thickness. Although, if the cost effectiveness of allowable heat loss over the additional thickness of effective insulation is higher, an organization is considering the additional thickness.
Hopes above explanations have provided some insight into how thick emergency insulation is practically determined as the best practice.
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