An electric cable of 20 mm diameter is insulated with rubber, which is exposed to the atmosphere at 300 C. Calculate the most economical thickness of rubber insulation (k = 0.175 W/m.K). When cable surface temperature with and without insulation is at 700 C. Also calculate the percentage increase in heat dissipation and current carrying capacity when the most economical thickness is provided. Take the heat transfer W coefficient, h =9.3 m²K

An electric cable of 20 mm diameter is insulated with rubber, which is exposed to the atmosphere at 300 C. Calculate the most economical thickness of rubber insulation (k = 0.175 W/m.K). When cable surface temperature with and without insulation is at 700 C. Also calculate the percentage increase in heat dissipation and current carrying capacity when the most economical thickness is provided. Take the heat transfer W coefficient, h =9.3 m²K

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter3: Transient Heat Conduction

Section: Chapter Questions

Problem 3.10P: 3.10 A spherical shell satellite (3-m-OD, 1.25-cm-thick stainless steel walls) re-enters the...

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3.10 A spherical shell satellite (3-m-OD, 1.25-cm-thick stainless steel walls) re-enters the atmosphere from outer space. If its original temperature is 38°C, the effective average temperature of the atmosphere is 1093°C, and the effective heat transfer coefficient is , estimate the temperature of the shell after reentry, assuming the time of reentry is 10 min and the interior of the shell is evacuated.
Estimate the rate of heat loss per unit length from a 5-cm ID, 6-cm OD steel pipe covered with high-temperature insulation having a thermal conductivity of 0.11 W/(m K) and a thickness of 1.2 cm. Steam flows in the pipe. It has a quality of 99% and is at 150C. The unit thermal resistance at the inner wall is 0.0026(m2K)/W the heat transfer coefficient at the outer surface is 17W/(m2K) and the ambient temperature is 16C.
2.7 A very thin silicon chip is bonded to a 6-mm thick aluminum substrate by a 0.02-mm thick epoxy glue. Both surfaces of this chip-aluminum system are cooled by air at , where the convective heat transfer coefficient of air flow is . If the heat dissipation per unit area from the chip is under steady-state conditions, draw the thermal circuit for the system and determine the operating temperature of the chip.
2.42 A circumferential fin of rectangular cross section, 3.7-cm OD and 0.3 cm thick, surrounds a 2.5-cm- diameter tube as shown below. The fin is constructed of mild steel. Air blowing over the fin produces a heat transfer coefficient of K. If the temperatures of the base of the fin and the air are and , respectively, calculate the heat transfer rate from the fin.
A steam turbine casing is made of a 6mm thick steel plate. At one location it has an inside surfacetemperature of 100°C. The convective heat transfer coefficient on the outside is 10 W/m2K. Theambient temperature is 25°C. What is the outer surface temperature? If the outer surface iscovered with a 3cm layer of insulation (k = 0.04 W mK), will it be safe to touch? Explain.
In a quenching process, a copper plate of 3 mm thick is heated up to 350°C and then suddenly, it is dropped into a water bath at 25°C. (a) Calculate the time required for the plate to reach thetemperature of 50°C. The heat transfer coefficient on the surface of the plate is 28 W/m2 -K. The plate dimensions may be taken as length 40 cm and width 30 cm. (b) Also calculate the time required for infinite long plate to cool to 50°C. Other parameters remain same. (c) Calculate the heat transfer rate loss of the plate when it is at 50°C. Take the properties of copper as: CP = 380 J/kg-K, ρ = 8800 kg/m3 , k = 385 W/m-K
Calculate the heat transfer rate from a single-pane glass window with aninside surface temperature of approximately 20° C and an outside surfacetemperature of 5° C . The glass is 1 m tall, 1.8 m wide, and 8 mm thick, asshown . The thermal conductivity of the glass is approximatelyk = 1.4 W⁄m ·K .
A steam pipe of 120-mm outside diameter is covered with a 20-mm-thick layer of calciumsilicate insulation (k = 0.089 W/m-K). The pipe surface temperature is 800 K, and theambient air and surroundings temperatures are 300 K. The convection and radiationcoefficients for the outer surface of the insulation are estimated as 5.5 W/m2-K and 10W/m2-K, respectively. Determine the heat rate per unit length from the pipe (W/m) andthe insulation outer surface temperature.
A pipe carrying the liquid at -20°C is 10 mm in outer diameter and is exposed to ambient at 25°C with convective heat transfer coefficient of 50 W/m2-K. It is proposed to apply the insulation of material having thermal conductivity of 0.5 W/m-K. Calculate the heat loss for 2.5 mm, 7.5 mm and 15 mm thickness of insulation over 1 m length. Which one is more effective thickness of insulation? note: include the diagram for solutions
A 25 mm diameter copper pipe is used to carry heated water, the external surface of the pipe is subjected to a convective heat transfer coefficient of h= 6.8 W/m2 K, fin the heat loss by convection per meter length of the pipe when the external surface temperature is 90oC and the surroundings are at 25oC. Assume black body radiation what is the heat loss by radiation? And also conclude your results?
Question 1:A glass window of width W = 1 m and height H = 2 m is 5 mm thick and has a thermal conductivity of kg=1.4 W/m.K. If the inner and outer surface temperatures of the glass are 15oC and -20oC, respectively, ona cold winter day, what is the rate of heat loss throughthe glass? To reduce heat loss through windows, it iscustomary to use a double pane construction in whichadjoining panes are separated by an air space. If thespacing is 10 mm and the glass surfaces in contact withthe air have temperatures of (last two digit of your Registration number) oC and -15oC, what isthe rate of heat loss from a 1 m x 2 m window? Thethermal conductivity of air is ka=0.024 W/m.K.
A transformer is to be cooled via forced air convection. To increase the effectiveness of thecooling, Alloy 1095 cast aluminum fins are attached to the top of the transformer overwhich air is blown (negligible thermal contact resistance between the transformer and thefins). The fins are 2 cm thick and 7 cm long and span the entire length of the transformer,and the transformer is 7 cm wide and 10 cm long. Air leaves the cooling fan at 25°C .a) Decide how to model the fins before performing any real analysis. Youare willing to accept some error in your analysis and so you will either model thefins as having adiabatic tips or being infinitely long. You expect the average convectioncoefficient over the heat transfer surfaces for this system to be around 50 W m2– K⁄ .Which tip condition should you use for the fins in your analysis? b) Modeling the transformer as being isothermal, determine the average convectioncoefficient over the heat transfer surfaces. Account for heat transfer from the…