A commercial grade cubical freezer, 2 m on a side, has a composite wall consisting of an exterior sheet of 8 mm-thick plain carbon steel, an intermediate layer of 90-mm- thick cork insulation, and an inner sheet of 8 mm-thick aluminum alloy (2024). Adhesive interfaces between the insulaticn and the metallic strips are each characterized by a thermal contact resistance of Rt,c-4.5*10 m.K/w. What is the steady-state cooling load that must be maintained by the refrigerator under conditions for which the outer and inner surface temperatures are 45°C and 8°C, respectively? Thermal conductivities of aluminum, cork, and carbon steel are 200 W/mK, 0.5 W/mK, and 70 W/mK, respectively. Adheşive Adhesive Aluminum Carbon steel Cork insulation

A commercial grade cubical freezer, 2 m on a side, has a composite wall consisting of an exterior sheet of 8 mm-thick plain carbon steel, an intermediate layer of 90-mm- thick cork insulation, and an inner sheet of 8 mm-thick aluminum alloy (2024). Adhesive interfaces between the insulaticn and the metallic strips are each characterized by a thermal contact resistance of Rt,c-4.5*10 m.K/w. What is the steady-state cooling load that must be maintained by the refrigerator under conditions for which the outer and inner surface temperatures are 45°C and 8°C, respectively? Thermal conductivities of aluminum, cork, and carbon steel are 200 W/mK, 0.5 W/mK, and 70 W/mK, respectively. Adheşive Adhesive Aluminum Carbon steel Cork insulation

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.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.45P: An electronic device that internally generates 600 mW of heat has a maximum permissible operating...

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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.
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.
An electronic device that internally generates 600 mW of heat has a maximum permissible operating temperature of 70C. It is to be cooled in 25C air by attaching aluminum fins with a total surface area of 12cm2. The convection heat transfer coefficient between the fins and the air is 20W/m2K. Estimate the operating temperature when the fins are attached in such a way that (a) there is a contact resistance of approximately 50 K/W between the surface of the device and the fin array and (b) there is no contact resistance (in this case, the construction of the device is more expensive). Comment on the design options.
1.1 On a cold winter day, the outer surface of a 0.2-m-thick concrete wall of a warehouse is exposed to temperature of –5°C, while the inner surface is kept at 20°C. The thermal conductivity of the concrete is 1.2 W/m K. Determine the heat loss through the wall, which is 10-m long and 3-m high. Problem 1.1
1.4 To measure thermal conductivity, two similar 1-cm-thick specimens are placed in the apparatus shown in the accompanying sketch. Electric current is supplied to the guard heater, and a wattmeter shows that the power dissipation is 10 W. Thermocouples attached to the warmer and to the cooler surfaces show temperatures of 322 and 300 K, respectively. Calculate the thermal conductivity of the material at the mean temperature in W/m K. Problem 1.4
The handle of a ladle used for pouring molten lead is 30 cm long. Originally the handle was made of 1.9cm1.25cm mild steel bar stock. To reduce the grip temperature, it is proposed to form the handle of tubing 0.15 cm thick to the same rectangular shape. If the average heat transfer coefficient over the handle surface is 14 W/m K, estimate the reduction of the temperature at the grip in air at 21C.
2.9 In a large chemical factory, hot gases at 2273 K are cooled by a liquid at 373 K with gas-side and liquid-side convection heat transfer coefficients of 50 and , respectively. The wall that separates the gas and liquid streams is composed of a 2-cm thick oxide layer on the gas side and a 4-cm thick slab of stainless steel on the liquid side. There is a contact resistance between the oxide layer and the steel of . Determine the rate of heat loss from hot gases through the composite wall to the liquid.
One end of a 0.3-m-long steel rod is connected to a wall at 204C. The other end is connected to a wall that is maintained at 93C. Air is blown across the rod so that a heat transfer coefficient of 17W/m2 K is maintained over the entire surface. If the diameter of the rod is 5 cm and the temperature of the air is 38C, what is the net rate of heat loss to the air?
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.
J 5 A cylindrical steel tank with a diameter of 1.8m and a height of 2m, is insulated with polyurethane on its lateral faces and its lid. The insulation is 40 mm thick and covered with a thin layer of metal. The tank contains water that is kept warm by an electric heater in such a way as to maintain the inner surface of the tank at a temperature of 55C. The ambient air temperature is 10C and the external skin coefficient is 10 w/m2K. If the energy cost is $0.15/kwh. What is the monthly energy cost for water storage?
Circular cooling fins of diameter D = 1 mm andlength L = 25.4 mm, made of copper (k = 400 W/m·K), areused to enhance heat transfer from a surface that is maintainedat temperature Ts1 = 132°C. Each rod has one end attached tothis surface (x = 0), while the opposite end (x = L) is joinedto a second surface, which is maintained at Ts2 = 0°C. Theair flowing between the surfaces and the rods is also at T` =0°C, and the convection coefficient is h = 100 W/m2·K.(a) Express the function u(x) = T(x) - T` along a fin, andcalculate the temperature at x = L/2. (b) Determine the rate of heat transferred from the hot surface through each fin and the fin effectiveness. Is the use of fins justified? Why? (c) What is the total rate of heat transfer from a 10-cm by 10-cm section of the wall, which has 625 uniformly distributed fins? Assume the same convection coefficient for the fin and for the unfinned wall surface.