Two very long, solid pipes are to be soldered together using a torch (flame applied at the solder). The pipes have 3 cm diameters and are made of a material with thermal conductivity k = 100 W/m-K. Surrounding the pipes is air at T = 26°C and the convection coefficient between the pipe and air can be approximated as h = 10 W/m²-K. a) At least how long must these pipes be if we are willing to treat them as being "very long". b) The temperature at the junction of the two pipes must reach 200°C for the solder to melt. Determine the minimum heat transfer rate the torch must supply to melt the solder between the pipes. Hint: This problem (and part c) is most straightforward if you leverage the concept of thermal resistance. Bars Solder c) Unfortunately, your torch can only supply half the necessary heat transfer rate you determined in part (a). You wrap each pipe near the junction with near-perfect insulation in hopes of increasing the junction temperature. Determine how long each insulating sleeve must be for the junction temperature to reach 200°C. Hint: The heat transfer through the sleeved portion of the pipes is purely due to conduction. Solder -Lins Insulating sleeves

Two very long, solid pipes are to be soldered together using a torch (flame applied at the solder). The pipes have 3 cm diameters and are made of a material with thermal conductivity k = 100 W/m-K. Surrounding the pipes is air at T = 26°C and the convection coefficient between the pipe and air can be approximated as h = 10 W/m²-K. a) At least how long must these pipes be if we are willing to treat them as being "very long". b) The temperature at the junction of the two pipes must reach 200°C for the solder to melt. Determine the minimum heat transfer rate the torch must supply to melt the solder between the pipes. Hint: This problem (and part c) is most straightforward if you leverage the concept of thermal resistance. Bars Solder c) Unfortunately, your torch can only supply half the necessary heat transfer rate you determined in part (a). You wrap each pipe near the junction with near-perfect insulation in hopes of increasing the junction temperature. Determine how long each insulating sleeve must be for the junction temperature to reach 200°C. Hint: The heat transfer through the sleeved portion of the pipes is purely due to conduction. Solder -Lins Insulating sleeves

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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1.10 A heat flux meter at the outer (cold) wall of a concrete building indicates that the heat loss through a wall of 10-cm thickness is . If a thermocouple at the inner surface of the wall indicates a temperature of 22°C while another at the outer surface shows 6°C, calculate the thermal conductivity of the concrete and compare your result with the value in Appendix 2, Table 11.
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.
3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.
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
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.
3.17 A 1.4-kg aluminum household iron has a 500-W heating element. The surface area is . The ambient temperature is 21°C, and the surface heat transfer coefficient is . How long after the iron is plugged in does its temperature reach 104°C?
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.
Determine the heat flux for a furnace wall made of fire clay if the furnace wall thickness is 6’’ and the thermal conductivity of the furnace clay is 0.3 W/m-K. The furnace wall temperature can be taken to be same as furnace operating temperature which is 650°C, and temperature of the outer wall is 150°C.
A pipe 30 m long with an outer diameter of 75 mm is used to deliver steam at a rate of 1500 kg / hour. The steam pressure is 198.53 kPa entering the pipe with a quality of 98%. The pipe that needs to be insulated with a thermal conductivity of 0.2 W / (m K) so that the quality of the steam only decreases slightly to 95%. The temperature of the outer surface of the insulation is assumed to be 25 ° C. The conductive of the pipe material and the situation of no pressure drop in the pipe. A. Determine the enthalpy of incoming vapor = Answer kJ / kg. b. Determine the enthalpy of steam that comes out = Answer kJ / kg. c. Determine the change / loss of steam heat along the flow = Answer watt. d. Determine the minimum required insulation thickness = Answer cm.
The wall of a furnace consists of 200 mm thick refractory brick (k = 3 Btu / hr-ft2-0F), 100 mm thick bricks (k = 0.050 Btu / hr-ft2-0F) and 6 mm thick steel. The temperature on the side of the refractory brick layer of the furnace is 1150 C, and the temperature on the outer steel surface is 30C. There is a heat loss of 300 W / square meter at the wall of the furnace. It is difficult to prevent the thin air additions between the brick layers and the brick layer and the steel wall. How many mm of brick layers is the total thickness of these air layers equivalent?
A very long fin ( k= 372 W/m .o C ) 2.5 cm diameter has one end maintained at 90 oC . The rod is exposed to a fluid whose temperature is 40oC . The heat transfer coefficient is 3.5 W/m2 . oC . How much heat is lost by the rod .
A plane wall (0.8 m x 0.6 m) at 10 oC is to be heated by attaching 7-cm-long, 3-mm-diameter pin fins (k=200 W/mK, ρ=5000 kg/m3) to it. nfin=200 fins are to be used in total. Temperature of the surrounding fluid is 100 oC, and the heat transfer coefficient on the surfaces is 35 W/m2K. Total (material + manufacturing) cost of the fins is 850 TL/kg. Fill the table below by neglecting heat transfer from the fin tips. nfin=? a) Fin efficiency b) Fin effectiveness c) Overall fin effectiveness d) Total cost of fins