A domestic pitched roof of tiles on felt sacking with a plasterboard ceiling has an existing u-value of 1.9w/m2k. Calculate the minimum thickness of fibre glass insulation in the roof space required to give the roof a new u-value of 0.25w/m2k. The thermal conductivity of the fibre glass used is 0.04w/mk

A domestic pitched roof of tiles on felt sacking with a plasterboard ceiling has an existing u-value of 1.9w/m2k. Calculate the minimum thickness of fibre glass insulation in the roof space required to give the roof a new u-value of 0.25w/m2k. The thermal conductivity of the fibre glass used is 0.04w/mk

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 1.4P: 1.4 To measure thermal conductivity, two similar 1-cm-thick specimens are placed in the apparatus...

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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
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?
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
A small metal building is to be constructed of corrugated steel sheet walls with a total wall overall surface area of about 300 m2. The air conditioner consumes about 1kW of electricity for every 4kW of cooling supplied. Two wall constructions are to be compared on the basis of cooling costs. Assume that electricity costs 10.15 Php/kWh. Determine the electrical energy savings of using 260mm of fiberglass insulation instead of 159 mm of fiberglass insulation in the wall. Assume an overall temperature difference across the wall of 20 ° (Uo (260-mm fiberglass)=0.17W/m2 °C and Uo (159-mm fiberglass)=0.31W/m2 °C)
A steel fin (k = 54 W/m-K) with a cross-section ofan equilateral triangle, 5 mm in side is 80 mmlong. It is attached to a plane wall maintained at400°C. The ambient air temperature is 50°C andthe heat transfer coefficient is 90 W/(m2-K). Calculate the fin efficiency and fin effectiveness, when the fin is considered as a very long fin
In order to reduce the heat loss through a large furnace wall, the decision has been made to add external insulation. Calculate the thickness of insulation required to reduce the heat loss by 75%. Before the change is made, no outer steel shell is used.Data: Refractory brick and wall brick:k = 0.87 W m-1 K-1Insulation: k = 0.090 W m-1 K-1•Steel: k = 43 W m-1 K-1h = 55 W m-1 K-1 (inside furnace).h = 11 W m-1 K-1 (outside furnace).
Finned heat sinks are used to keep computer chips in an acceptable temperature range. The heat sink base is 53 mm x 57 mm, and the maximum allowable chip temperature is 75 C. An array of 54 aluminum fins (k = 237 W/m*K) which have a square cross-section (3 mm x 3 mm) are attached to the heat sink. The length of each fin is 30 mm. The cooling air supplied to the heat sink is 25 C. If the convection heat transfer coefficient is 125 W/m2*K, what is the heat transfer rate (in W) from the heat sink, assuming adiabatic fin tips? (Hint: heat transfer does not solely occur from the fins.)
Lagged and unlagged pipes Hot water at 160 ⁰C is passing through a pipe, and it is lagged with a plastic covering. The coefficient of thermal conduction of the plastic is 0.2 Wm-1K-1 and the heat transfer coefficient is 16 Wm-2K-1. The ambient temperature surrounding the pipe is 25 ⁰C. Determine the critical radius and hence the maximum heat loss. Calculate the heat loss for varying thicknesses of lagging and plot this against the thickness. From the graph, estimate an optimal thickness of lagging and explain your reasons. You can use Excel for the calculations and plotting.
A steam pipe [k = 65 W/(m °C)] having 80 mm inside diameter and 105 mm outside diameter is lagged with two insulation layers; the layer in contact with the pipe is 35 mm asbestos [k = 0.15 W/(m °C)] and it is covered with 25 mm thick magnesia insulation [k = 0.075 W/(m °C)]. The heat transfer coefficients for the inside and outside surfaces are 220 W/ (m2 °C) and 6.5 W/ (m2 °C) respectively. If the temperature of steam is 350 °C and the ambient temperature is 30 °C, calculate a) The steady loss of heat for 50 m length of pipe. b) The overall heat transfer coefficient based on inside and outside surfaces of the lagged steam pipe.
A thick wallet tube of stainless steel (k+10 W/M°C) with 2 cm inside diameter and 4 cm outer diameter is covered with a 3 cm layers of asbestos insulation (K= 0.2 W/M°C), if the inside wall temperature of the pipe is maintained at 600 degree C , Calculate the heat loss per meter of length, Also calculate the tube - insulation interface temperature. Draw the Coresponding diagram. Ambient temperature is 2°C.
A steel fin (k= 54 W/m-K) with a cross-section ofan equilateral triangle, 5 mm in side is 80 mm long. It is attached to a plane wall maintained at 400°C. The ambient air temperature is 50°C andthe heat transfer coefficient is 90 W/(m2-K).Calculate the heat transfer rate (W), the fin effectiveness, and the fin efficiency when: a. the fin is considered a very long fin; b. there is negligible heat loss at the tip; c. the tip of the fin losses heat via convection
A straight fin of rectangular profile ( k = 14 W/m . oC ) , thickness of t= 2 mm and L= 23 mm . The base temperature Tb = 220 oC while the fin is exposed to the environment at 23 oC with h= 25 W/m2 .oC Calculate the heat lost peer meter of fin depth. ( use the chart to calculate the efficiency )