A thermal window consisting of two panes of 0.25" glass (R-value 0.08 hr · ft2. °F/BTU each), separated by a layer of air 0.75" thick is now used as a horizontal window in the ceiling (a skylight) of an air conditioned room. Find its R-value in the summer, when the outside is warmer than the inside, and in the winter, when the inside is warmer than the outside (in hr · ft2. °F/BTU). (Assume that the air is moving outside the window.) R (summer) _hr ft2. °F/BTU R (winter) hr ft2. °F/BTU

A thermal window consisting of two panes of 0.25" glass (R-value 0.08 hr · ft2. °F/BTU each), separated by a layer of air 0.75" thick is now used as a horizontal window in the ceiling (a skylight) of an air conditioned room. Find its R-value in the summer, when the outside is warmer than the inside, and in the winter, when the inside is warmer than the outside (in hr · ft2. °F/BTU). (Assume that the air is moving outside the window.) R (summer) _hr ft2. °F/BTU R (winter) hr ft2. °F/BTU

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.

Chapter2: Steady Heat Conduction

Section: Chapter Questions

Problem 2.60P

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1.60 Two electric resistance heaters with a 20 cm length and a 2 cm diameter are inserted into a well-insulated 40-L tank of water that is initially at 300 K. If each heater dissipates 500 W, what is the time required for bringing the water temperature in the tank to 340 K? State your assumption for your analysis.
A cooling system is to be designed for a food storage warehouse for keeping perishable foods cool prior to transportation to grocery stores. The warehouse has an effective surface area of 1860 m2 exposed to an ambient air temperature of 32C. The warehouse wall insulation (k=0.17W/(mK)) is 7.5 cm thick. Determine the rate at which heat must be removed (W) from the warehouse to maintain the food at 4C.
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.
1.29 A spherical interplanetary probe with a 30-cm diameter contains electronic equipment that dissipates 100 W. If the probe surface has an emissivity of 0.8, what is its surface temperature in outer space? State your assumptions in the calculations.
1.19 A cryogenic fluid is stored in a 0.3-m-diameter spherical container is still air. If the convection heat transfer coefficient between the outer surface of the container and the air is 6.8 , the temperature of the air is 27°C, and the temperature of the surface of the sphere is –183°C, determine the rate of heat transfer by convection.
A pipe carrying superheated steam in a basement at 10C has a surface temperature of 150C. Heat loss from the pipe occurs by radiation (=0.6) and natural convection (hc=25W/m2K). Determine the percentage of the total heat loss by these two mechanisms.
2.38 The addition of aluminum fins has been suggested to increase the rate of heat dissipation from one side of an electronic device 1 m wide and 1 m tall. The fins are to be rectangular in cross section, 2.5 cm long and 0.25 cm thick, as shown in the figure. There are to be 100 fins per meter. The convection heat transfer coefficient, both for the wall and the fins, is estimated to be K. With this information determine the percent increase in the rate of heat transfer of the finned wall compared to the bare wall.
1.67 In beauty salons and in homes, a ubiquitous device is the hairdryer. The front end of a typical hairdryer is idealized as a thin-walled cylindrical duct with a 6-cm diameter with a fan at the inlet that blows air over an electric heating coil as schematically shown in the figure. The design of this appliance requires two power settings, with which the air blown over the electric heating coil is heated from the ambient temperature of to an outlet temperature of and with exit air velocities of 1.0 m/s and 1.5 m/s. Estimate the electric power required for the heating coil to meet these conditions, assuming that heat loss from the outside of the dryer duct is neglected.
Consider a 1.2-m-high and 2-m-wide glass windowwith a thickness of 6 mm, thermal conductivity k = 0.78W/m·K, and emissivity « = 0.9. The room and the wallsthat face the window are maintained at 25°C, and the averagetemperature of the inner surface of the window is measuredto be 5°C. If the temperature of the outdoors is 25°C,determine (a) the convection heat transfer coefficient on theinner surface of the window, (b) the rate of total heat transferthrough the window, and (c) the combined natural convectionand radiation heat transfer coefficient on the outer surface ofthe window. Is it reasonable to neglect the thermal resistanceof the glass in this case?
In a new residential project, you strongly believethat double-paned windows are ‘better’ than singlepaned windows. Compare the rate of heat loss betweensingle and double-paned windows (1.5 m x 1 m) if thethickness of each pane is (th = 0.4 cm) and (k = 0.9W/m.K). The indoor and outdoor temperatures are 18°C and 2 °C, respectively. Thickness of the air gapbetween the double-paned windows is (th = 1 cm), and(k = 0.022 W/m.K). Image credit: Windowwhirl
Why does heat transfer increase initially with radius of insulation and then decrease beyond critical radius? Explain in details.
Consider a double-paned window consisting of two panes of glass, each with a thickness of 0.500 cm and an area of 0.795 m2 , separated by a layer of air with a thickness of 1.50 cm. The temperature on one side of the window is 0.00 ∘C∘C; the temperature on the other side is 21.0 ∘C∘C. In addition, note that the thermal conductivity of glass is roughly 36 times greater than that of air. Approximate the heat transfer through this window by ignoring the glass. That is, calculate the heat flow per second through 1.50 cmcm of air with a temperature difference of 21.0 ∘C∘C. (The exact result for the complete window is 25.6 J/sJ/s .)