The emissivity of galvanized steel sheet, a commonroofing material, is
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Fundamentals of Heat and Mass Transfer
- 1.26 Repeat Problem 1.25 but assume that the surface of the storage vessel has an absorbance (equal to the emittance) of 0.1. Then determine the rate of evaporation of the liquid oxygen in kilograms per second and pounds per hour, assuming that convection can be neglected. The heat of vaporization of oxygen at –183°C is .arrow_forward1.11 Calculate the heat loss through a glass window 7-mm thick if the inner surface temperature is 20°C and the outer surface temperature is 17°C. Comment on the possible effect of radiation on your answer.arrow_forwardA greenhouse 20m long and 15m wide has a nearly flat roof 3m above the floor level. When the sun is directly overhead the solar flux is 1,000 W/m^2. If the glass roof has an emissivity of 0.9 and the convective losses are 0.8 times the radiation losses, what is the temperature in the greenhouse?arrow_forward
- A room of length 5m height 5m and width 5m is heated through the floor by maintaining it at a uniform temperature of 350 K. while side walls are well insulated. The heat loss takes place through the ceiling at 300 K. Assuming that all surfaces have an emissivity of 0.8. determine the rate of heat loss by radiation through the ceiling. BIU 非 Σ Given-arrow_forwardA furnace wall is constructed of an inner layer of 12.7 mm of refracter material, a middle layer of 101.6 mm of insulation material and outer layer of 76.2 mm of steel. The wall surface temperature is 1370 °C inside the refracter material (T0) and 37.8 °C at the outside surface of the steel (T3). Use thermal conductivity for refractor kref = 6.64 W/mK, for insulation, kins= 2.34 W/mK and for steel kste = 45 W/mK. Calculate the heat loss in W for 1 m2 (heat flux)? The temperature at the inter face between the insulation material and steel board?arrow_forwardAn electrical cable with 20 mm in diameter and emissivity equal to 0.85 is installed inside a conduit whose inner surface and air in the its interior is at 30oC. The electrical resistivity of the cable, ρe (µΩ.m), is a function of its temperature, given by ρe=a[1+b(T-T0)], where a=0.0171 µΩ.m, b=0.00396 K-1 and T0= 25oC. The natural convection heat transfer coefficient is expressed by the relation h=cD-0.25(T-Tꚙ)0.25where c=1.21 W/(m1.75.K1.25) and D is the cable diameter. Electrical resistance per unit of cable length is R’e=ρe/Ac (Ac is the cross-sectional area). (a) For steady state operating conditions, estimate the maximum current that can be dissipated in the wire so that its temperature does not exceed 65oC;arrow_forward
- A small sphere (emissivity =0.503 radius=r1) is located at the center of a spherical abestos shell ( thickness =1.74 cm, outer radius= r2; thermal conductivity of abestos is 0.090 J/ (sm c degrees) The thickness of the shell is small compared to the inner and outer radii of the shell. The temperature of the small sphere is 695 degrees Celsius while the temperature of the inner surface of the shell is 352 degrees Celsius, both temperatures remaining constant. Assuming that r2/r1 =8.75 and ignoring any air inside the shell, find the temperature in degrees Celsius of the outer surface of the shell.arrow_forwardLiquefied natural gas (LNG) is transported around the globe using ships similar to thatshown in Figure QA3. This ship has four pressurised cylindrical steel tanks each ofradius of 20 m. The tanks are internally insulated with 30 cm of polyurethane foamwhich keeps the LNG at a constant -162 ºC. Take the effective sky temperature is 265K and the net radiative thermal energy exchange with the sky as 1x106 W. Calculate the surface temperature of the end (facing the sun) of a tank. Calculate the conductive heat transfer through the end (facing the sun)of a tank.arrow_forwardLiquefied natural gas (LNG) is transported around the globe using ships similar to thatshown in Figure QA3. This ship has four pressurised cylindrical steel tanks each ofradius of 20 m. The tanks are internally insulated with 30 cm of polyurethane foamwhich keeps the LNG at a constant -162 ºC. Take the effective sky temperature is 265K and the net radiative thermal energy exchange with the sky as 1x10^6 W. (a) Calculate the surface temperature of the end (facing the sun) of a tank.(b) Calculate the conductive heat transfer through the end (facing the sun)of a tank. answers: a) 375K b) 22.1kWarrow_forward
- Liquefied natural gas (LNG) is transported around the globe using ships similar to thatshown in Figure QA3. This ship has four pressurised cylindrical steel tanks each ofradius of 20 m. The tanks are internally insulated with 30 cm of polyurethane foamwhich keeps the LNG at a constant -162 ºC. Take the effective sky temperature is 265K and the net radiative thermal energy exchange with the sky as 1x10^6 W. (a) Calculate the surface temperature of the end (facing the sun) of a tank.(b) Calculate the conductive heat transfer through the end (facing the sun)of a tank. DATA FOR QUESTION: Thermal conductivity, polyurethane foam = 0.02 W/mKStefan’s Constant = 5.67x10^-8 W/m^2K^4Emissivity, steel = 0.95 answers: a) 375K b) 22.1kWarrow_forwardA spherical radiator has a radius of 3ft and emissivity of 0.5. If radiates 6000 Btu/hr of heat at 3000R, what is the temperature of a surface affected by its radiation?arrow_forwardA radiator plate of 0.2-m (wide) x 0.4-m (height) is in a room of 22°C. One side (the hot side facing the room) of the radiator plate is maintained at a temperature of 80°C, the other sides are assumed to be insulated, as shown in Figure 1. If the following assumptions can be made: 1) steady operating conditions exist; 2) the air an ideal gas; 3) the local' atmospheric pressure is 1.00 atm: (a) Briefly and qualitatively analyze the four different locations for the radiators marked A), B), C) and D), as shown in Figure 1, and rank them in terms of heat transfer efficiency by natural convection. (b) Determine the rate of heat transfer from the radiator plate by natural convection if the plate is A) vertical and C) horizontal with the hot surface facing down. Compare the two and provide a brief discussion of the findings. wall A 0.4 mi C 0.4 m 0.4 m B floor ceiling 0.4 m wall Nu Nu=0.59Ra Nu-0.1 Raa 0.387Ral Nu- - {0.825 + 11 + (0.492/Pr)162) (complex but more accurate) Use vertical plate…arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning