A steam-carrying steel-pipe has inner and outer diameter of 85 mm and 110 mm, respectively. It is covered with an insulation having a thermal conductivity of 1.8 W/mK. The convective heat transfer coefficient between the surface of insulation and the surrounding air is 14.5 W/m²K. Determine the critical radius of insulation.
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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 11.2.38 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.A 2 m X 1.5 m section of wall of an industrial furnace burning gas is not insulated, and the temperature at the outer surface of this section is measured to be 80oC. The temperature of the furnace room is 30oC, and the combined convection and radiation heat transfer coefficient at the surface of the outer furnace is 10 w/oC. It is proposed to insulate this section of the furnace wall with glass wool insulation (K=0.038 W/moC) in order to reduce the heat loss by 90 percent. Assuming the outer surface temperature of the section remains at about 80oC, determine i) the thickness of the insulation that is needed and ii) the outer surface temperature of the insulation after installation.
- Because you forgot to let the pipes drip during a freezing night, a section of an outdoor pipe is now frozen. The frozen section is L = 1 m long and the inner pipe diameter is D = 1.8 cm. During the day, the pipe is exposed to the cold air, the Sun, and the radiating surroundings. The cold air temperature is T∞ = -10°C, and has convection heat transfer coefficient h = 20 W/m2·K. The Sun provides solar irradiance of Gsun = 1350 W/m2. The steel pipe surface has absorptivity α = 0.6 and emissivity ε= 0.1. The surroundings, such as vegetation, houses, ground, etc. can be assumed to be blackbody held at Tsur = 280 K. A) Using the energy conservation system illustrated below, establish an equation that describes the stored energy in the section of frozen water (Est). B) Determine the amount of time needed to melt the ice in the pipe. Ice has density ρ= 920 kg/m3, and latent heat of fusion hsf = 334 kJ/kg. The ice is Tw = 0°C. Ignore conduction through the pipe walls – assume the pipe…The walls of the refrigerated storage room are 10 m long and 3 m high and constructed with 90 mm concrete blocks (k = 0.935 W / [m ° C]) and 12 cm fiber insulation board (k = 0.048 W). / [m ° C]). The temperature of the cold room is -10 ° C and the convective hot coupling is 40 W / (m² K); The outside air temperature is 30 ° C with a convection heat transfer coefficient of 10 W / (m² K). Calculate the total heat wave. Total heat transfer coefficient = Answer W / (m² K).The wall of a refrigerator is made of fiberglass insulation (k = 0.035 W/mK) glued between two 1 mm thick layers of metal plate (k = 15.1 W/mK). The refrigerated space is kept at 2°C, and the average heat transfer coefficients on the inner and outer wall surfaces are 4 W/m²K and 9 W/m²K, respectively. The average temperature in the kitchen is 24°C. It is observed that condensation occurs on the external surface of the refrigerator when the temperature of the external surface drops to 20°C. Determine the minimum thickness of fiberglass insulation that should be used on the wall to prevent condensation on the outside surface.
- A flat plate solar collector glass with a surface area of 1.9 m² has direct solar radiation of 789 W / m². If the effective absorptance-transmittance factor is 0.95, the average collector absorber surface temperature is 62 ° C, the water outlet temperature of the collector is 40 ° C and the collector efficiency is 55%, calculate the average heat loss coefficient of the collector [W / (m² ° C)].It is required to insulate a kitchen oven with cork board (k = 0.043 W/mK) so that the heat losses from the oven does not exceed 400 W/m2 when the inner surface of the oven is at 225°C and the outer surface of the oven is at 40°C. The thickness of insulation required is nearlyAn 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;
- In a food industry, pieces of meat with approximately 30 cm in diameter and 20 mm in thickness are stored in a industrial freezer and thawed by exposure to ambient air at 15°C with convective heat transfer coefficient equal to 10W/(m2.K). Consider a piece of meat that, when removed from the freezer, had a temperature of -12°C. For defrosting, the piece is hung on a "line", so that both larger surfaces are exposed to the ambient air. For frozen meat, the following properties can be assumed ρ=1090 kg/m3, cp=3.54 kJ/(kg.K), k=0.47 W/(m.K). Ask: (1) Indicating the control volume and the simplifications adopted, obtain the differential energy balance equation for the cooling the piece of meat and describe the conditions of outline and initial that apply to the process, justifying them; (2) Use the appropriate analytical solution to determine the time necessary for the complete defrosting of the part. The part is considered to be completely thawed when a minimum temperature of 5oC is reached…A domestic refrigerator with inner dimensions of 0.7 m by 0.7 m at the base and height 1 m was designed to maintain a set temperature of 6 ˚C. The bodies consist of two 10-mm-thick layers of Aluminium (k = 225 W/mK) separated by a 30 mm polyurethane insulation (k=0.028 W/mK). If the average convection heat transfer coefficient at the inner and outer surfaces are 11.6 W/m2K and 14.5 W/m2K respectively, calculate: the outer surface temperature of the Aluminum in 0C to 2 decimal placesConvective heat transfer coefficient is 15 W/(m*K), surface area of the fin is 2 m^2, the temperature at the base is 70 C and the ambient temperature is 20 C. Determine the fin efficiency if the actual heat transfer rate from the fin is 500 W? not sufficient information 66 % none of the given 15 % 33 %