PBEQ/additional probs Flashcards
(13 cards)
A 50 mm diameter pipe of circular cross section and with walls 3 mm thick is covered with two concentric layers of lagging, the inner layer having a thickness of 25 mm and a thermal conductivity of 0.08 W/m-k. What is the rate of heat loss per meter length of pipe if the temperature inside the pipe is 550 K and the outside surface termperature is 330K?
- 62.7 W/m
- 48.9 W/m
- 55.3 W/m
- 71.1 W/m
62.7 W/m
A composite insulating wall has a layer of red brick and another insulating brick, hekd together by an aluminum rivet of 30 mm diameter per 0.1 m^2 of the surface. The red brick layer is 100 mm thick with the hot surface at 225°C, and the insulating brick is 250 mm thick, with cold surface at 37°C. The thermal conductivities of materials are: k aluminum = 203.6, k red brick = 0.93 and k insulating brick = 0.116 W/m-k. Estimate the percentage increase in heat transfer due to the rivets.
- 930%
- 660%
- 840%
- 510%
930%
929.72%
pbeq
A plane wall that is 20 cm thick and has a constant thermal conductivity k = 3 W/m-k undergoes one dimensional steady conduction. The surface temperature of the left side of the wall is known (T1 = 50°C) and the right side of the wall is exposed to air (Tair = 10°C) and convective heat transfer (h = 60 W/m^2-k). What is the temperature at the interface between the wall and the air on the right hand side (T2)?
- 27°C
- 18°C
- 35°C
- 44°C
18°C
pbeq
Saturated steam at 267°F is flowing inside a 1-in. steel pipe having an ID of 0.824 in. and an OD of 1.050 in. The pipe is insulated with 1.5 in. of insulation on the e pipe is estimated as hi = 1000 BTU/h-ft^2-°F. The mean thermal conductivity of the metal is 45 W/m-k and 0.064 W/m-k or 0.037 BTU/h-ft^2-°F for the insulation. Calculate the heat loss for 1 ft of pipe using resistances if the surrounding air is at 80°F.
- 25 BTU/hr
- 19 BTU/hr
- 48 BTU/hr
- 30 BTU/hr
30 BTU/hr
29.76 BTU/hr
may 2023 beq
A spherical, thin-walled metallic container is used to store liquid nitrogen at 77 K. The container has a diameter of 0.5 m and is covered with an evacuated. reflective insulation composed of silica powder. The insulation is 25 mm thick, and its outer surface is exposed to ambient air at 300 K, The convection coefficient is known to be 20 W/m2-K. The latent heat of vaporization and the density of liquid nitrogen are 2x10^5 J/kg and 804 kg/m3, respectively. What is the rate of liquid boil-off?
- 7.19 kg/d
- 9.32 kg/d
- 3.08 kg/d
- 5.64 kg/d
5.64 kg/d
The critical radius ‘r’ of insulation on a pipe is given by
- r = 2k/h
- r = k/2h
- r = k/h
- r = h/k
k/h
Indiabix
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
- 1.66
- 3.66
- 88.66
- dependent on NRe only
3.66
P5-9
pbeq
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow for a uniform heat flux through the tube wall
- 4.36
- 3.66
- 88.66
- dependent on NRe only
4.36
P5-9
oct 2023 beq
For turbulent flow in a tube, the heat transfer coefficient is obtained from the Dittus-Boelter correlation. If the diameter is halved and the flow rate is doubled, the the heat transfer coefficient will change by a factor of
- 1
- 6.1
- 1.74
- 37
6.1
oct 2023 beq
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is
- 11.2 kW/m2
- 14.6 kW/m2
- 12.0 kW/m2
- 16.5 kW/m2
16.5 kW/m2
An uninsulated steam pipe passes through a room in which the air and walls are at 25°C. The outside diameter of the pipe is 70 mm, and its surface temperature and emissivity are 200°C and 0.8, respectively. If the coefficient associated with free convection heat transfer from the surface air is 15 W/m2, what is the rate of heat loss from the surface per unit length of pipe.
- 650 W/m
- 998 W/m
- 835 W/m
- 726 W/m
998 W/m
A single effect evaporator is being used to concentrate a feed of 10000 lb/h of a cane sugar solution at 80
