Question: Two infinitely long parallel plates of width
Two infinitely long parallel plates of width w are located at w distance apart, as shown in Fig. P21-67. Using the Hottel’s crossed-strings method, determine the view factor F12.
> Complete this table for H2O:
> Consider two rectangular surfaces perpendicular to each other with a common edge that is 1.6 m long. The horizontal surface is 0.8 m wide, and the vertical surface is 1.2 m high. The horizontal surface has an emissivity of 0.75 and is maintained at 450 K
> A double-pipe parallel-flow heat exchanger is to heat water (cp = 4180 J/kg⋅K) from 25°C to 60°C at a rate of 0.2 kg/s. The heating is to be accomplished by geothermal water (cp = 4310 J/kg⋅K) available at 140°C at a mass flow rate of 0.3 kg/s. The inner
> Two long, parallel 20-cm-diameter cylinders are located 30 cm apart from each other. Both cylinders are black and are maintained at temperatures 425 K and 275 K. The surroundings can be treated as a blackbody at 300 K. For a 1-m-long section of the cylin
> A stream of hydrocarbon (cp = 2.2 kJ/kg⋅K) is cooled at a rate of 720 kg/h from 150°C to 40°C in the tube side of a double-pipe counterflow heat exchanger. Water (cp = 4.18 kJ/kg⋅K) enters the heat exchanger at 10°C at a rate of 540 kg/h. The outside dia
> Two infinitely long parallel plates of width w are located at w distance apart, as shown in Fig. The two plates behave as black surfaces, where surface A1 has a temperature of 700 K and surface A2 has a temperature of 300 K. Determine the radiation heat
> A double-pipe parallel-flow heat exchanger is used to heat cold tap water with hot water. Hot water (cp = 4.25 kJ/kg⋅K) enters the tube at 85°C at a rate of 1.4 kg/s and leaves at 50°C. The heat exchanger is not we
> Two parallel disks of diameter D = 3 ft separated by L = 2 ft are located directly on top of each other. The disks are separated by a radiation shield whose emissivity is 0.15. Both disks are black and are maintained at temperatures of 1350 R and 650 R,
> For specified inlet and outlet temperatures, for what kind of heat exchanger will the ΔTlm be greatest: double-pipe parallel-flow, double-pipe counterflow, crossflow, or multipass shell-and tube heat exchanger?
> Two parallel black disks are positioned coaxially at a distance of 0.25 m apart in surroundings with a constant temperature of 300 K. The lower disk is 0.2 m in diameter and the upper disk is 0.4 m in diameter. If the lower disk is heated electrically at
> In the heat transfer relation Q = UAsFΔTlm for a heat exchanger, what is the quantity F called? What does it represent? Can F be greater than 1?
> A perfectly fitting pot and its lid often stick after cooking, and it becomes very difficult to open the lid when the pot cools down. Explain why this happens and what you would do to open the lid.
> Consider a hemispherical furnace of diameter D = 5 m with a flat base, as shown in Fig. The dome of the furnace is black, and the base has an emissivity of 0.7. The base and the dome of the furnace are maintained at uniform temperatures of 400 and 1000 K
> Explain how the LMTD method can be used to determine the heat transfer surface area of a multipass shell-and-tube heat exchanger when all the necessary information, including the outlet temperatures, is given.
> Two parallel disks of diameter D = 0.6 m separated by L = 0.4 m are located directly on top of each other. Both disks are black and are maintained at a temperature of 450 K. The back sides of the disks are insulated, and the environment that the disks ar
> Can the outlet temperature of the cold fluid in a heat exchanger be higher than the outlet temperature of the hot fluid in a parallel-flow heat exchanger? How about in a counterflow heat exchanger? Explain.
> A furnace is of cylindrical shape with R = H = 3 m. The base, top, and side surfaces of the furnace are all black and are maintained at uniform temperatures of 500, 700, and 1400 K, respectively. Determine the net rate of radiation heat transfer to or fr
> The temperature difference between the hot and cold fluids in a heat exchanger is given to be ΔT1 at one end and ΔT2 at the other end. Can the logarithmic temperature difference ΔTlm of this heat exchanger be greater than both ΔT1 and ΔT2? Explain.
> How do ultraviolet and infrared radiation differ? Do you think your body emits any radiation in the ultraviolet range? Explain.
> How does the log mean temperature difference for a heat exchanger differ from the arithmetic mean temperature difference? For specified inlet and outlet temperatures, which one of these two quantities is larger?
> A dryer is shaped like a long semicylindrical duct of diameter 1.5 m. The base of the dryer is occupied with water soaked materials to be dried. The base is maintained at a temperature of 370 K, while the dome of the dryer is maintained at 1000 K. If bot
> What is a regenerative heat exchanger? How does a static type of regenerative heat exchanger differ from a dynamic type?
> Is iced water a pure substance? Why?
> The room shown in Fig. P21–78E is 20 ft by 20 ft wide and 9 ft high. The floor is at 100°F, the walls are at 60°F, and the ceiling is at 40°F. All surfaces are assumed to be black. Calculate the net radi
> In the heat transfer relation Q = UAs ΔTlm for a heat exchanger, what is ΔTlm called? How is it calculated for a parallel-flow and a counterflow heat exchanger?
> Two black parallel rectangles with dimensions 3 ft × 5 ft are spaced apart by a distance of 1 ft. The two parallel rectangles are experiencing radiation heat transfer as black surfaces, where the top rectangle receives a total of 180,000 Btu
> Under what conditions will the temperature rise of the cold fluid in a heat exchanger be equal to the temperature drop of the hot fluid?
> Reconsider Prob. 21–75. Using appropriate software, evaluate the effect of the distance L between the black coaxial parallel disks (D = 1 m) on the radiation heat transfer coefficient. By varying the distance L between the disks from 0.
> What is the heat capacity rate? What can you say about the temperature changes of the hot and cold fluids in a heat exchanger if both fluids have the same capacity rate? What does a heat capacity of infinity for a fluid in a heat exchanger mean?
> Consider two black coaxial parallel circular disks of equal diameter D that are spaced apart by a distance L. The top and bottom disks have uniform temperatures of 500°C and 520°C, respectively. Determine the radiation heat transfer
> Consider a condenser in which steam at a specified temperature is condensed by rejecting heat to the cooling water. If the heat transfer rate in the condenser and the temperature rise of the cooling water are known, explain how the rate of condensation o
> Consider a person whose exposed surface area is 1.9 m2, emissivity is 0.85, and surface temperature is 30°C. Determine the rate of heat loss from that person by radiation in a large room whose walls are at a temperature of (a) 295 K and (b) 260 K.
> Under what conditions is the heat transfer relation valid for a heat exchanger?
> In the absence of compressed liquid tables, how is the specific volume of a compressed liquid at a given P and T determined?
> What are the two methods used in radiation analysis? How do these two methods differ?
> What are the common approximations made in the analysis of heat exchangers?
> What is a reradiating surface? What simplifications does a reradiating surface offer in the radiation analysis?
> Hot engine oil with a heat capacity rate of 4440 W/K (product of mass flow rate and specific heat) and an inlet temperature of 150°C flows through a double-pipe heat exchanger. The double-pipe heat exchanger is constructed of a 1.5-m-long copper pipe (k
> What are the radiation surface and space resistances? How are they expressed? For what kinds of surfaces is the radiation surface resistance zero?
> Reconsider Prob. 22–21. Using appropriate software, plot the overall heat transfer coefficient based on the inner surface as a function of fouling factor as it varies from 0.0001 m2⋅K/W to 0.0008 m2⋅K/W, and discuss the results. Data from Prob. 22-21: R
> How does radiosity for a surface differ from the emitted energy? For what kinds of surfaces are these two quantities identical?
> Repeat Prob. 22–20, assuming a fouling factor Rf, i = 0.0005 m2⋅K/W on the inner surface of the tube. Data from Prob. 22-20: Water at an average temperature of 110°C and an average velocity of 3.5 m/s flows through a 7 m-long stainless steel tube (k = 1
> Why do skiers get sunburned so easily?
> Water at an average temperature of 110°C and an average velocity of 3.5 m/s flows through a 7 m-long stainless steel tube (k = 14.2 W/m⋅K) in a boiler. The inner and outer diameters of the tube are Di = 1.0 cm and Do = 1.4 cm, respectively. If the convec
> It is well known that warm air in a cooler environment rises. Now consider a warm mixture of air and gasoline on top of an open gasoline can. Do you think this gas mixture will rise in a cooler environment?
> Why is the radiation analysis of enclosures that consist of black surfaces relatively easy? How is the rate of radiation heat transfer between two surfaces expressed in this case?
> When is a heat exchanger classified as being compact? Do you think a double-pipe heat exchanger can be classified as a compact heat exchanger?
> Two infinitely long parallel cylinders of diameter D are located a distance s apart from each other. Determine the view factor F12 between these two cylinders.
> A counterflow heat exchanger is stated to have an overall heat transfer coefficient based on outside tube area of 50 Btu/h⋅ft2⋅°F when operating at design and clean conditions. After a period of use, scale buildup in the heat exchanger gives a fouling fa
> A jacketed-agitated vessel, fitted with a turbine agitator, is used for heating a water stream from 10°C to 54°C. The average heat transfer coefficient for water at the vessel’s inner wall can be estimated from Nu = 0.76Re2/3Pr1/3. Saturated steam at 100
> For the internal surfaces of the right circular cylinder shown in Fig. P21–66, determine F13 and F33.
> Water at an average temperature of 180°F and an average velocity of 4 ft/s flows through a thin walled 3/4-in-diameter tube. The water is cooled by air that flows across the tube with a velocity of 12 ft/s at an average temperature of 80°F. Determine the
> Consider a cylindrical enclosure whose height is twice the diameter of its base. Determine the view factor from the side surface of this cylindrical enclosure to its base surface.
> Reconsider Prob. 22–15. Using appropriate software, plot the overall heat transfer coefficient as a function of the limestone thickness as it varies from 1 mm to 3 mm, and discuss the results. Data from Prob. 22-15: Repeat Prob. 22–14 by assuming a 2-mm
> In what kind of pot will a given volume of water boil at a higher temperature: a tall and narrow one or a short and wide one? Explain.
> Determine the view factor F12 between the rectangular surfaces shown in Fig. P21–64.
> Repeat Prob. 22–14 by assuming a 2-mm-thick layer of limestone (k = 1.3 W/m⋅K) forms on the outer surface of the inner tube. Data from Prob. 22-14: A long, thin-walled double-pipe heat exchanger with tube and shell diameters of 1.0 cm and 2.5 cm, respec
> Determine the view factors F13 and F23 between the rectangular surfaces shown in Fig. P21–63.
> A long, thin-walled double-pipe heat exchanger with tube and shell diameters of 1.0 cm and 2.5 cm, respectively, is used to condense refrigerant-134a with water at 20°C. The refrigerant flows through the tube, with a convection heat transfer coefficient
> Determine the view factors from the base of a cube to each of the other five surfaces.
> The tube in a heat exchanger has a 2-in inner diameter and a 3-in outer diameter. The thermal conductivity of the tube material is 0.5 Btu/h⋅ft⋅°F, while the inner surface heat transfer coefficient is 50 Btu/h⋅ft2⋅°F and the outer surface heat transfer c
> Consider a cylindrical enclosure with A1, A2, and A3 representing the internal base, top, and side surfaces, respectively. Using the length-to-diameter ratio, K = L/D, determine (a) the expression for the view factor between the base and the side surface
> The mass flow rate, specific heat, and inlet temperature of the tube-side stream in a double pipe, parallel-flow heat exchanger are 3200 kg/h, 2.0 kJ/kg⋅K, and 120°C, respectively. The mass flow rate, specific heat, and inlet temperature of the other str
> Determine the view factors from the very long grooves shown in Fig. P21–60 to the surroundings without using any view factor tables or charts. Neglect end effects.
> Air at 18°C (cp = 1006 J/kg⋅K) is to be heated to 58°C by hot oil at 80°C (cp = 2150 J/kg⋅K) in a crossflow heat exchanger with air mixed and oil unmixed. The product of the heat transfer surface area and the overall heat transfer coefficient is 750 W/K,
> Which process requires more energy: completely vaporizing 1 kg of saturated liquid water at 1 atm pressure or completely vaporizing 1 kg of saturated liquid water at 8 atm pressure?
> What is the cause of color? Why do some objects appear blue to the eye while others appear red? Is the color of a surface at room temperature related to the radiation it emits?
> A single-pass crossflow heat exchanger with both fluids unmixed has water entering at 16°C and exiting at 33°C, while oil (cp = 1.93 kJ/kgâ‹…K and ρ = 870 kg/m3) flowing at 0.19 m3/min enters at 38Â&
> Determine the four view factors associated with an enclosure formed by two very long concentric cylinders of radii r1 and r2. Neglect the end effects.
> In a chemical plant, a certain chemical is heated by hot water supplied by a natural gas furnace. The hot water (cp = 4180J/kg⋅K) is then discharged at 60°C at a rate of 8 kg/min. The plant operates 8 h a day, 5 days a week, 52 weeks a year. The furnace
> Consider a conical enclosure of height h and base diameter D. Determine the view factor from the conical side surface to a hole of diameter d located at the center of the base.
> A crossflow heat exchanger with both fluids unmixed has an overall heat transfer coefficient of 200 W/m2⋅K, and a heat transfer surface area of 400 m2. The hot fluid has a heat capacity of 40,000 W/K, while the cold fluid has a heat capacity of 80,000 W/
> Consider a hemispherical furnace with a flat circular base of diameter D. Determine the view factor from the dome of this furnace to its base.
> Geothermal water (cp = 4250 J/kg⋅K) at 75°C is to be used to heat fresh water (cp = 4180 J/kg⋅K) at 17°C at a rate of 1.2 kg/s in a double-pipe counterflow heat exchanger. The heat transfer surface area is 25 m2, the overall heat transfer coefficient is
> Consider an enclosure consisting of 13 surfaces. How many view factors does this geometry involve? How many of these view factors can be determined by the application of the reciprocity and the summation rules?
> Consider a water-to-water counterflow heat exchanger with these specifications. Hot water enters at 90°C while cold water enters at 20°C. The exit temperature of the hot water is 15°C greater than that of the cold water,
> Is it true that it takes more energy to vaporize 1 kg of saturated liquid water at 100°C than it would at 120°C?
> Cylindrical heaters are spaced equally at 5 cm apart in a row, and the heaters are positioned between two large parallel plates. If the diameter of the cylinders is 35 mm, determine the view factors between the plate and the row of cylinders, F12 and F32
> Under what conditions can the overall heat transfer coefficient of a heat exchanger be determined from U =(1/hi + 1/ho)−1?
> Consider two coaxial parallel circular disks of equal diameter D that are spaced apart by a distance L. If the view factor is F12 = 0.1, without altering the diameter of the disks, determine a solution that would increase the view factor F12 by a factor
> Oil in an engine is being cooled by air in a crossflow heat exchanger, where both fluids are unmixed. Oil (cph = 2047 J/kg⋅K) flowing with a flow rate of 0.026 kg/s enters the tube side at 75°C, while air (cpc = 1007 J/kg&aci
> Consider two coaxial parallel circular disks of equal diameter D = 1 m spaced apart by 1 m, and two aligned parallel square plates (1 m × 1 m) also spaced apart by 1 m. Determine the view factors F12 between the circular disks and the square
> A single-pass crossflow heat exchanger uses hot air (mixed) to heat water (unmixed), flowing with a mass flow rate of 3 kg/s, from 30°C to 80°C. The hot air enters and exits the heat exchanger at 220°C and 100°
> What is the crossed-strings method? For what kinds of geometries is the crossed-strings method applicable?
> Saturated water vapor at 100°C condenses in the shell side of a one-shell and two-tube heat exchanger with a surface area of 0.5 m2 and an overall heat transfer coefficient of 2000 W/m2â‹…K. If cold water (cpc = 4179 J/kgâ
> What are the summation rule and the superposition rule for view factors?
> A shell-and-tube heat exchanger with two shell passes and four tube passes is used for cooling oil (cp = 2.0 kJ/kg⋅K) from 125°C to 55°C. The coolant is water, which enters the shell side at 25°C and leaves at 46°C. The overall heat transfer coefficient
> Does hfg change with pressure? How?
> How can you determine the view factor F12 when the view factor F21 and the surface areas are available?
> A two-shell-pass and four-tube-pass heat exchanger is used for heating a hydrocarbon stream (cp = 2.0 kJ/kg⋅K) steadily from 20°C to 50°C. A water stream enters the shell side at 80°C and leaves at 40°C. There are 160 thin-walled tubes, each with a diame
> What is visible light? How does it differ from the other forms of electromagnetic radiation?
> A shell-and-tube heat exchanger is used for cooling 47 kg/s of a process stream flowing through the tubes from 160°C to 100°C. This heat exchanger has a total of 100 identical tubes, each with an inside diameter of 2.5 cm and negligible wall thickness. T
> What does the view factor represent? When is the view factor from a surface to itself not zero?
> Cold water (cp = 4180 J/kg⋅K) enters the tubes of a heat exchanger with two shell passes and 20 tube passes at 15°C at a rate of 4 kg/s, while hot oil (cp = 2200 J/kg⋅K) enters the shell at 130°C
> A horizontal plate is experiencing uniform irradiation on both upper and lower surfaces. The ambient air temperature surrounding the plate is 290 K with a convection heat transfer coefficient of 30 W/m2â‹…K. Both upper and lower surfaces
> Hot water at 60°C is cooled to 36°C through the tube side of a one-shell-pass and two-tube-passes heat exchanger. The coolant is also a water stream, for which the inlet and outlet temperatures are 7°C and 31°C, respectively. The overall heat transfer co
> Consider an opaque plate that is well insulated on the edges and heated at the bottom with an electric heater. The plate has an emissivity of 0.67 and is situated in an ambient surrounding temperature of 7°C where the natural convection heat transfer coe
> A shell-and-tube heat exchanger with one shell pass and 14 tube passes is used to heat water in the tubes with geothermal steam condensing at 120°C (hfg = 2203 kJ/kg) on the shell side. The tubes are thin-walled and have a diameter of 2.4 cm a
> What is the physical significance of hfg? Can it be obtained from a knowledge of hf and hg? How?
> What is the value of the engineering software packages in (a) engineering education and (b) engineering practice?
