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. The average properties of the process stream are: ρ = 950 kg/m3, k = 0.50 W/m⋅K, cp = 3.5 kJ/kg⋅K, and μ = 0.002 kg/m⋅s. The coolant stream is water cp = 4.18 kJ/kg⋅K at a flow rate of 66 kg/s and an inlet temperature of 10°C, which yields an average shell-side heat transfer coefficient of 4.0 kW/m2⋅K. Calculate the tube length if the heat exchanger has (a) one shell pass and one tube pass and (b) one shell pass and four tube passes.
> 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
> 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.
> 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?
> 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?
> Consider an opaque horizontal plate that is well insulated on the edges and the lower surface. The plate is uniformly irradiated from above while air at T∞ = 300 K flows over the surface, providing a uniform convection heat transfer coe
> Water (cp = 1.0 Btu/lbm⋅°F) is to be heated by solar heated hot air (cp = 0.24 Btu/lbm⋅°F) in a double-pipe counterflow heat exchanger. Air enters the heat exchanger at 190°F at a rate of 0.7 lbm/s and leaves at 135°F. Water enters at 70°F at a rate of 0
> Irradiation on a semitransparent medium is at a rate of 520 W/m2. If 160 W/m2 of the irradiation is reflected from the medium and 130 W/m2 is transmitted through the medium, determine the medium’s absorptivity, reflectivity, transmissivity, and emissivit
> What are the common causes of fouling in a heat exchanger? How does fouling affect heat transfer and pressure drop?
> An opaque horizontal plate is well insulated on the edges and the lower surface. The irradiation on the plate is 3000 W/m2, of which 500 W/m2 is reflected. The plate has a uniform temperature of 700 K and has an emissive power of 5000 W/m2. Determine the
> By taking the limit as ΔT2 → ΔT1, show that when ΔT1 = ΔT2 for a heat exchanger, the ΔTlm relation reduces to ΔTlm = ΔT1 = ΔT2.
> The reflectivity of aluminum coated with lead sulfate is 0.35 for radiation at wavelengths less than 3 μm and 0.95 for radiation greater than 3 μm. Determine the average reflectivity of this surface for solar radiation (T ≈ 5800 K) and radiation coming f
> Consider a water-to-water double-pipe heat exchanger whose flow arrangement is not known. The temperature measurements indicate that the cold water enters at 20°C and leaves at 50°C, while the hot water enters at 80°C and leaves at 45°C. Do you think thi
> The variation of the spectral absorptivity of a surface is as given in Fig. P21–42. Determine the average absorptivity and reflectivity of the surface for radiation that originates from a source at T = 2500 K. Also, determine the averag
> Consider the flow of saturated steam at 270.1 kPa that flows through the shell side of a shell-and tube heat exchanger while the water flows through four tubes of diameter 1.25 cm at a rate of 0.25 kg/s through each tube. The water enters the tubes of th
> Does the reference point selected for the properties of a substance have any effect on thermodynamic analysis? Why?
> The emissivity of a surface coated with aluminum oxide can be approximated to be 0.15 for radiation at wavelengths less than 5 μm and 0.9 for radiation at wavelengths greater than 5 μm. Determine the average emissivity of this surface at (a) 5800 K and (
> Consider a closed-loop heat exchanger that carries exit water (cp = 1 Btu/lbm⋅°F and ρ = 62.4 lbm/ft3) of a condenser side initially at 100°F. The water flows through a 500-ft long stainless steel pipe of 1 in inner diameter immersed in a large lake. The
> The variations of the spectral emissivity of two surfaces are as given in Fig. P21–40. Determine the average emissivity of each surface at T = 3000 K. Also, determine the average absorptivity and reflectivity of each surface for radiati
> Reconsider Prob. 22–104. Using appropriate software, investigate the effects of the condensing steam temperature and the tube diameter on the rate of heat transfer and the rate of condensation of steam. Let the steam temperature vary fr
> How does microwave cooking differ from conventional cooking?
> Steam is to be condensed on the shell side of a one-shell pass and eight-tube-passes condenser, with 50 tubes in each pass, at 30°C (hfg = 2431 kJ/kg). Cooling water (cp = 4180 J/kg⋅K) enters the tubes at 18°C at a
> Saturated water vapor at 100°C condenses in 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/m2⋅K. Cold water (cpc = 4179 J/kg⋅K) flowi
> The emissivity of a tungsten filament can be approximated to be 0.5 for radiation at wavelengths less than 1 μm and 0.15 for radiation at greater than 1 μm. Determine the average emissivity of the filament at (a) 1500 K and (b) 2500 K. Also, determine th
> Ethanol is vaporized at 78°C (hfg = 846 kJ/kg) in a double-pipe parallel-flow heat exchanger at a rate of 0.04 kg/s by hot oil (cp = 2200 J/kg⋅K) that enters at 115°C. If the heat transfer surface area and the over
> The spectral emissivity function of an opaque surface at 1000 K is approximated as; Determine the average emissivity of the surface and the rate of radiation emission from the surface, in W/m2.
> Steam at 400°C has a specific volume of 0.02 m3/kg. Determine the pressure of the steam based on (a) the ideal-gas equation, (b) the generalized compressibility chart, and (c) the steam tables.
> Water (cp = 4180 J/kg⋅K) enters the 2.5-cm-internal diameter tube of a double-pipe counterflow heat exchanger at 20°C at a rate of 2.2 kg/s. Water is heated by steam condensing at 120°C (hfg = 2203 kJ/kg) in the shell. If the overall heat transfer coeffi
> A furnace that has a 40-cm × 40-cm glass window can be considered to be a blackbody at 1200 K. If the transmissivity of the glass is 0.7 for radiation at wavelengths less than 3 μm and zero for radiation at wavelengths greater than 3 μm, determine the fr
> Consider the operation of a single-pass crossflow heat exchanger with water (cp = 4193 J/kg⋅K) (mixed) and methanol (cp = 2577 J/kg⋅K) (unmixed). Water entering and exiting the heat exchanger at 90°C and 60°C, respectively is used to heat the methanol, w
> What is the greenhouse effect? Why is it a matter of great concern among atmospheric scientists?
> In a thin-walled double-pipe heat exchanger, when is the approximation U = hi a reasonable one? Here U is the overall heat transfer coefficient and hi is the convection heat transfer coefficient inside the tube.
> Define the properties reflectivity and transmissivity, and discus the different forms of reflection.
> Classify heat exchangers according to flow type, and explain the characteristics of each type.
> Define the properties emissivity and absorptivity. When are these two properties equal to each other?
> What is a gray body? How does it differ from a blackbody? What is a diffuse gray surface?
> Consider the sun, which is considered to be a blackbody with a surface temperature of roughly 5800 K. Determine the percentage of solar energy (a) in the visible range, (b) at wavelengths shorter than the visible range, and (c) at wavelengths longer than
> Ethane at 10 MPa and 100°C is heated at constant pressure until its volume has increased by 60 percent. Determine the final temperature using (a) the ideal-gas equation of state and (b) the compressibility factor. Which of these two results is the more a
> A 3-mm-thick glass window transmits 90 percent of the radiation between λ = 0.3 and 3.0 μm and is essentially opaque for radiation at other wavelengths. Determine the rate of radiation transmitted through a 3-m × 3-m glass window from blackbody sources a
> The sun can be treated as a blackbody at an effective surface temperature of 10,400 R. Determine the rate at which infrared radiation energy (λ = 0.76 − 100 μm) is emitted by the sun, in Btu/h⋅ft2.
> What is thermal radiation? How does it differ from the other forms of electromagnetic radiation?
> It is desired that the radiation energy emitted by a light source reach a maximum in the blue range (λ = 0.47 μm). Determine the temperature of this light source and the fraction of radiation it emits in the visible range (λ = 0.40 − 0.76 μm).
> Reconsider Prob. 21–27. Using appropriate software, investigate the effect of temperature on the fraction of radiation emitted in the visible range. Let the surface temperature vary from 1000 K to 4000 K, and plot the fraction of radiation emitted in the
> The temperature of the filament of an incandescent lightbulb is 2800 K. Treating the filament as a blackbody, determine the fraction of the radiant energy emitted by the filament that falls in the visible range. Also, determine the wavelength at which th
> The temperature of the filament of an incandescent lightbulb is 2500 K. Assuming the filament to be a blackbody, determine the fraction of the radiant energy emitted by the filament that falls in the visible range. Also, determine the wavelength at which
> We wish an incandescent lightbulb to emit at least 15 percent of its energy at wavelengths shorter than 0.8 μm. Determine the minimum temperature to which the filament of the lightbulb must be heated.
> A circular ceramic plate that can be modeled as a blackbody is being heated by an electrical heater. The plate is 30 cm in diameter and is situated in a surrounding ambient temperature of 15°C where the natural convection heat transfer coeffic
> A thin vertical copper plate is subjected to a uniform heat flux of 1000 W/m2 on one side, while the other side is exposed to ambient surroundings at 5°C. The surface of the plate is oxidized black and can be treated as a blackbody. The heat t
