JNTU
JNTU B.Tech II PROCESS HEAT TRANSFER Previous Question Paper (December - 2018 - R13)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
Part A
Define the term conduction.
Answer
Conduction is the process of heat transfer through a solid or stationary fluid due to the kinetic energy exchange between adjacent molecules or atoms, occurring in the direction of decreasing temperature without bulk motion of the material.
Write the equation for heat transfer through a cylinder.
Answer
The steady-state heat transfer rate through a hollow cylinder of length L, inner radius r1, outer radius r2, and thermal conductivity k is given by:
Define critical radius of insulation for a sphere.
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Define overall heat transfer coefficient and explain.
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What is the physical significance of Prandtl number? Discuss.
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What is the difference between natural and forced convection? Explain.
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What is Grashoff number? Discuss its importance.
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What is Wein’s displacement law? Discuss.
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What is LMTD? Explain.
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Define capacity and economy of an evaporator.
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Part B
Estimate the rate of heat loss through the wall of a spherical boiling pan with an inner diameter of 1.0 m and total boiler wall and insulation thickness of 100 mm. The temperature of the inner surface is 140°C and that of the outer surface is 50°C. The equivalent thermal conductivity of material of the spherical wall is 0.12 W/mK.
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A furnace wall is made up of a steel plate 1.0 cm thick, lined on inside with silica bricks 10 cm thick and on the outside with magnesite bricks 10 cm thick. The temperature on the inside edge of the wall 800°C and on the outside of the furnace is 30°C. Find the amount of heat transmitted through composite wall of the furnace in kilocalories per hour per m2 of the furnace wall, surface and the temperature at the interface of steel and magnesite brick. Thermal conductivity for steel, silica brick and magnesite brick are 14.5, 1.5 and 4.5 kcal/hr m °C respectively.
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Explain the physical significance of Nusselt number, Prandtl number and discuss their role in convection.
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An aluminium ball, 7.5 cm in diameter, is cooled in an ambient at 25°C. When the temperature of the ball is 120°C, it is found to cool at the rate of 4°C per minute. Calculate the surface heat transfer coefficient. Given ρ=2700 kg/m3, CP= 0.89 kJ/kg °C.
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Derive the equation to calculate critical radius of insulation of a cylinder.
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One side of a 15 cm thick layer of insulation (k=0.75 W/m°C) has a temperature of 200°C while the other loses due to an ambient at 27°C at the rate of 600 W/m2. Calculate the skin temperature of the insulation and the air film coefficient?
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Describe Anology between momentum transfer and heat transfer.
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Air at 70°C and essentially atmospheric pressure is flowing through a 15cm × 10 cm rectangular duct at a velocity of 12 m/s. If the wall temperature of the duct is 32°C, and the length of the duct is 50 m, calculate the drop in the air temperature.
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Water must be heated from 15°C to 50°C in a simple double-pipe heat exchanger at a rate of 4 m3/hr. The water is flowing inside the inner tube with steam condensing at 110°C on the outside. The tube wall is so thin that the wall resistance may be neglected. Assume that the steam film coefficient ho is 11 kW/m2°C. What is the length of the shortest heat exchanger that will heat the water to the desired temperature? Average properties of water: ρ = 993 kg/m3, k = 0.61 W/m°C, μ = 0.78 cP, CP = 4.19 J/g°C.
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Derive the equation for natural convection to air from vertical plates.
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A laboratory water bath has an immersed horizontal heating element, 2.54 cm in diameter and 30 cm in length, with a power input of 500 W. If the bulk water temperature is 38°C and heat transfer occurs by free convection only, calculate the surface temperature of the heater.
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Write brief note on the following relevant to radiation heat transfer:
- Monochromatic radiating power
- Monochromatic emissivity
- Stefan-Boltzmann law
- Planck’s law
- Kichoff’s law.
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Define ‘fin efficiency’. Give the equation to calculate it.
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What is extended surface equipment? Discuss about different types of extended surfaces with neat figures.
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An aqueous solution of a solute is concentrated from 5% to 20% (mass basis) in a single-effect short tube evaporator. The feed enters the evaporator at a rate of 10 kg/s and at a temperature of 300 K. Steam is available at a saturation pressure of 1.3 bar and the corresponding saturation temperature of steam is 320 K. If the overall heat transfer coefficient is 5000 W/m2K, calculate the:
- steam economy
- Heat transfer surface area.
The boiling point elevation is 5 K.
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