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On a clear night there is radiation from earth’s surface to space. On such a night, the water particles on the plant leaves radiate to the sky whose temperature may be taken as 200 K. The water particles receive heat by convection from the surrounding air, the convection heat transfer coefficient has a value of 30 W/m2 K. If the water should not freeze, make calculations for the air temperature

Correct Answer: 280.474 K
Heat radiated to sky = Heat received by convection. So, temperature of air = 224.22/30 + 273 = 280.474 K.

In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives. Let us examine the facts in the case more ____________. First of all, language is no more ____________ a medium; it is like air to the creatures of the land or water to fishes. If it is perfectly clear and pure, we do not notice it any more than we notice pure air when the sun is shining ________ a clear sky, or the taste of pure cool water when we drink a glass __________ a hot day. Unless the sun is shining, there is no brightness; unless the water is cool, there is __________ refreshment. sun is shining ________ a clear sky,

A polished metal pipe 5 cm outside diameter and 370 K temperature at the outer surface is exposed to ambient conditions at 295 K temperature. The emissivity of the surface is 0.2 and the convection coefficient of heat transfer is 11.35 W/m2 degree. Calculate the overall coefficient of heat transfer by the combined mode of convection and radiation

In a surface condenser, the water flowing through a series of tubes at the rate of 200 kg/hr is heated from 15 degree Celsius to 75 degree Celsius. The steam condenses on the outside surface of tubes at atmospheric pressure and the overall heat transfer coefficient is estimated at 860 k J/m2 hr degree. Find the number of transfer units of the heat exchanger. At the condensing pressure, stream has saturation temperature 0f 100 degree Celsius and the latent heat of vaporization is 2160 k J/kg. Further, the steam is initially just saturated and the condensate leaves the exchanger without sub-cooling i.e. only latent heat of condensing steam is transferred to water. Take specific heat of water as 4 k J/kg K

A stream of air at 100 kPa pressure and 300 K is flowing on the top surface of a thin flat sheet of solid naphthalene of length 0.2 m with a velocity of 20 m/sec. The other data are: Mass diffusivity of naphthalene vapor in air = 6 * 10 –6 m2/sec Kinematic viscosity of air = 1.5 * 10 –5 m2.sc Concentration of naphthalene at the air-solid naphthalene interface = 1 * 10 – 5 kmol/m3 Calculate the overage mass transfer coefficient over the flat plate. Note: For heat transfer over a flat plate, convective heat transfer coefficient for laminar flow can be calculated by the equation. Nu = 0.664 ReL1/2 Pr1/3 you may use analogy between mass and heat transfer.

In a surface condenser, the water flowing through a series of tubes at the rate of 200 kg/hr is heated from 15 degree Celsius to 75 degree Celsius. The steam condenses on the outside surface of tubes at atmospheric pressure and the overall heat transfer coefficient is estimated at 860 k J/m2 hr degree. Find the effectiveness of the heat exchanger. At the condensing pressure, stream has a saturation temperature 0f 100 degree Celsius and the latent heat of vaporization is 2160 k J/kg. Further, the steam is initially just saturated and the condensate leaves the exchanger without sub-cooling i.e. only latent heat of condensing steam is transferred to the water. Take specific heat of water as 4 k J/kg K

The rear window of an automobile is made of thick glass i.e. AB = 5 mm and thermal conductivity is 0.8 W/m degree. To defrost this window, a thin transparent film type heating element has been fixed to its inner surface. For the conditions given below, determine the electric power that must be provided per unit area of window if a temperature 5 degree Celsius is maintained at its outer surface. Interior air temperature and the corresponding surface coefficient are 20 degree Celsius and 12 W/m2 degree. Surrounding air temperature and the corresponding surface coefficient are – 15 degree Celsius and 70 W/m2 degree. Electric heater provides uniform heat flux

Consider the following statements and choose the correct answer:
1. Soil gets heat energy from different sources such as solar radiation, decomposing organic matter and heat formed in the interior of earth
2. The temperature of soil is not affected by its colour and altitude of the land
3. Black soils absorb more heat than white soils
4. Sandy soils absorb more heat and radiate it out quickly at night than clay or loam soils

A horizontal heated plate at 200 degree Celsius and facing upwards has been placed in still air at 20 degree Celsius. If the plate measures 1.25 m by 1 m, make calculations for the heat loss by natural convection. The convective film coefficient for free convection is given by the following empirical relation h = 0.32 (α) 0.25 W/m2 K Where α is the mean film temperature in degrees kelvin

A heating unit is made in the form of a vertical tube of 50 mm outside diameter and 1.2 m height. The tube is fitted with 20 steel fins of rectangular section with height 40 mm and thickness 2.5 mm. The temperature at the base of fin is 75 degree Celsius, the surrounding air temperature is 20 degree Celsius and the heat transfer coefficient between the fin as well as the tube surface and the surrounding air is 9.5 W/m2 K. If thermal conductivity of the fin material is 55 W/m K, find the amount of heat transferred from the tube without fin

A heating unit is made in the form of a vertical tube of 50 mm outside diameter and 1.2 m height. The tube is fitted with 20 steel fins of rectangular section with height 40 mm and thickness 2.5 mm. The temperature at the base of fin is 75 degree Celsius, the surrounding air temperature is 20 degree Celsius and the heat transfer coefficient between the fin as well as the tube surface and the surrounding air is 9.5 W/m2 K. If thermal conductivity of the fin material is 55 W/m K, find the amount of heat transferred from the tube with fin