# Heat and Mass Transfer

• ## A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100°C and leaves at 60°C. A cold fluid enters the heat exchanger at 40°C. The mean temperature difference between the two fluids is_____.

• A. 20°C
• B. 40°C
• C. 60°C
• D. 66.7°C
Explanation:

tm = mean temperature difference between the two fluids for counter flow heat exchanger,
tm = th1-tc2 = th2-tc1.
tm = th2-tc1 = 60-40 = 20.

• ## In heat exchangers, degree of approach is defined as the difference between temperatures of_____.

• A. Cold water inlet and outlet
• B. Hot medium inlet and outlet
• C. Hot medium outlet and cold water inlet
• D. Hot medium outlet and cold water outlet
Explanation:

This term refers to the temperature difference between the leaving process fluid and the entering service fluid.

• ## In regenerator type heat exchanger, heat transfer takes place by_____.

• A. Direct mixing of hot and cold fluids
• B. A complete separation between hot and cold fluids
• C. Flow of hot and cold fluids alternately over a surface
• D. Generation of heat again and again
Explanation:

A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid..

• ## In a shell and tube heat exchanger, baffles are provided on the shell side to_____.

• A. Improve heat transfer
• B. Provide support for tubes
• C. Prevent stagnation of shell side fluid
• D. All of these
Explanation:

In a shell and tube heat exchanger, baffles are provided on the shell side to Improve heat transfer And Provide support for tubes and Prevent stagnation of shell side fluid.

• ## Fouling factor is used_____.

• A. In heat exchanger design as a safety factor
• B. In case of Newtonian fluids
• C. When a liquid exchanges heat with a gas
• D. None of the above
Explanation:

The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger.

• ## Heat flows from one body to other when they have_____.

• A. Different heat contents
• B. Different specific heat
• C. Different atomic structure
• D. Different temperatures
Explanation:

Heat Flow occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature.

• ## Fourier’s law of heat conduction gives the heat flow for_____.

• A. Irregular surfaces
• B. Nonuniform temperature surfaces
• C. One dimensional cases only
• D. Two dimensional cases only
Explanation:

Fourier’s law of heat conduction is valid for One dimensional cases only
Fourier equation is valid for all matter solid, liquid or gas. The vector expression indicating that heat flow rate is normal to an isotherm and is in the direction of decreasing temperature.

• ## The natural convection air cooled condensers are used in_____.

• A. Domestic refrigerators
• B. Water coolers
• C. Room air conditioners
• D. All of these
Explanation:

The natural convection air cooled condenser are used in small capacity applications such as domestic refrigerators, freezers, water cooler and air-conditioners.

• ## The ratio of the thickness of thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to (Prandtl number) n, where n is equal to_____.

• A. -1/3
• B. -2/3
• C. 1
• D. -1
Explanation:

The ratio of the thickness of thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to -1/3.

• ## Radiation is the process of heat transfer in which heat flows from a ________, in a straight line, without affecting the intervening medium.

• A. Cold body to hot body
• B. Hot body to cold body
• C. Smaller body to larger body
• D. Larger body to smaller body