ISRO 2019 Refrigeration and Air-Conditioning Questions, Answers and Explanation
1. The highest value of thermal conductivity is expected for
a) Solid ice
b) Steam
c) Water
d) Superheated steam
Answer
c) Water
Explanation
As water is heated, much of the energy is used to bend the hydrogen bonds; a factor not available in the solid or gaseous phase. This extra energy causes the specific heat to be greater in liquid water.
4. Starting friction is low in which type of lubrication
a) Hydrostatic lubrication
b) Hydrodynamic lubrication
c) Mixed (semi fluid) lubrication
d) Boundary lubrication
Answer
a) Hydrostatic lubrication
Explanation
In hydrodynamic lubrication the outer surface of journal and inner surface of bearing always remain in contact.
In hydrostatic lubrication when the bearing starts to move, the pump supplies oil at a pressure thereby creating a clearance between the journal and the bearing. This reduces the starting friction.
8. A circular ring of radius 42 cm is cut and bent into the form of a rectangle whose sides are in the ratio 6:5. The small side of the rectangle is
a) 80 cm
b) 30 cm
c) 120 cm
d) 60 cm
Answer
d) 60 cm
Explanation
Radius of the circular ring = 42 cm
Perimeter of the circular ring = Perimeter of the rectangle formed.
Perimeter of the circular ring = 2 pi 42 cm
Let the sides of the rectangle formed be 5x and 6x, 5x being the smaller side
Perimeter of the rectangle = 22 x
84 pi = 22 x (Substitute pi = 22/7)
84 x 22/7 = 22x
x = 84/7 = 12 cm
Smaller side = 5x = 5 x 12 = 60cm
9. A tank is normally filled in 8 hours but takes 2 hours longer to fill because of a leak in the bottom. If the tank is full and due to leakage alone, the tank will get empty in ------- hours. (Assume no further filling happens)
a) 20
b) 40
c) 30
d) 50
Answer
b) 40
Explanation
Let the capacity of the tank be C units.
When there is no leak tank fills at the rate of C/8 units an hour.
When there is leak the tank fills at the rate of C/10 units an hour.
Reduction in filling rate of tank when there is leak is C/8 - C/10 = (10C - 8C)/80 = 2C/80 = C/40
Hence if there is leakage, it happens at the rate of C/40 units an hour. So it takes 40 hours for the tank to be emptied from full if there is only leakage.
46. Crowning of pulley is done to
a) Increase the tightness of the belt on the pulley
b) Preventing the belt running off the pulley
c) Increase the torque transmitted
d) Improve the shape and strength of pulley
Answer
b) Preventing the belt running off the pulley
Explanation
Crowning of pulley increases the cross sectional area of the pulley as we move from the edges to the center.
When a flat belt is wound on the pulley, the highest tension occurs at the highest cross section area, that is closest to the center of the pulley. This causes the belt to move further towards the center till the center of the belt coincides the crown. Once reaching this center position it remains fairly stable thereby preventing the belt running off the pulley.
Crowning of pulley is cheaper than the side guide pulley.
47. Flow of a liquid at a constant rate through a conical pipe is classified as
a) Unsteady, non uniform flow
b) Unsteady, uniform flow
c) Steady, non uniform flow
d) Steady, uniform flow
Answer
c) Steady, non uniform flow
Explanation
When the velocity at a particular point changes with time, it is said to be unsteady flow.
When the velocity remains the same at all points in a flow it is called a uniform flow.
As the flow given in the question is at constant rate, velocity at a particular point does not change with time. Also as the cross section changes, velocity varies from one plane to another.
Hence the flow is steady and non uniform.
b) Steam
c) Water
d) Superheated steam
Answer
c) Water
Explanation
As water is heated, much of the energy is used to bend the hydrogen bonds; a factor not available in the solid or gaseous phase. This extra energy causes the specific heat to be greater in liquid water.
4. Starting friction is low in which type of lubrication
a) Hydrostatic lubrication
b) Hydrodynamic lubrication
c) Mixed (semi fluid) lubrication
d) Boundary lubrication
Answer
a) Hydrostatic lubrication
Explanation
In hydrodynamic lubrication the outer surface of journal and inner surface of bearing always remain in contact.
In hydrostatic lubrication when the bearing starts to move, the pump supplies oil at a pressure thereby creating a clearance between the journal and the bearing. This reduces the starting friction.
8. A circular ring of radius 42 cm is cut and bent into the form of a rectangle whose sides are in the ratio 6:5. The small side of the rectangle is
a) 80 cm
b) 30 cm
c) 120 cm
d) 60 cm
Answer
d) 60 cm
Explanation
Radius of the circular ring = 42 cm
Perimeter of the circular ring = Perimeter of the rectangle formed.
Perimeter of the circular ring = 2 pi 42 cm
Let the sides of the rectangle formed be 5x and 6x, 5x being the smaller side
Perimeter of the rectangle = 22 x
84 pi = 22 x (Substitute pi = 22/7)
84 x 22/7 = 22x
x = 84/7 = 12 cm
Smaller side = 5x = 5 x 12 = 60cm
9. A tank is normally filled in 8 hours but takes 2 hours longer to fill because of a leak in the bottom. If the tank is full and due to leakage alone, the tank will get empty in ------- hours. (Assume no further filling happens)
a) 20
b) 40
c) 30
d) 50
Answer
b) 40
Explanation
Let the capacity of the tank be C units.
When there is no leak tank fills at the rate of C/8 units an hour.
When there is leak the tank fills at the rate of C/10 units an hour.
Reduction in filling rate of tank when there is leak is C/8 - C/10 = (10C - 8C)/80 = 2C/80 = C/40
Hence if there is leakage, it happens at the rate of C/40 units an hour. So it takes 40 hours for the tank to be emptied from full if there is only leakage.
46. Crowning of pulley is done to
a) Increase the tightness of the belt on the pulley
b) Preventing the belt running off the pulley
c) Increase the torque transmitted
d) Improve the shape and strength of pulley
Answer
b) Preventing the belt running off the pulley
Explanation
Crowning of pulley increases the cross sectional area of the pulley as we move from the edges to the center.
When a flat belt is wound on the pulley, the highest tension occurs at the highest cross section area, that is closest to the center of the pulley. This causes the belt to move further towards the center till the center of the belt coincides the crown. Once reaching this center position it remains fairly stable thereby preventing the belt running off the pulley.
Crowning of pulley is cheaper than the side guide pulley.
47. Flow of a liquid at a constant rate through a conical pipe is classified as
a) Unsteady, non uniform flow
b) Unsteady, uniform flow
c) Steady, non uniform flow
d) Steady, uniform flow
Answer
c) Steady, non uniform flow
Explanation
When the velocity at a particular point changes with time, it is said to be unsteady flow.
When the velocity remains the same at all points in a flow it is called a uniform flow.
As the flow given in the question is at constant rate, velocity at a particular point does not change with time. Also as the cross section changes, velocity varies from one plane to another.
Hence the flow is steady and non uniform.