Practice Pakistan's most useful MCQs for competitive exams

A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions? Assume that the conductive heat transfer co-efficient is infinitely large ?

• (A)  5 minutes
• (B)  10 minutes
• (C)  20 minutes
• (D)  40 minutes

Answer

Submitted By :Ali Uppal

Presence of a non-condensing gas in a condensing vapour___________________?

• (A)  Increases the rate of condensation
• (B)  Decreases thermal resistance
• (C)  Is desirable to increase the film co-efficient
• (D)  None of these

Answer

Submitted By :Ali Uppal

The separation of liquid droplets from the vapor is done by a/an _____________ in the evaporators?

• (A)  Steam ejector
• (B)  Entrainment separator
• (C)  Compressor
• (D)  Vacuum pump

Answer

Submitted By :Ali Uppal

For the same heat transfer area and the terminal conditions, the ratio of the capacities of a single effect evaporator to a triple effect evaporator is_________________?

• (A)  3
• (B)  0.33
• (C)  1
• (D)  1.33

Answer

Submitted By :Ali Uppal

According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor) ?

• (A)  2f
• (B)  f
• (C)  f/2
• (D)  f/4

Answer

Submitted By :Ali Uppal

The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)?

• (A)  αT
• (B)  α × 1/T
• (C)  α T4
• (D)  Independent of T

Answer

Submitted By :Ali Uppal

The equivalent diameter for the annulus of a double pipe heat exchanger, whose inner pipe has fins on the outside is ______________ compared to the same size pipes without fins?

• (A)  More
• (B)  Less
• (C)  Same
• (D)  Unpredictable

Answer

Submitted By :Ali Uppal

A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because__________________?

• (A)  Total heat transfer area of all the effects is -less than that in a single effect evaporator system
• (B)  Total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect
• (C)  Boiling point elevation in a single effect system is much higher than that in any effect in a multi-effect system
• (D)  Heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system

Answer

Submitted By :Ali Uppal

Grashoff number, which is defined as g . β (Ts – T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to ____________ force?

• (A)  Viscous
• (B)  Elastic
• (C)  Inertial
• (D)  None of these

Answer

Submitted By :Ali Uppal

The critical radius ‘r’ of insulation on a pipe is given by____________________?

• (A)  r = 2k/h
• (B)  r = k/h
• (C)  r = k/2h
• (D)  r = h/k

Answer

Submitted By :Ali Uppal