A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
(A)
Lower heat transfer area
(B)
Lower steam economy
(C)
Higher steam economy
(D)
Higher solute concentrations in the
product
(GATE-1998)
Answer: (C)
The advantage of using a 1 – 2 shell and
tube heat exchanger over a 1 – 1 shell and tube heat exchanger is
(A)
Lower tube side pressure drop
(B)
Lower shell side pressure drop
(C)
Higher tube side heat transfer
(D)
Higher shell side heat transfer
(GATE-1998)
Answer: (C)
For a counter-current heat exchanger with `T_h^i=80℃,T_c^0=60℃,T_h^0=50℃andT_c^i=30℃` and the temperature difference between the two streams being the same everywhere along z, the direction of flow of the hot fluid, the temperature profile should satisfy
(A) d2T/dZ2>0
(B) d2T/dZ2=0
(C) d2T/dZ2<0
(D) dT/dZ=0
(GATE-1999)
Answer: (B)
In a pipe flow, heat is transferred from hot wall to the liquid by
(A)
Conduction only
(B)
forced convection only
(C)
forced convection and conduction
(D)
free and forced convection
(GATE-1999)
Answer: (C)
As the difference between the wall
temperature and bulk temperature increases, the boiling heat transfer
coefficient
(A)
Continues to increase
(B)
Continues to decrease
(C)
Goes through a minimum
(D)
Goes through a maximum
(GATE-1999)
Answer: (D)
Gibb’s phase rule finds application when
heat transfer occurs by
(A) conduction
(B) convection
(C) radiation
(D) condensation
(GATE-1999)
Answer: (D)
Heat transfer occurs by natural
convection because change in temperature causes differences in
(A) Viscosity
(B) density
(C) thermal conductivity
(D) heat
capacity
(GATE-1999)
Answer: (B)
The Grashof number is defined as the
ratio of
(A)
Buoyancy to inertial force
(B)
Buoyancy to viscous forces
(C)
Inertial forces to viscous forces
(D)
Buoyancy to surface tension forces
(GATE-1999)
Answer: (B)
For turbulent flow in a tube, the heat
transfer coefficient is obtained from Dittus-Boelter correlation. If the tube
diameter is halved and the flow is doubled, then the heat transfer coefficient
will change by a factor of
(A) 1
(B) 1.74
(C) 6.1
(D) 37
GATE
2000
Answer: (C)
The Sieder-Tate correlation for heat
transfer in turbulent flow in a pipe gives Nu α Re0.8 where Nu is the Nusselt number and Re is
Reynolds number for the flow. Assuming that this relation is valid, the heat
transfer coefficient varies with the pipe diameter (D) as
(A) D-1.8
(B) D-0.2
(C) D0.2
(D) D1.8
GATE
2001
Answer: (B)
Heat transfer by natural convection is
enhanced in systems with
(A)
High viscosity
(B)
High coefficient of thermal expansion
(C)
Low temperature gradients
(D)
Low density change with temperature
(GATE-2001)
Answer: (B)
The overall heat transfer coefficient
for a shell and tube heat exchanger for clean surface is U0=400 W/m2K.
The fouling factor after a year of operation is found to be hdo =
2000 W/m2K. The overall heat transfer coefficient at this time is
(A) 1200 W/m2K
(B) 894 W/m2K
(C) 300 W/m2K
(D) 287
W/m2K
(GATE-2001)
Answer: (C)
The heat transfer by radiation from a
mild steel surface is to be reduced by reducing the emissivity of the surface.
This can be best achieved by
(A)
Painting the surface black
(B)
Painting the surface white
(C)
Giving the surface a mirror finish
(D)
Roughening the surface
(GATE-2001)
Answer: (C)
In thermal radiation, for a black body
(A)
α = 1, ε ≠ 1
(B)
α ≠ 1, ε = 1
(C)
α ≠ 1, ε ≠ 1
(D) α = 1, ε = 1
(GATE-2001)
Answer: (D)
If the baffle spacing in a shell and
tube heat exchanger increases, then the Reynolds number of the shell side fluid
A.
Remains unchanged
B.
Increases
C.
Increases or decreases depending on number of shell passes
D.
Decreases
GATE
2002
Answer: (D)
Air flows through a smooth tube, 2.5 cm
diameter and 10 m long, at 37°C. If the pressure drop through the tube is 10000
Pa, estimate
a.
The air velocity through the tube and the friction factor
b.
The heat transfer coefficient using Colburn Analogy
jH=(St)(Pr)0.67
Where St is the Stanton Number and Pr is the Prandtl
Number. Gas constant, R = 82.06 cm3 atm/mol.K Darcy friction factor,
f = 0.184Re0.2. Other
relevant properties of air under the given conditions: viscosity = 1.8 × 10-5
kg/m s, density = 1134 kg/m3, specific heat capacity, C1, = 1.046
kJ/kg °C, thermal conductivity = 0.028 W/m °C.
GATE
2002
Answer: (a)
47.34 m/s, (b) 178.224 W/m2°C
A double pipe countercurrent heat
exchanger is designed to cool 3500 kg/hr of benzene flowing in the inner pipe
from 80°C to 35°C. Water enters at 20°C and exits at 37°C in the annular space.
The inside pipe has an inner diameter of 3.5 cm and wall thickness of 3.56 mm.
The outer pipe has an inner diameter of 5.25 cm and is insulated. Neglecting
the wall resistance to heat transfer from the inner pipe, and assuming the
individual film heat transfer coefficient for water to be 6600 W/m2
°C. calculate:
a.
the individual heat transfer coefficient for benzene flowing in the inner pipe.
b.
the overall heat transfer coefficient based on inside diameter of inner pipe.
c.
the total length required for the heat exchanger.
Nu
= 0.023 (Re)0.8(Pr)0.3
Where
Nu is the Nusselt Number and Pr is the Prandtl Number. Average properties of
benzene: viscosity = 4 × 10-4 kg/m s, thermal conductivity = 0.147
W/m °C, specific heat capacity = 1880 J/kg °C, density = 837 kg/m3.
GATE
2002
Answer: (a) 1428.37 W/m2°C, (b) 1210.65
W/m2°C, (c) 23.24 m
A 10 cm diameter steam pipe, carrying
steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m°C). It
loses heat to the surroundings at 30°C. Assume a heat transfer coefficient of
8.0 W/m2 °C for heat transfer from surface to the surroundings.
Neglect wall resistance of the pipe and film resistance of steam. If the insulation
thickness is 2 cm, the rate of heat loss from this insulated pipe will be
A.
greater than that of the un-insulated steam pipe
B.
less than that of the un-insulated steam pipe
C.
equal to that of the un-insulated steam pipe
D.
less than the steam pipe with 5 cm insulation
GATE
2002
Answer: (D)
1000 kg of liquid at 30°C in a
well-stirred vessel has to be heated to 120°C, using immersed coils carrying
condensing steam at 150°C. The area of the steam coils is 1.2 m2 and
overall heat transfer coefficient to the liquid is 1500 W/m2 °C.
Assuming negligible heat loss to surrounding and specific heat capacity of the
liquid to be 4 kJ/kg °C, the time taken for the liquid to reach desired
temperature will be
A.
15 min
B.
22 min
C.
44 min
D.
51 min
GATE
2002
Answer: (D)
A composite wall consists of two plates A and B placed in series normal to the
flow of heat. The thermal conductivities are kA and kB
and the specific heat capacities are CPA and CPB, for
plates A and B respectively. Plate B has twice the thickness of plate A. At
steady state, the temperature difference across plate A is greater than that
across plate B when
A.
CPA > CPB
B.
CPA < CPB
C.
kA < 0.5kB
D.
kA > 2kB
GATE
2002
Answer: (C)
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