A canteen requires hot water for its dish-washing. For this purpose, the canteen draws 0.4 kg/s of water at 298 K and heats it to 353 K in a fired-heater. The wastewater leaving the dishwasher is at 333 K. In order to save energy, it is proposed to recover heat from the waste water to partially heat up the incoming water in a counter-current exchanger as shown schematically below:
Assume that
there is no loss of water in the dishwasher and a minimum approach temperature
of 10 K should be maintained in the exchanger.
a. Determine the
maximum temperature to which the incoming water can be heated in the exchanger.
b. If the
overall heat transfer coefficient in the exchanger is 1200 W/m2 K
and the specific heat capacity of water is 4186 J/kg K, compute the area of the
heat exchanger.
c. If the cost
of the exchanger in 1982 was 2 × A0.41 (in lakh rupees), where A is
the exchanger area in m2 and the Marshall and Swift cost indices in
1982 and 2000 are 315 and 400 respectively, determine the cost of the exchanger
in the year 2000.
GATE
2002
Answer: (a) 323 K, (b) 3.488 m2,
(c) 4.238 lakhs
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 vapour produced per kg of feed steam in a multieffect 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 multieffect system.
D.
heat transfer coefficient ma single effect is much lower than that in any
effect in a multieffect system
GATE
2003
Answer: (B)
The units of resistance to heat transfer are
A. Jm-2K-1
B. Jm-1K-1
C. Wm-2K-1
D. W-1m2K
GATE
2003
Answer: (D)
A process stream
of dilute aqueous solution flowing at the rate of 10 kg s-1 is to be
heated. Steam condensate at 95°C is available for heating purpose, also at a
rate of 10 kg s-1. A 1-1 shell and tube heat exchanger is available.
The best arrangement is
A. counterflow with process stream on
shell side
B. counterflow with process stream on
tube side
C. parallel flow with process stream on
shell side
D. parallel flow with process stream on
tube side
GATE
2003
Answer: (B)
The inner wall
of a furnace is at a temperature of 700°C. The composite wall is made of two
substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 Wm-1°C-1
respectively. The ambient air is at 30°C and the heat transfer coefficient
between the outer surface of wall and air is 20 W m-2 °C-1.
The rate of heat loss from the outer surface in W m-2 is
A. 165.4
B. 167.5
C. 172.8
D. 175
GATE
2003
Answer: (A)
Steam is to be
condensed in a shell and tube heat exchanger; 5 m long with a shell diameter of
1 m. Cooling water is to be used for removing the heat. Heat transfer:
coefficient for the cooling water, whether on shell side or tube side, is same.
The best arrangement is
A. vertical heat exchanger with
steam on tube side
B. vertical heat exchanger with
steam on shell side
C. horizontal heat exchanger with
steam on tube side
D. horizontal heat exchanger with
steam on shell side
GATE
2003
Answer: (B)
A fluid is
flowing inside the inner tube of a double pipe heat exchanger with diameter ‘d’.
For a fixed mass flow rate, the tube side heat transfer coefficient for
turbulent flow conditions is proportional to
A. d0.8
B. d-0.2
C. d-1
D. d-1.8
GATE
2003
Answer: (D)
Air
is to be heated by condensing steam. Two heat exchangers are available: (i) a
shell and tube heat exchanger, and (ii) a finned tube heat exchanger. Tube side
heat transfer area is equal in both cases. The recommended arrangement is
A. finned tube heat exchanger with air inside and
steam outside
B. finned tube heat exchanger with air outside and
steam inside
C. shell and tube heat exchanger with air inside
tubes and steam shell side
D. shell and tube heat exchanger with air on shell
side and steam inside tubes
GATE
2003
Answer: (B)
For
a given ambient air temperature with increase in the thickness of insulation of
a hot cylindrical pipe, the rate of heat loss from the surface would
A. decrease
B. increase
C. first decrease and then increase
D. first increase and then decrease
GATE
2003
Answer: (D)
In force convection, the Nusselt number Nu is a function of
(A) Re and Pr
(B) Re and Gr
(C) Pr and Gr
(D) Re and Sc
GATE
2004
Answer: (A)
For an ideal black body
(A) Absorptivity = 1
(B) Reflectivity = 1
(C) Emissivity = 0
(D) Transmissivity = 1
GATE
2004
Answer: (A)
The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/(m.K) and the heat transfer coefficient from the right face is 10 W/(m2.K). At steady state, the temperature of the right face in °C is
(A) 77.2
(B) 71.2
(C) 63.8
(D) 48.7
GATE
2004
Answer: (D)
A metal bar of radius 0.1 m at uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 Kg/m3 and 0.4 KJ/(kg.K), respectively. The heat transfer coefficient is 50 W/(m2.K). Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is
(A) 555
(B) 55.5
(C) 0.55
(D) 0.15
GATE
2004
Answer: (D)
Hot water (0.01 m3/min) enters the tube side of a concurrent shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of density 800 kg/m3 and specific heat 2 KJ/(kg.K) enters at 20°C. The log mean temperature difference in °C is approximately
(A) 32
(B) 37
(C) 45
(D) 50
GATE
2004
Answer: (A)
It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporization is 2100 KJ/Kg and the specific heat of solution is 4 KJ/(kg.K). The rate at which heat has to be supplied (in KJ/min) to the evaporator is
(A) 3.06×105
(B) 6.12×105
(C) 7.24×105
(D) 9.08×105
GATE
2004
Answer: (A)
The thermal boundary layer is significantly thicker than the hydrodynamic boundary layer for
(A) Newtonian liquids
(B) Polymeric liquids
(C) Liquid metals
(D) Gases
GATE
2005
Answer: (C)
An electrically heated element is submerged in a pool of water at its saturation temperature. As the temperature of the element increases, the maximum heat transfer coefficient is observed
(A) In the free convection regime
(B) Between the nucleate boiling and partial nucleate boiling mixed with unstable film boiling regimes
(C) In the incipient nucleate boiling regime
(D) In the stable film boiling regime without significant radiation effects
GATE
2005
Answer: (B)
Baffles are used in heat exchanger is order to
(A) Increase the tube side fluid’s heat transfer coefficient
(B) Promote vibration in the heat exchanger
(C) Promote cross flow and turbulence in the shell side fluid
(D) To prevent shell expansion due to thermal effects
GATE
2005
Answer: (C)
In film type condensation of liquid along a vertical tube, the thickness of the condensate layer increases towards the bottom. This implies that the local heat transfer coefficient
(A) Increases from top to bottom
(B) Decreases from top to bottom
(C) Remains constant from top to bottom
(D) First increases and then decreases from top to bottom.
GATE
2005
Answer: (D)
A black body at a high temperature TH transfers energy by radiation to a black body at a lower temperature TL. Initial TH = 1850°C, TL = 500°C and the net rate of heat transfer is 25 W. After some time, when TH = 1500°C and TL = 750°C, what is the net rate of energy transfer
(A) 8.73 W
(B) 9.60 W
(C) 13.89 W
(D) 11.01 W
GATE
2005
Answer: (D)
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