Heat Exchangers and Evaporators - Heat Transfer - Chemical Engineering Previous Year Questions

1

2007 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2007

A hot fluid entering a well-stirred vessel is cooled by feeding cold water through a jacket around the vessel. Assume the jacket is well-mixed. For the following data,
mass flowrates of the hot fluid = 0.25 kg/s,
mass flow rate of cold water = 0.4 kg/s,
specific heats of oil = 6000 J/kgK
specific heat of cold water = 4184 J/kgK
the inlet and exit temperature of the hot fluid is 150 °C and 100 °C respectively,
inlet temperature of cold water = 20 °C
the overall heat transfer coefficient is 500 W/m²K.
the heat transfer area in m², is

A

1.82

B

2.1

C

3

D

4.26

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2

2007 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2007

The following list of options P, Q, R and S are some of the important considerations in the design of a shell and tube heat exchanger.
P) square pitch permits the use of more tubes in a given shell diameter
Q) the tube side clearance should not be less than one fourth of the tube diameter
R) baffle spacing is not greater than the diameter of the shell or less than one-fifth of the shell diameter
S) The pressure drop on the tube side is less than 10 psi
Pick out the correct combination of ‘TRUE’ statements from the following:

A

P, Q and R

B

Q, R and S

C

R, S and P

D

P, Q, R and S

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3

2007 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2007

In a double pipe heat exchanger the ID and OD of the inner pipe are 4cm and 5cm respectively. The ID of the outer pipe is 10cm with a wall thickness of 1cm. Then, the equivalent diameters (in cm) of the annulus for heat transfer and pressure drop respectively are

A

15, 5

B

21, 6

C

6, 19

D

15, 21

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4

2008 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2008

In a shell and tube heat exchanger, if the shell length is \( L_S \), the baffle spacing is \( L_B \) and the thickness of baffle is \( t_b \), the number of baffles on the shell side, \( N_b \), is

A

\(\frac{L_S}{L_B + t_b} \)

B

\(\frac{L_S}{L_B + t_b} - 1 \)

C

\(\frac{L_S}{L_B + t_b} + 1 \)

D

\(\frac{L_S}{L_B + t_b} + 2 \)

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5

2009 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2009

A double-pipe heat exchanger is to be designed to heat 4 kg/s of a cold feed from 20 to 40 °C using a hot stream available at 160 °C and a flow rate of 1 kg/s. The two streams have equal specific heat capacities and the overall heat transfer coefficient of the heat exchanger is 640 W/m²·K. Then the ratio of the heat transfer areas required for the co-current to counter-current modes of operation is

A

0.73

B

0.92

C

1.085

D

1.25

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6

2010 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2010

Which ONE of the following statements about baffles in a shell and tube heat exchanger is FALSE? Baffles

A

Act as a support to the tube bundle

B

reduce the pressure drop on the shell side

C

Alter the shell-side flow pattern

D

Help in increasing the shell-side heat transfer coefficient

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7

2010 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2010

Hot oil at 150°C is used to preheat a cold fluid at 30°C in a 1 : 1 shell and tube heat exchanger. The exit temperature of the hot oil is 110°C. Heat capacities (product of mass flow rate and specific heat capacity) of both the streams are equal. The heat duty is 2 kW.
Under co-current flow conditions, the overall heat transfer resistance (1/UA) is

A

0.4 °C/W

B

0.04 °C/W

C

0.36 °C/W

D

0.036 °C/W

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8

2010 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2010

Under counter-current flow conditions, the overall heat transfer resistance (1/UA) is

A

0.4 °C/W

B

0.04 °C/W

C

0.36 °C/W

D

0.036 °C/W

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9

2011 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2011

Oil at 120 °C is used to heat water at 30 °C in a 1–1 co-current shell and tube heat exchanger. The available heat exchange area is \( S_1 \). The exit temperatures of the oil and the water streams are 90 °C and 60 °C respectively. The co-current heat exchanger is replaced by a 1–1 counter-current heat exchanger having heat exchange area \( S_2 \). If the exit temperatures and the overall heat transfer coefficients are same, the ratio of \( S_1 \) to \( S_2 \) is

A

∞

B

1.1

C

0.91

D

0

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10

2012 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2012

In a counter-flow double pipe heat exchanger, oil (\( \dot{m} = 2 \) kg/s, \( C_p = 2.1 \) kJ/kg.°C) is cooled from 90 °C to 40 °C by water (\( \dot{m} = 1 \) kg/s, \( C_p = 4.2 \) kJ/kg.°C) which enters the inner tube at 10 °C. The radius of the inner tube is 3 cm and its length is 5 m. Neglecting the wall resistance, the overall heat transfer coefficient based on the inner radius, in kW/m².K, is

A

0.743

B

7.43

C

74.3

D

2475

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11

2012 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2012

In a 1-1 pass floating head type shell and tube heat exchanger, the tubes (od = 25 mm; id = 21 mm) are arranged in a square pitch. The tube pitch is 32 mm. The thermal conductivity of the shell side fluid is 0.19 W/m.K, and the Nusselt number is 200. The shell-side heat transfer coefficient (in W/m².K), rounded off to the nearest integer, is

A

1100

B

1400

C

1800

D

2100

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12

2013 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2013

The effectiveness of a heat exchanger in the ε-NTU method is defined as

A

increase in temperature of the cold fluid / decrease in temperature of the hot fluid

B

actual exit temperature attained by the cold fluid / maximum exit temperature attainable by the cold fluid

C

actual exit temperature attained by the hot fluid / minimum exit temperature attainable by the hot fluid

D

actual heat transfer rate / maximum possible heat transfer rate from hot fluid to cold fluid

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13

2013 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2013

In a double pipe counter-current heat exchanger, the temperature profiles shown in the figure were observed. During operation, due to fouling inside the pipe, the heat transfer rate reduces to half of the original value. Assuming that the flow rates and the physical properties of the fluids do not change, the LMTD (in °C) in the new situation is

A

0

B

20

C

40

D

Indeterminate

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14

2014 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2014

Steam economy of a multiple effect evaporator system is defined as

A

kilogram of steam used per hour

B

kilogram of steam consumed in all the effects for each kilogram of steam fed

C

kilogram of steam used in all the effects for each kilogram of water vaporized per hour

D

kilogram of water vaporized from all the effects for each kilogram of steam fed to the first effect

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15

2014 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2014

An oil with a flow rate of 1000 kg/h is to be cooled using water in a double-pipe counter-flow heat exchanger from a temperature of 70 °C to 40 °C. Water enters the exchanger at 25 °C and leaves at 40 °C. The specific heats of oil and water are 2 kJ kg⁻¹ K⁻¹ and 4.2 kJ kg⁻¹ K⁻¹, respectively. The overall heat transfer coefficient is 0.2 kW m⁻² K⁻¹. The minimum heat exchanger area (in m²) required for this operation is __________

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16

2016 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2016

In a 1-1 pass shell and tube exchanger, steam is condensing in the shell side at a temperature (\( T_3 \)) of 135°C and the cold fluid is heated from a temperature (\( T_1 \)) of 20°C to a temperature (\( T_2 \)) of 90°C.
The energy balance equation for this heat exchanger is
\[ \ln \frac{T_3 - T_1}{T_3 - T_2} = \frac{UA}{\dot{m} c_p} \]
where \( U \) is the overall heat transfer coefficient, \( A \) is the heat transfer area, \( \dot{m} \) is the mass flow rate of the cold fluid and \( c_p \) is its specific heat. Tube side fluid is in a turbulent flow and the heat transfer coefficient can be estimated from the following equation:
\[ Nu = 0.023 (Re)^{0.8} (Pr)^{0.3} \]
where \( Nu \) is the Nusselt number, \( Re \) is the Reynolds number and \( Pr \) is the Prandtl number. The condensing heat transfer coefficient in the shell side is significantly higher than the tube side heat transfer coefficient. The resistance of the wall to heat transfer is negligible. If only the mass flow rate of the cold fluid is doubled, what is the outlet temperature (in °C) of the cold fluid at steady state?

A

80.2

B

84.2

C

87.4

D

88.6

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17

2017 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2017

In a heat exchanger, the inner diameter of a tube is 25 mm and its outer diameter is 30 mm. The overall heat transfer coefficient based on the inner area is 360 W/m².°C. Then, the overall heat transfer coefficient based on the outer area, rounded to the nearest integer, is ______ W/m².°C.

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18

2018 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2018

Economy of evaporators used for concentrating sugarcane juice is

A

\(\frac{kg \text{ of concentrated juice produced}}{kg \text{ of steam supplied}}\)

B

\(\frac{kg \text{ of steam supplied}}{kg \text{ of sugarcane juice fed}}\)

C

\(\frac{kg \text{ of water vaporized}}{kg \text{ of steam supplied}}\)

D

\(\frac{kg \text{ of sugarcane juice fed}}{kg \text{ of water vaporized}}\)

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19

2018 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2018

Segmental baffles in a 2-4 shell and tube heat exchanger

A

change the flow pattern of the tube side fluid and increase the overall heat transfer coefficient

B

increase the heat transfer coefficient in the shell side and support the tubes

C

help to reduce the thermal expansion of the tubes and increase the heat transfer coefficient in the tube side

D

increase the number of passes in the shell side and increase the heat transfer coefficient in the tube side

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20

2018 · Chemical Engineering · Heat Transfer · Heat Exchangers and Evaporators

Chemical Engineering (CH) 2018

An insulated storage tank contains 1000 kg liquid of specific heat 10 kJ kg\(^{-1}\) K\(^{-1}\). The liquid is heated by saturated steam, condensing in a helical coil at a temperature of 180 \(^{\circ}C\). The heat transfer area of the coil is 0.1 m\(^2\). If the overall heat transfer coefficient is constant at 1000 W m\(^{-2}\) K\(^{-1}\), then the time (in hours) required to raise the temperature of the liquid in the tank from 20 \(^{\circ}C\) to 80 \(^{\circ}C\) is __________ (rounded off to second decimal place).

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