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VISVESVARAYA VISVESVARAYA NATIONAL INSTITUTE OF TECHNOLOGY FIFTH SEMESTER B.TECH. (CHEMICAL) End Semester Examination  –  November,  November, 2015 SUBJECT: ADVANCE HEAT TRANSFER (CML 341)

Time: 3 hours 

Q.1

Max. Marks: 50

Instruction to students Wherever not specified, assume counter current flow Missing Data, if any, may be suitably assumed. Useful formulae and figures are provided at the end of the t he question paper

A shell and tube heat exchanger is to be used to heat milk, at a flow rate of 1 kgs -1, from 20°C to 60°C. The heating medium is hot water at 95°C, available at a flow rate of 1.3 kgs -1 . The density of the milk is constant at 1040 kgm-3. The heat capacity of the milk is 4.2 kJkg -1K -1. The bulk viscosity of the milk is 0.5 mPa s, while the viscosity of the milk near the tube surfaces is 0.45 mPa s. The density of the water is constant at 960 kgm-3. The heat capacity of the water is 4.26 kJkg -1K -1. The thermal conductivity of the water is 0.677 Wm -1K -1. The bulk viscosity of the water is 0.3 mPa s, while the viscosity of the water near the tube surfaces is 0.4 mPa s. (a) Calculate the exit temperature of the water and the log mean temperature difference across the heat exchanger.

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(b) In the workshop there is available a shell and tube heat exchanger (specifications below) that has been salvaged from a decommissioned plant. It is already configured for two tube side passes and one shell side  pass. The water will will travel through the shell side. side. Calculate Calculate the shell shell side heat heat transfer transfer coefficient. coefficient.

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(c) Given that the milk will travel through the tubes of the heat exchanger and that the tube side heat transfer coefficient is known to be 5000 Wm -2K -1, will the heat exchanger perform the required duty? Assume that FT = 0.875 and neglect fouling.

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(d) What will be the pressure drop on the tube side? Take value of m as 0.25 for laminar flow and 0.14 for turbulent flow. Heat Exchanger specifications

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Pitch Type Tube Pitch Tube Inner Diameter Tube Wall Thickness Baffle Spacing Baffle cut Shell Diameter Tube Length  Number of Tubes Tubes Thermal Conductivity (tube metal) Q.2

Square 0.03 m 0.02 m 0.003 m 0.5 m 25% 0.2 m 2m 36 18 Wm-1K -1

A cross-flow heat exchanger (fluids flow perpendicular to each other) is used in cardiopulmonary bypass  procedures to cool the blood (density = 1050 kg m -3 , specific heat capacity = 3740 Jkg -1°C-1 ) from body temperature (37°C) down to 25°C in order to induce hypothermia. Hypothermia reduces the metabolic and oxygen requirements which protect the major organs from damage during surgical procedures. Both fluids

are unmixed in this heat exchanger and the overall heat transfer coefficient is 780 Wm -2°C-1 . Cold water (specific heat capacity = 4200 J kg -1 °C-1 ) at 3°C is used to cool the blood and the water outlet temperature needs to be maintained at 22°C. The flow rate of the blood is 5 L min -1 . (a) Calculate the rate of heat removed from the blood.

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(b) Calculate the flow rate of cold water required. (c) Using the log mean temperature difference method, calculate the area of the heat exchanger. (d) Using the NTU-ε method, calculate the area of the heat exchanger.

Q.3

(e) For this type of application, what considerations may be necessary when designing or selecting a heat exchanger.

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a) Rate the limpet coil, spiral baffled jacket, dimpled jacket and plain jacket in terms of pressure rating, cost and heat transfer rates.

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b)  Estimate the steam requirement as you start to heat 50 kg of pea soup in a jacketed pan, if the initial temperature of the soup is 18°C and the steam used is at 100 kPa gauge. The pan has a heating surface of 1 m2 and the overall heat transfer coefficient is assumed to be 300 J m -2 s-1 °C-1. The saturation temperature of steam at 100 kPa gauge = 120°C and latent heat = 2202 kJkg -1 . c) Derive an expression for time required to bring the temperature of the batch from initial temperature T 1 to final temperature T 2 when the heating fluid do not undergo any temperature change. Using the derived expression, calculate the time needed to bring the stirred pea soup in part b) up to a temperature of 90°C, assuming the specific heat is 3.95 kJ kg -1 °C-1. Q.4

In a process design, the following process streams must be cooled or heated: a) Using the minimum temperature of approach of 10 °C, determine the pinch (5) temperatures.  b) Determine the minimum cooling and heating utilities required. (1) c) Determine the minimum number of heat exchangers required below and above the pinch. (2)

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Stream  Number

Heat Capacity kW/°C

Source Temperature °C

Target Temperature °C

1 2 3 4

2 3 4 2

250 240 130 190

120 140 230 240

d) Determine a valid heat exchanger network below and above the pinch. (2) Q.5

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Each sub question carries 2 marks a)  Oil flows through the tube of double pipe heat exchanger at the rate of 0.2 kg/s. The oil is cooled by counter flow of water which passes through the annulus. The water flow rate is 0.15 kg/s. The oil is cooled from 70 oC to 50 oC. The cooling water is available at 12 oC. If the tube mean diameter is 15 mm and its wall resistance is neglected, find the length of the tube required for this duty. Data: hi (oil side) = 2270 W/m2K; ho (water side) = 5670 W/m2K; C p (oil) = 2.18 kJ/kg K, C p (water) = 4.18 kJ/kg K

b) A heat exchanger has been designed on the basis of calculated U c (clean heat transfer coefficient) equal to 50 Btu/h.ft 2 oF and a dirt factor of 0.005. No overdesign or excess area is provided. Do you think that the exchanger will have an ‘apparent overdesign’ for some period of time after installation? What is the apparent  percent excess area at the start? c) Draw the stream flow diagram for a 2-2 pass, 5 channels per pass counter current plate heat exchanger, clearly depicting the two streams. Q.6)

a) Choose the most appropriate answer for the following:

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i) Concentration of viscous liquid is handled effectively in a (a) short tube evaporator (b) forced circulation evaporator (c) falling-film evaporator (d) none of the above ii) Liquor with foaming tendency can be concentrated in a (a) horizontal tube evaporator (b) long-tube evaporator (c) agitated film evaporator (d) short film evaporator iii) In multiple effect evaporator, if the feed is cold then the method of feeding is of (a) Forward type (b) backward type (c) mixed type (d) parallel type iv) In case of evaporators, liquid entrainment results primarily due to (a) high vacuum in the evaporator (b) high evaporation rate (c) foaming of the solution (d) high heat transfer rate b) A 1.5 wt% aqueous salt solution is concentrated to 4 wt% in a single effect evaporator. The feed rate to the evaporator is 7500 kg/h and the feed is at 85 °C. The evaporator operates at 1.0 bar. Saturated steam at 170 kPa heats the evaporating solution.

i) What heat transfer area is required if the heat transfer coefficient is 2500 W/m 2 K? ii) Calculate the steam flow rate assuming that there was no sub-cooling of the condensate?

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iii) Calculate the steam economy of the evaporator?

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Additional data Saturation temperature of steam: 115 °C

Specific enthalpy of concentrated liquid: 419 kJ/kg

Specific enthalpy of vapor exiting the system: 2675 kJ/kg Specific enthalpy of feed : 356 kJ/kg

Latent heat of steam: 2215 kJ/kg

Useful Formulae and Figures

 =

1.2 

(2 − 0.7852 )