INSTRUCTIONS SLIDE

WelcomeThis is a template to create an Instructional Design Document of the concept you have selected for creating animation.

This will take you through a 5 section process to provide the necessary details to the animator before starting the animation.

The legend on the left will indicate the current status of the document. The Black coloured number will denote the current section, the Turquoise color would denote the completed sections, and the Sky blue color would denote the remaining sections.

The slides having 'Instructions' would have a Yellow box, as shown on the top of this slide.

1

5

3

2

4

Heat Exchangers

http://en.wikipedia.org/wiki/File:Tubular_heat_exchanger.png

Created by:Shri Vardhan NuwalGaurav Jasoriya

Definitions and KeywordsHeat Exchangers:

Heat exchanger is a device for heat exchange between two fluids; say hot and cold fluids. Heat exchangers are classified based on the (geometrical) construction or flow arrangement. For instance heat exchangers can be concentric pipe heat exchangers, cross-flow heat exchangers, and shell-tube heat exchangers. Our main discussion will be around concentric pipe/tubular heat exchangers.

5

3

2

4

1

Definitions and KeywordsHeat transfer coefficient:

The heat transfer coefficient(h) is the proportionality coefficient between the heat flux, Q/(AΔt), and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ΔT).

For tubular heat exchanger

Inside heat transfer coefficient : h0

Outside heat transfer coefficient : hi

Heat transfer coefficient of pipe wall : k/x

5

3

2

4

1

Definitions and KeywordsReynolds number

Re is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces

Prandtl numberPr is a dimensionless number approximating the ratio of momentum diffusivity (kinematic viscosity) and thermal diffusivity.

5

3

2

4

1

Definitions and Keywords

Nusselt numberNu is a dimensionless number that gives ratio of convective to conductive heat transfer across (normal to) the boundary.

Typically the average Nusselt number is expressed as a function of the Reynolds number and the Prandtl number, written as: Nu = f(Re, Pr).

5

3

2

4

1

Definitions and KeywordsThe overall heat transfer coefficient(U):

The overall heat transfer coefficient U is a measure of the overall ability of a series of conductive and convective barriers to transfer heat. The common resistances offered for heat exchange are those between hot fluid and the wall, between cold fluid and the wall, and wall resistance.

*Additionally, during the operation of HE, scales and films may form on the walls of the HE tubes. Formation of these is called fouling. Fouling of the

HEs can offer certain resistance which is termed fouling resistances.

5

3

2

4

1

Definitions and KeywordsNeglecting the fouling resistance ,we can write the above equation for tubular heat exchangers as follows

Since the thermal conductivity of pipe material is very high and we assume the thickness of pipe to be very small as compared to other dimensions, we can simplify the above equation to

5

3

2

4

1

Definitions and KeywordsLog-mean temperature difference:

According to Newton's law of cooling, the overall heat exchanged by the two fluids (using the overall heat transfer coefficient) is given by

Where the log-mean temperature difference ΔTlmtd depends on the flow configuration.

5

3

2

4

1

Concept details:

Double Pipe Heat Exchanger( Parallel Flow)

Parallel-flow heat exchanger Parallel-flow temperature profile

1

5

3

2

4

2

Concept details:

Double Pipe Heat Exchanger( Counter Flow)

Counter-flow heat exchanger Counter-flow temperature profile

1

5

3

2

4

2

Concept details:

Special Operating Conditions

Reference : Fundamentals of heat and mass transfer Frank P. IncroperaDavid P. Witt

1

5

3

2

4

Concept details: Nusselt Number Correlations

1

5

3

2

4

Circular Pipes

Turbulent Flow Dittus-Boelter equation

where:

D is the inside diameter of the circular duct Pr is the Prandtl number n = 0.4 for heating of the fluid, and n = 0.3 for cooling of the fluid

Laminar Flow 𝑁𝑢𝑑 = 1.86∗𝑅𝑒1/3 ∗𝑃𝑟1/3 ∗൬𝐷𝑒𝑙 ൰1/3

Where: De is equivalent diameter

Pr is the Prandtl number The above equation can be used for ReD <2100

Concept details: Nusselt Number Correlations

1

5

3

2

4

Annulus

Turbulent Flow Dittus-Boelter equation

where:

D is the inside diameter of the circular duct Pr is the Prandtl number n = 0.4 for heating of the fluid, and n = 0.3 for cooling of the fluid

Laminar Flow 𝑁𝑢𝑑 = 1.86∗𝑅𝑒1/3 ∗𝑃𝑟1/3 ∗൬𝐷𝑒𝑙 ൰1/3

Where: De is equivalent diameter

Pr is the Prandtl number For annulus equivalent diameter is d0-di The above equation can be used for ReD <2100

Interactivity and Boundary limits1

5

2

4

3

Interactivity and Boundary limits1

5

2

4

3

Interactivity and Boundary limits1

5

2

4

3

Interactivity and Boundary limits1

5

2

4

3

Questionnaire1. Tc,o can exceed Th,o in parallel-flow.

Answers: a)True b)False

2. Tc,o can exceed Th,o in counter-flow.

Answers: a)True b)False

3. Which configuration was more efficient at transferring heat, countercurrent or parallel flow?(Given same inlet and outlet temperatures)

Answer: Counter-flow.

for the same temperatures of the hot and cold fluid streams and heat transfer surface area , the counter-flow configuration will result in a higher heat exchange when compared with the parallel-flow configuration. This is because,

ΔTlmtd;CF > ΔTlmtd;PF

1

5

2

4

3

Questionnaire4. Consider a counter current concentric pipe heat exchanger of axial length

L with hot fluid entering the tube at a mass flow rate of and the temperature Th,i and the cold fluid entering the outside tube at a mass flow rate of and enters at a temperature Tc,i. Assume the radial temperature variations to be negligible. If the specific heat of the cold fluid is twice as that of the cold fluid,

A) Sketch the axial temperature profiles in the inside and outside tubes if the hot fluid and the cold fluid leave the heat exchanger at the temperatures Th,o and Tc,o respectively.

Ans)

1

5

2

4

3

Questionnaire

B) What is the log mean temperature difference for this system?

Ans) ΔTlmtd = ΔTh= ΔTc

As per formula ΔTlmtd attains 0/0 form . The above answer provides the logical reason for the value of ΔTlmtd .

1

5

2

4

3

Thank you