heat exchangers

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Heat exchangers

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Heat exchangers. Heat exchangers. Device that facilitate the exchange of heat between fluids that are at different temperatures while keeping them from mixing with each other. Heat exchanger involves convection in each fluid and conduction through wall that separating the two fluids - PowerPoint PPT Presentation

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Page 1: Heat exchangers

Heat exchangers

Page 2: Heat exchangers

Heat exchangers

• Device that facilitate the exchange of heat between fluids that are at different

temperatures while keeping them from mixing with each other.

• Heat exchanger involves convection in each fluid and conduction through wall that

separating the two fluids• It is convenient to use an overall heat

transfer coefficient (U)

Page 3: Heat exchangers

HEAT EXCHANGERS

Hot In Hot Out

Cold Out Cold In

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Page 5: Heat exchangers

Heat Exchangers

• Parallel flow

• Counterflow

• Crossflow

Ref: Incropera & Dewitt (2002)

Page 6: Heat exchangers

Cross Flow Heat Exchanger

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HEAT EXCHANGERS, Shell and Tube

Page 14: Heat exchangers

HEAT EXCHANGERS, U-Tube

Page 15: Heat exchangers

HEAT EXCHANGERS, Plate Design

Page 16: Heat exchangers

HEAT EXCHANGERS, Condenser

Watt Equipment, Inc., Equip. No. E-203-TR, 2690 Sq. Ft. Surface, 400 PSI Shell, 400 PSI Tubes, 3/4" Stainless Steel Tubes, Stainless Steel Shell.

Page 17: Heat exchangers

HEAT EXCHANGERS

Heat exchanger stainless steel, serial no. 3410, 170 tubes 1 inch diameter x 10 foot long,

overall length of 124 inches.

APV condenser/shell and tube heat exchanger. The shell side is 316L and the tube side is

Titanium. It is 18 foot 10 in. and has a tube surface of 1,752 square feet. There are (2)16 in,

shell side inlets. There are (558) 3/4 in. od tubes, and the shell is 31 in. inside diameter.

Page 18: Heat exchangers

Temperature profile in double-pipe heat exchanger

Page 19: Heat exchangers

Heat Exchanger Analysis

Page 20: Heat exchangers

Heat Exchanger Analysis

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ln

Counterflow

b

a

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t

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Parallel

Page 21: Heat exchangers

Counterflow Heat Exchangers

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icoc

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Page 22: Heat exchangers

Counterflow Heat Exchanger

• What about crossflow heat exchangers?

• Δtm= F Δtm,cf

RPP

Rttt icocm

11

ln

1,,

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Overall heat transfer coefficient in heat exchanger

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Example

• Hot oil is to be cooled in a double-tube counter-flow heat exchanger. The copper inner tubes

have a diameter of 2 cm and negligible thickness. The inner diameter of the outer tube (the shell) is 3 cm. Water flows through the tube at a rate of 0.5 kg/s, and the oil through the shell

at a rate of 0.8 kg/s. Taking the average temperatures of the water and the oil to be 45C

and 80C, respectively, determine the overall heat transfer coefficient of this heat exchanger.

Page 36: Heat exchangers

Estimation of h

• For turbulent flow

Nu = 0.023Re0.8Pr0.4

• For laminar flow

Di/Do Nui Nuo

0.000.051.000.250.501.00

-17.4611.567.375.744.86

3.664.064.114.234.434.86

Page 37: Heat exchangers

Analysis of heat exchangers

• Select a heat exchanger when specified temperature change in a fluid stream of known mass flow rate log mean temperature difference method (LMTD)

• Predict the outlet temperature of hot and cold fluid streams in a specified heat exchanger effectiveness-NTU method

Page 38: Heat exchangers

Log mean temperature difference

• c = cold fluid, h = hot fluid

=

Page 39: Heat exchangers

=

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Example-the condensation of steam in a condenser

• Steam in condenser of a steam power plant is to be condensed at a temperature of 30C with cooling water from a nearby

lake, which enters the tube of the condenser at 14C and leaves at 22C.

The surface area of the tubes is 45 m2 and the overall heat transfer coefficient is 2100

W/m2.C. Determine mass flow rate of cooling water needed and the rate of the

steam in condenser.

Page 42: Heat exchangers
Page 43: Heat exchangers

Example-heating water in a counter-flow heat exchanger

• A counter-flow double-pipe heat exchanger is to heat water from 20C to 80C at a rate of 1.2 kg/s. The heating is to be accomplished by

geothermal available at 160C at a mass flow rate of 2 kg/s. The inner tube is thin-walled and

has a diameter of 1.5 cm. If the overall heat transfer coefficient of the heat exchanger is 640 W/m2.C, determine the length of the heat

exchanger required to achieve the desired heating.

Page 44: Heat exchangers

The effectiveness-NTU method• Dimensionless parameter

• Actual heat transfer rate

• Maximum possible heat transfer rate

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Example-heating water in a counter-flow heat exchanger

• A counter-flow double-pipe heat exchanger is to heat water from 20C to 80C at a rate of 1.2 kg/s. The heating is to be accomplished by

geothermal available at 160C at a mass flow rate of 2 kg/s. The inner tube is thin-walled and

has a diameter of 1.5 cm. If the overall heat transfer coefficient of the heat exchanger is 640 W/m2.C, determine the length of the heat

exchanger required to achieve the desired heating.

Page 51: Heat exchangers

Example-cooling hot oil by water in a multipass heat exchanger

• Hot oil is to be cooled by water in a 1-shell-pass and 8-tubes passes heat exchanger. The tube are thin walled and are made of copper with an internal diameter of 1.4 cm. The length of each tube pass in the heat exchanger

is 5 m, and the overall heat transfer coefficient is 310 W/m2C. Water flows through the tube at a rate of 0.2 kg/s, and the oil through the shell at a rate of 0.3 kg/s. The water and the oil enter at temperature of

20C and 150C, respectively. Determine the rate of heat transfer in the heat exchanger and the outlet

temperature of the water and the oil.

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