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)

HEAT EXCHANGERS

Hot In Hot Out

Cold Out Cold In

Heat Exchangers

• Parallel flow

• Counterflow

• Crossflow

Ref: Incropera & Dewitt (2002)

Cross Flow Heat Exchanger

HEAT EXCHANGERS, Shell and Tube

HEAT EXCHANGERS, U-Tube

HEAT EXCHANGERS, Plate Design

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.

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.

Temperature profile in double-pipe heat exchanger

Heat Exchanger Analysis

Heat Exchanger Analysis

ocih

icoh

ocihicohm

tt

tt

ttttt

,,

,,

,,,,

ln

Counterflow

b

a

bam

t

t

ttt

ln

Parallel

Counterflow Heat Exchangers

ocih

icoh

ocihicohm

tt

tt

ttttt

,,

,,

,,,,

ln

icoc

ohih

tt

ttR

,,

,,

icih

icoc

tt

ttP

,,

,,

Counterflow Heat Exchanger

• What about crossflow heat exchangers?

• Δtm= F Δtm,cf

RPP

Rttt icocm

11

ln

1,,

Overall heat transfer coefficient in heat exchanger

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.

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

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

Log mean temperature difference

• c = cold fluid, h = hot fluid

=

=

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.

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.

The effectiveness-NTU method• Dimensionless parameter

• Actual heat transfer rate

• Maximum possible heat transfer rate

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.

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.