Cross Flow Heat Exchangers

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

A Major Element for the Success of Combustion based

Power Plants!!!

Correlations are developed from experimental data to compute Nu as a f(Rem,Prn)

Overall Average Nusselt number

6

4131

10Re1500Pr7.0

:for Valid

Pr

Pr Pr Re

D

s

mDD C

k

DhNu

•All properties are evaluated at the freestream temperature, except Prs which is evaluated at the surface temperature.

Cross Flow Past A Cylinder

Cylinder in Cross Flow

3/1PrRemDD C

k

DhNu

The empirical correlation due to Hilpert

ReD C m

0.4 -4 0.989 0.330

4 - 40 0.911 0.385

40 -- 4000 0.683 0.466

4000 -- 40000 0.193 0.618

40000 -- 400000 0.027 0.805

Draught Systems for Steam Generators

Square Cylinder in Cross Flow

D

3/1588.0 PrRe246.0 DDk

DhNu

Valid for 5 X 103 < ReD < 105

D3/1675.0 PrRe102.0 DD

k

DhNu

Valid for 5 X 103 < ReD < 105

Hexagonal Cylinder in Cross Flow

D

3/1638.0 PrRe160.0 DDk

DhNu

Valid for 5 X 103 < ReD <1.95X104

3/1782.0 PrRe0385.0 DDk

DhNu

Valid for 1.95X104 < ReD < 105

D3/1638.0 PrRe153.0 DD

k

DhNu

Valid for 5 X 103 < ReD < 105

Convection heat transfer with banks of tubes

• Typically, one fluid moves over the tubes, while a second fluid at a different temperature passes through the tubes. (cross flow)

• The tube rows of a bank are staggered or aligned.

• The configuration is characterized by the tube diameter D, the transverse pitch ST and longitudinal pitch SL.

Inline Tube Bundle Staggered Tube Bundle

Characteristic Dimension of External Flow

•Definition of Parameters for Reynolds number

DV

Dmax

max,Re

VDS

SV

T

T

max V

DS

SV

D

T

)(2max

If staggered and 2

DSS T

D

or

•For tube bundles composed of 10 or more rows

3/11 PrRe13.1 max,

mD DCNu

10

0.7r

104Re2000

:for valid

4max,

L

D

N

P

All properties are evaluated at the film temperature.

If number of tubes are less than 10, a correction factor is applied as:

)10(2

)10(

LL ND

ND NuCNu

And values for C2 are from table

Power Plant Heat Exchangers

FSH

Platen S

HT

R

RHTR

LTSH

Economiser

APH ESP ID Fan

drum

Furnace

BCWpump

Bottom ash

stack

screentubes

Thermal Structure of A Boiler Furnace

Thermal Balance in Convective SH.

• The energy absorbed by steam )( sup,sup,, inoutsteamconabs hhmQ

meanSHlosscon TUAQ

,

• The convective heat exchange in the super heater:

• Overall Coefficient of Heat Transfer, U

Mean Temperature Difference• The average temperature difference for parallel flow and counter flow is

expressed as

min

max

minmax

log3.2TTTT

Tmean

2minmax TT

T

• When Tmax /Tmin < 1.7, the average temperature may be expressed as:

• Generally, the flow direction of the flue gas is perpendicular to the axes of tubes.

• Cross flow creates a conditions close to Tmax /Tmin 1.7.

Platen SH, U (W/m2 K) 120 – 140

Final SH, U (W/m2 K) 120 – 140

LTSH, U (W/m2 K) 60 – 80

Typical Values of U

Thermal Ratings of CHXs

FSH

Platen S

HT

R

RHTR

LTSH

Economiser

APH ESP ID Fan

drum

Furnace

BCWpump

Bottom ash

stack

screentubes

Thermal Structure of A Boiler Furnace

Gas Temperatures

• Platen Super Heater:• Inlet Temperature: 1236.4 0C• Outlet Temperature: 1077 0C• Final Super Heater:• Inlet Temperature: 1077 0C• Outlet Temperature: 962.4 0C• Reheater:• Inlet Temperature: 962.4 0C• Outlet Temperature: 724.3 0C• Low Temperature Super

Heater:• Inlet Temperature: 724.30C• Outlet Temperature: 481.3 0C• Economizer:• Inlet Temperature: 481.3 0C• Outlet Temperature: 328.5 0C

Steam Temperatures

• Platen Super Heater:• Inlet Temperature: 404 0C• Outlet Temperature: 475 0C• Final Super Heater:• Inlet Temperature: 475 0C• Outlet Temperature: 540 0C• Reheater:• Inlet Temperature: 345 0C• Outlet Temperature: 5400C• Low Temperature Super

Heater:• Inlet Temperature: 3590C• Outlet Temperature: 404 0C• Economizer:• Inlet Temperature: 254 0C• Outlet Temperature: 302 0C

LMTD

Two Pass Tube Bank

Multi Pass Tube Bank

Counter Cross & Parallel Cross

Real Mean Temperature Differences

• Three dimensionless parameters are introduced and used to compute real mean temperature difference.

incoldinhot

incoldouthot

TT

TT

,,

,,

cold

hot

C

C

hotC

UA

pc ZZ 1

324.01136.05.0 Z

1

1ep

1

1

ec

CHX for Low LMTD

Economizer

• The economizer preheats the feed water by utilizing the residual heat of the flue gas.

• It reduces the exhaust gas temperature and saves the fuel.• Modern power plants use steel-tube-type economizers.• Design Configuration: divided into several sections : 0.6 – 0.8 m

gap

Tube Bank Arrangement

Thermal Structure of Economizer

• Out side diameter : 25 – 38 mm.• Tube thinckness: 3 – 5 mm• Transverse spacing : 2.5 – 3.0• Longitudinal spacing : 1.5 – 2.0• The water flow velocity : 600 – 800 kg/m2 s• The waterside resistance should not exceed 5 – 8 %.

Of drum pressure.• Flue gas velocity : 7 – 13 m/s.

Extended Surfaces to Economizer