More Ideas for Compact Double Pipe HXs

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

Ideas for Creation of Compact HX!!!

Helical Double-tube HX

Secondary Flow in Helical Coils

• The form of the secondary flow would depend on the ratio of the tube diameters and other factors.

• A representative secondary flow pattern is shown below:

• Thirdly, this configuration should lead to a more standard approach for characterizing the heat transfer in the exchanger.

• The ratio of the two tube diameters may be one of the ways to characterize the heat transfer.

Heat Transfer in Helical Tubes

Acharya et al. (1992, 2001) developed the following two correlations of the Nusselt number, for Prandtl numbers less than and greater than one, respectively.

Heat Transfer in Helical Annulus

Nusselt numbers for the annulus have been calculated and correlated to a modified Dean number.

The modified dean number for the annulus is calculated as it would be for a normal Dean number, except that the curvature ratio used is based on the ratio of the radius of the outer tube to the radius of curvature of the outer tube, and the Reynolds number based on the hydraulic radius of the annulus.

Thus the modified Dean number is:

Helical Coils: Laminar flow

• De is Dean Number. De=Re (a/R)1/2.

• Srinivasan et al. (7 < R/a < 104):

• Manlapaz and Churchill:

• Correction for vp:

0.275

0.5

1 for 30

0.419 for 30 300

0.1125 for 300

c

s

Def

De Def

De De

0.5

2

0.52

0.18 /1.0 1.0

3 88.331 35 /

m

c

s

f a R De

f De

0.25

0.91c w

cp b

f

f

Helical coils: turbulent flow

0.250.5 2 2

0.00725 0.076 Re for 0.034 Re 300c

R R Rf

a a a

0.20.5 2 2

0.0084 Re for Re 700 and 7 10c

R R R Rf

a a a a

0.33Pr

Prc m

cp w

f

f

Classification of Heat Exchangers

Creation of Variety in Anatomy of Heat Exchanger!!!

Creative Ideas for Techno-economic Feasibility of a HX.

• For a viable size of a HX:

• How to maximize Effective area of heat communication?.

• How to maximize Overall Heat transfer coefficient?

• How to modify the effective temperature difference?

Heat Exchanger : An Effective Landlord

• Creates a housing for both donor and Receiver.• How to accommodate both in a single housing?• Space Sharing & Time sharing • Space sharing: Donor and Receiver are present always.• Develop partition(s) in the house(HX).• Time Sharing : Donor And Mediator for sometime and

Mediator and Receiver for sometime : Repeat!• Time Sharing : Regenerators• Space Sharing : Recuperators• Central Limit Theorem : It is impossible to have time and

space sharing in one system.

A Train of External HXs in A Power Plant

S

A

B

0

Di

i-1

C

T

CBcondcond hhmQ

DASGSG hhmQ

T-s Diagram of A Modern Power Plant

Train of Shell & Tube HXs.

6

5

4

3

21

DCGSC

6 5 4 3 2 1

DC

GSC

Sequence of Energy Exchange from Flue Gas to Steam

FLUE GAS

PLATEN SH

PENDENT SH

COVECTIVE SH

ECONOMIZER

RHEVAPORATOR

Fuel Power

Furnace absorption

Platen SH

Final SHLTSH

Reheater

Economizer

Combustion Losses C & R losses Hot Exhaust Gaslosses

~4000C

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

    DesignCalculated

1 Adiabatic Flame Temp (K) 1957 1966

2 FEGT (0C) 1102 1117

3 Platen SH-I Outlet (0C) 932 951

4Platen SH-II Outlet-I outlet (0C) 859 878

5 RH 3rd & 2nd outlet (0C) 595 604

6 RH 1st Stage outlet (0C) 510 531

7 Economiser outlet (0C) 385 398

8 APH Outlet (0C) 138 151

Flue Gas Temperature At different regions of Furnace:210 MWe)

The concept of Time Sharing

• At any time:

• The overall heat transfer coefficient, U

me

me

as

as

g

gas

h

U

1

1

adust

dust

me

meair

h

U1

1

OR

• At stead operation:

condg

gas

Rh

U

1

1

acond

air

hR

U1

1

OR

Stockholm 1920The Ljungström Air Preheater

Economic Impact of the Landmark

• The use of a Ljungström Air Preheater in a modern power plant saves a considerable quantity of fuel.

• So much that the cost of the preheater is generally recovered after only a few months.

• It has been estimated that the total world-wide fuel savings resulting from all Ljungström Air Preheaters which have been in service is equivalent to 4,500,000,000 tons of oil.

• An estimate shows that the Ljungström Air Preheaters in operation annually saves about $30 Billion US.

• The distribution of thermal power capacity in which Ljungström Air Preheaters are installed over the world is shown in the table below.