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THERMAL PARAMETERS EVALUATION OF SHELL-AND-TUBE

HEAT EXCHANGERS WITH MULTIPLE COUNTER FLOW

Branislav Ja}imovi} Srbislav Geni}***Mihai Nagi

ABSTRACT

This paper discusses a new way to improve heat transfer performances of shell-and-tube

heat exchangers, namely, heat transfer driving force. This could be achieved by using only

counter-current passes in heat exchanger, while outer tubes are added to turn fluid from one

side of apparatus to another. Given example show that apparatuses built in this manner can

overcome the major problem of heat exchangers used in district heating systems which is

small mean temperature difference.

1. INTRODUCTION

Shell-and-tube heat exchangers are still the most common used type of heat transfer

apparatus. Last few decades engineers spent trying to improve heat transfer performances and

their efforts were mainly based on additional agitation/turbulization of fluid flow in order to

magnify overall coefficient of heat transfer. This approach is known as internal heat transfer

augmentation technique.Heat exchanger performances can also be improved using more sophisticated fluid flow

organization across the heat transfer surface. Roetzel and Spang suggested that the mean

temperature difference could be raised by using greater number of tubes for counter-current

than for co-current passes. In [1], they have derived performance evaluation equations in the

form ( )P f R NTU= ; 2 for heat exchanger with only one tube in co-current flow direction.This paper discusses another possible solution: heat exchanger with only counter-

current passes, while outer tubes are added to turn fluid from one side of apparatus to another.

Professor at the Faculty of Mechanical Engineering, University of Belgrade

Assistant at the Faculty of Mechanical Engineering, University of Belgrade*** Professor at the Faculty of Mechanical Engineering, University Politehnica Timisoara

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2. DESCRIPTION OF THE MODIFIED E AND F TYPE OF HEAT EXCHANGER

Common classification of shell-and-tube heat exchanger types is given in TEMA

standards [2]. Heat exchanger type E (so called TEMA-E type) refers to heat exchanger with

one shell pass and type F (TEMA-F) is apparatus with two shell passes, both with, one or

multiple (usually even number) tube passes (figure 1). Multiple tube fluid flow arrangementhas both co-current and counter-current passes, which means that their thermal performance is

worse than for strictly counter flow apparatus.

Figure 1 TEMA-E and TEMA-F heat exchangers

A new solution (here designated as MOD type) for fluid flow organization is shown on

figure 2. MOD-E shell now has two counter-current passes due to addition of the outer tube

that returns tube side fluid to the front header. MOD-F type has four tube passes (two outer

tubes) and two shell passes.

Figure 2 MOD-E and MOD-F heat exchangers

3. THERMAL PARAMETERS OF MOD-E AND MOD-F HEAT EXCHANGERS

Commonly used methodology for the surface heat exchanger design is based on thermal

parameters: heat capacity ratio R , efficiency parameter P, number of transfer units NTU and correction factor for mean temperature difference . According to this approach in

thermal design, heat exchanger performance is completely defined if the one of the followingequations can be established [3]:

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( )F R P NTU; ; =0 (1)( )F R P; ; =0 (2)

Equations for evaluation of thermal parameters for MOD-E type of exchanger are:

for hot fluid on the tube side

( )

( )

P

R

NTU

R R

RNTU

RNTU

RNTU

R

NTU

NTUNTU

R

=

+

+

+

+ +

=

1 12

2 2

1 12

12

12

2

1

1 11

2

2

2

2

2

2

2

2

exp

exp

exp

exp

exp

exp

for

for

(3)

for cold fluid on the tube side

( )P

R NTU

R

R NTU

NTUR NTU

R

NTU

NTU

NTUR

=

+

+

+

+ +

+

=

11

2

1

2

11

2

1

1

2

1

2

1

2

12

1

2

1

2

11

2

1

2

2

2

22

2

2

2

exp

exp

expexp

exp

exp

for

for

(4)

For theoretical case of infinite heat transfer surface, maximal temperature efficiency

depends only on heat capacity ratio. Equations for evaluation of Pmax are:

for hot fluid on the tube side

P

RR

+R

Rmax

=