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Optimization of Air Preheater for compactness of shell by

evaluating performance for different Baffle structure

Nemish Kanwar1* and Dr P Srinivasan2*

1BE(Hons) Mechanical Engineering, 2Associate Professor

*Department of Mechanical Engineering, Birla Institute of Technology and Science, Pilani,

Rajasthan 333031, India 1Corressponding author, email: nemishkanwar@gmail.com

ABSTRACT:

Designing of an Air Preheater with increased performance from an existing design

through alteration in baffle placement. Analysis of 4 Baffle designs for segmented Baffle

case was done using Ansys Fluent. The net heat recovery rate was computed by

subtracting pump work from heat recovered. Based on the result, Air Preheater design

was recommended.

Keywords: Heat Transfer, Numerical Analysis, Simulation, Baffle Arrangement

1. INTRODUCTION Air Preheater is designed to heat air before another processes. The purpose of Air Preheater is

to recover heat from boiler flue gas which increases thermal efficiency of boiler by reducing

useful heat lost in flue gas 1. As a result, Flue gas temperature is reduced in the stack. It also

allows temperature control over temperature of gases leaving stack. Air Preheaters are

generally divided into 2 types: Regenerative and Recuperative. Regenerative type has an

energy storage matrix which is alternatively exposed to hot and cold fluids. In Recuperative

type, heat is transferred between hot gases to air across heat exchanging surface2. They are

plate type and tubular, although tubular type is generally used. Tubular units are essentially

counter-flow shell and tube in which hot gases flow inside the tubes and the air flows outside

in the shell. Baffles are provided to maximize air contact with tubes.

Shell and tube heat exchangers are probably the most widespread and commonly used

equipment in the process industries. They are essential equipment for all the major industries

like chemical and petrochemical plants, oil refineries, power plants and metallurgical

operations. They are employed for several applications such as heating, cooling, condensation

and boiling. There are several reasons for their general acceptance. Firstly, these equipment

provide a comparatively large ratio of heat transfer area to volume as well as weight. It is

relatively easy to construct in a wide range of sizes and which is mechanically rugged enough

to withstand the normal shop fabrication stresses, shipping and field erection stresses, and

normal operating conditions. There are many versions of the basic configuration, which can be

used to solve special problems. The shell and tube exchanger can be cleaned reasonably easily.

Further, the components which are most subject to failure, gaskets and tubes, can be easily

replaced. Finally, fairly good design methods and standards exist, and the expertise and shop

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facilities for the successful design and construction of shell and tube exchangers are universally

available 3.

The basic principle of a shell and tube heat exchanger is that two fluids flow at different

temperatures separated by a wall. Owing to the temperature difference, heat transfer from fluid

at higher temperature to lower temperature occurs by mechanism of conduction and

convection. The shell side flow in a shell and tube heat exchanger is quite complicated. The

inlet and outlet being perpendicular to general flow direction brings complexities in shell flow.

In addition, the presence of baffles makes the flow further complex. It is thus desirable to

understand the flow field and hence, in turn, be able to predict the heat transfer mechanism

over a wide range of heat duty and mass velocities on both the sides.

During the past 25 years, (CFD) computational fluid dynamics is being increasingly used

because of the developments in the computational power as well as numerical techniques. It is

being used as a quick and efficient tool to assess the flow distribution and heat transfer in

various types of heat exchangers. 4investigated the overall heat transfer rate and the pressure

drop through shell side with helical baffles by using commercial CFD code FLUENT and the

authors obtained good agreement between the CFD predictions and the experimental

measurements.

5 also used FLUENT 6.3 for the CFD simulations. The simulations were performed using

standard k–ε,2nd order k–ε, realizable k–ε model and Spallart–Allmaras turbulence models.

The number of baffles was varied from 6 to 12. Heat transfer was evaluated by log mean

temperature difference (LMTD) from the temperature profiles obtained from the simulations.

The studies were focused on the effects of various types of baffle and their cut, spacing, etc. on

the pressure drop and heat transfer.

Figure 1: Effect of baffle cut and baffle spacing on the shell side main stream: (a) small baffle cut, (b)

large baffle cut, (c) small baffle spacing, (d) large baffle spacing, (e) ideal 5

This report focuses on cuboidal shell design for a shell and tube Air Preheater. Air flows in

the shell and Flue gas flows inside the tube in counter flow.

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The geometry is a 970𝑚𝑚 × 650𝑚𝑚 × 4200𝑚𝑚 cuboidal shell with 12 × 8 quantity of tubes

with external diameter 64𝑚𝑚 and thickness 1.63 𝑚𝑚. Mass flow rate of air and gas supplied

are 3.07 𝑘𝑔/𝑠 and 3.24𝑘𝑔/𝑠 with inlet temperatures 27°𝐶 and 360°𝐶 respectively.

The main aim of this report is to illustrate change in heat recovery rate and pressure changes

for different baffle arrangements. Baffle spacing between 4 baffles in an arrangement is kept

constant. The transverse view of APH is divided into 4 quadrants and named 1,2,3,4

respectively.

The order of baffle position in the direction of air flow is used to name a baffle arrangement.

A total of 4 baffle arrangements have been chosen keeping all the other parameters fixed- 1-3-

1-3, 1-2-3-4, 12-34-12-34 and 0-0-0-0. semi baffle and no baffle names are used instead of 12-

34-12-34 and 0-0-0-0 respectively throughout this document for simplification.

2. CFD MODELLING

1.1 PHYSICAL MODELLING

The geometry was drawn and meshed by using GAMBIT 2.2.30. Original Air Preheater is

compared with 3 different segmented Air Preheaters with alternative Baffle designs.

The original APH had rectangular baffles in 1-3-1-3 format. The three other designs are: No

baffle, 1-2-3-4 format and semi-baffles.

Governing equations and boundary conditions The commercial CFD code Fluent 6.3 was used

to perform the simulation. Since the Re number of fluid in this article was in the range from

dozens to hundreds of thousands, the simulations were solved employing the laminar flow

model and the renormalization group (Standard) k–𝜖 model. Choosing (Standard) k–𝜖 model

as the turbulence model was because it involved the effect of swirl on turbulence, which could

enhance the simulation accuracy of swirling existed in the heat exchanger; plus, the (Standard)

k–𝜖 model had already gained excellent utilization in some published articles about heat

exchanger simulation. The governing equations for continuity, momentum, energy, k and 𝜖

were used in the computation.

Table 1: Thermo physical properties of Air (polynomial equation)

Property 𝑥0 𝑥−1 𝑥−2

Density (kg/m3) 2.35 -0.0051 3.57E-06

Cp (J/kg K) 1014 -0.1153 0.000296

Thermal Conductivity

(W/m K)

0.002753 8.58E-

05

-2.28E-

08

Dynamic Viscosity

(kg/Pa s)

2.95E-06 5.79E-

08

-1.91E-

11

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Table 2: Thermo physical properties of Flue Gas (polynomial equation)

Property 𝑥0 𝑥−1 𝑥−2 𝑥−3 𝑥−4 𝑥−5 𝑥−6 𝑥−7

Density

(kg/m3)

3.892 -0.017 3.98E-

05

-

5.38E

-08

4.36E

-11

-

2.10E

-14

5.41E

-18

-

5.90E

-22

Cp (J/kg K) 965.9 0.646 -

0.00398

1.16E

-05

-

1.66E

-08

1.29E

-11

-

5.34E

-15

9.17E

-19

Thermal

Conductivity

(W/m K)

0.00017

4

0.00010

4

-7.01E-

08

4.38E

-11

-

1.17E

-14

- - -

Dynamic

Viscosity

(kg/Pa s)

6.62E-

07

7.41E-

08

-5.77E-

11

3.60E

-14

-

9.59E

-18

- - -

1.2 MESH SELECTION

Mesh generation for segmented baffle design was performed using Gambit. The whole

volume was meshed using Hex/Wedge Cooper scheme with mesh size 10. Mesh was

successfully generated error-free.

1.3 NUMERICAL METHOD

The governing equations along with the boundary conditions were iteratively solved by the

finite volume method using SIMPLE pressure–velocity coupling algorithm. The QUICK

scheme with first-order precision was utilized for convective formulation and the SIMPLE

algorithm was for pressure-velocity coupling

The convergence criteria were 10−3 for energy, x-y-z velocities and 10−2 for Turbulent Kinetic

Energy, 𝑘 and 10−1 for turbulent dissipation, 𝜖. The calculation was carried out in Dell

Workstation in IPC, BITS Pilani with 8GB RAM and 3.1 GHz CPU.

Air and flue gas property was taken as polynomial equation with coefficients in Table 1 and

Table 2 respectively.

3. RESULTS The 4 segmented Baffle case were simulated in Fluent. It is crucial to determine flue gas outlet

temperature profile (see Figure 2) Pressure drop and average temperature at outlet should be

least for the best case. In this case, Temperature profile of A shows many regions with

temperature 310°C.

The flue gas domain is above sulphur dew point temperature (170°𝐶) which marks it safe from

sulphur corrosion.

.

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Figure 2: Flue Gas Outlet temperature of A. 1-3-1-3 baffle geometry, B. 1-2-3-4 baffle geometry, C.

Semi Baffle geometry, D. No Baffle geometry

Figure 3 illustrates the streamlines in the air domain, where dark blue lines shows dead zone

formation where flow is stagnant. Heat transfer coefficient is least in these zone because of low

Reynolds number. 2 out of 4 quarters mostly comprise of dead zones in 1-3-1-3 arrangement;

resulting in very low heat transfer from these tube bundles.

Figure 3: Streamlines Air Domain of A. 1-3-1-3 baffle geometry, B. 1-2-3-4 baffle geometry, C. Semi

Baffle geometry, D. No Baffle geometry (air flow is from left to right)

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Table 3: Summary of average temperature at all the inlets and outlets

1-2-3-4 1-3-1-3 no Baffle Semi

Baffles

Airin °𝐶 26.85 26.85 26.85 26.85

airout1 °𝐶 133.59 126.92 131.08 135.29

airout2 °𝐶 149.88 150.45 160.29 187.04

Fluein °𝐶 340.07 339.01 343.97 339.50

Flueout °𝐶 234.38 238.27 235.78 229.73

The overall heat transfer coefficient and effectiveness needs to be computed for each case for

comparison.

Heat transfer coefficient can be calculated using LMTD approach

𝑈𝐴(𝐿𝑀𝑇𝐷) = 𝑚𝑐𝑝Δ𝑇

Effectiveness of a Heat exchanger is used to evaluate performance.

𝜖 =𝐶𝑓𝑙𝑢𝑒(𝑇𝑓𝑙𝑢𝑒𝑖𝑛

− 𝑇𝑓𝑙𝑢𝑒𝑜𝑢𝑡)

𝐶𝑚𝑖𝑛(𝑇𝑓𝑙𝑢𝑒𝑖𝑛− 𝑇𝑎𝑖𝑟𝑜𝑢𝑡

)

Table 4 shows the heat recovery rate from the exiting flue gas and fan work required for air

and gas circulation. Assuming, 40% efficiency of thermal plant and 75% FD fan efficiency and

90% motor efficiency, Net heat recovered is calculated. Heat recovered is maximum for No

Baffle structure, followed by 1-2-3-4, 1-3-1-3, Semi Baffles.

Table 4: Net Heat Recovered from different designs

1-2-3-4 1-3-1-3 No

Baffle

Semi

Baffles

Fan work

Total, W

3247 3576 2562 12036

Heat

recovered,

W

358116 341450 366787 371748

Net heat

recovered

138436 131282 142919 130868

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Table 5: Performance Parameters of Air preheaters and Pressure drops in flow stream

1-2-3-4 1-3-1-3 no Baffle Semi Baffles

U, 𝐖/𝐦𝟐𝐊 21.09 19.86 20.90 23.21

Effectiveness 36.86% 35.27% 37.28% 38.32%

Pressure drop_air 𝑷𝒂 841.190198 841.2309693 576.4366207 2391.179035

Pressure drop_flue 𝑷𝒂 116.82776 118.55953 118.06029 115.98264

4. CONCLUSIONS Formation of large amounts of wake region (see Figure 3) in 1-3-1-3 arrangement explains the

temperature profile, in Figure 2, which shows several regions with 310°𝐶 temperature.

The overall heat transfer coefficient and effectiveness for different baffle arrangements are

determined via numerical analysis using a method based on convective mass transfer. Results shown

in

Table 5, dictates that arrangements 1-2-3-4 and semi baffle gives better results than commercial

Air preheater model with 1-3-1-3 arrangement. Pressure drop is same for 1-2-3-4, while it is

increased for semi baffle case.

Referring Table 4, Air preheater without baffles is the clear choice; but it is not used for

vibrational stability of tubes. So, the next better 1-2-3-4 arrangement should be used instead of

conventional 1-3-1-3 arrangement as it has more heat recovery rate.

5. REFERENCES 1. Kakac, S. & Liu, H. Heat Exchangers Selection, Rating and Thermal Design. (CRC

PRESS, 2002).

2. Nag, P. K. Power Plant Engineering. (Tata McGraw-Hill Publishing Company Limited,

2008).

3. Pal, E., Kumar, I., Joshi, J. B. & Maheshwari, N. K. CFD simulations of shell-side flow

in a shell-and-tube type heat exchanger with and without baffles. Chem. Eng. Sci. 143,

314–340 (2016).

4. Wang, Q., Chen, Q., Chen, G. & Zeng, M. Numerical investigation on combined

multiple shell-pass shell-and-tube heat exchanger with continuous helical baffles. Int. J.

Heat Mass Transf. 52, 1214–1222 (2009).

5. Ozden, E. & Tari, I. Shell side CFD analysis of a small shell-and-tube heat exchanger.

Energy Convers. Manag. 51, 1004–1014 (2010).