design and analysis of heat exchanger

DESIGN AND ANALYSIS OF HEAT EXCHANGERAkash Behl, 09117003, Mechanical Engineering

Mentor: Mr. Parag Bhandarkar, Senior Managar, R&D, CG - BHOPAL

Crompton Greaves Ltd.Large and Traction Machine DivisionD-5, Ind. Area MPAKVN, Mandideep

MP – 462064Ph: +91-7840-40-0109

Website: www.cgglobal.com

Crompton Greaves

1878: REB Crompton Co. 1937: Indian Subsidiary, Crompton

Parkinson Ltd. 1947: Owned by Thapar Group. 1966: The name “Crompton Greaves”

was adopted 2005: Acquisition of Belgium based

Pauwels Group

Crompton Greaves - Bhopal

Established in 1993. Division: Transformers, Motors,

Generators. HV/LV Motors, Energy efficient motors,

motors for hazardous areas, DC Motors, Special Application Motors

Major production: HV Motors

Project

Design an Air Cooled Heat Exchanger, preferably of shell and tube type, that can be used to transfer the heat energy generated in the windings of 3-phase Induction motor.

Scope is to develop the prototype by the R&D division.

Figure

Specifications

Air Flow Rate from External Fan (Cold Fluid) = 30 m3 /s

Inlet temperature of cold fluid (external fan) = 45o C External temperature of cold fluid (external fan) =

61o C Inlet Temperature of Hot Fluid (Internal Fan) = 84o C Outlet temperature of Hot Fluid (Internal Fan) = 64o

C Heat Exchange Capacity = 500 KW Dimensions of Heat Exchanger = 2400 x 2250 x

3900 (in mm)

What to be done

Material of Tubes and its cost consideration

Pattern of Tubes Air Flow rate from Internal Fan Fin Selection on surface Flow of Fluid (cross, parallel and counter) Factor of Fouling and Corrosiveness

Selection of Tubes

Tube Framing and Spacing

Baffle Cut

Reynolds No.

Nusselt No.

Calculation of heat

coefficients

Total Resistance and NTU

Effectiveness, and Q (max)

NTU Method, Q = ε x Q (max)

Fin Specification

Total Heat Dissipation

Flow Sheet

Available Methods: Logarithm Mean Temperature Difference Method or NTU Method

Step 1: Basics

LMTD• Inlet and Exit Temperatures•Calculate ΔTln

•Calculate Fouling factor if necessary•Calculate Heat transfer coefficient•Q= UA ΔTln

NTU• Inlet Temperatures are needed•Calculate Cmax & Cmin

•Qmax = (Tmax – Tmin)Cmin

•Calculate Effectiveness using appropriate expression•Q= ε*Qmax

Division of HE in six different zones (graphix add)

Calculation of total no. of pipes in each region- Tube Dimensions BWG- Optimally transverse pitch (St) to outer diameter (OD) ratio is 1.25 – 1.5 (1.5 for 1st Iteration)- Linear Configuration or Staggered. - 60o for maximum density.

Total no. of pipes can now be calculated.

Step 2: Piping Configuration

Step 2: Piping Configuration

Ratio of length of cut to total inner length

It can range from 20% - 49%. For first iteration, 25% is selected. So total no. of pipes in Cross Flow =

0.75* Total no. of pipes (Nt) And total no. of pipes in Parallel/ Counter

Flow = 0.25 * Total No. of Pipes (Nt)

Step 3: Baffle Cut

Reynolds No. = Critical Length * V (max.) / Kinematic Viscosity

For Inside the tubes: a) Q is known and total no. of tubes is knownb) Inner cross-section area through BWGc) V (max) is calculatedd) Critical length is inner diameter

For outside the tubes: a) Q=A x Vb) Area is approx. to be consist of 1/6th of total length of Heat Exchanger i.e. 0.4m (Region 1). So total area is 0.4m * 2.25mc) Q can be obtained from fan calculations

Step 4: Calculation of Reynolds No.

Laminar or Turbulent (Different for Inside and outside flow)

Flow inside the tube- Laminar Flow (Re < 10,000) Nu = constant= 3.66 or 4.36

- Turbulent Flow (Re >= 10,000) Gnielinski equation,

F is Darcy’s friction factor given as,

Step 5: Nusselt No.

Flow outside the tubea) Cross Flow (Region 1 & 3)

b) For parallel or Counter Flow

Nu = 0.2 * (Re) ^0.6 * (Pr)^0.33

Convection heat transfer coefficient can be calculated using Nu = h*l/k where, l = critical length

k = Conduction heat transfer coefficient of air

Step 5: Nusselt No. & Convection Heat Transfer Coefficient (ho and hi)

Total resistance (R x Total no. of Tubes in region)

Using the data we calculate no of transfer units (NTU) = UAs/Cmin.

Step 6: Calculate Total Resistance and hence total no. of transfer units (NTU)

Qmax= Cmin(Thin - Tcin)

Q = Qmax x ε

= Chot(Thin - Thout)

= Ccold (Tcout - Tcin)

Step 7: Evaluate Effectiveness, Q(maximum), Q(actual) and Outlet Temperature

Fin Specifications

Fin Height (in inch) 0.035

Fin Height (in metre) 0.000889

Fin thickness (in inch) 0.01

Fin thickness (in metre) 0.000254

Fin density (inches) 3

Fin Material's thermal conductivity (Alluminium) 270

Step 8: Fin Selection

•Fin Analyzed: Circular•€=(Fin height+0.5*Fin Thickness)*Sqrt(Convection Heat Transfer Coeff./Fin Material’s thermal conductivity*Fin Thickness)•Total heat capacity=(Efficiency*Fin Surface Area*No. of fins in that region+ Ext. surface area-Area no fin*No. of fins in that region)*Temp. Diff. in that region*Convection heat transfer Coeff. *Pipes in Cross Flow

Calculation of efficiency

Calculations and ResultMicrosoft Office Excel Worksheet

Material Properties of Pipe

Total Length of Pipe in One Circuit of HE 1.2

Baffle position from External Fan 0.333

Length of Pipe in region 1 and 4 (in m) 0.3996



Outer Diameter(in inch) 1.5

Outer Diameter(in m) 0.0381

Thickness (in inch) 0.065

Thickness (in m) 0.001651

Inner Diameter (in m) 0.034798

Inner Cross Section Flow Area of Pipe 0.000950557

External Surface Area per unit meter(in m^2) 0.119634

External Surface Area(in m^2) (region 1 and 4) 0.047805746

Internal Surface Area per unit meter (in m^2) 0.10926572

Internal Surface Area (in m^2) (region 1 and 4) 0.043662582

Outer Cross Section Area of Pipe 0.001139514

Volume flow rate from External Fan(in m^3/sec) 30

Volume flow rate in each pipe (in m^3/sec) 0.011432927

Mass flow rate in each pipe (Kg/sec) 0.012884909

Maximum Velocity inside the pipe(m/sec) 12.02760618

Volume

Length (in m) 2.4

Breath (in m) 2.25

Height (in m) 3.9

Diagonal Pitch 0.05715

Longitudinal Pitch (in m) 0.0494919

Transverse Pitch (in m) 0.05715

No. of Longitudinal Pipes in each column 64

No. of columns 41

Total No of Pipes 2624

Baffle Cut 0.25

Pipes in Cross Flow 1968

Pipes for Parallel/Couter Flow 656

Fouling Factor of Air, Rf (m2 · °C/W) 0.0004

Thermal Conductivity of Mild Steel 40

Losses Consideration Fan calculations Graphical Plotting through the iterations Formulation of Heat dissipation against

all the variables. Cost Analysis

Scope of the project

Heat and Mass Transfer: Cengel ABB fan discharge motor guide Research paper on “Baffle selection for

HE”, Salem Bouhairie BWG Tube Charts CAIN Industries: Finned Tubing Brochure

Sources

Thank You!