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Preliminary Double Pipe Heat Exchanger Design (S.I. units) Estimation of Heat Transfer Area Needed

Inputs

Calculations

Fluid1 mass flow

Overall heat transf.

rate, m1 =

11,300

Fluid1 temp. in, T1in =

90

Fluid1 temp. out, T1out =

60

kg/hr

coeff. estim., U =

o

C

Heat Transfer Rate, Q =

o

C

Log Mean Temp Diff, DTlm =

Fluid1 sp. heat, Cp1 =

3.1

1,050,900

44.8

kJ/hr-m2-K

kJ/hr

o

C

kJ/kg-oC Heat Transfer Area, A =

Fluid2 temp. in, T2in =

2448

o

10

9.58

m2

6270

kg/hr

C Fluid2 mass flow

o

C

Fluid2 temp. out, T2out =

50

Fluid2 sp. heat, Cp2 =

4.2

kJ/kg-oC

680

J/sec-m 2-K

rate, m2 =

Overall heat transf. coeff. estim., U =

Equations used for calculations: Q = + (m1)(Cp1)(T1in - T1out) Q = + (m2)(Cp2)(T2in - T2out) DTlm = [(T1in - T2out) - (T1out - T2in)]/ln[(T1in - T2out)/(T1out - T2in)] Q = U A DTlm

Preliminary Double Pipe Heat Exchanger Design Determination of pipe length needed (for known heat transfer area) Inputs

Calculations

Heat Transfer Area, A =

9.58

m2

Pipe Diam. in m, D =

0.075

m

41

m

(from calculations above) Pipe length needed, L = pipe Diameter, Dmm =

75

mm

(in mm)

Equations used for calculations: D = Dmm/1000 A = pDL

Calculation of Frictional Head Loss through Pipe for given flow rate, Q, pipe diam., D, pipe length, L, pipe roughness, e, and fluid properties, r & m. 1. Determ. Frict. Factor, f, assuming completely turbulent flow

Inputs

{ f = [1.14 + 2 log10(D/e)]-2 }

Calculations

Pipe Diameter, D Pipe Roughness, e Pipe Length, L Pipe Flow Rate, Q Fluid Density, r

75

mm

Pipe Diameter, D

0.075

0.15

mm

Friction Factor, f

0.02339

41

m

Cross-Sect. Area, A =

0.004418

m2

0.00345

m3/s

Ave. Velocity, V

0.781

m/s

910

kg/m3

Reynolds number, Re

0.001787

N-s/m2

29,819

(tubeside fluid) Fluid Viscosity, m (tubeside fluid) 2. Check on whether the given flow is "completely turbulent flow" (Calculate f with the transition region equation and see if it differs from the one calculated above.)

f = {-2*log10[((e/D)/3.7)+(2.51/(Re*(f1/2))]}-2 Transistion Region Friction Factor, f:

f=

0.0285

Repeat calc of f using new value of f:

f=

0.0281

f=

0.0281

Repeat again if necessary:

3. Calculate hL and DPf, for straight pipe flow, using the final value for f calculated in step 2 2

(hL = f(L/D)(V /2g)

and DPf = rghL)

Frictional Head Loss, hL

0.47

m

Frictional Pressure Drop, DPf

4223

N/m2

Frictional Pressure Drop, DPf

4.22

kN/m2

m

4. Calculate hL and DPf, for the 180o bends

Inputs

Calculations

Pipe length between bends, Lsect =

No. of 180o bends, NB = L/Lsect =

4

m

Minor Loss Coefficient for 180o bends, K =

1.5 ( K = 1.5 for threaded pipe or 0.2 for flanged pipe. )

Head loss due to bends, hB = NBK(V2/2g) =

0.47

m

Pressure Drop due to bends, DPB =

4229

N/m2

Pressure Drop due to bends in psi =

4.23

kN/m2

5. Add the results from part 3 and part 4 to get total hL and DPf Total Frictional Head Loss, hL =

0.947

m

Total Frictional Pressure Drop, DPf =

8452

N/m2

Total Frictional Pressure Drop in kN/m2 =

8.45

kN/m2

10