Design of heat exchanger

design of heat exchanger

1. DESIGING OF HEAT EXCHANGER

2. The main basic Heat Exchanger equation is: Q = U x A x ΔTm The log mean temperature difference ΔTm is: ΔTm = (T1 – t2) – (T2 – t1)/ln ( (T1 – t2 )/ (T2 – t1) ). Where: T1 = Inlet tube side fluid temperature. t2 = Outlet shell side fluid temperature. T2 = Outlet tube side fluid temperature. t1 = Inlet shell side fluid temperature. When used as a design equation to calculate the required heat transfer surface area, the equation can be rearranged to become: A = Q/ (U x ΔTm ) Where: A = Heat transfer area (m²) (ft² ) Q – Heat transfer rate ( kJ/h] U - Overall heat transfer coefficient (kJ/h.m².°C) ΔTm - Log mean temperature difference (°C) .

3. STEPS FOR DESIGing HEAT EXCHANGER Step-1 Assume tube diameter and BWG, Assume tube length, For Diameter Longitudinal stresses Hoop stresses Joints Corrosion factor(Material) Stresses depending upon the velocity. BRIMINGHAM WIRE GUAGE(BWG) is used to find thickness.

4. FOR LENGTH Heat transfer required. Shell size distribution. Step 2 Assume fouling factor based on inside and outside tubes, hdi and hdo Resistance to heat flow Decrease the overall heat transfer coefficient Pressure drop Mechanical performance

5. Rd = 1 / Ud - 1 / U (1) where Rd = fouling factor. Ud = T hermal conductance of heat exchanger after fouling U = Thermal conductance of clean heat exchanger .

6. Step 3 Assume material of construction for the tubes Æ thermal conductivity? Amount of heat transfer required Expansion factor Stresses

7. Step 4 Assume temperature at three points and find fourth one. Q=m c * CP c *  (∆T) C  =M H * CP H *  (∆T ) H Let t1=20C t2=15C Mc= Mh CP=CP T1=10C Then T2=…? T2=5c

8. Step 5 Based on the type of flow, calculate Log Mean Temperature Difference , LMTD Use above Value and Calculate LMTD

9. Step 6 Correction factor It is function of shell and tube temperature difference “S” is measure of temperature efficiency

10. Step 7 Calculate the mean temperature difference D Tm =Ft*LMTD Step 8 Find overall heat transfer coefficient from the table. U Step 9 Now use heat transfer coefficient and find the area .

11. Step 10 Based on the assumed tube diameter and tube length, L, calculate number of tubes Step 11 Calculate tube pitch and the bundle diameter

12. Step 12 Provide/Assume the type of floating head of the exchanger and obtain the bundle diameter clearance, BDC.

13. Step 13 Calculate the shell diameter Ds= Db+BDC Step 14 Calculate the baffle spacing Bs =.4Ds The minimum baffle spacing is the greater of 50.8mm or one fifth of the inner shell diameter. The maximum baffle spacing is dependent on material and size of tubes.

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