design of heat exchanger

2. DESIGING OF HEAT
EXCHANGER

3. 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) .

4. STEPS FOR DESIGING HEAT
EXCHANGERStep-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.

5. 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

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

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

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

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

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

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

12. 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

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

14. 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.

15. THANK YOU