Summary of lmtd and e ntu

Summary of lmtd and e ntu. The Log Mean Temperature Difference Method (LMTD)  The Logarithmic Mean Temperature Difference(LMTD) is valid only for heat exchanger with one shell pass and one tube pass.  For multiple number of shell and tube passes the flow pattern in a heat exchanger is neither purely co-current nor purely counter-current.  The temperature difference between the hot and cold fluids varies along the heat exchanger.  It is convenient to have a mean temperature difference Tm for use in the relation. s mQ UA T 
3.  The mean temperature difference in a heat transfer process depends on the direction of fluid flows involved in the process. The primary and secondary fluid in an heat exchanger process may  flow in the same direction - parallel flow or cocurrent flow  in the opposite direction - countercurrent flow or perpendicular to each other - cross flow

1. Summary of LMTD and E-NTU NURUL AFIFAH BINTI MOHD YUSOFF 2013275464 EH110 4A FACULTY OF CHEMICAL ENGINEERING

2. The Log Mean Temperature Difference Method (LMTD) The Logarithmic Mean Temperature Difference(LMTD) is valid only for heat exchanger with one shell pass and one tube pass. For multiple number of shell and tube passes the flow pattern in a heat exchanger is neither purely co-current nor purely counter-current. The temperature difference between the hot and cold fluids varies along the heat exchanger. It is convenient to have a mean temperature difference T m for use in the relation.

3. The mean temperature difference in a heat transfer process depends on the direction of fluid flows involved in the process. The primary and secondary fluid in an heat exchanger process may flow in the same direction -  parallel flow or cocurrent flow in the opposite direction -  countercurrent flow or perpendicular to each other -  cross flow

4. Co-current flow ∆ T 1 ∆ T 2 ∆ A A 1 2 T 1 T 2 T 4 T 5 T 6 T 3 T 7 T 8 T 9 T 10 P ara ll e l Fl ow

5. Counter-current flow T 1 A 1 2 T 2 T 3 T 6 T 4 T 6 T 7 T 8 T 9 T 10 Wall T 1 T 2 T 4 T 5 T 3 T 7 T 8 T 9 T 10 T 6 Co un t e r - C u r re n t F l ow

6. LMTD Counter-Flow HX D T lm,CF > D T lm,PF FOR SAME U: A CF < A PF

7. LMTD- Multi-Pass and Cross-Flow Apply a correction factor to obtain LMTD t: Tube Side

8. LMTD Parallel-Flow HX

9. In a multi-pass exchanger , in addition to frictional loss the head loss known as return loss has to be taken into account. The pressure drop owing to the return loss is given by- Where, n=the number of tube passes V=linear velocity of the tube fluid The total tube-side pressure drop is ∆P T = ∆P t + ∆P r

10. THE EFFECTIVENESS-NTU METHOD LMTD method is useful for determining the overall heat transfer coefficient U based on experimental values of the inlet and outlet temperatures and the fluid flow rates . A more convenient method for predicting the outlet temperatures is the effectiveness NTU method. This method can be derived from the LMTD method without introducing any additional assumptions. Therefore , the effectiveness-NTU and LMTD methods are equivalent. An advantage of the effectiveness-NTU method is its ability to predict the outlet temperatures without resorting to a numerical iterative solution of a system of nonlinear equations. The heat-exchanger effectiveness ε is defined as

11. Heat exchanger effectiveness, : Maximum possible heat rate : Will the fluid characterized by C min or C max experience the largest possible temperature change in transit through the HX? Why is C min and not C max used in the definition of q max ?

12. Number of Transfer Units, NTU A dimensionless parameter whose magnitude influences HX performance:

13. Heat Exchanger Relations Performance Calculations : C r

14. Design Calculations : For all heat exchangers, For C r = 0, to all HX types.

15. References http://www.engineeringtoolbox.com/arithmetic-logarithmic-mean-temperature-d_436.html http:// www-old.me.gatech.edu/energy/laura/node5.html http :// www.che.ufl.edu/unit-ops-lab/experiments/HE/HE-theory.pdf http://web.iitd.ac.in/~prabal/MEL242/(30-31)- Heat-exchanger-part-2.pdf