1. Heat Exchangers
Heat Exchangers

2. Heat Exchangers
A heat exchanger is used to exchange heat between two fluids of different temperatures, which are separated by a solid wall.
Heat exchangers are used to carry out energy conversion and utilization. They utlize a wide range of flow configurations.
Applications in heating and air conditioning, power production, waste heat recovery, chemical processing, food processing, sterilization in bio-processes.
Heat exchangers are classified according to flow arrangement and type of construction.
Heat Exchangers

3. Heat Exchangers

4. . Flow in a double-pipe heat exchanger.

5. Heat Exchangers
Chee 318

6. The RODbaffle heat exchanger design (Phillips Petroleum Co.)

7. Tube Bundles

8. Tube Pitch

9. Figure Shell-and-tube heat exchangers: (a) 1 shell pass and 1 tube pass (1-1 exchanger); (b) 1 shell pass and 2 tube passes (1-2 exchanger).

10. Shell-and-Tube Heat Exchangers
Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection.
The number of tube and shell passes may be varied
One Shell Pass and One Tube Pass
One Shell Pass,
Two Tube Passes
Two Shell Passes,
Four Tube Passes
Heat Exchangers

11. Heat Exchangers
Chee 318

12. Heat Exchangers

13. TEMA AES Exchanger

14. TEMA Designations

15. Heat Exchangers
Chee 318

16. Heat Exchangers
Chee 318

17. Heat Exchangers
Chee 318

18. Cross-Flow Heat Exchangers
Unfinned -
One Fluid Mixed
the Other Unmixed
Finned -
Both Fluids
Unmixed
Heat Exchangers

19. Compact Heat Exchangers
Widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas.
Characterized by large heat transfer surface areas per unit volume (>700 m2/m3), small flow passages, and laminar flow.
Heat Exchangers

20. Heat Exchangers
Chee 318

21. Heat Exchangers
Chee 318

22. Baffles
How do baffles help? Where are they installed and which fluid is directly affected? Common practice is to cut away a segment having a height equal to one-fourth the inside diameter of the shell. Such baffles are called 25 percent
baffles.

23. Baffle Arrangement

24. Tube sizes Tubes Standard tube lengths are 8, 12, 16 and 20 ft.
Tubes are drawn to definite wall thickness in terms of BWG and true outside diameter (OD), and they are available in all common metals.

25. Tube Pitch
The spacing between the tubes (center to center) is referred to as the tube pitch (PT). Triangular or square pitch arrangements are
used. Unless the shell side tends to foul badly, triangular pitch is Used.

26. •Heat Exchanger (HEX) Rating
Checkingthe existing design for compatibility with the user requirements (outlet temperature, heat load etc.)
given: flow rates, inlet temperatures, allowable pressure drop; thus HT area and passage dimensions
find: heat transfer rate, fluid outlet temperatures, actual pressure drop
•HEX Sizing
Thermal and pressure drop considerations, maintenance scheduling with fouling consideration.
given: inlet and outlet temperatures, flow rates, pressure drop
find: dimensions -type and size of HEX
Heat Exchangers

27. steady-state, steady flow• no heat generation in the HEX
Assumptions for Basic Design Equations for Sizing
steady-state, steady flow•
no heat generation in the HEX
• negligible ΔPE, ΔKE
• adiabatic processes
• no phase change (later)
• constant specific heats and other physical properties.
Heat Exchangers

28. Heat Exchanger Analysis
LMTD Method
Expression for convection heat transfer for flow of a fluid inside a tube:
For case 3 involving constant surrounding fluid temperature:
Heat Exchangers

29. Heat Exchanger Analysis
In a two-fluid heat exchanger, consider the hot and cold fluids separately:
(11.1)
and
(11.2)
Need to define U and DTlm
Heat Exchangers

30. Concentric Tube Construction• • - : :
Heat Exchangers

31. Heat Exchangers

32. Heat Exchangers

33. Heat Exchangers

34. Heat Exchangers

35. Heat Exchangers

36. Heat Exchangers

37. Special Operating Conditions
Condenser:
Hot fluid is condensing vapor (eg. steam)
Evaporator/boiler:
Cold fluid is evaporating liquid
Heat Exchangers

38. Heat Exchangers

39. Heat Exchangers

43. Overall Heat Transfer Coefficient
For tubular heat exchangers we must take into account the conduction resistance in the wall and convection resistances of the fluids at the inner and outer tube surfaces.
(11.3a)
Note that:
where inner tube surface
outer tube surface
Heat Exchangers

44. Heat Exchangers

45. Fouling
Heat exchanger surfaces are subject to fouling by fluid impurities, rust formation, or other reactions between the fluid and the wall material. The subsequent deposition of a film or scale on the surface can greatly increase the resistance to heat transfer between the fluids.
An additional thermal resistance, can be introduced: The Fouling factor, Rf.
Depends on operating temperature, fluid velocity and length of service of heat exchanger. It is variable during heat exchanger operation.
Typical values see Heat Transfer for Kern .
The overall heat transfer coefficient can be written:
(11.3b)
Heat Exchangers

46. Heat Exchangers

47. Heat Exchangers

48. Fin (extended surface) effects
Fins reduce the resistance to convection heat transfer, by increasing surface area.
Expression for overall heat transfer coefficient includes overall surface efficiency, or temperature effectiveness, ho, of the finned surface, which depends on the type of fin (see also Ch )
(11.3c)
where c is for cold and h for hot fluids respectively
Heat Exchangers

49. Example 1
Heat Exchangers

50. Heat Exchangers

51. Heat Exchangers

52. ُExample 2
Heat Exchangers

53. Heat Exchangers

54. Heat Exchangers