1. HEAT EXCHANGERS
石 岩
March 21, 2005

2. Content Instruction Heat Transfer and Fluid Flow Heat Exchangers
Heat Exchangers and Systems
Summary

3. Instruction
Heat transfer is an important function of many industrial processes
Transfer heat from one process to another
Types of heat exchangers
pipe coil exchangers, double-pipe heat exchanger, shell and tube heat exchanger, reboiler, plate and frame heat exchanger, air cooled heat changer, spiral heat exchangers

4. Heat Transfer and Fluid Flow
Conduction
Convection
Radiant
Laminar
Turbulent

5. Conduction Heat energy is transferred through solid objects
Molecules absorb heat energy from a hotter source and then vibrate, so transfer the energy from the hot side to the cooler side
What is more, there are free electrons moving in the metal, so metal have better capability of conduction

7. Convection
Convection occurs in fluids when warmer molecules move toward cooler molecules
The movement of the molecules sets up currents in the fluid that redistribute heat energy
The result is that the energy is equally distributed

8. Radiant Heat Transfer Heat is conveyed by electromagnetic waves
Radiant heat transfer is a line-of-sight process
Radiant heat transfer is not used in a heat exchanger

9. Laminar Flow Liquid flowing are in parallel fashion
Laminar flow usually exists at low flow rate
Laminar flow promotes the development of static film, which acts as an insulator

10. Turbulent Flow Turbulent flow is random movement or mixing of fluids
Turbulent flow allows molecules of fluid to mix and absorb heat more readily than does laminar flow
Turbulent flow decreases the thickness of static film, increasing the rate of heat transfer

11. Heat Exchangers Double-pipe heat exchanger
Shell and tube heat exchanger
Plate and frame heat exchanger
Spiral heat exchanger
Pipe coil exchanger
Air-cooled heat exchangers

12. Double-Pipe Heat Exchanger
A double-pipe exchanger has a pipe inside a pipe
The outside pipe provides the shell. And the inner pipe provides the tube
The warm and cool fluids can run parallel flow or countercurrent,because it is more efficient
Flow rates are very low in a double-pipe heat exchanger

14. Double-Pipe Heat Exchanger
The chemical processing industry commonly uses hairpin heat exchangers
Hairpins are typically rated at 500psig shellside and 500 psig tubeside
Fins can be added to the internal tube’s external wall to increase heat transfer

15. Double-Pipe Heat Exchanger
Advantages:
Its excellent capacity for thermal expansion
It is easy to install and clean
Its modular design makes it easy to add new sections
Replacement parts are inexpensive and always in supply
Disadvantages
It is not as cost effective as most shell and tube exchangers
it requires special gaskets

16. Shell and Tube Heat Exchangers
The shell and tube heat exchanger is the most common style found in industry
As the tubeside flow enters the exchanger, flow is directed into tubes that run parallel to each other. these tubes run through a shell that has a fluid passing through it
Heat energy is transferred through the tube wall into the cooler fluid
Heat transfer occurs primarily through conduction and convection

19. Shell and Tube Heat Exchanger ---Head
The head can be classified as front-end or rear-end types, the front-end head has five primary designs, the rear-end has eight possible designs

20. Shell and Tube Heat Exchanger ---Shell
In most cases, the shell is designed to withstand the greatest temperature and pressure condition
The shell is the largest single part of the heat exchanger
The shell can be classified as single-pass, double-pass, split flow, double split flow, divided flow, cross-flow

22. Shell and Tube Heat Exchanger ---Tube
Tube can be plain or dinned
Plain tubes are commonly used in fabrication
Finned tube are starting to make an impact, fins can be located externally or internally
Tube materials include brass, carbon, …

24. Shell and Tube Heat Exchanger ---Tube Sheet
Tube sheets are often described as fixed or floating, single or double
Tube sheets have carefully drilled hole, the ends of the tubes in a heat exchanger are fixed by rolling, welding, or both
Double tube sheets are used to prevent tubeside leakage of highly corrosive fluids

25. Shell and Tube Heat Exchanger ---Tube Sheet
Engineering specifications take into account thermal tube expansion
If the tube sheet is welded or bolted to the shell, it is called fixed
If the tube sheet is independently secured to the tub head and is allowed to move freely inside the shell, it is called floating

26. Shell and Tube Heat Exchanger ---Baffle
Baffles provide the framework to support and secure the tubes and prevent vibration
The baffle layout increases or decreases fluid and directs flow at specific points
Tubeside baffles are built into the heads to direct tubeside flow
In multipass exchangers, cost goes up with each pass, provide adequate fluid velocities to prevent fouling and to control heat transfer

28. Shell and Tube Heat Exchanger ---Baffle
Each segmental baffle supports half of the tubes
Baffles are evenly spaced
Segmental baffles may be horizontal or vertical cut
Systems transferring large quantities of suspended solids may use vertical arrangement, which allows liquid and solids to flow around baffles
Horizontal baffles are used in clean service with notches at the bottom to allow liquid drainage on removal from serve

29. Shell and Tube Heat Exchanger ---Baffle
Impingement baffles are used to protect tubing from direct fluid impact
Longitudinal baffles are used inside the shell to split or divide the flow, increase velocity, and provide superior heat transfer capabilities, longitudinal baffles do not extend the entire length of the exchanger since at some point the fluid must flow around it

30. Shell and Tube Heat Exchanger ---Tie Rod
Tie rods and concentric tube spacers keep the baffles in place and evenly spaced
Each hole in the baffle plates is 1/64” larger than the tube’s outside diameter

31. Shell and Tube Heat Exchanger ---Nozzles and Accessory Part
Inlet and outlet nozzles are sized for pressure drop and velocity considerations
Thermowells, pressure indicator connections, safety and relief valves, product drains, vents, block valves and control valves

32. Shell and Tube Heat Exchanger ---Fixed Head
In a fixed head, single pass shell and tube heat exchangers, the tubes are connected to two tube sheets which are firmly attached to the shell
in the multipass Shell and tube heat exchangers, the baffle added to the channel head and the lack of a tube side outlet on the discharge head
Temperature differential is less to 200℃

33. Shell and Tube Heat Exchanger ---Floating Head
One side of the tube bundle is fixed to the channel head, the other side is unsecured
Floating head exchangers, with their high cross-sectional areas(fins), are designed for high temperature differentials and high flow rate, produces the highest heat transfer efficiency

34. Shell and Tube Heat Exchanger ---U-Tube
The tube sheet connects a series of tubes bent in a U shape, the ends of the tubes are secured to the tube sheet
The total number of tubes is limits
Large temperature differentials
Each complete U tube has a single fundamental frequency

35. Plate and Frame Heat Exchanger
It consist of a series of gasketed plates, sandwiched together by two end plates and compression bolts

37. Plate and Frame Heat Exchanger
Easy to disassemble and clean and distribute heat evenly so there are no hot spots
Plates can easily be added or removed
Low fluid resistance time, low fouling, high heat transfer

38. Plate and Frame Heat Exchanger
If gaskets leak, they leak to the outside, and gasket easy to replace
Prevent cross-contamination of products
High turbulence and large pressure drop and small
This device is best suited for vicious or corrosive fluid slurries
High-pressure and high-temperature limitations (2.5MPa and 180℃) for protect internal gasket
Gaskets are easily damaged and may not be compatible with process fluids

39. Spiral Heat Exchanger
Spiral heat exchangers are characterized by a compact concentric design that generates high fluid turbulence
Type 1: spiral flow on both sides
Type 2: spiral flow-cross-flow

41. Pipe Coil Exchanger
Pipe coils are submerged in water or sprayed with water to transfer heat
This type of operation has a low heat transfer coefficient and requires a lot of space

43. Air-Cooled Heat Exchanger
Air-cooled heat exchangers provide plain or dinned tubes connected to an inlet and return header
Air is used as the outside medium to transfer heat away from the tubes
Fans are used in a variety of arrangements to apply forced convection for heat transfer coefficients
Provides a 40℃ temperature differential between the ambient air and the exiting process fluid

45. Air-Cooled Heat Exchanger
simple to construct and cheaper to maintain
Cannot fouling or corrosion
Low operating costs and superior high temperature removal
Limited in use
High outlet fluid temperature and high initial cost of equipment
In cases of loss of containment, they would be fire or explosion

46. Heat Exchangers and Systems ---Parallel and Series Flow
in series flow, the tubeside flow in a multipass heat exchanger is discharged into the tubeside flow of the second exchanger
In parallel flow, the process flow goes through multiple exchangers at the same time

47. Heat Exchangers and Systems ---Cooling Towers
The system consists of a cooling tower, heat exchanger, pump
Cooling water is pumped into the shellside of a heat exchanger and returned (much hotter) to the top of the cooling tower

48. Heat Exchangers and Systems ---Reboilers, Distillation Column
Rebolier---add heat to a liquid that was once boiling until the liquid boils again
Associated with the operation of a distillation column, which is energy of heat balance, reboilers are used to restore this balance by adding additional heat for the separation processes

49. Heat Exchangers and Systems --- Reboilers, Distillation Column
Take kettle reboiler for example, reboilers take suction off of the bottom products and pump them through their system, so column temperatures are controlled at established set-points
Other type of reboilers:
vertical and horizontal thermosyphon reboilers
stab-in reboilers
hot oil jacket reboilers

51. summary
The methods of heat transfer are conduction, convection, and radiation
The fluid flow are laminar and turbulent
The best conditions for heat exchangers are large temperature difference,high heating or coolant flow rates, large cross-sectional area of the exchanger, and others, so which heat exchanger is choose, it depends

52. Thank for your lessening