HEAT EXCHANGERS

石 岩

March 21, 2005

Content  Instruction  Heat Transfer and Fluid Flow  Heat Exchangers  Heat Exchangers and Systems  Summary

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

Heat Transfer and Fluid Flow  Conduction  Convection  Radiant  Laminar  Turbulent

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

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

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

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

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

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

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

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 to increase heat transfer tube’s external wall

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

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

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

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

    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, …

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

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

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

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

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

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

Shell and Tube Heat Exchanger ---Nozzles and Accessory Part  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

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 200 ℃ differential is less to

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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