HEAT EXCHANGERS 石 岩 March 21, 2005 Content l Instruction l Heat Transfer and Fluid Flow l Heat Exchangers l Heat Exchangers and Systems l Summary.

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 flow promotes the development of static film, which acts as an insulator

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 tube’s external wall to increase heat transfer

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 s

olids 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 differential is less to 200 ℃

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 freq

uency

Plate and Frame Heat Exchanger It consist of a series of gasketed plates, sandwiched t

ogether 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 that generates high fluid turbulence

Type 1: spiral flow on both sides

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 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

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 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

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 tak

e 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

Thank for your lessening