What are Heat Exchangers?

HEAT EXCHANGERS:

Heat Exchangers are devices designed to transfer or exchange heat between two or more fluids (liquids, vapors or gases), which are flowing at two different temperature levels.

Depending on the type of heat exchanger, the heat transferring process can be gas-to-gas, liquid-to-gas or liquid-to-liquid and occur through a solid separator. Which prevents mixing of the fluids, or direct fluid contact.

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I.???Principle of Heat Exchanger:

·????????The heat energy will transfer naturally from hot to cold.

·????????The difference in temperature will allow a flow of heat to take place until a uniform temperature reaches.

In production and process facilities the transfer of heat energy takes place in equipment called heat exchangers. The addition of heat takes place in heaters and the extraction of heat takes place in coolers

?I.???????Heat Transfer Rate:

The basic equation of Heat Transfer Rate is:

Q = U A ▲T LMTD

Q = Heat Transfer Rate

U = Over all Heat Transfer Coefficient

▲T LMTD = Log Arithmetic Mean Temperature Difference

The rate at which heat is transferred between the fluids must be considered. The main factors affecting heat transfer are:

·????????Temperature Difference

·????????Surface Area

·????????Transfer Coefficient

·????????Velocity of Flow

?II.???????Factors Affecting Heat Exchange:

1.???Surface Area:

Surface are affecting the flow of heat is the area of heat exchanger.

The basic principle is expressed as follows:

·????????The larger the surface area available for exchange, the greater the amount of heat exchange.

·????????An exchanger with 10 tubes would obviously transfer twice as much heat as one having 5 tubes. The area of an exchanger is the total external area of the tubes in the unit.

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2.???Temperature Differentials:?

The cold medium is entering from the one end and hot medium is entering from the other side.

·????????Temperature gain by cold medium = t2 - t1

·????????Temperature loss by hot medium = T2 - T1

The two-temperature difference between hot and cold is arrange in a such a way that Q = U A ▲T LMTD, influence on the total heat transfer rate.

3.???Temperature Difference:

The amount of heat transfer varies directly with the temperature difference of the hot and cold fluid.

To achieve better heat transfer use different flow patterns

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4.???Heat Transfer Coefficient:

Some materials conduct heat better than others. The thinner the material the better the heat transfer. The properties of the material from which the heat exchanger is made are used to calculate the coefficient of an exchanger. It is a measure of the amount of heat that can transfer in one hour across 1ft2 of exchanger area for each oF temperature difference between the hot and cold fluid.

The above table describes thermal conductivity of various metals and how the various temperature varies will affect the overall heat transfer coefficient.

5.???Velocity of Flow:

If the flow of each liquid is slow, the temperature will be about the same and heat transfer slow. If we increase the velocity so that the flow is turbulent. We have the maximum temperature difference and great heat transfer.

Exchangers are designed so that flow will be turbulent region.

Types of velocity of flow:

·????????Laminar flow

·????????Flows in straight line until a critical velocity is reached.

·????????Turbulent flow region

·????????Inner layer is stationary.

·????????Inner flow is turbulent.

?Exchangers are designed to that flow will be turbulent.

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6.???Flow Pattern:

Parallel Flow: It is a most used patten.

Both mediums flow through their respective containers in the same direction.?

Counter current flow: In counter current flow both fluids pass through the other directions.

The two mediums flow through their respective containers in the opposite direction.

Process Heat Duty:

All heat exchangers used in the oil and gas industry are required to transfer a certain amount of heat per unit of time, depending on the application. This is called the Heat Duty of exchanger and is usually expressed in British Thermal Units per hour (Btu/hr).

Heat Transfer = Mass x Temp Change x Heat Capacity

Heat Exchanger Types – Overview:

Different types of heat exchangers are using for heating and cooling purposes.

Shell and Tube Heat Exchanger:

The fluid will pass from shell side and other fluid will passes through the tube side. Arranging and exchanging of heat will causes efficient mechanism. Applications are Heating and Cooling purpose.

Forced Draft Air Cooler:

This type of heat exchanger is commonly known as a fin fan cooler.

It pushes ambient air up through the exchanger tubes.

Mainly on cooling purpose using air as cooling media.??

Radiator Heat Exchanger:

A plate type heat exchanger, in this case using sea water is used to cool plant rotating the equipment.

Re-Boiler Heat Exchanger:

This is used to heat products used in columns. Heating in high temperatures. Most of the cases one fluid changing its phase to vapour.

Used HTF as the hot medium. Normally thermo-syphon type principle is using.

Condenser:

It is just opposed to free boiler i.e. hot fluid is condensing high temperature vapor phase to liquid phase and cold fluid is from the low temperature to high temperature is increasing. Heat is receiving from the condensing fluid to cold fluid.

The function of the condenser is to change a process fluid from the vapor to the liquid phase. The cooling fluid may be water, air or another special auxiliary fluid, or even another process fluid which needs preheating.

Heat Exchanger Passes:

For efficient process of heat exchanger, the number of passes used through the tube and shell side for better efficiency of the heat exchanger.

·????????One fluid of flows through the tube side.

·????????The other fluid flows through the shell side.

·????????Various flow patterns and passes increased better heat transfer.

·????????Selection fluids through shell side or tube is based on various factors.

Heat Exchangers – Tube Side Flow Passes:

One pass circulation:

The tube medium passes once only through the tubes.

?Two pass circulation:

The tube medium passes travel the length of the tubes twice. This gives twice the opportunity to be cooled against the shell medium.

Multi pass circulation:

The tube medium now receives four exchanges of heat from the shell medium.

?Heat Exchangers – Shell Side Flow Passes:

Shell two pass circulation:

The longitudinal baffle forces the shell medium to travel twice over the length of the tubes.

Two pass split circulation:

Additional turbulence is gained when the flow strikes the baffle and splits left and right.

Five pass split circulation:

This enables the shell flow to change direction five times creating turbulence and giving extra gain in heat transfer.

Vertical baffles are using create multi passes and turbulence in this case.

Multi shell and tube passes:

·????????3 Tube passes

·????????9 Shell passes

·????????Much more turbulence is created

Both mediums pass many times over each other

?Shell and Tube Heat Exchangers:?

Shell and Tube heat exchanger is most common type of heat exchanger in industrial applications.

They contain many tubes (sometimes several hundred) packed in a shell with their axes parallel to that of the shell. Heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes through the shell.

Shell and Tube heat exchangers are further classified according to the number of shell and tubes passes involved

Shell and tube exchangers have two major components.

A tube bundle containing the tubes and through which the tube side flows.

A shell that encases the tube bundle and through which the shell side flows.

Shell:

The shell is the support framework of the tube bundle and consists of;

·????????Cylindrical casing

·????????Flanges

·????????Nozzles

·????????Supports

·????????Cover

????U-Tube Heat Exchanger:

It has one U-Tube shell. The flow is entering into the inlet and will pass through the U-Tube section and going back to outlet. It is a two pass and bundle of U-Tubes are arranged inside the shell such a way tubes can be removed for cleaning purpose.

???Straight Tube (1-Pass) Heat Exchanger:

In this exchanger Tube bundles are in a straight-line tube sheet and associated with Inlet and Outlet.

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????Straight Tube (2-Pass) Heat Exchanger:

In this exchanger Tube bundles are also in a straight-line tube sheet and associated with 2-Pass Outlet.

????Double Pipe Heat Exchanger:

In this heat exchanger one fluid flows inside the inner tube. The second flows in the space between the larger outer tube and the smaller inner tube which contains the first fluid.

Advantages:

·????????Low cost

·????????Ease of maintenance

·????????Ease of adding additional units

·????????Multi-purpose

Disadvantages:

·????????May not perform to process requirements – unit is a standard package.

·????????Limited selection of metals – limited application.

·????????Prone to blockages.

????Design Details of Shell and Tube Heat Exchanger:

·????????2-fixed tube sheets

·????????Locked at both ends

·????????Bundle not removable

·????????Tube externals can only be cleaned chemically (acid cleaning standard method).

Fixed tube-sheet:

·????????Tube-sheet is fixed between the channel cover flange and the shell flange.

·????????Cannot thermally expand or contract.

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Shell and Tube types:

Fixed tube-sheet, Shell expansion bellows

·????????Tube sheets cannot move.

·????????The shell expansion joint allows the shell to thermally expand and contract.

Shell and Tube types:

Fixed tube-sheet, Shell expansion bellows

·????????Tube sheet is fixed.

·????????The shell can be expanded and contact because of the bellows movement.

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Shell and Tube types:

Floating head

·????????One tube sheet is fixed.

·????????One tube sheet is free to thermally expand and contract.

·????????Wasted space between shell and tubes.

Shell and Tube types:

Floating head

·????????The outer diameter of the tube sheet is smaller than the shell inside diameter.

·????????The tube bundle is free to thermally expand and contract.

Shell and Tube types:

Floating head, Split ring type

·????????Free to thermally expand and contract.

·????????No wasted space between the shell and tubes.

Shell and Tube types:

Split ring type

·????????The ring splits in half.

·????????The floating head cover is removed.

·????????The bundle can be withdrawn from the opposite end.

Shell and Tube types:

Gaskets are used in various types for Shell and Tube heat exchanger.

1.???Floating head

2.???Shell cover

3.???Shell to tube sheet

4.???Tube sheet to channel box

5.???Channel box cover

Shell and Tube types:

Floating head, Gland ring type.

·????????The tube sheet floats along the gland ring which is sealed from both mediums by two dynamic O rings.

Shell and Tube types:

U tube (hairpin) type.

·????????Fixed at one end

·????????Free to float at the other end

·????????One bolted header box.

·????????One welded header box.

Application of Shell and Tube heat exchangers.

The process equipment and heat exchanger are connected, and liquid is entering, and vapors are leaving from heat exchanger and entering to the process vessel.

Shell and Tube types:

Re-boiler types

Horizontal types

·????????Used to heat column products

·????????Use a thermo-syphon affect

Materials used in Heat Exchangers:

Materials for construction of heat exchangers and corrosion explained as follows:

·????????Low coefficient of thermal expansion.

·????????High heat transfer coefficient.

·????????High corrosion resistance.

·????????Low initial cost.

·????????Low production cost.

·????????Low maintenance cost.

A high corrosion resistance of material will be a necessity.

Generally, the two most commonly selected materials for heat exchangers are aluminum copper.

Both metals have the optimum thermal properties and corrosion resistance to make them ideal choices.

Furthermore, copper has lower flexibility than aluminum, making it more difficult to form into certain shapes.

Aluminum is a better fit for the fins and also far more lightweight, malleable and flexible than copper.

However, aluminum does typically have lower sag resistance than copper. So aluminum is used to bring the metal’s properties close to that of copper, without significantly increasing the price.

Other materials used in heat exchangers:

·????????Alloy steel.

·????????Low Carbon Steel.

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Alloy Steel: Alloy steel is used in case of high temperature (500 °C).

?? Low Carbon Steel: It is used for handling corrosive fluids.

Codes and Standards:

A standard can be defined as a set of technical definitions and guidelines.

The following codes and standards are legal requirements form an integral part of the specification. Codes and standards are of the latest editions specified.

1.???Design, Fabrication, and Inspection

a)??????????ASME Boiler and Pressure vessel section VIII, Div.1.

b)??????????TEMA 9th edition, class R.

2.???Materials

a)??????????ASME Boiler and Pressure vessel section II.

b)??????????ASTM Standards

c)??????????NACE MR 0130

d)??????????Other equivalent codes & standards will be subject to owner’s approval.

3.???Threads

a)??????????ISO Metric system shall be used except for bolting ANSI Standard flange, which can be unified thread.

????I.???????Advantages of Heat exchangers:

·????????High heat transfer - turbulence on both sides

·????????Cost - low because plates are thin

·????????Accessibility - can easily be opened for inspection and cleaning

·????????Flexibility - Extra plates can be added

·????????Short retention time with low liquid inventory hence good for heat sensitive or expensive liquids

·????????Less fouling - low r values often possible

?II.???????Disadvantages of Heat exchangers:

·????????Pressure - maximum value limited by the sealing of the gaskets and the construction of the frame.

·????????Temperature - limited by the gasket material.

·????????Capacity - limited by the size of the ports

·????????Block easily when solids in suspension unless special wide gap plates are used

·????????Corrosion - Plates good but the gaskets may not be suitable for organic solvents

·????????Leakage - Gaskets always increase the risk

·????????Fire resistance - Cannot withstand prolonged fire (usually not considered for refinery duties)

For more information please contact JMS-Johar Manufacturing Services;?[email protected] ?or call +971545759109