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Heat Exchanger

heat exchanger banner 4Heat exchanger, abbreviated as HE or HEX, is a device used to transer heat from one medium to another at different temperatures.  The heat transfer can be air or a liquid such as water or oil.

A heat exchanger has many functions.  They are critical to the process industries as they used to condense, heat, cool, or chill a fluid.  It also can be used to change the phase of a fluid (liquid to vapor or vapor to liquid) like a boiler.  Heat exchangers can be a direct contact HEX or an indirect contact type HEX. 

In a direct contact heat exchanger, heat transfer continuously occurs by the mixing of two process streams.  An example of this is a cooling tower, commonly found in power plants where a stream of hot air is consistently mixed with cooler water.  The water cools the hot air and exits the top of the cooling tower.  Warm water exits the bottom of the tower, is recollected, cooled and used again. 

An indirect contact type heat exchanger transfers heat by maintaining a separation between the two process streams.  Examples of an indirect contact type HEX are Shell and Tube Heat Exchangers and Air Cooled Heat Exchanger (also known as a fin-fan)

 

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Heat Exchanger Index

 

Heat Exchanger Design Classification

The construction of the heat exchanger is to exchange or transfer heat between the fluids used.  The function of heat exchanging device depends on its purpose of use.  It can be used either to heat a fluid or even to cool down a fluid.

  • Extended Surface Heat Exchanger  -  The design of finned surfaces enables to improve the heat transfer coefficient by distributing the fluid flow apart from increasing the surface area.  Gas flow patterns separates moisture efficiently.  Selection of the best combination of materials suitable for process and required duty conditions ensures most economic design allied with the longer service life.  Additional surface protection against corrosion can also be incorporated in the form of galvanizing on steel.
  • Finned Plate Heat Exchanger  -  This exchanger consist of a series of fin surfaces sandwiched between sheets stacked together.  The fin can increase efficient heat transfer area and transfer heat to fluid through the sheets.
    • Fin-fan Heat Exchanger (Air Cooled Heat Exchanger)  -  This heat exchanger forces air over a set of coils to cool the process.
      • Horizontal Forced Draft Air Cooled  -  The fans are located below the bundle and push air upwards.
      • Horizontal Induced Draft Air Cooled  -  The fan is located above the tube bundle and air is induced or pulled through the finned tubes.
      • Vertical Air Cooled  -  The fan is located at the front of the bundle and the air is forced through vertically.
      • A Frame Forced Draft Air Cooled  -  The fans are located below the bundle, the bundle is split in two, the left and right side, an A shaped frame, then the air is forced towards the bundles.
      • V Frame Induced Draft Air Cooled  -  The fans are located above the bundle, the bundle is split in two, the left and right side, in a V shaped frame, then the air is pulled through the finned tubes.
  • Finned Tube Heat Exchanger  -  This is a tube with extended outer surface area or fins to enhance the heat transfer rate from the additional area.  Finned tubes with extended outer surface area enhance the heat transfer rate by increasing the effective heat transfer area between the tubes and surrounding fluid.  The fluid surrounding finned tubes maybe process fluid or air.
    • Longitudinal Finned Tube  -  These tubes are best suited for applications where the flow outside the tubes is expected to be streamlined along the tube length.
      • Double Pipe Heat Exchanger
    • Traverse Finned Tube  -  They normally used for gas flows or turbulent flows and for cross flow type exchangers or shell and tube heat exchangers.
      • Shell and Tube Heat Exchanger
  • Plate Heat Exchanger  -  Metal plates are used to transfer heat between two fluids. The plate is a metal shell, with spaces inside each plate that act as hallways for fluids to travel through.  With a plate heat exchanger, there is a greater surface area in contact with the fluids, so it has better rates of heat transfer compared to all other types.  Although plate heat exchangers can be more expensive, the efficiency gained by the design is a big plus.
    • Brazed Plate  -  These are used in many industrial and refrigeration applications.  Since they use stainless steel plates with copper brazing, they are highly resistant to corrosion.
    • Gasketed Plate  -  These exchangers use high-quality gaskets to seal plates together and protect against any leaks.  Plates can easily be removed for cleaning, expansion, or replacement.
    • Welded Plate  -  The welded plate is similar to gasket plate heat exchangers, but all the plates are welded together.  Since the plates are welded together, cleaning the plates isn’t possible with plate and frame heat exchangers.
    • Semi-welded Plate  -  This exchanger consist of a mixture of welded and gasketed plates.  The two pairs of plates are welded together and then gasketed to the other joints.
  • Regenerative Heat Exchanger  -  A type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid.  It has a single flow path in which the hot and cold fluids alternately pass through.  This exchangers normally increase in efficiency with the time allowed for the temperature exchange to occur and so it can run at lower temperatures.  A recouperator exchanger normally requires a higher temperature metallic or plastic heat exchanger but can be more effective over shorter exchange periods.
    • Static (Fixed Matrix or Fixed Bed)  -  This type transfers heat between two fluid streams.  The fluid streams flow in and out of the matrix through different ports, with the direction of flow being controlled by valves or other mechanisms.
    • Dynamic (Rotary)  -  This use a rotating wheel or drum to transfer heat between two fluid streams.
  • Spiral Plate Heat Exchanger (Coil)  -  The spiral exchanger is made by rolling two long metal plates around a center core to form two concentric spiral flow passages, one for each fluid.  The plate edges are welded shut so that each fluid stays within its own passage and there is no flow bypassing or intermixing.  Channel plate width and spacing are optimized for the specified duty, maximum heat transfer, and ease of access.  The plate gap is maintained by welded spacer studs although some designs do not require them.
    • Spiral Flow Both Sides
    • One Side in Cross Flow, One Side in Spiral Flow
    • Combined Cross - Spiral Flow
  • Tubular Heat Exchanger  -  This exchanger is used to warm or cool a product by exchanging heat between the process fluid and a service fluid.  The hot or cold service fluid flows around the tubes within the exchanger and heat is transferred between the product and the media.  The rate of heat transfer is dependent on the tubular heat exchanger configuration, temperature differential, size of tubes, turbulence in the flow and flow velocity.
    • Double Pipe Heat Exchanger  -  Used for the transfer or exchange of heat between two liquids without mixing them.  They consist of two or more concentric, cylindrical pipes or tubes, one larger and the other smaller.  One fluid flows inside the tube and the other fluid flows outside.
    • Pipe Coil Heat Exchanger  -  Coil exchangers are frequently used for heating or cooling product in processing plant tanks and vessels.  Tube coils provide an economical and straightforward method of obtaining heat transfer surface area.  Tube coils are made by rolling lengths of tubing into helixes or double helixes in which the inlet and outlet are located side by side.
    • Shell and Tube Heat Exchanger  -  Has multiple tubes in which the two working fluids exchange heat by thermal contact which is placed within a cylindrical shell.  As such, one fluid flows inside the tubes and the other through the shell.
      • One Pass Shell
      • Two Pass Shell
      • Split Flow
      • Double Split Flow
      • Divided Flow
      • Kettle - Reboiler
      • Cross Flow
    • Spiral Tube Heat Exchanger  -  A coil assembly fitted in a compact shell that optimizes heat transfer efficiency and space.  Spiral coil assembly has welded tube to manifold joints and uses stainless steel as a minimum material requirement for durability and strength.  The coil assembly is welded to a head and fitted in a compact shell.  The spaces or gaps between the coils of the spiral tube bundle become the shell side flow path when the bundle is placed in the shell.  Tube side and shell side connections on the bottom or top of the assembly allow for different flow path configurations.

 

Heat Exchanger Design Consideration

  • Transfer Process  -  Heat exchangers are classified into direct and indirect contact type exchangers.
    • Direct Contact  -  The two fluids are not separated by a wall.  The fluids come into direct contact to exchange heat between each other and exit through their respective outlet nozzles or line.  Due to direct heat exchange, very close temperatures can be attained.
    • Indirect Contact  -  The fluid streams remain separate, and the heat transfer takes place continuously through a separating wall.
  • Flow Arrangement  -  The choice of a particular flow arrangement is dependent upon required exchanger effectiveness, exchanger construction type, upstream and downstream ducting, packaging envelope, and other design criteria.
    • Counter Flow  -  The direction of the fluid is parallel but opposite to the other are known as counter current type heat exchangers.  They usually have the highest heat exchange and greatest temperature change.
    • Cross Flow  -  The flow of the fluids inside the heat exchanger is perpendicular to each other, they are called cross flow.  In terms of efficiency, a cross-flow heat exchanger gives better efficiency than the parallel flow configuration type heat exchanger.
    • Parallel Flow  -  A double pipe heat exchanger can be operated in parallel flow.  Similarly a shell and tube heat exchanger can be operated in approximately parallel flow by having both fluids enter at one end and exit at the other end.  With parallel flow the temperature difference between the two fluids is large at the entrance end, but it becomes small at the exit end as the two fluid temperatures approach each other.
    • Spiral Flow  -  These exchangers use thin plates or tubes to keep two fluids of different temperatures apart while allowing energy to flow from hot to cold through the wall.  The energy transfer acts to change the temperatures of the two media.  The hotter one becomes cooler and the colder one becomes hotter.
  • Pass Arrangement  -  Refers to the movement of a fluid from one end of the heat exchanger to the other.
    • Single Pass  -  The fluid flows through the heat exchanger along its length once.
    • Multiple Pass  -  The fluid flows through the heat exchanger along its length and then it’s reversed to flow again through the whole length again.  In multi pass arrangements the flow can pass along the length of heat exchanger 2 or more times.  Multi pass arrangements are used when design of heat exchanger results in extreme length, significantly low velocities or low effectiveness.
  • Flow Rate  -  Flow rate determines the flow area.  The higher the flow rate, the higher will be the crossflow rate area.  Higher flow area is required to limit the flow velocity through the conduits and flow passage.  The higher velocity is limited by pressure drop, impingement, and erosion.  Sometimes, a minimum flow velocity is necessary to improve heat transfer to eliminate stagnant areas and to minimize fouling.
  • Pressure  -  The design pressure is important to determine the thickness of the pressure components.  The higher the pressure, the greater the required thickness.  The pressure level of the fluids has a significant effect on the type of unit selected.  At lower pressures, the vapor phase volumetric flow rate is higher and the low allowable pressure drops may require a design that maximizes the area available for flow.  At higher pressures, the vapor phase volumetric flow rates are lower and allowable pressure drops and greater.  Normally, higher heat transfer rates are obtained by placing the low pressure gas on the outside of tubular surfaces.
  • Tempereature  -  The effective temperature driving force is a measure of the actual potential for heat transfer that exists at the design conditions.  With a counterflow arrangement, the effective temperature difference is defined by the log mean temperature difference.  For flow arrangements other than counterflow arrangement, the log mean temperature difference must be corrected by a correction factor.
  • Temperature Differential  -  The difference between the hot fluid and coolant is very important when designing a heat exchanger.  The coolant always needs to be at a lower temperature than the hot fluid.  Lower coolant temperatures will take more heat out of the hot fluid than warmer coolant temperatures.
  • Tempereature Cross Over  -  This is a term used to describe the scenario where the temperatures of both circuits in a liquid cooled heat exchanger begin to cross over.  This can be an important factor in a heat exchanger design as the efficiency of a cooler will be significantly reduced when the temperatures cross over.
  • Heat Transfer  -  The transfer of heat in heat exchangers is a combination of both conduction and convection.  The driving force of heat transfer is the temperature difference between the inlet and outlet temperatures minus the inlet and outlet temperature of the process stream.
    • Temperature  -  The amount of heat or cold, but it is neither heat or cold.  Temperature is expressed as a number that is related to energy and porportional to a type of energy, but it is not energy.  Temperature is a number related to the average kineric energy, but is not kinetic energy.
    • Conduction  -  Heat transfer by conduction is the second most efficient form of heat transfer after mixing.  It is the flow of energy between two objects, or within one object, where there is a temperature differential.  Energy flow is from high energy to low (hot to cold).
    • Convection  -  Heat transfer by convection is the energy transfer of heat by air, water, or any other fluid to and object.  It includes energy transferred by random molecular motion as well as energy transferred by the bulk motion of the fluid.  There are two types of convecion: natural convection and forced convection.  Natural convection is fluid flow caused by changes to the fluid, such as change of density or temperature variations of the fluid.  Forced convection is convection that is caused by a a piece of equipment, such as a pump or a fan.
  • Phase of the Process Fluid  -  The phase within a unit is an important consideration in the selection of the heat exchanger type.  Liquid phase fluids are generally the simplest to deal with.  The high density and the favorable values of many transport properties allow high heat transfer coefficients to be obtained at relaticely low pressure drops.
  • Fin Surface Geometry  -  Transferring heat from a hot fluid into a colder fluid through a tube wall is the reason to use finned tubes.  The fluid with the lowest heat transfer coefficient will dictate the overall heat transfer rate.  When the heat transfer coefficient of the fluid inside the tube is several times larger than that of the fluid outside the tube, the overall heat transfer rate can be greatly improved by increasing the outside surface area of the tube.  Finned surfaces are manufactured by extruding, welding, or wrapping a thin metal sheet on a surface.  Fins enhance heat transfer from a surface by exposing a larger surface area to convection and radiation.  There are many different types of fins.
    • Longitudinally
    • Transverse
    • Herringbone
    • Louvered
    • Offset Strip
    • Perforated
    • Rectangular
    • Serrated
    • Triangular
    • etc.

 

Heat Exchanger Standards

API Standards

  • API Std 530 - Calculation of Heater-tube Thickness in Petroleum Refineries
  • API Std 660 - Shell-and-Tube Heat Exchangers
  • API Std 661 - Petroleum, Petrochemical, and Natural Gas Industries - Air-cooled Heat Exchangers

ASTM Standards

  • ASTM B395 / B395M - Standard Specification for U-Bend Seamless Copper and Copper Alloy Heat Exchanger and Condenser Tubes
  • ASTM B543 / B543M - Standard Specification for Welded Copper and Copper-Alloy Heat Exchanger Tube
  • ASTM B743 - Standard Specification for Seamless Copper Tube in Coils
  • ASTM B903 - Standard Specification for Seamless Copper Heat Exchanger Tubes With Internal Enhancement
  • ASTM B919 - Standard Specification for Welded Copper Heat Exchanger Tubes With Internal Enhancement
  • ASTM B944 - Standard Specification for Copper-Beryllium Welded Heat Exchanger and Condenser Tube (UNS No. C17510)
  • ASTM B956 / B956M - Standard Specification for Welded Copper and Copper-Alloy Condenser and Heat Exchanger Tubes with Integral Fins

 

Heat Exchanger Glossary

A

B

  • Baffle Plate  -  Tubes pass through this plate for support.  It provides a blocked path for the shell-side medium, forcing the medium across the tubes for better heat exchanger performance.
  • Baffle Spacing  -  The space between baffle plates on a tube bundle.
  • Batch Furnace  -  A heat treating furnace where the load enters the quench tank at predetermined times, all at once.
  • Bundle Assembly  -  The tube assembly in removable bundle heat exchangers and consists of tubes, tubesheets, baffles, spacers, and tierods.
  • Burnout Point  -  At this point in the boiling process, vapor will form to such an extent that it hinders contact between the liquid refrigerant and the heat transfer area, leading to lower heat transfer efficiency.

C

  • Cavitation  -  The creation and collapse of bubbles in a liquid.  Localized cavities of vapor form on the low pressure side of the pump (suction) and collapse on themselves as energy is added to the fluid.
  • Circulation Number  -  In flooded systems, the number of times a specific volume has to pass the evaporator to be fully evaporated.
  • Closed Temperature Program  - When the primary and the secondary fluid temperatures are close to each other.
  • Compressor  -  A device that forces air or gas into a smaller area increasing the volume and creating a usable force of energy.
  • Condenser  -  A device that transfers unwanted heat out of a refrigeration system to a medium (either air, water, or a combination of air and water) that absorbs the heat and transfers it to a disposal point.
  • Condenser Coil  -  A series or network of tubes filled with refrigerant, normally located outside the home, which removes heat from the hot, gaseous refrigerant so that the refrigerant becomes liquid again.
  • Corrosion  -  The thinning of a pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.  Examples of non metallic corrosion include the dissolution of ceramic materials or the discoloration and weakening of polymers by the sun's ultraviolet light.
  • Critical Heat Flux  - At this point, vapor will form to such an extent in the evaporator that it hinders contact between the liquid refrigerant and the heat transfer area, leading to lower heat transfer efficiency.
  • Critical Pressure  -  The pressure of substance at the critical point.
  • Critical Temperature  -  The temperature of substance at the critical point.

D

  • A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

  • Dew Point  -  The temperature at which air must be cooled to become saturated with water vapor.  It is porportional to the amount of water vapor in a given amount of air and when the dew point is raised the more water vapor present, also the opposite.

  • Downstream Pressure  -  The pressure exiting the outlet side of the heat exchanger.
  • Dry Bulb Temperature  -  The temperature of air measured by a thermometer shielded from radiation and moisture.

E

  • End Plate  -  Covers that have been welded to the end of the heat exchamnger.  Most end plates are used on bonnet assemblies.
  • Expansion Valve  -  A valve that maintains the pressure difference between the high-pressure and low-pressure sides in a condenser/evaporator system.
  • External Pressure Equalixation  -  An expansion valve with external pressure equalization compares the bulb pressure with the suction line pressure in order to control the superheat. Recommended in connection with BPHE evaporators due to the extra V-ring pressure drop.

F

  • Fin-fan Heat Exchanger  -  A fin-fan is a type of heat exchanger that forces air over a set of coils to cool the process.  It is also referred to as an air cooled heat exchanger.
  • Flow Rate  -  The amount of fluid that flows in a given time past a specific point.
  • Flue  -  Any vent or passageway that carries the products of combustion from a furnace.
  • Fluid  -  A substance that deforms and changes position when put under stress.
  • Fluid Pressure  -  Fluid at rest, exerts a force perpendicular to any surface in comes in contact with.
  • Foaming  -  Droplets of refrigerant entering the oil sump, where they are immediately evaporated.
  • Forced Circulation System  -  The forced-flow system is similar to a thermosiphon, but the forced-flow system has a pump installed before the evaporator.
  • Forward Temperature  -  Temperature of the hot fluid when it enters the device, radiator, or heat exchanger.

G

  • Gasket  -  A seal between two components, usually bolted.

H

  • Heat  -  A form of energy that causes physical change in what is being heated.  The lack of heat is cold.
  • Heat Gain  -  Heat added to the conditioned space by infiltration, solar radiation, occupant respiration and lighting.
  • Heat Transfer  -  The exertion of power that is created by heat, or the increase in temperature.  It is the transfer of heat from one system to another.  There are four ways to transfer heat: conduction, convection, mixing, and radiation.
  • Heat Transfer by Conduction  -  It is the flow of energy between two objects, or within one object, where there is a temperature differential.
  • Heat Transfer Rate  -  The amount of hear transfered per unit of time per fluid or material.
  • Hydraulic Block  -  A boiler, block consisting of connections to the Brazed Plate Heat Exchanger, instead of pipes, in order to save space. It contains a number of functions, such as threeway valve, bypass valve, fill-up valve and temperature sensors.

Il

  • A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

  • Inlet Pressure  -  See upstream pressure

J

K

L

  • Latent Heat  -  The energy absorbed or released by a substance during a constant temperature or phase change from a solid to liquid, liquid to gas or vise versa.
  • Line Pressure  -  The pressure in the supply line.
  • Liquid Hammer  -  In compressors, if a considerable amount of liquid enters the compressor house, a very large pressure can be built up when the piston reaches its top position. This phenomenon is called liquid hammer, and may cause severe damage to the valves or crankshaft.

M

N

  • Nipple  -  A short length of pipe either fully threaded (close nipple) or  thread both ends.

O

  • Oil Separator  -  Separates oil from refrigerant and returns it to the compressor.
  • Open Compressor  -  The motor and compressor house are mounted without a shell, although the shaft has a seal.
  • Operasting Point  -  In a refrigerant system: the equilibrium point, where the performance of the evaporator matches the performance of the compressor.
  • Operating Pressure  -  The operating pressure of equipment occurs when the plant or process is operating at steady state conditions.
  • Outlet Pressure  -  See downstream pressure

P

  • Packed End  -  The end of a heat exchanger which contains the packing joint.
  • Pressure  -  It is the force exerted perpendicular to the surface of an object and is expressed as force per unit area.

Q

  • Quad Layout  -  A quadrant type layout.  Four separate quarters of a tube layout with pass lanes between them.

R

S

  • Saturated Vapor  -  A vapor at a temperature of the boiling point on the verge of condensing.
  • Shell and Tube Heat Exchanger TEMA Designation  -  There are many different types of shell and tube heat exchanger configurations.  To help standardize the types of configurations, the Tubular Exchanger Manufactures Association has devised a standard nomenclature to briefly describe heat exchangers.

  • Shell Assembly  -  The assembly into which the tube bundle fits. The shell also contains the shell-side connections.

  • Shell Head  -  A formed plate welded to the shell, or bonnet, pipe. It can be many styles or shapes, including flanged and dished, elliptical, ellipsoidal and hemispherical.  Generally, as a head gets flatter, it gets weaker; therefore, designers can use a flat-end plate or a thinner formed head to do the same job.

  • Specific Heat  -  The amount of heat required to raise the temperature of a material 1 degree.
  • Steam  -  The invisible vapor (gas) when water is heated to its boiling point and passes from a liquid to a gaseous state.
  • Steam Density  -  Has a higher density than water vapor, the higher the pressure the higher the steam density.
  • Superheated Gas  -  When a dry saturated gas is heated at constant pressure, its temperature rises and it becomes superheated.

 T

  • Temperature  -  The amount of heat or cold, but it is neither heat or cold.  Temperature is expressed as a number that is related to energy and porportional to a type of energy, but it is not energy.
  • Tensile Strength  -  The maximum stress a material can resist before it starts to elongate.
  • Thermal Resistance  -  Measures the temperature difference by which an object or material resists a heat flow.
  • Thermocouple  -  The junction between two metals generates a voltage that is a function of temperature.  Thermocouples rely on this effect. Almost any metals can be used to make a thermocouple. 
  • Tie Rod  -  Bars mounted between the tube sheets to support the baffles.
  • Tie Tube  -  A tie tube takes the place of the tie rod in a small diameter heat exchanger, and serves the same purpose.
  • Tube Bundle  -  Heat exchangers consist basically of housing and tube bundle.  The housing is a welded construction and includes all connecting, joining and fastening elements.

  • Tube Layout  -  Shows the positioning of the tubes inside the heat exchanger and the locations of the tie rods.

  • Tube Side  -  The fluid that circulates through the inside of the tubes of the heat exchanger.
  • Two-phase Static Head  -  A pressure head created by a liquid/gas mixture.

U

  • Upstream Pressure  -  The pressure entering the inlet side of the heat exchanger.

V

  • Viscosity  -  The measure of the internal friction/resistance to the flow of a liquid.

W

  • Water Hammer  -  A valve is suddenly opened or closed.  This can creates a repeating pressure wave of the liquid in the pipe that could cause a rupture to the pipe or even damage equipment.
  • Water Hammer Arrester  -  Absorbs unstable pressure due to sudden closing of gate, and moderates the impact of the fluid inside the pipe.
  • Wet Bulb Temperature  -  A temperature that takes into account the relative humidity and the dry bulb temperature.  At any given ambient temperature, less relative humidity results in a greater difference between the dry-bulb and wet-bulb temperatures.

X

Y

Z

 

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Shell and Tube Heat Exchanger TEMA Designation

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