Heat Exchanger Types and Selection

In order to achieve optimum process operations, it is essential to use the right type of process equipment in any given process.  Heat exchangers, commonly used to transfer energy from one fluid to another, are no exception.       The selection of the proper type of heat exchangers is of critical importance.  Selecting the wrong type can lead to sub-optimum plant performance, operability issues and equipment failure.     The following criteria can help in selecting the type of heat exchanger best suited for a given process: Application (i.e. sensible vapor or liquid, condensing or boiling) Operating pressures & temperatures (including startup, shutdown, normal & process upset conditions) Fouling characteristics of the fluids (i.e. tendency to foul due to temperature, suspended solids ...) Available utilities (cooling tower water, once through cooling water, chilled water, steam, hot oil...) Temperature driving force (i.e. temperature of approach or cross and available LMTD) Plot plan & layout constraints Accessibility for cleaning and maintenance Considerations for future expansions Mechanical considerations such as: 1) material of construction; 2) thermal stresses (during startup, shutdown; process upset and clean out conditions); 3) impingement protection Shell-and-tube heat exchangers accounts for more than 50% of all heat exchangers installed.  However, in many cases, there are more attractive alternatives in terms of cost and energy recovery.  Any time a heat exchanger is being replaced, the opportunity should be taken to re-assess if the type used is best for the given process.  Operating changes since initial installation as well as advancements in the field of heat transfer may point towards a different type as being optimal.  Heat Exchangers Types       Shell & tube heat exchangers           Baffle types                   Segmental baffles                   Double segmental baffles                   No-tube-in-window (NTIW) baffles                   Rod baffles                   EM baffles                   Helical baffles          Tube Enhancements                   Twisted tubes                   Low finned tubes                   Tubes inserts (twisted tapes, Cal Gavin)              Compact type heat Exchangers          Plate & frame heat exchangers (gasketed, semi-welded, welded)          Spiral          Blazed plate & frame          Plate-fin heat exchanger          Printed circuits           Air-cooled heat exchangers      Heat Exchangers Selection      Past experience, is always the best place to start to guide the selection of heat exchanger types.  Understanding the reasons behind both successes and failures will lead to better equipment selection.       When comparing different types of heat transfer equipment, one must take into consideration the total cost of the equipment which includes:     purchase cost installation cost operating cost (pumping, fan…) maintenance cost In order to make the best selection, it is important to have some knowledge of the different types of heat exchangers and how they operate.   The tables below offer the advantages and disadvantages of common types of heat exchangers.  They can be used to arrive at a type that is best suited for a given process.         Shell & tube heat exchangers  Advantages Disadvantages Widely known and understood since it is the most common type  Less thermally efficient than other types of heat transfer equipment  Most versatile in terms of types of service Subject to flow induced vibration which Can lead to equipment failure  Widest range of allowable design pressures and temperatures   Not well suited for temperature cross conditions (multiple units in series must be used)  Rugged mechanical construction - can withstand more abuse (physical and process)   Contains stagnant zones (dead zones) on the shell side which can lead to corrosion problems Subject to flow mal-distribution especially with two phase inlet streams                  Compact Heat Exchangers  Advantages Disadvantages Low initial purchase cost (plate type) Narrower rage of allowable pressures and temperatures Many different configurations are available (gasketed, semi-welded, welded, spiral) Subject to plugging/fouling due to very narrow flow path High heat transfer coefficients (3 or more times greater than for shell & tube heat exchangers, due to much higher wall shear stress) Gasketed units require specialized opening and closing procedures Tend to exhibit lower fouling characteristics due to the high turbulence within the exchanger Material of construction selection is critical since wall thickness very thin (typically less than 10 mm) True countercurrent designs allow significant temperature crosses to be achieved   Require small footprint for installation and have small volume hold-up               Air Cooled Heat Exchangers  Advantages Disadvantages Attractive option for locations where cooling water is scarce or expensive to treat High initial purchase cost   Well suited for cooling high temperature process streams (above 80oC when using cooling water should be avoided)  Require relatively large footprint Low maintenance and operating costs (typically 30-50% less than cooling water)  Higher process outlet temperature (10-20 oF above the ambient dry bulb temperature)

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