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  • Selecting proper heat exchanger for process applications

    A heat exchanger is a device used in transfer heat between a solid object and a fluid, or between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heatingrefrigerationair conditioningpower stationschemical plantspetrochemical plantspetroleum refineriesnatural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.

     

    In order to achieve optimum performance, it is essential to use the right type of thermal heat exchanger. With so many options at hand, selecting the proper heat transfer solution is hard.

      

     

    When it comes to process applications involving heat transfer, heat exchangers are critical components that directly affect the overall performance and reliability of the system.

    As key parts of most heat transfer applications, from petrochemical plants to refineries and even some domestic application, understanding how different types of heat exchangers work and which one is appropriate for each individual application is paramount for energy and operational efficiency.

    So how can you select the type of heat exchanger that is best suited for your application?

    The following categorization of heat exchanger may help you understand the working principle of different heat exchangers, their advantages and limitations and discover the applications they are suited for Graphite heat exchanger

     

    Graphite is a widely used material in heat exchangers due to its great refractory and mechanical properties. Offering better corrosion resistance than most types of heat exchangers, the graphite heat exchanger is used in a wide range of chemical applications, such as food and beverage, petrochemical, chemical processing, metal finishing, and more.

    Key benefits:

    • The graphite heat exchanger is highly thermal efficient;
    • Flexible and compact design;
    • Rapid replacement of damaged block due to the compact design;
    • Corrosive resistance.

     

    Brazed heat exchangers

    Brazed plate heat exchangers accommodate a set of plates that are brazed together, but without the gasket and frame parts. The stainless steel composition with copper brazing make brazed heat exchangers resistant to high temperatures and corrosion.  Compact and lighter than regular plate and frame heat exchangers, they can complete the thermal exchange process quicker.

    Key benefits:

    • The compact design makes them perfect for tight spaces;
    • Brazed heat exchangers offer maximum performance at low installation and maintenance costs;
    • Easy to clean and maintain;
    • Environmentally friendly.

     

    Radiator heat exchanger

    A radiator is a heat exchanger that transfers thermal energy from one medium to another. In a radiator heat exchanger design air is blown within tubes, picking up heat from the stream on the tube-side, which is cooled. The resulting hot air is dispersed into the atmosphere or is used to heat buildings or other equipment.

    Key benefits:

    • Environmentally friendly: radiator heat exchangers don’t require water;
    • Flexible design;
    • Prevention of thermal and chemical pollution of cooling fluids;
    • Easy installation;
    • Low maintenance costs.

     

     

    Shell & Tube heat exchanger

    Shell and tube heat exchangers have been used for more than 150 years in almost every industrial application, including chemical process industries. Probably the best known and understood heat exchanger, the shell and tube design is widely popular, mostly for its versatility in terms of types of services.

    Key benefits

    • Able to meet process requirements in almost every industry or application;
    • Highly efficient and reliable;
    • Built from solid materials, they can handle a wide range of pressures and temperature requirements;
    • Easy maintenance.

     

    Plate heat exchangers

    Unlike shell and tube, a plate heat exchanger uses metal plates to transfer heat between two fluids. This gives the design a major advantage in that the overall heat transfer coefficient is higher than in a traditional shell and tube heater.

    Key benefits

    • The compact configuration allows for the same thermal efficiency as a shell and tube heat exchanger, but on a smaller area;
    • High thermal efficiency;
    • Easy maintenance: plate can easily be removed to be cleaned or repaired.

     


    Heat exchangers are the equipments that perform the heating or cooling operations. Their performance and longevity have an impact not only on the capital cost but also on operating cost. We all know that breakdown maintenance is costly and annoying, thereby necessitating a regular maintenance schedule for process equipments like heat exchangers. A well designed heat exchanger will not only give a good performance but also have low maintenance requirement. 

    Shell and Tube Heat Exchangers (STHE) and Plate Type Heat Exchangers (PHE) are two prominently used heat exchangers in the process industry. Over the years, many units have been designed based on scale up of earlier design or a ‘need to use’ design, which can sometimes lead to improper performance and higher maintenance.

    The heat exchangers normally have process fluids in liquid / vapour (gaseous) form as the primary fluid and the secondary fluid normally is a utility, viz., cooling water / hot water / chilled water (brine, steam, refrigerant gases or process fluids). The fluids or the process can have / can generate material that adheres to the heat transfer surface thereby impacting heat transfer. This is termed generically as 'fouling', which can be of the following types:

    a) Fouling due to high dissolved solid content
    b) Fouling due to suspended colloidal solids
    c) Localised precipitation of dissolved solid
    d) Biological fouling due to algae / fungi typically in cooling water
    e) Deposition due to chemical reaction
    f) Corrosion due to non compatible material
    g) Solidification due to temperature or property of fluids, etc.

    Hence, in a process plant, it is inevitable that some of the equipments would need servicing or maintenance. 

    Analysis / Measuring Aids
    it’s important to be able to measure the drop in performance of the heat exchanger or in other words the need for maintenance of heat exchanger. There are indicators, which become handy at times:
    • Monitoring the equipment with the help of temperature and pressure gauges helps to identify a drop in performance 
    • Reduction / elevation in temperature over design with no change in parameters like fluid flow rate and properties 
    • When the unit does not cool or heat as designed or specified (loss of heat transfer occurs) 
    • When pressure drop rises and exceeds or original design, with no change in parameters like fluid flow rate and properties 
    • In some systems, choking of filter can also be an indication of these symptoms.
    • Some failures can be detected during regular visual checks through visible leaks or cross contamination between fluids. 

    Types of maintenance
    In chemical process and most of the other process industries, normally two types of maintenance problem are encountered with heat exchangers:
    • Problems associated with various types of fouling in heat exchangers
    • Problems associated with corrosion / leakage 

    In cases where there is failure of material due to corrosion or any other reason, there are options for replacement of component (tubes or plates) to set right the equipment or attended to these by other methods like welding, brazing, etc.

    Maintenance of heat exchangers with various types of fouling can be done by the following means:

    • Manual cleaning methods 
    • Cleaning in place (CIP) 
    • Combination of both these types 
    • Other special type-as recommended by manufacturer 

    In manual cleaning, one could use the simplest method of cleaning whereby the equipment needs to be opened / dismantled for cleaning. The tubes or plates are cleaned by scrubbing with a soft wire brush or by high-pressure water jet (adjusted for the cleaning operation) depending on fouling. In some cases, special tools like bullet / pig cleaning is also used. In cases where there is a hard scaling on the surface (especially in tubular heat exchanger), sometimes drill rods are also used. 

    CIP is a mechanism whereby one circulates a cleaning fluid which can soften / dissolve / dislodge / remove the fouling material without opening the heat exchanger. There are special chemicals (and temperature) to be used, which will help in this cleaning process. The fluid is circulated at high velocity and preferably in reverse direction of process fluid.


    Proactive measures
    Prevention is better than cure, hence we must ensure the following when we are selecting or buying a heat exchanger: 

    Design: The design of heat exchanger must take into consideration the fluid properties and the behaviour to enable a right heat exchanger selection. A right type of heat exchanger selection would ensure proper performance with low maintenance. This is because each type of heat exchanger has some key advantages as well as limitations. Understanding these for the fluid is very critical to enable a good design and efficient operation. 

    Material of Construction (MoC): Selecting the right material which is compatible with the fluid, enhances performance as it reduces corrosion / failure and expensive emergency maintenance. A good design can always work as an effective configuration to reduce the cost of ownership whereby one can enhance the material say from carbon steel to stainless steel.