BOILERS & HOT WATER Know your way around heat exchangers


Heat exchangers come in all shapes and sizes and are designed for a wide variety of applications. Car radiators and refrigerators are examples of heat exchangers as are cooling towers, and a range of air conditioning systems and heating appliances contain them


H


owever, the heat exchangers I am going talking


about in this article are those used in the industrial and


commercial sectors for HVAC- related functions. Here, there are essentially two types to choose from – shell & tube, and flat plate.


The main operating principle for heat exchangers in HVAC applications (other than cooling towers, which use evaporation to transfer heat) is simply to move heat by conduction through the exchanger materials (shells and tubes, or plates) which separate the mediums being used. As the name suggests, shell & tube heat exchangers place two fluids in thermal contact using tubes housed within an outer cylindrical shell. The pathways in these units are typically constructed from thermally conductive metals such as steel or aluminium which facilitate the easy transfer of heat.


The number of tubes (also called the tube bundle) in this type of heat exchanger determines how much heat is transferred because it governs how much surface area is exposed to the flow of fluids. Shell & tube heat exchangers


are often used in comfort applications and industrial settings to heat and cool fluids. They can commonly be found in power generation, oil refineries and other chemical processes, metals and mining, chilled water systems and refrigeration, cooling for turbines and compressors, and pulp and paper applications.


Flat plate heat exchangers employ the same heat conducting principles as shell & tube units but are profoundly different in design. They are constructed from a series of corrugated metal plates stacked one after another and housed within a large frame. Hot and cold (usually) liquid or (sometimes) gas is passed between the plates. Each plate typically has an alternating gasket pattern. The


hot medium flows into the space between a pair of plates but will not flow into the space between the next pair of plates because the gaskets prevent it. The process continues so that every second set of plates is filled with the hot flow medium. At the same time, the cold medium enters a heat exchanger through the cold medium inlet, but this time the gaskets are positioned to allow the cold medium to flow into the space where no hot medium is present. This means the heat exchanger is filled with both hot and cold flowing mediums between each alternate plate. Heat is transferred from one side of the plate to the other, thereby removing heat from the hot medium.


While all flat plate heat exchangers feature corrugated plates, they differ in terms of how they seal the plates together. As mentioned earlier, nitrile rubber gaskets (which are inexpensive and easy to replace) running around the edges of the plates can maintain a seal between them. They have a very good coefficient of elasticity which means they can be heated and cooled without breaking. Using this type of seal means the flat plate heat exchanger offers modularity so it can be disassembled, and plates added to increase capacity. Alternatively, the plates can be brazed or welded (although this tends to reduce flexibility because it is more of a fuss to add or remove plates).


Applications for flat plate heat exchangers are many and varied. They include supplying domestic hot water and swimming pools, and heat for industrial processes. They are also found in district heating and cooling schemes, in thermal ice storage systems, and for waste heat recovery. However, one of their most effective applications is in the healthcare sector. Here, flat plate heat exchangers can provide reliable hot water to facilitate


The benefits of flat plate heat exchangers


Efficiency – The corrugated plate gives the PHE a larger contact surface area than a traditional shell & tube heat exchanger of similar size; this increases the heat transfer rate and makes a heat exchanger more efficient.


The corrugated plate is stiffer than a flat plate, so a thinner plate can be used which offers a higher heat transfer rate. And the corrugations in the plate create a turbulent flow which prevents deposits forming under the plate and break down the boundary layer of liquids that may form on a surface.


Ease of maintenance and servicing – PHEs are smaller and lighter than the shell & tube alternative. They are simple to disassemble should a defect be discovered in a plate, making them particularly easy to clean and maintain. PHEs don’t need access space to dismantle. Should a defect be discovered in a plate, it can be removed, and the PHE can be returned to service at a marginally lower capacity until a replacement is installed.


Flexibility – The number of plates in these units can easily be increased or decreased depending on capacity requirements.


everything from hand washing and bathing through to laboratory functions and janitorial facilities. Moreover, they can help guard against pathogens. After all, a big risk in hospitals and other healthcare facilities is legionnaire’s disease a rare form of pneumonia) which can thrive in stored water. There is no need for stored hot water with a PHE, reducing the risk of a legionnaires’ disease outbreak. My own company installs and maintains plate heat exchangers as part of its mechanical engineering services. Working with manufacturer, Thermo Logistics (which has more than 25 years’ experience in heat exchanger


12 BUILDING SERVICES & ENVIRONMENTAL ENGINEER NOVEMBER 2022


servicing and maintenance), ECEX delivers plate heat exchanger services in a variety of commercial applications, including hospitals, industrial and manufacturing processes, district heating systems, and swimming pools. The company provides technical expertise on all aspects of heat exchangers, supplying the comprehensive service and support of flat plate heat exchangers, including accessories, loan packs, and certification of heat exchanger plates. Plate heat exchanger servicing includes cleaning, hole detection, reconditioning, or replacement where necessary.


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