Plate Heat Exchangers Explained
Heat exchangers are a ubiquitous, well-established, and proven technology that offers a host of solid benefits in a range of applications.
The purpose of a heat exchanger is simply to swap heat between two substances, usually water or gas, without letting the substances mix together. Car radiators and refrigerators are examples of heat exchangers, and many air conditioning systems and heating appliances contain them.
However, the heat exchangers we are concerned with here are those used in the industrial and commercial sectors as well as in healthcare settings. In these applications, there are broadly two types – shell & tube, and flatplate.
Shell & tube heat exchangers
As its name implies, the shell & tube heat exchanger comprises a large pressure vessel (or shell) with tubes inside it. Typically, hot liquid or gas runs through the tubes and cold liquid or gas flows through the shell and over the tubes to transfer heat between the two substances.
Flat plate heat exchangers (PHEs)
Flat plate heat exchangers (PHEs) operate on the same principle but are fundamentally 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 are passed between the plates.
Each plate 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.
Benefits of PHE
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.
On top of this, 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.
So, PHEs are smaller, lighter, and more efficient than the shell & tube alternative. Moreover, PHEs are simple to disassemble, making them particularly easy to clean and maintain; and they don’t need access space to dismantle.
Furthermore, 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.
While all flat plate heat exchangers feature corrugated plates, they can differ by 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 PHE 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 palaver to add or remove plates).
Plate heat exchanger applications
Applications for PHEs are many and varied, including for 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, they can provide reliable hot water to facilitate everything from hand washing and bathing through to laboratory functions and janitorial facilities.
Moreover, PHEs can help guard against pathogens. 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.
ECEX installs PHE’s from Thermo Logistics. This video show scale cleaning from heat exchanger plates at Fenchurch Street, London.