How plate heat exchangers work. In this video, we are going to look at plate heat exchangers and how they work. They are often called PHE, PHX, or sometimes just HX or HEX. Plate heat exchangers are very common. They are widely used in construction and manufacturing services. The reason why they are popular is that they are very compact, they are highly efficient, they are easy to maintain and require little maintenance.
Scroll down for YouTube video tutorial on how plate heat exchangers work
The purpose of a plate heat exchanger is to transfer heat energy between two fluids, without the fluids mixing. For example, in building services, you may want to transfer heat from a primary loop connected to a boiler to a separate secondary loop, perhaps in a district heating network. In manufacturing, you may want to quench some oil with water, but obviously you don't want to mix oil and water.
Looking at the main parts of the plate heat exchanger. We have the end plates on the front and rear deck which are usually mild steel. They are very strong, they are there to hold everything together. Next we have the nuts, these are joined and tightened on the clamping bolts. The clamp bolts fit into certain slots on the side of the unit and will run the full length of the heat exchanger. Bolts are attached to these bars and compress all plates and gaskets to form a tight seal. Between the end plates are the gaskets and heat transfer plates.
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| Examples of heat exchanger sizes |
Larger heat exchangers will also have a support bar at the top and bottom. This will support the weight of the heat exchanger. The plates can simply be slid out for maintenance once the end plate is removed.
Above is an example of a real world plate heat exchanger. They are usually made of steel or titanium, and you can see that they have a grooved or stamped pattern. These patterns will reinforce the plates and increase the heat transfer surface area by creating a highly turbulent flow within them. Turbulent flow is good because it mixes the fluid so that the heat is distributed or averaged. If it were a constant flow, heat would build up in some areas more than others.
Between the plates we have rubber gaskets. It is bonded to the face of the plate and the purpose of the gasket is to ensure a tight fit and prevent leakage. The gasket also allows or prevents the flow of fluid into the interior of the sheet.
In the illustration of the real world heat exchanger plate, you can see the black line running near the outer edge, this is the gasket. Notice that the two holes on the right have a diagonal rubber gasket running through them; this will prevent water from entering the plate. However, the holes on the left do not have this diagonal seal, so fluid can enter and exit the plate through these holes.
If you look at the heat exchanger size comparison photo example above. You will probably notice that the clamp bolts extend well past the heat exchanger. This is for several reasons. One of the reasons is to be able to mount all these plates during installation or during maintenance, but also, this gives you the possibility to expand the plate heat exchanger in the future. Suppose, for example, that the building hopes to expand in the future, then you can simply expand its cooling capacity by adding more plates. You can also remove the plates to reduce this.
There are several ways to connect plate heat exchangers.
The most common is where the inlets and outlets are all on the faceplate, so fluids enter, flow through their channels, and then return to the faceplate.
The other version is where one of the fluids enters the front plate but exits the back plate. The other fluid flows in the opposite direction.
The first version is the most common version and is because you do not need to modify the piping if you need to expand the plate heat exchanger in the future. With the second version, all connected pipes will need to be removed and rewired to fit, depending on how far you extend your plate heat exchanger.
So how does it work?
We stack several plates together, then use the seals on the inside to prevent fluids from entering alternate plates. The seal can be rotated to block the holes on the right or left side. The two fluids will then flow in any other channel between the plates. Example: Fluid 1, plate, fluid 2, plate, fluid 1, plate…
The holes line up to form a pipe-like channel through which fluids flow.
If you look closely at the image above, you'll notice that the seal alternates which side it locks on.
If the cooler fluid passes through the heat exchanger, it can be let in through the top left inlet. It will then flow into plates 2, 4 and 6. A higher temperature will then exit through the lower left outlet.
So we have the hot fluid entering the lower right inlet, it flows through channels 1, 3, 5 and 7. It will then exit the upper right outlet at a lower temperature.
The seals are what allow the fluid to flow through a certain channel.
What happens is that the channels between the plates are at different temperatures and the hot always turns cold. Therefore, the hot fluid will transfer some of its thermal energy through and into the cooler fluid. The two fluids never meet or mix, they are always separated by the wall of the metal plate. The heat is simply transferred through it. Therefore, the hot fluid will get colder and the cold fluid will get hotter.
You may also notice that these fluids flow against the current. This is the best setting for the highest efficiency, because the log mean temperature difference, the LMTD, is maximum.
If you find them in a building, make sure they are 1) insulated to retain as much heat energy as possible and 2) that the protective boot is in place over the threads of the grab bars.
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