Plate heat ex changers are used for transferring heat, they require inlets and outlets where the flowing mediums or fluids can enter and leave the heat exchange. A fluid may be a liquid or a gas.
As fluids are often assumed to be liquid only, we will use the term flowing medium to avoid confusion.
Gaskets and plates: The gaskets adhere to one side of each plate only.
Carry bar and Clamping Bolts: The plates hang upon a carrying bar and are pressed together using clamping bolts. When the plates are compressed together, they form a plate stack.
Guide bar: A guide bar ensures the plates are aligned correctly when the plate stack is opened or closed.
Moveable cover or fixed cover: One cover is moveable while the other is fixed. The moveable cover and fixed cover are referred to as the frame of plates and pressure plates. The inlets and outlets are mounted to the fixed cover only.
Plate heat ex changer main components:
It has two flowing mediums, one is cold and the other is hot. The hot medium needs to be cooled by the cold medium. The hot medium enters the heat ex changer through the hot medium inlet. Gaskets direct the hot medium as it flows through the heat ex-changer. Each plate has an alternating gasket pattern. The hot flow into the space between the pair of plates but does not flow in the space between the next pair of plates because gaskets prevent this. The process is continuous so that each second set of plates is filled with a hot flowing medium.
At the same time, the cold medium enters the heat ex-changer through the cold medium inlets but this time the gaskets are positioned to allow the cold medium to flow into the space where no hot medium is present.
How plate heat ex-changer works:
We know a heat exchange filled with both hot and cold flowing mediums. Each medium flows out of its associated outlet and the process is continuous. The two flowing mediums are always adjacent to each other throughout the heat ex-changer. The flowing mediums thus have a hot-cold, hot-cold flow pattern as they flow through the heat ex-changer. They are completely separated from each other by the gaskets and plates and they do not mix. Due to the proximity of the flowing mediums, heat is exchanged between them. The hot medium heats the plates and the plate passes some of this heat to the cold flowing medium. Thus the hot medium temperature decreases whilst the cold temperature increases.
What makes plate heat ex changer so efficient:
- The main reason plate heat ex changer sare so efficient. This plate may appear simple but it is full of interesting engineering design features. For example, when the plates are compressed together to form a plate stack, the gap between each other plate is very small which ensures good thermal contact between the two flowing mediums. The gap between the plates is also known as clearance.
- Plates are thin and have a large contact surface area which gives each plate a high heat transfer rate.
- Plates are manufactured from a material with high thermal conductivity which further increases the heat transfer rate.
- Corrugation on the plate surfaces prevent laminar flow and prevents turbulence which increases the heat transfer rate whilst also reducing the likelihood of deposit accumulating upon the plate surfaces. The corrugations also serve to stiffen the structure of the plate which allows a thinner plate to be used compared to a plate that has no corrugations. Corrugation is sometimes referred to as herringbone.
- The gaskets can maintain a seal between the plates even when the system pressure and temperature varies.
- Holes in each gasket known as telltales are used to identify leaking gaskets. This feature follows the operator to change the affected plates before the leaking medium leaks through the next gasket and contaminates the other flowing medium. Because the gaskets guide flow through the heat ex-changer.
- It is essential that they be installed in the correct order. For this reason, gaskets are often fitted with markings so that operator can check each plate is installed in the correct order throughout the entire plate stack. Another way to check the plates is to spray paint a diagonal line from the top left to the bottom right of the entire plate stack.
- Gaskets alternate them throughout the heat ex-changer except for the first and the last plates within the plate stack which press against the fixed, and moveable covers. The plate that presses against the fixed and moveable covers are known as start and end plates. Because of their position within the plate stack, the purpose of the start and end plates is to prevent flow into the space between the fixed cover and start plates and to prevent flow into the space between the moveable cover and end plate. This way the covers are not actively used to exchange heat. This make the sense because each cover is quite thick. There are no corrugations, they are poorly designed to exchange heat.
Varying cooling capacity:
- One way is to regulate the outlet valves so that flow is increased or decreased. This method is useful because no dismantling of the heat ex-changer
- Another method is to increase or decrease the number of plates in the plate stack. Increasing the number of plates in the plate stack gives the corresponding increase in cooling capacity. Decreasing the number of plates in the plate stack decreases the corresponding cooling capacity.
- The final method, a varying of plate heat ex changer’s cooling capacity is to use the single pass or multi-pass design.
A single-pass heat ex-changer allows the two flowing mediums to flow and past each other only once. Multi-pass heat ex-changers allow the flowing mediums to flow past each other several times.
Most heat ex-changers use the single pass design. Flow through plate exchange may be parallel, cross, or counter. Plate heat ex-changer usually uses counter-flow as the most efficient type of flow for heat transfer.