A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change.
Welded, semi-welded and brazed heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required. In place of a pipe passing through a chamber, there are instead two alternating chambers, usually thin in depth, separated at their largest surface by a corrugated metal plate.
The plates used in a plate and frame heat exchanger are obtained by one piece pressing of metal plates. Stainless steel is a commonly used metal for the plates because of its ability to withstand high temperatures, its strength, and its corrosion resistance.
The plates are often spaced by rubber sealing gaskets which are cemented into a section around the edge of the plates. The plates are pressed to form troughs at right angles to the direction of flow of the liquid which runs through the channels in the heat exchanger. These troughs are arranged so that they interlink with the other plates which forms the channel with gaps of 1.3–1.5 mm between the plates.
The plates produce an extremely large surface area, which allows for the fastest possible transfer. Making each chamber thin ensures that the majority of the volume of the liquid contacts the plate, again aiding exchange. The troughs also create and maintain a turbulent flow in the liquid to maximize heat transfer in the exchanger. A high degree of turbulence can be obtained at low flow rates and high heat transfer coefficient can then be achieved.
When aligned these ports form four distribution headers through the plates. Hot process fluid flows through every other plate, and cold process fluid flows through the other plates. Distribution of hot and cold fluids to alternating plates is achieved by the gaskets around each port. If the fluid is to flow through a plate, the gasket is removed from around the port. If the fluid is to bypass a plate and flow to the next plate, the gasket is left intact.
It is very important for the gaskets to be made of a material that is compatible with the
process fluids or leaks may develop between plates.
Advantages and disadvantages of plate heat exchangers
- Compactness- The units in a plate heat exchanger occupy less floor space and floor loading by having a large surface area that is formed from a small volume. This in turn produces a high overall heat transfer coefficient due to the heat transfer associated with the narrow passages and corrugated surfaces.
- Flexibility- Changes can be made to heat exchanger performance by utilizing a wide range of fluids and conditions that can be modified to adapt to the various design specifications. These specifications can be matched with different plate corrugations.
- Low Fabrication Costs- Welded plates are relatively more expensive than pressed plates. Plate heat exchangers are made from pressed plates, which allow greater resistance to corrosion and chemical reactions.
- Ease of Cleaning- The heat exchanger can be easily dismantled for inspection and cleaning (especially in food processing) and the plates are also easily replaceable as they can be removed and replaced individually.
- Temperature Control- The plate heat exchanger can operate with relatively small temperature differences. This is an advantage when high temperatures must be avoided. Local overheating and possibility of stagnant zones can also be reduced by the form of the flow passage
- The main weakness of the plate and frame heat exchanger is the necessity for the long gaskets which holds the plates together. Although these gaskets are seen as drawback, plate-and-frame heat exchangers have been successfully run at high temperatures and pressures.
- There is a potential for leakage. The leaks that occur are sent to the atmosphere and not between process streams.
- The pressure drop that occurs through a plate heat exchanger is relatively high and the running costs and capital of the pumping system should be considered.
- When loss of containment or loss of pressure occurs, it can take a long time to clean and reinitialise this type of exchanger as hundreds of plates are common in larger built.
- The narrow spacing between plates can become blocked by particulate contaminants in the fluid, for example oxide and sludge particles found in central heating systems.
Plate Cleaning Procedure.
- The plates can be cleaned, hanging in their frame, by hand with a brush of nylon or stainless steel, or mechanically using a rotary brush.
- The plates in some types of heat exchanger will have to be removed. Care must be taken not to damage gaskets.
- Steel wool or brushes made of carbon steel are not to be used. Lime deposits can mostly be removed without opening the apparatus using chemical cleaning provided that the cleaning is performed regularly to prevent build up of deposits.
- With stainless plates, and only when the deposits are practically free from chlorides, nitric acid with a concentration of 2% by volume may be used.
- The solution is to be circulated for 30 min. at 120F (50C). The heat exchanger shall then be carefully flushed out with clean water.
Before the movable pressure plate is pushed against the plate package, check the plates are located correctly in the frame. Viewing through the portholes and checking that plate edges are in line carries out this check.Furthermore, the symmetrical pattern of the outside edges of the plates should be checked. They should have a honeycomb appearance.
The exchanger may leak a little when cold, but normally the gaskets will expand when warm and the leakage then stops, thus making further tightening unnecessary.
Normally, it is only possible to tighten the plate package when it is not under pressure.
The function and the lifetime of the heat exchanger depend largely on the correct plate pack tightening procedures being adhered to.
- The plate package should be tightened to a definitive measurement between the inner surfaces of the fixed and the movable pressure plates. This measure, called the minimum tightening measure, varies with different plate sizes. Each manufacturer will specify this distance for any given number of plates. With a ten bolt plate pack do not utilise or tighten all ten bolts at the start. Begin with one bolt on each side, the two bolts in the middle of the pressure plates. By means of the two bolts the plate package may be tightened to approximately 20% above the minimum tightening measure.
- The tightening operation is continued with a total of four bolts, the bolts adjacent to the middle bolts. The four bolts are tightened diagonally. When the torsion moment of the nuts becomes high, all ten bolts are utilised. By symmetrical tightening of the ten bolts, the minimum tightening measure is obtained.
- When the gaskets are relatively new it is not necessary to obtain the minimum tightening measure. Stop at a measure about 5% larger than the minimum. As the gaskets get older, it may be necessary to increase bolt tension to avoid leakage.
A damaged plate can easily be exchanged for an identical spare plate. Two adjoining 4-port plates can be removed without being replaced. The capacity of the heat exchanger will usually only be slightly reduced. The length of two plates reduces the plate pack length.
Plates can be removed in either lateral direction depending on access available.
A gasket, which has parted from its groove, must be re-cemented. If the gasket is only partly loose it can often be re-cemented on the plate whilst it still hangs in the frame.
A gasket that has become separated must be removed from the heat exchanger and be re-cemented on to the plate.
Before assembling the plate pack, the rear of the gasket tracks, which is actually the surface that tightens against the gaskets on the next plate must first be coated with a film of silicon oil.
Care must be taken not to oil the gasket tracks that are to be cemented. If this does happen the surface must be cleaned with solvent to allow the cement to stick.