When it comes to liquid-to-liquid applications and evaporators and condensers, plate heat exchangers are the most efficient ones among other heat exchangers. The reason is the turbulence in the corrugated plate channels. Total heat transfer coefficient (k-value or u-value) of plate heat exchangers is mostly 3 to 4 times higher compared to the boiler-pipe type heat exchangers. The thickness of their heat transfer surfaces is a lot less compared to the other types and this advantage makes plate heat exchangers the most economic heat exchanger within the limits of pressure, heat and liquid subject to heat transfer. Plate heat exchangers are still used at 25 bar pressure and 180° temperature and even at higher temperatures. The largest units available in the market have 50 mm inlet-outlet connections and a flow rate of 4500 m³/hour can be achieved with one unit.
Intensive turbulence between the plates minimizes the fouling problems.
Plate heat exchangers have some standard components mainly being plate bundles inserted between the removable cover plates.
Components of a plate heat exchanger
|3||Top load-bearing pipe|
|4||Lower guiding bar|
Plates are available in dimensions from 0.02m2 to 4m2. A few hundred plates may be used in a single unit and total heat transfer surface of one unit may be higher than 2000 m2.
All plates that are available today are offered in a “fish-bone” like arrangement.
Conventional fish-bone pattern plates are manufactured with two arrow angles.
Heat exchanger plate
These two different types of plates are combined in three configurations showing different characteristics in terms of heat transfer and pressure loss.
Plates are produced in thicknesses that vary between 0.4 mm and 0.8 mm based on the design pressure and maintenance stability. Thin plates are not used if it is required to open the unit for cleaning or by other reasons.
Some plate materials;
There is a gasket around each plate and two inlet-outlet openings. In addition, other plates have gaskets around other holes. This allows passing the fluid through different channels in heat exchanger.
Two gaskets separate two fluids thanks to the arrangements of all plates in all suppliers. The reason of taking this safety measure is the fact that the fluid would flow out instead of leakage in case of seldom experienced gasket failures.
Gaskets are produced from various elastomer materials. Today, the following materials are used commonly;
NBR(Nitrile butadien rubber)
Rubberr cured with peroxide T < 140 °C
It is suitable for animal fats, mineral oils (petroleum derivatives), vegetable oils, food and pharmaceutics industry.
Ethylene propylene diene monomer rubber T < 160 °C
It is suitable for water-based fluid and steam applications.
They can be used with organic solvents, acids with certain concentrations and all alkaline materials.
They are never used with oil.
Fluorocarbon rubber (FKM-Viton): T < 200 °C
It is suitable for the majority of acid and solvent applications. It is suitable for water, water-based fluids and steam applications (Tmax: 200 °C).
At present, the types of elastomer treated with peroxide are the commonly-used ones and the aforementioned temperatures are specified for gaskets treated with the highest quality peroxide.
Gaskets wear in time. As we have mentioned above, minimum operating temperature applies based on the water resistance of the gasket material and using it under stable conditions for a certain period of time. It is also dependent on the operating pressure. High operating temperature requires maintaining the original flexibility of the gasket, however, a slightly worn gasket shall be sufficient when the operating pressure is low.
Gaskets economic life is extended when the operating temperature is lower. To put it simply, having an operating temperature that is 10°C will double the economic life of a gasket. Therefore, it will be advantageous to use a gasket with a quality conforming to high minimum operating temperature. This will extend the potential economic life of the gasket.
Frame of the plate heat exchangers is usually made of carbon steel. Covers and bolts are designed in accordance with different pressure codes and standards. PED standard is used in Europe and ASME standard is used in the U.S.A.
Tightening bolts must be made of steel that is resistant against high tension forces. For example, it should be protected with zinc plating. Parts used for top and bottom bars that are in contact with the plate bundle are usually made of stainless steel.
Carbon steel surfaces should be subjected to a good surface protection program; sand blasting and degreasing treatments are applied to surfaces in order to prevent wearing. Then they are painted.
There are various connection types. At present, grooved or welded pipes are the most commonly used pipes for large units in industrial applications.
Connections must be coated with a water-proof material so as to ensure that carbon steel does not come into contact with the frame. Coating is made with stainless steel, titanium or rubber.
The most common and popular flow configuration is single-pass pattern. It means that fluid passes through the plates for one time and all connections are made on the fixed cover plater. In this case, four connection holes are opened to all plates except for the end plate End plate does not have any holes for the purpose of preventing the contact between the fluid and carbon steel.
If the temperature program range is small, then multipass heat exchanger should be used. It means that the fluid passes through a part of the plate bundle, it is collected in the outlet and then directed to the next pass. In this case, rotating plates are required for achieving the right flow. Please see the figure below illustrating the
A disadvantage of the multipass configuration is the requirement of having connections on the removable plate. This situation poses a challenge during maintenance operations as it required to remove the pipes on the removable plate in order to open the unit.
Fouling means the formation of a layer that reduces the heat transfer capacity of the heat transfer surface and/or increasing the pressure loss.
Fouling may reduce the heat transfer substantially, therefore it should be minimized. The following are the main methods used for this purpose;
Chemical production processes
Food production processes