Introduction to Vahterus Plate & Shell Heat Exchanger Technology
Vahterus is the inventor, pioneer and market leader in Plate & Shell Heat Exchanger (PSHE) technology. Our unique Plate & Shell Heat Exchanger is at the forefront of heat exchanger technology. The PSHE combines the best features of Plate & Frame and Shell & Tube exchangers, offering a durable, compact and gasket-free solution with high heat transfer, high pressure and high temperature capability.
Table of Contents
General overview of the construction
The functional principle of a heat exchanger is to transfer heat from the primary to the secondary side through a heat transfer plate so that the flows are not mixed with each other. Vahterus heat exchangers are used for e.g. heating, cooling, condensing and recovery of process heat. Vahterus heat exchangers comprise a fully welded pack of circular heat transfer plates assembled into a cylindrical steel shell. This principle is known as Plate & Shell®. The overall construction and a number of the component parts are patented. The nature of the construction means that the distance for the product flow is very short and the pressure drop is very low. This heat exchanger type is thus ideal for applications with viscous fluids. There are no gaskets used in the construction; all connections are welded.
The heat exchanger is a combination of many parts and features, which are summarised below. Note that not all available details are shown in this drawing.
Summary of construction:
- Plate side connection
- Flow directors
- Lifting point
- Shell side connection
- Name plate support
- Threaded connections (≤ DN 50)
Key features of Plate & Shell Heat Exchangers
A Plate & Shell Heat Exchanger are 60–70% smaller than traditional shell & tube heat exchangers. Thanks to their compactness, PSHEs are an excellent choice for retrofitting. PSHEs are also light, and thus easy to transport and install. They can be used in locations where weight limits apply and require no additional support structures.
Fully welded – no gaskets
PSHEs are constructed to be strong, compact and safe. They withstand extremely high pressure, stress variation and thermal shocks. Like the plate pack, the surrounding shell is fully welded. The plate pack and shell are not, however, welded together, which allows them to expand and contract with heat variation. The plate pack consists of circular plates, which guarantees even distribution of stress.
The strong turbulent flow between the plates in a Plate & Shell Heat Exchanger (PSHE) makes the process highly energy-efficient. Unlike with traditional plate heat exchangers, circulation is possible at higher temperatures and pressure. PSHEs are also compatible with strict temperature programmes.
The welded construction of our PSHEs makes them reliable and easy to maintain. They have no gaskets, so there’s no risk of them breaking or leaking. In addition, PSHEs are not susceptible to fouling. We can customise efficient heat exchanger cleaning systems for even the toughest of environments.
Benefits of Plate & Shell technology over traditional heat exchangers
Our PSHE has numerous advantages over traditional heat exchangers.
- small volume(~1/5 S&T),
- low weight (~1/10 S&T),
- large surface;
- fully welded, no gaskets;
- cylindrical shell, easy to insulate;
- high system pressure admissible;
- flexibility to build into vessels.
- high pressure (200bar)
- high temperature (-196°C~+600°C)
- low fouling
- close approach temps
- big temperature difference
- batch process
Advantages & disadvantages: Plate & Frame Heat Exchangers
+ low fouling
+ close approach temps
- low pressure (<15bar)
- low temperature (<150°C)
- gasket (hydrocarbons corrosion)
Advantages & disadvantages: Shell & Tube Heat Exchangers
+ high pressure
+ high temperature
+ no gasket
- large size and weight
- high fouling
Plate pack construction
The basic heat exchanger is assembled from a plate pack made of stainless steel plates and a shell made of carbon steel or stainless steel. However, the heat exchanger can also be made in other materials.
Figures below shows the basic assembly of the plate pack.
Main shell arrangements
In refrigeration, the openable version is rarely used. In total, fewer than 1% of units supplied for refrigeration applications are openable, and then only following special customer request and mostly for chemical plant systems. In openable units the plate pack is fixed to the end plate flange (not as shown in the diagram).
In the compact design, all connections are placed on the end plates – there are no connections on the shell. The compact heat exchangers can be arranged in multi-pass on the plate side as well on the shell side. This construction can be very attractive for liquid/liquid and oil cooling applications in narrow spaces and/or skid systems. Flanged valves or pipe line flanges may be directly connected to the endplates.
The plate pack can be placed concentrically or eccentrically inside the shell, depending on the application. The eccentric version is often used in flooded evaporators
The space between the shell and the plate pack can be partly filled with flow directors. In this way it is possible to create an optimal flow through the plates. The standard angle α of the axial flow directors is 90°. In condensers, an angle of 45° is used as a standard solution. For specific applications, i.e. low Δp applications, α < 90° might be helpful. In some circumstances no flow directors are used, for example in Vahterus PSHE combined evaporators. For unit type C, flow directors are turned through 90°.
Stoppers for multi-pass units
The flow on the plate side is often designed as one-pass, but it is possible to create more passes by using stoppers. Flow with more passes gives the following benefits:
- the larger pressure drop allows a better distribution over the plates;
- a higher flow velocity between the plates allows increased heat transfer and reduces the possibility of fouling;
- large temperature differentials can be better handled compared to a single-pass heat exchanger;
- functional separation, such as evaporating and superheating, is achieved in direct expansion systems.
When using more passes on the shell or plate side, attention must be paid to correct purging and draining of the different sections within the exchanger. Trapped air or non-condensables may have a negative effect on the heat exchanger’s performance. The flow directors on the shell side are assembled using fabricated metal sheet parts and flexible seal materials to create the passes. Care needs to be taken in the following areas:
- draining the shell side. It can take a long time to drain the space between the flow directors;
- cleaning the shell side. The cleaning detergent must be suitable for the flow-directors. Flushing after cleaning must be done very carefully to clean all the spaces. For this reason, use of the plate side is recommended for a dirty medium.
Plate side stoppers may consist of metal with gasket material. Please check the technical data sheet for the applicability with the plate side medium. Plate side stoppers may be provided with a small purge hole on top (A) or a small drain hole at the lowest point (B). The correct location of stoppers must be carefully designed. It is not possible to move them after the plate pack has been constructed and it is very difficult to remove them.
The flow direction of plate side flow in comparison with shell side flow might be:
- counter-current flow;
- co-current flow;
- counter-current and co-current flow in one heat exchanger.
The selection for a certain flow direction is determined by:
- the application;
- the temperature differences;
- the best flow direction in relation to draining or purging the heat exchanger.
Normally the heat exchangers are installed with plates in the vertical direction as shown above. If it is necessary for the process or system layout, the plates may be installed horizontally.
The heat exchanger can be mounted on feet or saddles constructed from carbon or stainless steel, wood or other insulating materials. Some examples are given in the drawings below. Type D, E and F are not standard. Vahterus supports are designed to accommodate the exchanger weight only. They do not incorporate specific requirements such as transport loading, wind loading and earthquake forces, which are not permitted unless otherwise shown by strength calculations.
Other supports available on request. Please tell us your specific requirements.
Name plate supports
Two different supports for name plates are available (as shown in the figure below). Type A should be used for heat exchangers with insulation at high or low temperature. Type B can be used for cases where no insulation or small insulation thickness is necessary. The direction and the location of the supports should be confirmed by the customer according to the specific installation on site and the requirement for nameplate visibility. This should be considered during construction drawing approval.
Each heat exchanger leaves the factory in a sturdy wooden box. The boxes are made from Finnish pine harvested from a sustainable forest. The wood is free from quarantine pests and practically free from other injurious pests. The wood is ISPM15 compliant and is supplied with the saw-mill stamp shown in the image below.