Double-wall heat exchangers: enhancing safety and efficiency

The need to protect the environment is ever increasing, Funke is playing its part in many ways and one is through the development of a double-wall safety range of heat exchangers. These units are designed to prevent cross-contamination between fluids inside the heat exchanger.

Text & images by Funke Wärmeaustauscher

How it works

Safety heat exchangers are designed as three chamber devices with straight internal tubes and fixed (F) or removable (P) tube bundle. In each individual internal tube of the tube bundle a further smaller internal tube is installed. These concentric internal tube pairs form radial gaps which are interconnected by the design of the double tubesheets and thus produce the enclosed safety space. A mixing of the flowing operating media is prevented in this way. This sealed safety space is filled with a special barrier fluid. It is responsible for the heat transfer in the radial gap and functions as a transmitter in the leak monitoring system. The safety area is sealed off and permanently monitored for pressure changes. In the case of a leakage of the outer or inner tube, the corresponding pressure change within the safety space will be detected by the pressure control device and signalled accordingly. The safety area is also equipped with an expansion tank which prevents false alarms due to thermal volume or pressure changes.

Design features:

• Media separated by double tube bundles, media cannot mix;
• Special sealing liquid for immediate leak indication;
• Leakage system works independently of the installation position;
• Leakage detection is possible through process control;
• Suitable for a wide range of liquid media.

From this, Funke has gone on to develop a new generation of safety heat exchangers. A structured inner tube is hydraulically expanded to a smooth outer tube and the gap created between the tubes is charged with a vacuum for leak detection. The hydraulic expansion gives maximum contact between the two tubes and ensures optimum heat transfer characteristics.

Specialties and advantages

In conventional technical solutions with double tubes, the outer tube has a relatively rough structure on its interior. FUNKE, however, shifts the structure to the outside of the inner tube and uses a finer structure in a pyramid shape.

The advantages of moving the structure to the inner tube are:

• Contact area between inner and outer tube of a double tube increased by up to 80%;
• Optimized distribution of safety channels;
• Heat transfer with or without transmitter fluid in the safety area;
• Optimum tube/tube sheet connection by rolling in with groove and/or welding;
• Crevice corrosion prevented by applying the inner tubes after tubes are welded in;
• Compact sizes and reduced weight.

In double tube systems, the required safety space is created by a pair of tubes – a smooth outer tube and an inner tube with a surface texture which is plastically deformed (expanded) under pressure.

Applications

In case of leakage the application of FUNKE safety shell-and-tube heat exchangers prevents from
transferring pressure and temperature to the other side and avoids other undesirable consequences such as:

• A chemical reaction when the media mix together;
• Possible damage to the low pressure side of the heat exchangers;
• Damage to downstream system components from mixed media or increased temperatures and pressure;
• Hazards for human beings and the environment;
• Removing, replacing and disposing of mixed media;
• Downtime due to the need of purchase a replacement or make repairs;

The Funke safety shell-and-tube heat exchangers with expanded double pipes are mainly used in the chemical industry, pharmaceutical technology, solar industries, natural gas pre-heating and transformer oil cooling (according to DIN EN 501216-9). The safety technology has also been developed in the company’s plate type heat exchanger range with a series of double wall plates. More information can be found at www.funke.de.

About this Technical Story

This Technical Story was first published in Heat Exchanger World Magazine in September 2024. To read more Technical Stories and many other articles, subscribe to our print magazine.

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