About Double Wall Heat Exchangers
Double wall heat exchangers are extremely cost effective solutions for eliminating the possibility of contamination of the process fluid into the heating medium. This is particularly relevant when the heating medium is not an isolated system and used for other purposes, such as cooling or heating other systems. AF Pipe Solutions can offer a range of double walled heat exchangers, designed in house, and produced according to EN or ASME standards in co-operation with an authorised and approved contractor.
Double wall heat exchangers have existed for some time for both shell and tube , and plate type exchangers. Not to be confused with a double tube type (or double pipe) heat exchanger, which is simply a tube-in-tube exchanger whereby the heating medium is contained in one tube and the process medium is in the other tube, itself the simplest form of heat exchanger. Double wall heat exchangers are used in many applications , typically to completely avoid risk of contamination between the heating medium and the process medium. This is critical in many applications (for example potable water, nuclear systems) but also highly relevant when the cooling or heating circuit is shared across many systems. In this case it is very important that any contamination of the heating medium does not travel around the system, therefore a double wall system is applicable. This is where it becomes far more economical to apply double walled heat exchangers, compared to other solutions such as applying an intermediate heating loop, as shown in the figures below:
Without a double walled heat exchanger, an intermediate heating loop is required in order to isolate the central heating medium from the process medium in order to completely avoid any risk of contamination of the primary heating fluid. This requires a significant amount of additional equipment as shown in the diagram above. For each heating circuit, at least one extra heat exchanger, one expansion tank, one pump, leakage detection means, and additional control valves and sensors to regulate the intermediate heating circuit.
When a double wall heat exchanger is applied, in this case a Shell & Double tube type, then the intermediate heating circuits can be removed, as the process fluid is already securely isolated from the central heating fluid in the heat exchanger itself, by means of the double walled heating tubes and heat exchanger design itself. Therefore the only additional expense is the double wall design of the heat exchanger, compared to all the additional equipment required for the intermediate heating circuits.
The working method is fairly simple in that the tubes or plates with the process medium are encased by a secondary tube or plate, creating a vacant volume for any leaking process fluid to leak into, thereby preventing contamination of the heating medium. For high pressure (up to 420bar) or high temperature (up to 400deg C) applications then shell & tube type exchangers are applied, and plate type heat exchangers are normally used for less demanding applications. The leaking fluid collects in the leakage space, where it can be easily detected and contained, prior to any process fluid escaping causing an unsafe situation.
Whilst the construction of standard shell & tube heat exchangers is well covered under design codes, the shell & double tube heat exchanger requires additional design considerations both for heat transfer efficiency, and for thermal compensation in the double tubes themselves. A fundamental concept for maintaining high efficiency is to maximise heat conduction between the inner and outer pipe, therefore the AF Slim double wall pipe is perfect for this application. As previously described here, the inner pipe is expanded into the outer pipe, so there is maximum wall contact thereby minimizing heat losses between the inner and outer pipe, whilst allowing for leakage detection in the extremely compact annular space.
Special consideration must also be given to the temperature differences between the inner & outer pipes of the double wall system, and how the tubesheet design can compensate for these differences, if calculations show this is necessary. For example on smaller size exchangers with a low ∆T then a fully fixed construction may be permissible, but this won’t always be the case. Therefore it is recommended to consider the design criteria at an early stage in the design phase, to establish which type of shell & double tube heat exchanger is applicable. This also applies to the small reduction in efficiency of the exchanger, due to the gap that is introduced in between the inner & outer pipe. If the efficiency loss is too great (typically between 1 and 3%), then an inert thermal fluid can be used in the annular space to minimize efficiency loss; in this case pressure leakage detection can still be applied.