Today I hosted a presentation by Alfa Laval at Grangemouth covering a couple of their compact heat exchangers. What follows are my rough notes from the talk.
I went into the presentation with a little bit of experience of alfa laval exchangers, several were used in the sugar factories that I had worked in, though I have not had any experience with ones used at higher temperatures and pressures. The ones used in the sugar factories were the gasket type exchangers, where each plate can be removed and cleaned separately before being lined back up and pushed together.
I was aware that they can be welded, and I had assumed that if they were welded, then anything other than a chemical clean would be impossible. It turns out that this is not the case. Even though the plates are welded, it is possible to access them in situ and jet the surfaces clean.
In the session that lasted just over an hour, Alfa Laval covered two of their exchanger types, the compabloc and the spiral heat exchanger.
The compabloc is very similar to the gasket exchangers, except that the plates are welded to each other in a vertical stack. The plates are set up that the individual passes are cross flow, the welds alternate between being welded at both the back and front, and being welded down the sides. Therefore there is always metal between the two fluids, no gaskets. This stack of plates is stored within a box that has sides that are bolted up. Baffles can be placed in the void between the side of the box and the plates, forcing a multi pass arrangement and effectively creating a counter flow heat exchanger.
The sides can be removed, allowing access to the gap between the plates so they can all be cleaned with high pressure water jets. The welds are also all accessible so can be inspected and it is possible to plug an individual pass if required. I was initially worried about the fouling aspects but it has apparently been used on a crude heating duty without any filters and proven to be less prone than the shell and tube exchangers they replaced. They recommended that if you are expecting to have periods of significant turndown, it would be better to have two smaller exchangers and to isolate one of them during turndown periods. This would reduce the amount of fowling generated during turndown periods and also gives an opportunity to clean one of them if required.
The exchangers are suitable for stable temperatures (-46 to 343°C) and pressures. If the process is expected to be cyclic, then it is probably better to find a different style of exchanger.
The spiral exchangers are made from a sheet of metal that is wound around forming two channels that wind in together. This allows for try counter flow heat exchange. The edges are again welded on opposite sides to keep the materials separate but allows access to mechanically clean. The exchangers are described as self cleaning, because if material builds up, it restricts the surface area and causes the velocity to increase past the restriction. This localised increased velocity can then help dislodge the fowling material.
The spiral exchangers can be rated for temperatures between -100 and 400°C and can handle pressures between full vacuum and 100barg. The temperature limit seems to be because the end cap is made of carbon steel though can be lined with another material.
Overall it was good to learn more about the exchangers on offer and to re-evaluate some of my earlier preconceptions about the technology.Go Top