Shell-and-tube reboilers are most common but four compact heat exchangers — plate-and-frame, welded plate, spiral plate and spiral tube — have found some application as reboilers. In addition, double-pipe exchangers have seen limited use. So, let's look at each of these.
Plate-and-frame. These exchangers use a series of parallel plates. Gaskets between the plates keep the process fluid in. Gasket geometry, combined with flow passage openings in the plates, segregates different process fluids on each side of a single plate. A frame holds multiple plates, creating many parallel flow paths for each fluid. Corrugations on the plate increase strength and improve heat transfer. Plate-and-frame exchangers can offer very large surface areas in a relatively compact size.
While most of these exchangers handle liquid/liquid service, they can serve as reboilers. Operating conditions normally are restricted to below 375 psig and 300°F.
Advantages include large area at moderate cost, possible high heat-transfer coefficients, and small exchanger physical size for the area. Depending upon layout, they may provide true counter-current flow, allowing for close temperature approaches. Most only exchange between two fluids, but some highly heat-integrated plants rely on specially fabricated units to deal with up to five fluids. Many are fabricated from semi-standard components, making maintenance parts relatively easy to obtain. Because frame sizes are standard, adding additional plates may be straightforward.
Reboiler service complicates design. Momentum change effects in the parallel flow paths can require extensive design work or even modifications after initial results are checked in the field. While inconvenient, modifications are much simpler than for most other types of exchangers. Other disadvantages may include the need to have a pumped system, materials selections for the gaskets, thermal expansion, and the potential for fouling.
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Your maintenance department must know how to handle the exchangers. Plates and gaskets can be ruined if the bolting on the frame is set by torque instead of by position or plate assembly thickness.
Welded plate. Such exchangers eliminate the gaskets of the plate-and-frame. Welding plates together solves one problem but creates another — the plate pack can't be dissembled for cleaning. So, conventional welded-plate exchangers only suit clean services. Complex welded-plate exchangers also tend to suffer from thermal expansion problems. Because the plates usually are long, differential thermal expansion can create large stresses. Conventional welded-plate exchangers can provide true counter-current flow. They serve as reboilers in some cryogenic and ultra-clean applications.
New derivatives can act as cross flow exchangers. These often feed the process fluid in from edges 90° from each other. Along with simpler plate geometry, this creates an exchanger that possibly can be hydroblasted clean. A limited number of reboilers have used this newer design.
Spiral plate. These exchangers wrap a long continuous channel into a spiral — most often to provide cross flow but it's possible to create a true counter-flow reboiler, as well. Variants include both welded and flange-gasket units, as well as gravity-flow and pumped-flow versions.
Advantages include surface areas that are very large for the exchanger size and high heat-transfer coefficients. Spiral plate exchangers boast excellent records in services containing solids — and often are the exchanger of choice for such systems. Flange-gasket designs also typically are relatively easy to clean. Properly laid out, spiral plate exchangers can tolerate fairly large temperature variations without excessive thermal stress.
The main disadvantage is usually cost, especially for the flanged versions. The large flanges, as well as fabrication requirements, may make these the most expensive option.
Spiral tube. Such exchangers run a coil of tube from a distribution manifold to a collection manifold. The tube-and-manifold assembly is inserted into a shell or can. The design easily accommodates thermal stresses. The availability of bendable tubes in the correct materials may limit fabrication. Spiral tube reboilers often are very small and used in applications where fabrication limitations make other types impractical.
Double pipe. Also known as pipe-in-pipe exchangers, these have one or more pipes inside a larger pipe. As such, they don't have a conventional TEMA shell. Normally, the internal pipes have U-bends, so inlets and outlets of both sides are at the same end.
Advantages include true counter-current flow, ability to handle extremely high pressures, and easy cleaning. Disadvantages include relatively large plot space requirements for small surface areas, and multiple sealing areas that may lead to fugitive emissions or leaks. They almost always require a pumped system.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can e-mail him at [email protected]
