Selecting a Shell and Tube Condenser

Which Side Should the Vapor Be On?

This is the first question and it drives everything after it. Vapor on the shell side gives a large flow area, which is what you want when the volumetric flow is high — and under vacuum, volumetric flow is always high. It also lets you use the X or J shells that exist specifically to keep vapor-side pressure drop low. The drawback is that the shell side is the harder side to clean and the harder side to drain completely.

Vapor inside the tubes is the better answer for hygienic service, where the wetted side must be smooth, positively drained and cleanable, and for reflux and dephlegmator duty where condensate has to run back down the same passage the vapor came up. It is also the choice when the vapor is at high pressure, because containing pressure in tubes is cheaper than containing it in a large shell.

Shell Types That Exist for Condensing

The TEMA X shell, or crossflow shell, is the one most commonly used for vapor condensing. Vapor is distributed across the top of the bundle and flows down across the tubes, which produces a very low shell-side pressure drop and makes the X shell the standard answer for vacuum condensing. Proper X shell condensers reserve a tube-free lane along the top of the exchanger so vapor distributes evenly, and a clear area at the bottom of the bundle so condensate can run to the outlet nozzle without backing up into the tubes. Because the vapor path is short and the velocities are low, careful attention to noncondensable removal is essential in an X shell — there is little natural sweeping action to move gas along.

The J shell, or divided flow shell, is the other condensing-oriented design. For a condensing shell fluid it is arranged with two vapor inlets and one central outlet, so each half of the vapor travels half the length. Pressure drop falls by roughly an order of magnitude compared with the equivalent E shell. The same shell reversed, with one inlet and two outlets, is the conventional arrangement for thermosiphon reboilers.

Rear Head: The Cleaning and Expansion Decision

Fixed tubesheet construction welds the tubesheets to the shell. It is the simplest and least expensive design per square foot of surface, and the tube bores can be cleaned mechanically or chemically without difficulty. What you give up is access: the outside of the tubes can only be cleaned chemically, and there is no provision for differential thermal expansion, so a large shell-to-tube temperature difference requires an expansion joint.

U-tube construction has a single tubesheet with bent tubes, so every tube is free to expand and contract independently. That makes it the most tolerant design for thermal shock and large temperature differences, and the bundle lifts out for shell-side cleaning. The trade-off is the U-bend itself, which is awkward to clean mechanically on the inside.

Floating head designs let the rear tubesheet move axially inside the shell. They are the most flexible and the most expensive. The split-ring type is the usual choice where both thermal expansion and regular shell-side cleaning are required; the pull-through type is simpler to maintain because the whole bundle comes out as one piece, but it costs more and carries fewer tubes because of the annular dead space around the floating head. Packed and gasketed floating tubesheets are cheaper still, but the seal can leak, so they are limited to fluids that are neither toxic nor volatile.

Materials and Class

Condensate chemistry, not bulk vapor chemistry, should drive material selection. The first liquid to condense concentrates whatever acid or chloride is present into a small volume, and that film is what attacks the tube wall. 304L and 316L stainless cover a great deal of condensing duty. Duplex grades, titanium, high-nickel alloys such as Hastelloy and AL-6XN, and lined or graphite construction all exist for the services where stainless will not survive.

TEMA class sets the construction standard rather than the geometry. Class R is the most stringent, intended for refinery service, with the heaviest materials and tightest tolerances. Class B covers chemical process service. Class C is lighter construction for general commercial duty. Matching the class to the severity of the service is a straightforward way to avoid paying for robustness you do not need — or regretting the absence of it.