Thermal Transfer Systems TEMA Designations of Heat Exchangers - Manual (Page 6)

The TEMA X shell, or crossflow shell is most commonly used in vapor
condensing
applications, though it can also be used effectively in low pressure gas cooling or heating. It
produces a very low shell side pressure drop, and is therefore most suitable for vacuum
service condensing. In order to assure adequate distribution of vapors, X-shell designs
typically feature an area free of tubes along the top of the exchanger. It is also typical to
design X shell condensers with a flow area at the bottom of the tube bundle to allow free
condensate flow to the exit nozzle. Careful attention to the effective removal of non-
condensables is vital to X-shell constructions.