A. Inside tubes — vertical downflow

A vertical inline condenser is shown in Figure 1 as a shell-and-tube unit with an outside packed head and a separating head. A fixed tubesheet construction can be used if shellside cleaning is unnecessary or can be done chemically. The space between the upper tubesheet and the upper shell nozzle can entrap air; therefore, special vents are drilled into the upper tubesheet, which should be arranged to discharge continuously into an open drain. Thus we are assured that the tubesheet is wetted and cooled, and that the water is kept from stagnating. This reduces fouling and corrosion, which would otherwise occur at the air-water interlace. The lower separating head has either a funnel or a baffle designed to minimize re-entrainment of condensate into the vent gas stream. The condensate level is kept below the baffle or funnel.

Figure 1 Vertical in-tube downflow condenser

The vapor enters the top head usually through a radial nozzle, although an axial nozzle can also be used. For axial nozzles, the nozzle entrance velocity head pressure should be compared to the condenser tube pressure drop to ensure that maldistribution is not severe. If needed, a perforated impingement plate with 5–10% hole area placed 0.5–1.0 nozzle diameters downstream will help.

The vapors condense on the tube wall as an annular film and drain to the bottom. Tube diameters are usually 19 mm or 25 mm, although for low pressures larger tubes up to 50 mm are used to reduce pressure drop. Occasionally small diameters of 16 mm are used. At the end of the condensing zone there is a vapor-noncondensable gas interface, and below this interface the condensate is subcooled as a falling film. As the load on the condenser varies, the vapor-gas interface (and, hence, the ratio of condensing to subcooling lengths) will change. System pressure control is obtained by controlling the vent gas pressure. If the noncondensable gases in the incoming vapor are insufficient to maintain pressure control for a reduction in load, then for pressure operation an inert gas is bled in or for vacuum operation air is bled into the vacuum line. For atmospheric operation, the vent line exhausts to a stack.