Schmidt'sche® Bathtub Transfer Line Exchangers (TLEs) are multi-tube exchangers having several double tube registers welded together to form an oblong-shaped exchanger inlet thus allowing two or four gas inlet connections. Each inlet nozzle is connected to a set of furnace coils.

The "tube sheet" is formed by the oval headers and is installed in a step-like arrangement to reduce the inlet channel volume and enhance gas flow patterns. A typical furnace will have 3 to 8 exchangers as shown in the arrangement sketch.

The bathtub TLE inlet channel volume is about one half of that in a conventional Transfer Line Exchanger inlet minimizing the adiabatic residence time and enhancing olefins selectivity.

The residence time in a bathtub TLE is about 20 milliseconds compared to 40 milliseconds in a conventional Transfer Line Exchanger. The unique geometric shape of the bathtub offers better gas flow patterns which reduce recirculation and improve gas distribution to the tubes; hence, reducing residence time, which provides quicker cooling of the gas and improved steam generation.

The inlet channel is internally lined to protect the shell from high gas temperatures. The outlet channel is normally not insulated unless the temperature is in excess of 985°F (530°C) to 1,020°F (550°C).

The outlet flange made of high temperature resistant materials exits on top or laterally to connect directly to the downstream piping. As with the Schmidt'sche® Conventional TLE the ends of the double tube register are connected to the riser and downcomer manifolds. The "tube sheets" are connected to the inlet and outlet channel using specially designed flanges.

The Schmidt'sche® Bathtub TLE as shown in the arrangement sketch has gained wide acceptance in new grassroots plants. Thanks to its unique design it has found its way into the replacement market of conventional TLE where furnace coil configurations lend themselves to the advantages of this short residence time TLE. Using a multiple inlet TLE will eliminate unnecessary Y-pieces and manifolds.