Schmidt'sche® Linear Exchangers (SLEs) are closed-coupled to individual furnace radiant coil outlets, typically arranged in double tube register modules as shown on the right side. The number of double tubes per module is determined by the furnace coil outlet configuration and limited by overall transportation dimensions.

Modules can consist of a single double tube arrangement or as many as may be limited by furnace or transport dimension. Modules of up to 13 double tubes and more are not uncommon. Short inlet transition pieces connect individual radiant coils to one double tube. The inlet transition piece is designed to gradually reduce the skin metal temperature between the hot coil outlets and the cooled exchanger oval headers from approx. 1,650°F (900°C) to around 600°F (320°C). Thermal stresses that could cause premature failure are avoided by having a moderate temperature gradient in the transition piece.

The selection of the right metallurgy for the inlet transition piece and verification of the design using finite element analysis has resulted in long trouble-free operation. The top and bottom oval headers of the SLE are connected to the riser and downcomer piping respectively. There is no tube sheet and only a minimum adiabatic zone in the transition between the furnace and the exchanger tubes.

Over 5,000 TLEs use the Schmidt'sche® Oval Header design. Since 1980 over 800 SLEs have been put into operation without a single failure of this component. The high feed water flow velocity through the oval header and the annulus between inner and outer tubes not only assures effective cooling of the process gas but maintains the tube metal temperature uniform at relatively low temperature which guarantees longevity. The high flow velocity also prevents settlement of solids in the bottom oval header; thus preventing overheating and under deposit corrosion. The inner bore of the process tubes on a linear exchanger is matched to meet the inner diameter of the radiant coil outlets. As a result, linear exchangers typically have large bore tubes with inner diameters of up to 160 mm, (6.3").

The linear exchanger is typically called primary quench exchanger, PQE, whereas a second unit is referred to as the secondary quench exchanger or SQE. The SQE can be either a Schmidt'sche® Conventional Multi-Tube TLE operating in vertical position with gas flow from top to bottom or bottom-up or alternatively a horizontal shell and tube unit.

In some installations, a second linear exchanger leg is used in place of the SQE. The primary quench exchanger is then connected to the secondary leg using return elbows.