The other HEX design that we use is the counter flow design, which in our case is a 20' 1/2"X .049 wall 304SS tube inside a 3/4" 304SS tube, rolled into a convenient diameter, in our case a 10" coil. The heavier .049 wall is required my ASME for LPG service, in case of extrusion or butt welding defects in the tubing itself..
We used Swagelok reducing compression tees to finish the ends. One end of the base of the tee is 3/4" and the other is 1/2", with the perpendicular tee member 1/2" as well.
We drill through the 1/2" tube pocket on the base leg, so that the 1/2" tube passes through it, allowing us to clamp both the 3/4" and the 1/2" compression fittings on their respective tube.
The 3/4" jacket is where the coolant passes, with the LPG passing through the 1/2" center tube. The inlet of the center tube gets a 1/2" tube to JIC connection, but the outlet gets another 1/2" X 1/2" X 1/8" compression tee, and the perpendicular leg gets a 1/8" tubular thermocouple to track the HEX's performance.
We use 1/2" tube to JIC connections on the rest of the ports on the pump HEX. On the HEX for LPG injection cooling, we use a coil of 3/8" 304SS tubing from the LN2 Dewar liquid valve and the HEX, with compression fittings, because polymers perform poorly at those temperatures.