As the name indicates, a heat exchanger is a device that exchanges heat between two fluids (gas or liquid) which are at different temperatures. Two common heat exchangers are heating radiators where hot water heats cold air to heat a room, and in a car where the radiator cools the water coming from the engine with air from the outside.
Most heat exchangers work by passing a fluid or gas through tubes, while a second fluid or gas flows around those tubes allowing heat to be transferred without the two fluids or gases mixing.
There are two ways to make the process work more efficiently. One is to add devices like fins to increase surface area but these add weight. The second solution is to make the tubes smaller which, if the walls can be made thin enough, does not introduce a weight penalty.
Having smaller tubes means more tubes and therefore more surface area over which to transfer heat in a given volume and therefore a more compact and lightweight heat exchanger. It is this second approach that is used in the SABRE engine Pre-cooler which has tubes around 1 mm in diameter with wall thicknesses of 27 microns (that is thinner than human hair).
As the air passes thorough the Pre-cooler tube matrix it is evenly cooled and the temperature plummets from 1,000°C to minus 150°C.
The SABRE Pre-coolers incorporate two major new innovations. The first is the spiral arrangement of the tubes which makes the heat exchanger more efficient.
The other innovation is a frost control system that prevents water vapour in the air from forming frost during low temperature operation which if uncontrolled can completely block the Pre-cooler in a matter of seconds. The details of this frost control system are confidential.