How to Make a Tube-in-Tube Heat Exchanger

Choose the sizes of the tubes that will form the inner and outer shells of the exchanger. The outer shell should be approximately double the diameter of the inner shell. For example, if you choose a 1-inch-diameter tube for the outer shell, use a 1/2-inch-diameter tube for the inner shell. Put on appropriate personal protective equipment, including safety glasses and gloves.

Cut a piece of the large-diameter tube to the required length for the outer shell. This will usually be between 2 and 3 feet and will depend on the space you have available to mount the exchanger. Use a 3-foot length if you have space. If you need more heating or cooling capacity than a 3-foot exchanger will provide, use multiple exchangers. Use one exchanger initially and check the results to determine whether you need more capacity.

Select two equal tee fittings that are the same diameter as the tube. Solder one to each end of the tube, with the arms of the tees in line with the pipe and the leg of each tee sticking out at right angles to the pipe. Align the legs of the tees so they are diametrically opposite one another. Check this by viewing the pipe from one end – one tee leg should be pointing up and the other pointing down.

Select two identical reducer couplings, sized to join the two different diameters of tube that you are using. For example, if you chose 1-inch- and 1/2-inch-diameter tubes, select 1-inch to 1/2-inch reducers. Solder one reducer to the free arm of each tee. If necessary, use a short piece of the large-diameter tube as a jointing piece between the tee and the reducer. You should now have a cylindrical assembly that has tapered ends and two tee legs protruding on opposite sides of the assembly.

Check if the reducers have internal stops that prevent the small diameter tube from passing straight through the bore. If so, file the inside diameter of the reducers carefully to remove the stops. Check that the small-diameter tube passes snugly through each of the reducers.

Measure the length of the soldered assembly between the outer ends of the reducers. Cut a piece of the small-diameter tube that is 6 inches longer than the assembly. For example, if the assembly measures 30 inches in length, cut a 36-inch length of small-diameter tube.

Insert the small-diameter tube into the bore of one of the reducers and through the inside of the assembly. Feed it carefully out through the other reducer and position it so that 3 inches of tube protrude from each end. Solder it in this position.

Connect a hosepipe to one of the tee legs and seal the other leg. Turn on the hose and check all soldered joints. Disconnect the hose, drain the water and repair any leaks that you found. The two tubes will now function as a heat exchanger, transferring heat from fluid flowing through one tube to fluid flowing in the opposite direction through the other.