How Do You Warm One Thing With Another Without Mixing Them? Surface Area

The requirement is to move heat from one stream to another while keeping them separate, which matters because the streams are frequently incompatible, being at different pressures, chemically hostile, or simply things nobody wants mixed. The rate at which heat crosses depends on three factors, namely the area of the barrier, the temperature difference across it and how readily heat passes through the materials and the boundary layers at each surface. Design therefore consists of maximising area within a given volume, maintaining as large a temperature difference as possible along the whole path, and disrupting the sluggish layer of fluid that clings to each surface and does most of the resisting.

A few configurations cover most applications:

• •Shell and tube, with one fluid inside a bundle of tubes and the other around them
• •Plate exchangers, with thin corrugated plates stacked to form alternating channels
• •Finned tube, adding fins where one fluid is a gas and transfers heat poorly
• •Double pipe, one tube inside another, simple and used for small duties
• •Regenerative, where a rotating matrix is heated by one stream and then the other
• •Direct contact, where the streams do mix, used in cooling towers

Running the two streams in opposite directions rather than the same direction improves performance substantially, and the reason is worth following. With both flowing the same way, the hot stream is coolest where the cold stream is warmest, so the temperature difference collapses towards the far end and heat transfer there is poor, and the cold stream can never leave hotter than the hot stream leaves. Running them in opposite directions keeps a useful difference along the entire length, and the cold stream leaves at one end against the hottest incoming fluid, so it can emerge hotter than the other stream leaves. That single arrangement is why nearly every efficient design uses opposed flow.

The largest application is not transferring heat where it is needed but capturing heat that would otherwise be thrown away. Industrial processes discharge enormous quantities of warm exhaust and warm water, and passing that stream against an incoming one preheats the incoming material, so less fuel is needed to reach the working temperature, which is among the cheapest reductions in energy use available anywhere. Buildings do the same with ventilation, warming incoming fresh air against outgoing stale air and recovering most of the heat that opening a window would lose. Power stations use the technique throughout. The limit is that recovered heat is only useful if something nearby wants heat at that temperature, which is why so much low grade heat is still discarded.

These devices are everywhere once recognised, in the radiator of a car, the coil at the back of a refrigerator, a domestic boiler, an air conditioner, every power station, and the system that recovers heat from outgoing ventilation air in a well-built house. The persistent problem in all of them is fouling, where deposits build on the surfaces from scale, corrosion, biological growth or particles, adding an insulating layer that reduces performance progressively and restricts flow. Designs therefore include allowance for it, with extra area built in and arrangements for cleaning, and industrial operation involves scheduled cleaning that costs money and downtime. Leakage between the streams is the other serious failure and is why some applications use a double barrier.

Two streams pass either side of a barrier so heat crosses and nothing else does, and performance depends on area, temperature difference and the sluggish fluid layer at each surface. Shell and tube, plate and finned designs cover most uses. Running the streams in opposite directions maintains a useful difference along the whole length and lets the cold stream leave hotter than the hot stream does.