Page 17 Heat Exchange

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The Stirling engine is really just a set of heat exchangers.

The heat of the burner is transferred into the metal of the hot cap.
The hot metal then transfers the heat into the gas inside the engine.

The regenerator transfers heat from hot gas into its thin wires and then back into the gas.
For each stroke of the engine.

Then the cooler takes remaining heat from the gas and transfers it to the cooling water jacket.

Robert Stirling’s patent details four types of heat exchanger. The engine is just one example.

Inside the engine, heat only transfers from metal to gas, or vice versa, when molecules
touch the metal. Thus the need to maximise the surface area of the metal with as many
tightly packed fins as possible.

If the air is still, and next to the metal, very little heat is transferred.
It is only when there is a flow of of gas across the metal that heat flows.
This is why a Stirling will not self start.
The motion of the displacer is needed to throw molecules against the metal to cause
heat exchange.
Heating the top of the hot cap is no good. The air at the top of the hot cap is pretty stationary.
Heat must be applied to the side of the hot cap.
It is only the side of the hot cap that has a flow of air passing along the inside of the cap.

Making all these fins is arduous.
Thus the attraction of heater tubes – despite the disadvantage of dead space.
And the likelyhood of multiple leaks.

But fins are better. One of the UK boat engine makers converted a heater tube engine to
internal fins. The engine power was increased.

How to make internal fins?

Unfortunately it’s a slow process.

I used a motorised slitting disc mounted on the lathe tool post. The saw could be
worked in and out of the tube of the hot cap. It took me five minutes or so per slit.
My concentration on a hand operated lathe only allowed me to cut four or five slots
in a session. I spread the task out over many days.

Wire erosion is another method. When an electrical current is passed through
a wire resting against a metal surface, a spark will erode some of the metal.
I am told it takes approximately 20 minutes to erode one slot in stainless steel.
But at least it is mechanised.

The hope is that metal 3d printing will come to the rescue.

It too, is a slow process.
A 60 mm diameter plastic hot cap with internal and external fins took 8 hours to print.
Metal 3d printing is much the same.

One of the prime usages of metal 3d printing is for heat exchangers.
With computer design, we are not limited to simple straight line slots.
Far bigger heat transfer areas can be generated for the same volume.
The hope is that the cost will come down.

The Stirling engine will benefit greatly from 3d printing.