A kitchen experiment to demonstrate the power of condensing steam
What do you think about when you consider how a steam engine works?
Well you probably think about a fire used to heat water in a pressure vessel to form super-heated steam.
Then that steam is injected into cylinders in a controlled and sequenced manner to push a piston up the length of the cylinder. Piston rods and cranks are used to change the linear motion into rotary motion to drive wheels or gears.
Inertia causes the rotary motion, through the same cranks and rods, to expel the spent steam from the cylinder, before the whole cycle begins again.
But that's only half the story.
Steam engines also use the power of a partial vacuum created by condensing steam to produce linear motion in the same way, or to suck the hot products of combustion from the fire through pipes which pass through the pressure-vessel and heat the water.
What you will need for the experiment
- An empty drinks can, such as a 330ml soda can.
- A shallow container, such as a baking tray.
- A pair of barbecue tongs or kitchen tongs with which you will be able to grasp a hot can without burning your hand or getting too close to the heat.
- A source of heat, the cooker top in the kitchen is ideal.
How to do it
Thoroughly wash out the empty can. Any sugary residue might burn and smell.
Put about a centimetre (half an inch) of water into the can. Enough to cover the centre of the concave end of the can inside.
Place your shallow container next to your source of heat. This will most probably be a baking tray on the kitchen worktop next to the cooker where you will heat your can.
Put about a centimetre (half an inch) of cool water in the tray.
Place your can on the heat. It will quickly begin to boil because it only contains a small amount of water.
Wait until the water is really boiling and filling the can with steam.
Grasp the can with the tongs. You need to do this in an underhand grip which will allow you to, in one swift and smooth motion, remove the can from the heat, turn it over and plunge the open end into the cool water in the tray.
Do exactly that. As swiftly and smoothly as you can.
Here you will see the need for a vessel like a baking tray, the larger target makes the quick and smooth transfer of the can from the heat to the condensing cool water easier.
The can is full of steam, which will instantly condense when the open end of the can is plunged into the cool water.
The partial vacuum created in the can will cause it to be very quickly and impressively crushed.
The power of a partial vacuum is far easier to harness and is more powerful than pushing a piston along a cylinder with pressurised steam.
The mechanism of an efficient steam engine will inject pressurised steam into a cylinder during the phase of motion where the piston is pushed up the length of the cylinder, and inject a small amount of cold water into the cylinder fractionally after the full travel of the piston and as it is just beginning the return stroke. In this way the cylinder is both pushed and pulled up and down the cylinder.
But the condensing part of the cycle is far more efficient at pulling the piston than the steam injection phase is at pushing it.
The dramatic 'chugging' of a steam engine as it pulls away from a station is the part of the sequence in which condensing steam is used to create a partial vacuum in the smoke-box at the front of the engine. The hot products of combustion are drawn by this partial vacuum from the fire at the far end of the engine, through the pipes which pass through the boiler, increasing the efficiency of the heating of the water. The same partial vacuum is also used to increase the draft of air into the furnace.
It is mostly the rush of clean air into the furnace, drawn by the partial vacuum, which gives rise to the deep and pleasing heart-beat of a big powerful engine as it works hard to move a heavy train from a standing start.