💦 The Slayer Steam post in the forum
generated a lot of interest. It also reminded me that I’ve been meaning to geek out about steam. So here goes!
Have you ever wondered why we use steam to heat milk, rather than hot air or a stovetop arrangement? It might seem like a silly question, but there’s some really interesting science behind why steam is so great at heating up moo juice.
We know that steam is hot and that’s why the milk gets hot. We also know that steaming a jug of milk to drinking temperature will increase its weight by 10%. This is because the steam condenses and adds water to the milk. Duh.
The problem is, if we do a simple thermal equilibrium equation (remember high school chemistry?), there’s no way the heat energy of the water is doing all the work.
With a little math, if we wanted to heat 100ml milk at 4℃ (39℉) up to a temperature of 60℃ (140℉) with 10ml of water, that water (steam) would have to be at a temperature of 620℃ (1,148℉).
I for one am standing nowhere near that boiler.
Steam boilers are usually set around 130℃ (266℉). Nowhere near the above, but they still manage to heat up the milk. HOW?!
Enter: The Latent Heat of Condensation.
So it turns out that when a material changes phase, it releases or absorbs energy. If those phases are of the liquid/gas variety, then the energy can be massive!
As gas condenses -changing from gas to liquid- it releases an enormous amount of energy (exothermic). On the contrary, as a liquid evaporates -liquid to gas- it absorbs a similar amount of energy (endothermic).
This latent heat can be seen all around us:
- we cool down rapidly upon sweating (vaporisation)
- self-perpetuating thunderstorms (condensation)
- ice cools drinks down really well (melting)
- alcohol on the skin feels cool (low temp vaporisation)
- The list goes on…
So it’s the phase change that’s doing most of the heavy lifting. The 130℃ water is adding 16.8 kilojoules of energy, and the latent heat of condensation is providing another 25 kilojoules.
Electrically speaking, that’s almost 7 kilowatts of energy delivered in around 6 seconds. If you’re on 240 volts that’s 30amps: for those in the US and Japan it’s 58amps 😳 An entire 3 group espresso machine at peak-service might draw that much energy.
Steam is really great at dishing out energy as it condenses. Think of steam engines, your refrigerator, air conditioning, and even nuclear power. All of these things rely on latent heat to pull double its weight.
In essence, the steam boiler acts as a giant battery (or capacitor). It sips on the electrical juice throughout the day, storing it as latent heat within the steam, ready and waiting to be unleashed at a moment’s notice. It means we can heat milk wicked fast, without needing crazy electrical bandwidth to make it happen.
So that’s why! Thanks water! 🤓