Boiler Fill Level And Shot Temperature

HX Machine boiler fill level & effect on shot temperature

Also see:

This is a bit controvercial and counter intuitive, but I have found actually found works in practice over the years, the water level in the boiler affects the shot temperature and stability.

Low water=higher shot temps (but poorer steaming)
High Water levels = lower shot temps, but better steaming (unless it's far too high)

All for the same given pressure (or temperature)…..the reason as I understand it is because of the HX orientation within the boiler, passing through both water and steam. Steam is at least 4 times more efficient at transferring heat than water. This means brew temperatures are up overall with low water levels(after a cooling flush). You could confirm this by doing the same test with NO 50ml cooling flush and you should find the max shot temperatures would be the same whatever the boiler fill level as the water sitting in the HX should be exactly the same in all cases.

This efficiency of steam to transfer heat is why shot stability is improved with lower water levels. Many years ago I read about some manufacturers passing the HX only through steam in an attempt to improve shot stability and also make the HX more efficient for shot after shot… and I am sure they are propbably still doing this today. I wish I could find the study/paper from the manufacturer as it makes interesting reading.

The why

When you consider the latent heat of vaporisation/evaporation (which is pretty enormous as water is a dipole and really doesn't like being split at the molecular level….all those little magnets). When hot steam comes in contact with a cooler surface, it condenses. The change of state from a gas to a liquid releases much more energy than what would be released by just cooling the same weight of water by one degree.

In reverse it's also the reason why evaporating sweat is so much better at cooling you than completely wet skin.

"In summary: inside a heat pipe, "hot" vapor flows in one direction, condenses to the liquid phase, and migrates back in the other direction to evaporate again and repeat the cycle. One reason for the effectiveness of heat pipes is the amount of heat that an evaporating fluid absorbs and then returns when it condenses. For water for instance, to evaporate one gram of water takes as much heat as would be needed to raise the temperature of that same gram of water by 540 °C."

Or possibly a more relevant example as similar temperatures and pressures are used.

"Steam has six times the heat potential when it condenses on a cool food product. This increased heat transfer potential is why steam is such an effective cooking medium."

The HX pipe with it's cold water inside can be considered a cool food product in tranferring the example to a coffee machine.