BY BRADY BUTLER

Water is powerful and useful. Engineers have relied on water’s capacity to store and transfer energy for centuries: putting it to work driving steam locomotives and turning power plant turbines. Baristas also rely on water as a solvent to extract flavorful compounds when brewing coffee and as hot pressurized steam to quickly heat and texture milk.

Water can be dangerous and destructive too. Excessive steam pressure can cause a boiler to burst explosively and freezing water may rupture pipes or espresso machine components.

For these reasons, understanding water’s behavior is important for the coffee technician. Changes to water as it is heated or cooled or transitions between its solid, liquid, and gas phases can help a technician better understand the function of the hydraulic systems of an espresso machine.

1. The power of water as a solid and liquid

Like all matter, water has volume and mass. Understanding water’s most interesting behaviors as it’s heated and cooled often boils down to changes in the volume of a given mass of water.

Water is at its most dense at 4ºC (39ºF). That means that the volume that 1g of water occupies is at its lowest. At 4ºC (39ºF), 1g of water will occupy exactly 1mL of volume and be in its liquid state.1 This physical parameter, the amount of space that 1 gram of matter occupies, is called specific volume.2 Specific volume is the inverse of a more commonly known parameter: density.

Water is incompressible in its solid and liquid state. This means that a mass of water’s specific volume is fixed at a given temperature and it will resist even very large amounts of pressure that attempt to compress it into a smaller volume.

One unusual characteristic of water is that, unlike almost all other liquids, water expands as it freezes. When frozen, its specific volume increases by 9%.1 That may not sound like much, but this expansion can be enough to burst household pipes in cold weather or beverage cans forgotten in a freezer. This potential for expansion is also why espresso machines must have all traces of water removed before they are subjected to freezing temperatures to avoid severe damage to heat exchangers, coffee boilers, pressure gages, and water lines.

Liquid water also expands slightly when heated. When heated to 93ºC (200ºF) brewing temperature this expansion is about 4%.1 Espresso machines’ brew water hydraulic circuits are equipped with expansion valves to accommodate this expansion pressure.

The expansion of water should also be kept in mind when developing brewing recipes or performing brew analysis. The common assumption that 1 gram of hot water occupies 1ml of space will introduce an error in calculations. For this reason, detail-oriented brewers are encouraged to remember that 1 liter of 93ºC (200ºF) water only weighs 963g.1

2. The power of steam

Steam is useful in beverage preparation. Espresso machines trace their roots to early brewing devices which used steam pressure to force water through a bed of finely ground coffee.3 Stovetop moka pots still use this pressure for brewing their thick and espresso-like elixir.

Baristas also rely on steam’s ability to store the energy that it took in when it boiled and then transfer that energy when it condenses back to a liquid. This phenomenon is essential for quickly heating and texturing milk. Baristas and technicians alike also learn to respect steam’s tendency to scald skin on contact. This stored energy is called latent heat of vaporization.4

The relationship between water’s boiling temperature and surrounding pressure also play an important role in the proper function of an espresso machine.

At sea level and during typical weather conditions, water in an open vessel will boil to create steam when heated to 100ºC (212ºF). Steam is much, much less dense than water - if allowed to expand freely, 1g of steam would occupy approximately 1600 milliliters of volume. This is comparable to the difference in volume between a 5 gallon bucket and a 11 ft x 12 ft room.5

However, when water boils to steam inside a closed vessel like an espresso machine steam boiler, the resulting steam cannot expand freely. It can only attempt to fill the “empty” space in the vessel. Like other gasses, steam is compressible, meaning that a fixed mass of steam can be contained within a smaller amount of space. As increasing amounts of steam attempt to fill this fixed space, the pressure inside the boiler rises. The steam also exerts more pressure on the surface of the remaining liquid water within the boiler.2

Water’s boiling temperature is directly related to the pressure at the surface of the water: higher at higher pressures and lower at lower pressures. If the pressure inside a sealed steam boiler increases to 1.0 bar gage pressure*, the boiling temperature increases to 120ºC (248ºF). At 1.5 bars of gage pressure, it increases to 127ºC (260ºF).7

So as the water boils, the pressure increases. Higher pressures then raise the boiling temperature of the water until an equilibrium is reached. This equilibrium represents a balance between superheated liquid water boiling to steam and saturated steam condensing back to liquid water. The equilibrium will persist until steam or water is removed, which reduces the pressure inside the vessel slightly and allows additional water to boil off as steam.

This relationship between temperature and pressure is direct and consistent, making it possible for espresso machine steam boiler temperature to be reliably controlled using a pressure switch.

*The pressure shown on coffee equipment gages is measured relative to atmospheric pressure.This parameter is called gage pressure. So when an espresso machine steam boiler pressure gage shows a pressure of 0 bars, the pressure inside the boiler is the same as atmospheric pressure. When the gage shows a pressure of 1 bars, the pressure inside the boiler is 1 bar higher than atmospheric pressure. Atmospheric pressure varies according to elevation and weather conditions but is generally considered to be 1 bar (14.5 psi) at sea level.6

3. Steam expansion and contraction

Though useful, the relationship between boiling temperature and pressure and the presence of superheated water also creates a potential hazard. During normal operation, a steam boiler heating element should cycle on and off to maintain the desired temperature and steam pressure. If this element controller fails and causes it to run continuously, steam pressure can build to dangerous levels, potentially causing the boiler or another system component to burst.5 For this reason, espresso machines are equipped with safety components including pressure safety valves on espresso boilers and thermal protection devices on heating elements.

The potential dangers of a steam boiler rupture are multiplied by the superheated state of the water in a hot steam boiler. This water, heated well above its normal boiling point, is held in its liquid state by positive pressure alone. If the boiler were to burst this positive pressure would disappear, allowing the superheated water to instantanty boil, expanding by a factor of approximately 1600 and creating a destructive shock wave in the process.5

Another interesting and potentially damaging behavior of steam is its capability to create a partial vacuum inside a vessel when it cools.

An espresso machine steam boiler in its operating state is typically only about half-full of superheated water. Unlike a pessimist’s glass, this boiler is not half empty - the remaining half is full of hot steam. When this hot espresso machine is powered off and the boiler is allowed to cool, the steam in its boiler will attempt to condense back to liquid water. This liquid water has a specific volume approximately 1600 times less than the steam it once was.

When steam condenses to liquid water in an open vessel, air from the atmosphere moves to fill the space the steam once occupied. If the boiler is sealed, though, nothing can replace the condensing steam. Since space can only be truly empty in a perfect vacuum, not all of the steam will be able to condense and the boiler will still be half full of steam even when cold. The pressure inside this cool sealed boiler will be much, much lower than atmospheric pressure:

0.98 bars of negative gage pressure at 20ºC (68ºF) - a partial vacuum.7 To prevent problems resulting from this vacuum, including introduction of milk into a steam boiler, espresso machines are equipped with anti-vacuum valves which open at 0 bars of gage pressure.

This ability to create a partial vacuum is put to more constructive use in a syphon or vacuum pot brewer. In these brewers, the much gentler vacuum created by condensing steam in a sealed lower chamber is used to draw liquid quickly down through a bed of coffee. Vacuum brewers must be used carefully, though, to prevent creating too much negative pressure which might cause a glass lower chamber to crack.

Water is So much more than the liquid in our cup. Whether expanding with heat, boiling to steam, or condensing to water, the behavior of water in its various states has a significant impact on coffee equipment design and operation. Understanding this behavior is key to understanding how this equipment and its components function properly and safely.