There's no such thing as 'head pressure'

Headnote

Head and pressure are not the same.

(ProQuest: ... denotes formulae omitted.)

Anyone who has worked on the technical side of the hydronics industry has used terminology such as: pressure, differential pressure, head and head loss. These terms all have legitimate and specific meanings. But when some of these words get scrambled into jargon, the result can be an undefined or meaningless term. One example of such a scramble is "head pressure."

To see why the phrase "head pressure" is not a valid technical term, it's important to understand both words.

Let's start with "head." I don't know who coined this word for use in fluid mechanics, but I do know where the concept that the word represents came from. It's attributable to a man named Daniel Bernoulli who, in 1738, published a book entitled Hydrodynamica. That book presented a concept that is now embodied in what's appropriately called Bernoulli's equation. That equation is used by everyone who studies fluid motion. It explains the workings of everything from a pipeline conveying water to a city from a reservoir that's miles away, to the shape of airplane wings. Suffice it to say that if engineers preceding us had not applied Bernoulli's equation, our "modern" lives would be very different.

Fundamentally, Bernoulli's equation describes the mechanical energy present in a fluid and how that energy can be transformed as the fluid changes height, pressure and speed. The "head" of a fluid refers to the total mechanical energy contained in that fluid.

In the case of a closed-loop hydronic system, head energy is added to the fluid by a circulator. Everything else the fluid flows through - piping, fittings, valves, heat emitters, etc. - removes head energy from the fluid due to the friction present between moving fluid molecules as well as between those molecules and the surfaces they come in contact with.

The strange part is that we can't "see" energy. We can't see it with our bare eyes, or under a microscope. Think about it - have you ever seen a Btu, or a Kilowatt-hour, or a Joule of energy? Neither have I.

Although we can't see it directly, we can still detect...