Why does the pressure have no energy but the head does?

Pressure vs Head

An object's energy is its capacity to produce a force that can do work and transfer heat. When one object acts upon another, altering its momentum, the former's energy (its capacity to perform work) diminishes and the latter's energy increases, through the transfer of heat.

Work is a process of energy transfer in which temperature is not considered during energy transfer. Unit [SI] of energy is always in Joules. It is also a result of force acting through the distance expressed as W = Force x Distance. The dimensional formula of energy is [M1L2T-2], here M is mass. L is time and T is time.

Pressure

Pressure is force per unit area, P = F·A-1 [Pa]. The SI unit of pressure is Pascal [Pa]. The dimensional formula of pressure is M1 L-1 T-2. It is evident that pressure and energy have different dimensional formulas, so pressure is not the same as energy. But when pressure is multiplied by volume, PV becomes ‘work energy’ expressed as Joule, the dimensional formula for energy is PV = M1 L-1 T-2 x L3 = M1 L 2 T -2 = Joule

In summary, the SI unit of pressure is Pascal [Pa]. The SI unit of energy is Joule. Therefore, pressure is not energy.

It is noteworthy to mention that the expression of the pressure term in Bernoulli's equation is frequently given incorrectly as energy per unit area. Energy/volume equals pressure always.

Head

The head is energy. Put simply, the head is the height at which a pump can raise fluid and is measured in metres or feet. The advantage of ‘head ‘over the pressure is that pressure is fluid-dependent and influenced by gravity, whereas the head is fluid-independent, meaning that a pump will raise the fluid to the same height regardless of its relative density, making it irrelevant whether the fluid is heavy sludge or water. The head depends only on the rpm and impeller size of the pump.

One can find the best explanation of what is head in Bernoulli’s equation.

A simple form of Bernoulli’s equation between two points 1 and 2 in a fluid flow can be written as follows:

P1/rho g + V1^2/2g +Z1 = P2/rho g + V2^2/2g +Z2

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Z????????? =????????? height above reference (ft)

V???????? =????????? average velocity (ft/sec)

g????????? =????????? acceleration due to gravity (32.17 ft/sec2)

rho???? =???????? density

The first term represents the pressure head, the second term represents the velocity head and the third term is the elevation head.

The terms "heads" (pressure head, velocity head, and elevation head) refer to the units of distance for all the different forms of energy.

As said above, the term "head" refers to pressure; it is the height, usually in feet, of a column of water that a given pressure will support.

Head is a unit of measurement for all the energies possessed by a fluid; the elevation head represents the potential energy of a fluid due to its elevation above a reference level. The velocity head represents the kinetic energy of the fluid, and it is the height in feet that a flowing fluid would rise in a column. The total head of a fluid is the sum of its elevation head, velocity head, and pressure head.

Bernoulli's equation states that the total head of a fluid is constant. The velocity head is the height in feet that a flowing fluid would rise in a column if all of its kinetic energy were converted to potential energy. The pressure head is the flow energy of a column of fluid whose weight is equivalent to the pressure of the fluid.