terminology problem 1

[m777182]

I came across a term "number of inlet velocity heads", it has to do with a pressure drop and I do not quite understand it. Can somebody please explain the meaning of it to me?
m777182

 

[25362]

In the Bernoulli equation for incompressible fluids flowing between points 1 and 2:

p₁/[ρ]g + V₁²/2g + z₁ = p₂/[ρ]g + V₂²/2g + z₂​

the first term is called pressure head, and the second term is called velocity head, and is measured in dimensions of height or length. Multiplying it by [ρ]g one gets the dimensions of force per unit area, N/m², ie, pressure.

The above-given energy balance formula has to be modified for real fluids by adding to the right side a loss of mechanical energy converted to heat due to friction, [Δ]P_f/[ρ]g.

In an horizontal circular conduit (z₁=z₂) of constant diameter D, the pressure drop between two points separated by a distance L, at constant rate of flow, is totally due to friction, namely

[Δ]P_f prop. to (L/D)([ρ]V²/2)​

I omitted the demonstration of this formula for the sake of simplicity.

[Δ]P_f in pipe fittings is usually given either as the equivalent length of straight pipe, L_e, which gives the same pressure drop as the fitting in question, or as a factor, n (number of velocity heads) by which [ρ]V²/2 has to be multiplied to give the pressure drop.

The friction drop due to contractions or enlargements, as for inlets to vessels, are also expressed in this way.