Pressure drop
Encyclopedia
Pressure drop is a term used to describe the decrease in pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main determinants of resistance to fluid flow are fluid velocity through the pipe and fluid viscosity. The flow of any liquid or gas will always flow in the direction of least resistance(less pressure). Pressure drop increases proportional to the frictional shear forces within the piping network. A piping network containing a high relative roughness rating as well as many pipe fittings and joints, tube convergence, divergence, turns, surface roughness and other physical properties will affect the pressure drop. High flow velocities and / or high fluid viscosities result in a larger pressure drop across a section of pipe or a valve or elbow. Low velocity will result in lower or no pressure drop.

Pressure Drop can be calculated by 2 values: the Reynolds Number, NRe (determining laminar or turbulent flow), and the relative roughness of the piping, ε/D.
NRe = Dvρ/μ
Where D is the pipe diameter in metres, v is the velocity of the flow in metres per second, ρ is the density in kilograms per cubic metre, and μ is in kilograms per metre-second.
The relative roughness of the piping is usually known and then these two values can be cross referenced using:
http://www.fao.org/docrep/x5744e/x5744eee.gif
By cross referencing the Reynolds number with the relative roughness, the friction factor, f, is calculated.
f=(ΔpπR2/2πRΔL)/(ρv2/2)
Where solving for Δp will allow for the calculation of pressure drop through a pipe.

• Pressure measurement
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....

• Atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

• Voltage drop
Voltage drop
Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...

• Darcy–Weisbach equation ( To calculate pressure drop in a channel )