Pressure Difference – Delta P (ΔP)

Delta P (ΔP) or differential pressure usually refers to the drop of pressure in a piping system. The symbol Δ (delta) is the fourth letter of the Greek alphabet and is used as a mathematical symbol which represents the „difference“ between two values.

So ΔP stands for the difference between two measured values of pressure. This can be measured either on different times/dates or different points in a system.

In a piping system or a heat exchanger the pressure usually drops due to friction. The friction is in between the water and the contacting surfaces, the pipe wall.  This friction will be increased by limescale, suspended solids, biological growth or other kinds of fouling. The deposits build up in the pipe and interfere with the water flow. Therefore ΔP can be used to measure the flow resistance. One can calculate the amount of deposits in the pipe or heat exchanger. The unit is MPa which refers to force divided by area.

ΔP in the heat exchanger to the left can be calculated very simple. By subtracting the outlet pressure (P2) at the outlet A, from the inlet pressure (P1) at the point B.

The equation is: ΔP = P2 – P1

It is obvious that a high concentration of deposits, lead to a high ΔP. As the water is not flowing free in a pipe with a lot of scale in it way, hence the outlet pressure is much lower than the inlet pressure.


ΔP = Inlet pressure 3,99 MPa – outlet pressure 1,99 MPa = 2 MPa

We use ΔP for performance monitoring. After the installation of Merus Rings the delta P decreases, if all works well, to its design level.