Delta P (ΔP), pressure difference or differential pressure usually refers in the technical world to the drop of pressure in a piping system, a heat exchanger or another machine.

The delta symbol Δ is the fourth letter in the Greek alphabet. Delta Δ is used as a mathematical symbol which represents the „difference“ between two values.

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 with a moving fluid, the pressure usually drops due to friction. The friction is in between the water and the contacting surfaces, the pipe wall. This pressure drop will increase as more incrustations are in the way of flow. The deposits in are water system are caused 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.

The ΔP in the heat exchanger to the left can be calculated very simple. Subtract the outlet pressure (P2) at the exit A, from the inlet pressure (P1) at the point B and you will get delta P.

The equation for the pressure difference is: ΔP = P1 – P2

It is obvious that a high concentration of deposits, in pipe or exchanger, lead to a high ΔP. If there are deposits the water cant flow free. Hence the outlet pressure is much lower than the inlet pressure.

## Example about differential pressure

If you are sitting in an airplane and drink from a plastic bottle, then the pressure in the bottle is the same as in the airplane. Once you have landed you will notice that the bottle is pushed inwards. That’s because the pressure in the plane is much lower than on the ground. This pressure difference can be seen on the bottle.

The delta P would be the difference in between pressure in the airplane and the pressure on the ground.