Environment Product Guide

The basics of high pressure cleaning

The principles of high pressure cleaning – which type of cleaning is suitable for which type of dirt.

What makes dirt stick?

Dirt can bond to the surface of an object in three different ways:

1. Electrostatic attraction:
If a surface is electrically charged with the reverse polarity of the charge of the dirt, a bond occurs as a result of the inherent attractive forces.

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2. Chemical reaction:
A chemical reaction changes the composition of the surface which causes it to look dirty.

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3. Structural interlocking:
Dirt gets caught and trapped in rough surfaces.

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How are the principles of dirt adhesion applied to the cleaning process?

The objective of cleaning is to destroy the bond between the dirt and the surface. High pressure cleaning utilises mechanical, thermal and chemical energy to achieve this.


There are four primary variables which affect the performance of a high pressure cleaner: flow rate, pump pressure, spraying distance and spraying angle.

Impact force, or the penetrating force of the water jet in relation to the unit of surface area to be cleaned, is most important. This is dependent on

  • Spraying distance: A distance of 10 to 30 cm is recommended, as the pressure curve drops quickly at greater distances.
  • Spraying angle: This angle should be selected according to the degree of soiling. Stubborn dirt requires an angle of 0 - 25°, medium soiling 25 - 50° and light soiling an angle of 50 - 80°.
  • Nozzle pressure: As a function of water flow rate and the cross section of the nozzle, only a disproportionate increase of nozzle pressure can generate greater impact force.
  • Water flow rate: A quick increase of impact force is achieved by increasing the quantity of water. This also makes it easier to remove dirt, as there is minimal atomisation even at greater distances.


Chemical processes are accelerated when heat is applied, which means grease and oil can be dissolved more quickly. Since this also warms up the object being cleaned, drying time is also reduced. This leads to better results, and the cleaning time is reduced by as much as 40 %. However, there is a corresponding drop in temperature as the spraying distance is increased.


If an increase in impact force or temperature does not produce satisfactory results, the use of chemical cleaning agents will be necessary. This has a positive effect on wetting power, emulsification and direct chemical reactions with particles of dirt.

The suitability of a cleaning agent is determined by pH and the surface being cleaned. Acidic agents with a pH of 0 - 6 are used for calcium deposits, urine deposits, rust and other oxides, and alkaline agents with a pH of 8.5 to 14 are used for oils, greases, tar and soot. Neutral cleaning solutions are the right choice for sensitive surfaces which are contaminated with small amounts of oil or grease. Extremely acidic or alkaline levels can, however, result in damage to the high pressure cleaner and the object to be cleaned itself, and may even violate water protection laws. The prolonged application of chemical cleaning agents can also cause damage. Increasing the duration of the application time is generally only effective up to certain point, as cleaning intensity approaches a threshold with the progression of time. Pre-soaking coarse layers of dirt with water can have a positive effect on cleaning time, as this can reduce the amount of cleaning time required with the high pressure cleaner by as much as 50 %.

Information about pressure specifications in MPa (mega pascal)
As a result of the migration to safety testing standard IEC 60335/2/79, pressure values will no longer be specified in "bar", but rather in "MPa". This how to convert the values: 10 bar = 1 MPa.

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