Dirt and Air Separators: Introduction and Sizing Guide.


During fabrication and installation of a new system there will be a build-up of dirt and once the system is in operation there will be an accumulation of scale and particles from corrosion caused by entrained air.

Some of the many benefits of installing a dirt separator are:

  • More cost effective systems
  • Longer life-span of pumps and other equipment
  • Save time and money on commissioning and bleeding the system
  • Reduce water losses from flushing 
  • Energy savings 
  • Reduce emissions
  • Reduce carbon footprint
  • Reduce wastage of chemicals (removing dirt makes the chemicals more effective)
  • Reduce system noise
  • Lower maintenance costs

Air is always present before filling a system. When the system gets filled this air then gets trapped within it.  Unless this air is removed, this will result in inefficient system operation and causes high maintenance costs.

Some of the many benefits of installing an air separator are:

  • Reduced noise in radiators, heat exchangers, pipework and pumps
  • Decreased wear and tear of heat exchangers, controllers, shut-off valves and pumps
  • Greater heat emissions or cooling efficiency
  • Energy savings
  • Less regular manual system bleeding
  • Lower maintenance costs
  • Less system corrosion

Air in Heating Systems

The amount of air which is dissolved in water is dependent on both temperature and pressure within the system. As temperature rises or pressure falls, air that is dissolved in the water is released (Figure1). During boiler operation micro bubbles are released continuously.  When high temperatures occur at the combustion chamber wall of the boiler, micro bubbles are released in the water. Micro bubbles should be removed immediately upon leaving the boiler to experience the benefits listed above.

Graph showing effects of air in heating systems

Air in Cooling Systems

In a cooling system it is a reduction in system pressure that mostly causes the formation of air bubbles. Also, when the temperature of the water in the coil is reduced, some of the air bubbles will dissolve into the water. To prevent cavitation damage to the pump these air bubbles should be released from the system as early in the system as possible.

Sizing Guide

Separator sizing chart

The following two installation examples may help in selecting the correct size unit (Figure 1):

1. If the separator is installed in a low pressure, high temperature situation, the velocity should be 1.5m/s.

  • On the ‘Flow’ axis at point A (70m3/h), move vertically to point B on the 1.5m/s water velocity curve.
  • Move left from point B to point D on the ‘Dimensions’ axis.
  • Choose the nearest dimension that is higher than point D. In this example you would select DN125.

2. If the separator is installed in a high pressure, high temperature situation, for example in a basement, the velocity should be 1m/s.

  • On the ‘Flow’ axis at point A (70m3/h), move vertically to point B on the 1m/s water velocity curve
  • Move left from point C to point E on the ‘Dimensions’ axis.
  • Choose the nearest dimension that is higher than point D. In this example you would select DN150.

Use the chart below to determine the pressure loss across your system with your chosen separator:

Graph showing pressure loss with Dutypoint separators

Pressure Loss Example:

When the water flow is 70m3/h, the pressure loss for the DN150 dirt and air separator is 1kPa.

  • On the ‘Flow’ axis at point A (70m3/h), move vertically to point B on the DN150 line.
  • Move left from point B to point C on the ‘pressure loss’ axis. In this example the pressure loss would be 1 kPa.

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