When is a 24" Filter not a 24" Filter?

When you check the size of a filter in a suppliers catalogue, you will notice that the diameter (o/d) of the filter is listed. This dimension is usually taken from the outside diameter (o/d) of the filter vessel. This dimension will include the wall thickness of the vessel, however the important dimension that needs to be considered when selecting the correct filter is the filtration surface area!

You might think that this dimension would be derived from the o/d of the filter? Area of a circle = πr2

If we have a 24” filter, lets convert that to modern day units of measurement, metric. 24” = 609mm.

Using the formula above we calculate a filtration surface to be 0.29m2

A flowrate of 8.7m3/h through filter with a surface area of 0.29m2 would provide a filtration rate of 30m/h (30m3/h/m2).

The effective filtration area will be governed by the positioning of the filters collection system at the bottom of the filter under the filter media. Common to a domestic filter is an arrangement of laterals. Another method usually found in larger commercial filters are nozzle plate. It has to be said that the nozzle plate type of collection system for me is a better option with no potential for dead spots underneath. A well designed filter will have these collection systems spread as far as possible over the entire area at the base of the vessel to ensure efficient flow and minimal dead spots.

If the collection system is left short, then that filter will not be providing the surface area as promised in the technical specification provided by the manufacturer. In the images to the right you can see that dimension ‘A’ will be the diameter of the filter that is advertised and will appear on the filter specification but, dimension ‘B’ clearly shows a lack of coverage over the total base area, so what might be advertised as a 24” filter is in fact a 20” filter. To put this into perspective, our filtration rate of 30m/h at flow rate of 8.7m3/h in a 24” filter actually becomes 43.5m/h. The faster the filtration rate, the less efficient the media becomes.

PWTAG states that ‘high-rate filters do not handle particles and colloids as effectively as medium-rate filters. Such materials tend to pass through the sand bed. PWTAG research has shown that a high-rate filter with a sand depth of 300mm might remove as little as 10% of turbidity particles in one pass.’ This does not sound like a good choice of filter for use in any pool! European standard EN16713 define high rate filtration as 30 – 50m/h

Inadequate filtration leads to poor water quality. With the short comings of the collection system you will have severe dead spots around the outside circumference of the filter vessel and a higher than specified filtration rate. This is where bacterial biofilm will form, consuming disinfection (chlorine) and producing disinfection by-products in the form of chloramines again another contributing factor to poor water quality and continuous problematic pools.

Of course there are the costs incurred in the manufacturing of a filter with the correct collection system that provides the specified surface area and perhaps one of the reasons why we see such a difference in prices of filters that appear on face value to offer the same performance. Another area for closer attention is internal pipework and bulk head connections. If these aren’t sized according to the flowrate required to achieve the desired filtration and backwash rates, it could cause flow restrictions which, in turn increase the power consumption of the pump as it tries to force water through a smaller aperture. This could reduce the flow rate and circulation, again resulting in poor water quality. You can only truly tell if you are achieving the designed flow rate if a flow meter is installed. How many times do you see a flow meter installed on a domestic pool?