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Automatic Backwashing Filter Flow Rates
August 7, 2007 |
When choosing an auto-backwashing filter system to solve water problems, one of the main considerations are size and flow rate. The size of the system is determined by how much media you will need to solve the water problem, but it also directly affects the flow rate of the water. For example; if your maximum flow rate is 5 gallons-per-minute (gpm), then you should choose a filter tank that is large enough to provide 5 gpm of flow. The larger the diameter (not height) of a media tank, the less resistance to water, the result being higher flow rates. So many people assume that if they choose a flow rate that is higher than their plumbing system requires, they’ll be safe, but that’s not always the case.
Because filter media is so coarse and porous compared to water softener resin, the maximum flow rate is considerably less for a filter than a water softener. It is because of this higher density and adsorption characteristic of filter media that backwashing flow rates come into play. Water softeners generally require a lower flow rate for backwashing than their maximum service flow rate. That is why over sizing a water softener can be bad. If your plumbing system can not keep up with the minimum backwashing flow rate requirement, your softener will prove inefficient during the regeneration process. So too with the auto-backwashing filter. If you choose a filter system with a backwash flow rate requirement of 9 gpm, yet your plumbing system only provides 7 gpm, then your filter media will not be properly backwashed.
To measure your water system’s flow rate, use a stop watch and open both the hot and cold water faucets on your bath tub. In most cases, this will be the point of maximum water flow through your house. Use a 5 gallon bucket and time how long it takes to fill this bucket in seconds. For example, if it takes a full 60 seconds to fill the bucket, you are looking at a flow rate of 5gpm. If it only takes 30 seconds, 10gpm is your flow rate, etc.
Once you know your flow rate, you can properly size your auto-filter system. If you know the amounts of contaminants in your water, then you can also choose the proper filter media. So long as the media’s specifications provide for a flow rate that does not exceed your concentration and flow requirements (usually stated per cubic foot of media) then you’re all set.
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