What is a Filter Nozzle
A Filter Nozzle retains the media and also provides the means for good distribution of water and air.
This term is applied to the components which retain the media and also provide the means for good distribution of water and air (where used). Nozzles may be installed in lateral pipes or in suspended floors.
A nozzle has three main components, a strainer to exclude the media or packing gravel, a water control orifice and a tailpipe with an orifice(s) to control the airflow.
Nozzles may rely upon the open area of the strainer slots to provide the controlling head loss for good distribution or an independent orifice within the assembly is often preferred.
Strainer slots can become partially blocked with media. This will alter the head loss across the slots. If the control orifice is within the body of the nozzle and the slots have a larger open area, there will be less affect by partial blocked slots to the head loss through the nozzle.
Filter Nozzle slot sizes vary considerably from about 0.2mm to around 10mm, larger slots best suited for up flow filters. Slots of 0.2, 0.3 & 0.35mm are commonly used in direct contact with the filter media without the requirement of a gravel layer on top of the nozzles. This direct contact has an advantage for combined air scour and water backwash.
Care must be taken to avoid grit particles entering with the backwash water if slots less than 1.0 mm width are to be used. If filtrate is always used for backwashing, there is usually no problem.
Some floor suppliers promote nozzle-less floors, but these rely either on packing layers or porous compositions which would seem equally or more likely to block and more difficult to clear.
The function of the tailpipe of a filter nozzle is to control the distribution of air. If air is not used tailpipes are not required. Tailpipes are usually provided with an upper orifice (starter or air bleed hole) and lower orifices or slot (metering hole/slots). As air is applied to the floor whether suspended or of the lateral type, the air collects under the floor and depresses the water downward until the air reaches the metering hole or slot. The metering hole is sized accordingly to prevent the air reaching the end of the tail pipe, which would cause poor distribution of the air scour and localized aggressive aeration in turn causing uneven media, possible carryover of media into the drain channels and the mixing of filter media layers if used.
Filter nozzles can be made of many materials such as plastics and metals and are supplied with auxiliary parts to enable fitting in concrete, metal plate floors and pipe lateral systems.