filtration

The solids are removed over a period of time as they pass through the filter. Filtration is simply the mechanical process by which suspended solids are removed and particles become trapped within the filter membrane or element.

Hot tub cartridges are not paper but are made with special spun-bonded, non-woven media. This material is 100 percent polyester fiber and does not contain any resins or binders, which could break down over time. In addition, the fabric is rot and mildew-resistant.

There are 4 main variables to consider in regards to hot tub filters:

• Dirt Holding Capacity
• Efficiency
• Flow
• Cost

Dirt Holding Capacity: relates to the size of the filter and is measured in square feet. Essentially bigger is better to a degree as long as it doesn’t impede the flow of water, which could affect other components such as your heater. Larger filters generally last longer and don’t have to work as hard. By work hard, we mean handle both the particles and the constant flow of water.

Efficiency: The efficiency of the element relates to the size of pores within it which allow particles to pass, and are measured in microns. The average size of a particle in a tub will range from 10 to 50 microns, not counting leaves and larger objects. The naked eye is able to see a 20-micron particle. Unicel spa cartridges are rated at 20 microns nominal.

Cartridge elements use the filtered debris as a filter aid and will filter down to approximately 10 microns before cleaning is required. Cleaning too often does not allow the filter to work at an optimum filtration efficiency, while not cleaning enough will shorten the life of a cartridge element.

The elements can be cleaned by hand using a garden hose, followed by soaking overnight in a chemical cartridge degreaser. For this reason, cartridge filter manufacturers recommend having two sets of elements.

Flow: If nothing moves, nothing gets filtered. Then again, there is such a thing as too little or too much. Your filter’s ability to handle water flow can be calculated with the following equation:

# Square feet x .75 (an industry average calculation) = max. Recommended flow (measured in Gallons Per Minute) so a 50 square foot filter x .75 could handle an ongoing 37.5 gallons per minute.

Hot Tubs will create suction through the filter ranging from 20-30 gallons per minute leaving a very healthy margin for the filter. In regards to National Sanitation Foundation certification, the NSF allows a flow rate of 1 GPM for residential pools and .375 GPM on commercial installations. Unicel filters are rated at a maximum flow rate of 1 gallon/min. per square foot of filter. So a 50 Square foot Unicel filter has a maximum flow capacity of 50 GPM.

While cartridge filters can work at any flow rate, generally speaking, the lower the flow per square foot, the more efficient the system works given that there is enough agitation periodically to stir things up.

According to ANSI American National Standard for portable Spas and the National Sanitation Foundation, the hot tub should be designed to turn over the entire spa water capacity at a minimum of once every hour. In other words, the entire contents of the hot tub must pass through the filters 24 times a day. This is all about safety, both for the consumer and for the equipment.

Most hot tub designs are such that the water turns over 2-4 times the turn over rate recommended by ANSI and the NSF and in addition the system will purge twice a day, creating turbulence for a few minutes which agitates the water and increases the over all effectiveness of the filtration.

Cost: will be a reflection on both the quality and the size of the cartridge filter.