Filtration System
Filtration is a mechanical process to remove suspended matter such as mud, silt and microbial matter from water by collecting the solids on a porous medium. Removing suspended matter helps the overall water management program's success by eliminating deposit-causing substances, as well as chemical treatment products to work more effectively. There are two methods of filtration:
1. In-Line Filtration – In-line filtration allows all system circulating water to pass through a strainer or filter in order to remove impurities and suspended solids.
2. Side-stream filtration – Side-stream filtration means placing a filter in a bypass stream so that a portion of the total cooling water circulation rate (at least 5%) is filtered. Higher bypass portion leads to better water quality but also increase the filtration equipment capacity. The advantage of side-stream filtration includes lower capital and space requirement than in-line filtration using the same filtration method. In addition, side-stream filtration has the advantage of being able to process the recirculation cooling system and remove debris, which has been drawn in by the cooling tower, as well as impurities precipitated in the bulk water.
Both in-line filtration and side-stream filtration processes help in reducing suspended solids to an acceptable level.
Filtration Equipment
A number of mechanical filtration devices commonly used in cooling tower systems are:
1. Strainers – A strainer is a closed vessel with a cleanable screen to remove and retain foreign particles down to 25μm diameter inside cooling water. It shall only be used as pre-filtration to remove large particles in the system. Routine inspection and cleaning is necessary to ensure strainers are in good condition and normal function.
2. Cartridge filters – Cartridge filters can be used as final filters to remove nearly all suspended particles from about 100μm down to 1μm or less. Cartridge filters are typically disposable, which shall be replaced if required. Frequency of replacement depends on the concentration of suspended solids in water, the size of the smallest particles to be removed and the removal efficiency of the cartridge filter selected.
3. Sand filters (Permanent media filters) – The degree of suspended solids removal in sand filters depends on the combinations and grades of the medium being used in the vessel. Typical sand filter can remove suspended contaminants down to 10μm. Specialized fine sand media filters are designed to remove suspended particles down to less than 1μm. Multimedia vessels with each layer containing medium of different size may also be used for low suspended solids application. When the vessel has retained enough suspended solids to develop a substantial pressure drop, the unit must be backwashed either manually or automatically by reversing the direction of flow.
4. Centrifugal-gravity separators – Cooling water is drawn through tangential slots and accelerated into the separation chamber. Centrifugal action tosses the particles heavier than the liquid to the perimeter of the separation chamber. Efficiency of centrifugal-gravity
separator depends on the gravitational mass of suspended solids; performance data indicate that separator efficiency is about 40% for particles in the range of 20μm to 40μm.
5. Bag type filters – Bag filters are composed of a bag of mesh or felt supported by a removable perforated metal basket, placed in a closed housing with an inlet and outlet. Filter bags can be made of many materials (cotton, nylon, polypropylene and polyester) with a range of ratings from 0.01mm to 0.85mm. Mesh bag are generally coarser, but are reusable. However, periodic replacement of filters is required to ensure the efficiency of filters.
Magnetic devices
This method involves the exposure of incoming make up water under the intense magnetic field. Magnetic field affects the suspended particles or the ions in solution and prevents the deposition of a hardened deposit. The particles will then form a mobile suspension or do not precipitate at all. Also, existing deposits of scale can be converted into solution. Some magnetic devices use permanent magnets and hence do not require electrical power input to the device.
Electronic De-Scaling technology
Electronic de-scaling technology makes use of induced oscillating electric fields using time-varying magnetic fields generated in the solenoid wrapped around a water pipe. Dissolved ions are then charged and collided with each other. Collisions between positive and negative ions facilitate precipitation of the ions in the pipe-work.
Electronic de-scaling technology can be used to enhance chemical-based water treatment program but caution: selection of chemicals used in corrosion inhibition and micro-biological control shall be compatible with the technology.
