"Should filters be used on cooling water systems?" is a question I am often asked. So what do I say? Well, it depends on a number of considerations, but my first response is absolutely an unqualified "Yes." Filters are a cost-effective investment. Filters remove suspended solids from the cooling water and thus reduce many problems. So, what are these considerations?

Anyone considering filters must think about what type of cooling water system is involved. The decision to use filters depends on:

  • The quality of water being added to the cooling system.

  • What is getting into the cooling system or being generated within the system.

  • What is already in the system.

  • The operation of the system.

  • The impact water treatment has on the suspended solids.

    Once all of these considerations are reviewed, then the question is what type of filter should be used. There are many different types for different purposes.

    So, is it worthwhile to install and use filters in the cooling system? Again I say, absolutely -- they will save money in a number of ways. I'll take a closer look at some of the considerations.

    What Type of Cooling Water System Should Use a Filter? Cooling tower and closed cooling water systems both are excellent candidates.

    Cooling tower systems are constantly being contaminated by air and water entering these systems. Air contains dust, dirt, vegetation, insects, microbiological organisms such as fungus, spores and airborne bacteria, plus gases from auto/plant exhaust. Even kitchen and bathroom vents create suspended solids and eventually deposits. The water going into these systems can produce or contribute to suspended solids. A variety of dissolved solids such as iron salts, hardness and scales plus microbiological organisms all can cause potential deposits. These deposits often interfere with the efficient operation of the water-contacted cooling equipment. They also can contribute to causing corrosion and harboring airborne pathogenic microorganisms such as Legionella.

    The cooling tower water systems also can produce suspended solids from within such as corrosion products, scales and microbiological growths. And, the water treatment can contribute to suspended solids, forming sludge and inhibitor-degradation products that can form deposits. These deposits can greatly reduce the efficiency of the cooling system components.

    Filters can substantially reduce the suspended solids and potential deposits, thus improving cooling tower water system operating efficiency. It can reduce cleaning and maintenance costs and increase life expectancy of all of the system components.

    Key among these components are the cooling tower, the condenser portion of the chiller, and the piping. Deposits in the cooling tower can reduce heat rejection and, in severe cases, even cause the cellular fill to collapse. The condenser will lose cooling capacity, increasing cost of operation and often contributing to condenser tube corrosion. Piping deposits not only can cause underdeposit corrosion but restrict water flow, which increases the cost of pumping.

    Closed cooling water systems are also prone to suspended solids and deposits. They differ from cooling tower water systems in that they do not have much makeup water and no air entering that would contribute to suspended solids. However, any corrosion in these systems will produce corrosion products that continue to build up such as iron oxide. Microbiological growth also can develop, producing suspended solids and deposits -- particularly with nitrite based closed system water treatment.

    Removal of suspended solids is critical in closed cooling water systems, not only to prevent a loss in cooling system operating efficiency but to prevent underdeposit corrosion, premature pump seal failure and increased cleaning requirements. Increased water treatment chemical use is often needed when suspended solids and deposits occur.

    Which Types of Filters and Filter Media Should Be Used? Selection considers three key criteria: the type, density and size of the suspended solids.

    The type of suspended solids relates to their character and whether they are soft or hard. Microbiological organisms are gelatinous and very soft. Oil and greasy suspended solids also are very soft. Sand, corrosion products and scale compounds are quite hard.

    Density of the suspended solids is also a key for selecting an effective filter. Sand, corrosion products and scales are very dense. Vegetation, microbiological biomass and oily/greasy products float readily (actually float in the water) and are not dense. Bacteria are very small -- actually minute -- while corrosion products such as chips are very large.

    Selection of the filter and its media must identify these considerations prior to their selection. So check with your filter supplier and identify your needs, and consider having the filter supplier spec out your filter. PCE

  • Sidebar:
    Tips For Filter Use

    To determine the filter and the water flow for good suspended solids removal, consider the expected loading and other criteria mentioned. In addition, keep the following tips in mind:

  • If mainly sand and other dense particulates are present, a cyclone separator is ideal.

  • If the suspended solids are lightweight, then a sand filter or cartridge filter will work well.

  • For a cooling tower system operating 24 hours per day, a "rule of thumb" is to use a side-stream filtration system that filters the entire volume of the system four to five times per day. Thus, if the volume is 30,000 gal, filter about 85 to 105 gal/min. As an alternative, 3 percent to 5 percent of the recirculation rate is used. Using this calculation method, if the recirculation rate is 5,000 gal/min, filter 150 to 250 gal/min.

  • For a closed cooling water system, a "rule of thumb" is to filter the entire volume of the system two to three times per day. Thus, if the volume is 50,000 gal, filter 70 to 105 gal/min.

  • To conserve cooling system water and treatment chemicals, backwash filters with makeup water, not system water.

  • If oil and/or grease are present with suspended solids, use an oil water separator, then filter the separated water to remove suspended solids.

  • Dispersants often used for cooling tower water treatment break up suspended solids so fine that sand filters may not work very well. Media should be much finer or precoat used.

  • Filter media may harbor microbiological organisms and should be disinfected periodically.

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