Mar 05, 2015

Clean Cooling

Automatic self-cleaning filters ease the task of removing suspended solids in cooling towers

cooling tower filtration automatic self-cleaning filters suspended solids

When industry moves into a neighborhood, residents might say, “Not in my backyard.” Jobs, an infusion to local economy and ancillary businesses are great—“just don’t build it within sight of my yard.” 

But the naysayers should look on the bright side. Most industry requires cooling towers, and cooling towers scrub pollen, insects, dust, debris and other allergens from the air throughout the surrounding community. So industry can be a good neighbor after all, if it helps alleviate your hay fever. 

What is good for hay fever relief, however, can be the bane of industry maintenance staff’s existence. A continuous stream of suspended solids entering the cooling water through the cooling tower can result in a maintenance nightmare.  

Overcoming Inefficiencies

Chemicals can help control biological growth and mineral precipitation, but only mechanical filtration can remove the suspended solids that cooling towers grab from the atmosphere. Even a thin layer of mineral deposits can have deleterious effects on cooling efficiency. To overcome inefficiencies, more water must be pumped, using more horsepower, which means higher utility costs and lower profits. If spray nozzles plug, maintenance personnel must physically remove and clean or replace them by hand. Time and labor costs subtract from the bottom line. Automatic self-cleaning filters can solve most of these issues. Capital costs will be higher, but operational costs will be at a minimum and human error is taken out of the equation.  

Two basic filtration systems most often are utilized for suspended solids removal. The first is side-stream filtration. When protecting heat exchangers, chillers, compressors and cooling jackets, this method is the most economical and still meets performance expectations. Typically, 5% to 15% of the full flow is pulled off the main line as a side-stream and run through a filter. As a rule of thumb, 10% side-stream filtration will maintain the concentration of total suspended solids (TSS) at a steady low level to protect heat exchange surfaces from deposition. The percentage of full flow used in side-stream filtration is determined by cooling system volume, TSS loading, filtration degree required and type of suspended solids. Sometimes it is desirable to pull the side-stream off the main header after the supply pump, filter the side-stream and reintroduce the water into the main header.  

A booster pump is needed to add energy back to the side-stream flow to overcome system losses in the filtration process. These show up as pressure losses due to valves, piping, fittings, flow-stream directional changes and velocity losses.

If the main supply pump has excess capacity, the side-stream can be taken from the main header downstream of the pump, run through the filter and discharged back to the cooling tower basin.  

A third side-stream scenario, called a kidney loop, uses a small dedicated pump to take water from the cooling tower basin, run it through the filter and return it to the tower basin. The system is independent from the main cooling system and cannot negatively interfere with the process. 

When nozzles are involved in the downstream process, a single particle could plug a nozzle. For full protection, full-flow filtration is necessary. This is the second basic cooling tower filtration system. A bypass often is incorporated into the filter installation so flow can be maintained to process in times of filter maintenance. This bypass can be manually or automatically activated. The filter controller can recognize a filter fault and automatically open a bypass valve and send a warning signal to the operator. Even filters with a built-in automatic bypass system are available.

Case Study 

A large Midwest pharmaceutical plant for animal vaccines and medications uses sophisticated processes that are utilized to produce its products, and cooling is an important component. Plant engineers discovered that the 400-gal-per-minute (gpm) side-stream four-bag filtration system on its 5,300-gpm cooling tower system did not provide adequate protection for heat exchangers, condensers and vessel cooling jackets, in addition to an 800-ton chiller.  

A big design criterion is sufficient screen area to handle water quality conditions. The filter flux must be of appropriate value to meet the conditions of filtration degree, TSS loading and type of solids. Filter flux is defined as the flow rate per unit area of screen media; i.e., gallons per minute per square inch of usable screen surface. Filters with a small footprint and large screen area were chosen to meet the demands of this application.  

Five Orival Model ORG-080-LS automatic self-cleaning filters were mounted on a 16-in. manifold that included a blind flange. This enabled a sixth filter to be installed when a pre-scheduled cooling capacity increase was added. A 16-in. pneumatically actuated bypass valve was incorporated into the manifold system to open automatically if the filtration system controller sensed a fault. The controller also has a set of dry contacts for connecting an alarm system for fault situations. An 8-in. manual butterfly valve was located at each filter inlet and outlet to allow the isolation of any filter for maintenance. The filters were pre-mounted on the manifold system before shipping for easy installation.

Prudent use of chemical additives, routine blow-down and proper filtration has resulted in six years of exemplary performance with no maintenance issues or process interruptions.

About the author

Marcus Allhands, Ph.D., P.E., is vice president of business development for Orival Inc. Allhands can be reached at [email protected].

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