Full-Stream Cooling Tower Filtration
September 1, 2010
Water chemistry is routinely addressed in cooling systems. Scale formation, corrosion and biological growth are just some of the activities that can be abated by proper use of chemical additives. Foreign matter in the form of suspended solids though, is too often overlooked in the design stage. Control of this physical problem is generally addressed sometime after operation has begun when heat transfer problems arise.
A large Midwest pharmaceutical plant for animal vaccines and medications caters to cattle, swine, equine and companion animals such as cats and dogs. Highly sophisticated processes are utilized to produce pharmaceuticals; and cooling is a very important component of these processes. In-plant cooling means cooling towers and the twin-cell cooling tower at this site acts as a wet air scrubber trying to clean the atmosphere of the entire agricultural community. Plant engineers soon discovered that the small 400 gpm side-stream four-bag filtration system on their 5,300 gpm cooling tower system did not provide the protection they needed for heat exchangers, condensers and vessel cooling jackets scattered throughout their facility in addition to an 800 ton chiller. Plus, changing the bags based on a differential pressure alarm was labor intensive and not always attended to in a timely manner.
Their Capital Projects Engineering Department began looking for a full-stream filtration system to filter the 5,300 gpm flow with a 40 to 50 psi operating pressure down to 100 microns. Space on the outdoor concrete pad was very limited. To make matters worse, the concrete pad’s surface was partially flat and partially inclined. Two pumps moved water from the cooling tower basin up to an 18” header located about six feet above the concrete pad. A third pump was to be added when the filtration system was installed to provide a backup.
When designing mechanical filtration equipment one must provide adequate hydraulic capacity maintaining reasonable velocities through the inlet and outlet flanges to minimize static pressure losses, taking into account streamline turbulence caused by internal filter structures and components and numerous changes in direction of fluid flow. However, a bigger design criterion is sufficient screen area to handle water quality conditions. As at most Midwest settings, insects, sand, leaves, dust, pollen, cottonwood seeds and algae as well as manmade debris such as paper, cups and grass clippings influence the water quality greatly. The filter flux must be of appropriate value to meet the specific 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. gpm/in2 of usable screen surface. Filters with a small footprint and large screen area were chosen to meet the specific demands of this application (see Figure 1). Five Orival Model ORG-080-LS automatic self-cleaning filters were mounted on a 16" manifold that included a blind flange so that a sixth filter could be added at a later date should it become necessary (see Figure 2). A 16" pneumatically actuated by-pass valve was incorporated into the manifold system to be opened automatically should the filtration system controller sense a fault in the filtration process. The controller also has a set of dry contacts for connecting an alarm system for fault situations. An 8" manual butterfly valve was located at each filter inlet and outlet to allow the isolation of any individual filter for maintenance or repairs. The filters were pre-mounted on the manifold system before shipping to the site but the system had to be broken in half for shipping to prevent handling an oversize load on the highway.
Normally the hydraulic piston used to move the self-cleaning mechanism (dirt collector) linearly inside each filter and the rinse valve are operated by the water and pressure in the system. However, these filters would possible operate during the winter months when freezing would be a problem. Therefore, available industrial pressurized air is used to pneumatically operate the pistons and rinse valves. The piston and rinse valve actuator on each filter are filled with a glycol mixture that connects to an accumulator tank. This tank is partially filled with the glycol mixture and then the tank is pressurized with the industrial air supply. A solenoid valve bleeds air from the tank lowering pressure; thus, allowing the rinse valve to open and the piston to operate when signaled by the controller to initiate a cleaning cycle. Cleaning cycles automatically occur when a 7 psi pressure differential develops across the inlet and outlet manifolds or when a preset timer lapses. Filter No. 1 goes through its 15 second cleaning cycle and then Filter No. 2 and so on sequentially until all five filters have cleaned themselves. Each filter remains on-line at all times with no disruption of the filtration process.
A design/build firm installed the filtration system mounting the entire package about twelve feet above ground level on a mezzanine built on-site (see Figure 3). This overcame the uneven concrete pad problem and put the filters in proximity to where the cooling water enters the building. Each filter is equipped with a liquid-filled pressure gauge and a 3-way selector valve. This allows the inlet pressure, outlet pressure and rinse chamber pressure to be conveniently observed with one gauge on each filter unit eliminating variations between gauges. Using these pressures allows one to run a number of diagnostics on each filter.
Prudent use of appropriate chemical additives, routine blowdown and proper filtration has resulted in exemplary performance with no maintenance issues or process interruptions.
About the Author: Dr. Marcus Allhands has specialized in water quality for over twenty years with the last fifteen years devoted specifically to filtration technologies. For more information contact Dr. Allhands at email@example.com, (800) 567-9767 or visit www.orival.com.
About Orival Inc.: Providing automatic self-cleaning filtration systems for the removal of suspended solids from water is more than a job for Orival – it’s a way of life. Orival filters are installed on cooling towers, chillers, golf course and landscape irrigation systems, membrane systems, water & wastewater treatment systems, heat exchangers, spraying systems and water reuse systems. Municipal applications abound and Orival filters are found in industries such as Petrochemical, Food & Beverage, Steel, Mining, Automotive, Paper, Power Generation, Wastewater Treatment, Supply Water and Ethanol. Special emphasis has been placed recently on pretreatment for all types of membrane treatment systems.