By Dr. Marcus Allhands
Chemicals can help control biological growth and mineral precipitation, but only mechanical filtration can remove the tons of suspended solids that cooling towers grab from the atmosphere each year. A thin layer of mineral deposits, the thickness of only a few sheets of paper, can have dramatic negative effects on cooling efficiency. To overcome inefficiencies, more water has to be pumped, which means higher utility costs and lower profit. If spray nozzles plug, someone has to manually remove and replace or clean them.
Automatic self-cleaning filters can solve most of these issues. Capital costs will be higher than manual bag or cartridge filters, but operational costs will be at a minimum, and human error is taken out of the equation.
Filtration systems for cooling towers
Two basic filtration systems are most often utilized. The first is side stream filtration. When protecting heat exchangers, chillers, compressors and cooling jackets, this method is the most economical, while still meeting performance expectations.
Typically, 5-15% of the full flow is pulled off the main line as a side stream and run through a filter. Typically, 10% side stream filtration will usually maintain the concentration of total suspended solids (TSS) at a steady low level to protect heat exchange surfaces from deposition. The percent of full flow used in side stream filtration will be 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 then re-introduce the filtered water into the main header. A booster pump will be necessary to add energy back to the side stream flow to overcome system losses in the filtration process. (See Figure 1). These losses 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. This utilizes existing horse power and requires no booster pump. See Figure 2.
A third side stream scenario, often called a kidney loop, uses a small dedicated pump to take water from the cooling tower basin, run it through the filter and back to the tower basin. See Figure 3. This system is totally independent from the main cooling system and cannot negatively interfere with the cooling process. The particular filter skid shown will handle a cooling system of up 1000 gpm (100 gpm through the filter).
When nozzles are involved in the downstream process, a single particle of the right size could plug them. To prevent nozzle problems, full flow filtration is necessary. This constitutes the second basic cooling tower filtration system. Figure 4. shows a full flow schematic.
A bypass is often incorporated into the filter installation, so flow can be maintained to process in times of filter maintenance or repairs. This by-pass can be manually or automatically activated. The filter controller can recognize a filter fault and automatically open a by-pass valve and send a warning signal to the operator. Filters with a built-in automatic by-pass system are available.
A large pharmaceutical plant for animal vaccines and medications caters to cattle, hogs, equine and companion animals such as cats and dogs. Highly sophisticated processes are utilized to produce the pharmaceuticals and cooling is a very important component of them.
Plant engineers 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, vessel cooling jackets and an 800 ton chiller.
An important design criterion is sufficient screen area to handle varying water quality conditions. 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. Five Orival Model ORG-080-LS automatic self-cleaning filters were mounted on a 16” manifold that included a blind flange. This flange enabled a sixth filter to be installed when a pre-scheduled cooling capacity increase was added at a later date. A 16” pneumatically actuated by-pass valve was incorporated into the manifold system to open automatically if the filtration system controller sensed 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. Filters were pre-mounted on the manifold system, before shipping to the site for ease of installation.
Prudent use of appropriate chemical additives, routine blow-down and proper filtration has resulted in six years of exemplary performance with no maintenance issues or process interruptions.
Dr. Marcus Allhands, PE, is with Orival Inc. This article appeared in ES&E’s March/April 2014 issue.