Cooling towers attract all kinds of airborne debris. Fine particles, sometimes smaller than 5 microns, greatly affect heat transfer surfaces.
Filtration is vital to cooling tower water treatment.
Cooling tower filtration systems exist to remove sediments, turbidity, organic matter, process oils, and silt from cooling tower water. Depending on the type of filtration chosen, particles as small as one micron can be captured and expelled from the system.
Because sustainability and energy savings became one of the primary goals of any building operation, a strategy commonly adopted in the HVAC market is to recirculate the cooling tower water throughout the system. In cases like this, a side-stream filtration unit can help eliminate potential contaminants that were carried into the tower as part of the process.
Side-stream filtration is a cost-effective solution that continuously filters a percentage of the water flow, constantly improving the water quality.
Think of a chain of events: side stream filtration systems will reduce suspended solids and debris in the cooling tower. Less debris means reduced biological growth, which results in less microbiological corrosion. The result of all these factors contribute to optimized performance, and the final consequence can be seen in energy and water savings. In addition, unlike the full flow filtration, a side stream filtration system does not require planned downtime.
What are your options when it comes to cooling tower water side stream filtration solutions available in the market? What are the advantages of each of them?
Our technical support team highlights three of them: disc filters, sediment separators, and sand filters.
1. Disc filters (disc filters):
Disc filters can trap sediments from 400 to 20 microns. When the correct disc is chosen for a particular application, you will reach satisfactory results. Because this system was developed in Israel, where water is considered a premium instead of a commodity, this system does not use much water to backwash.
But be aware: the success of this solution truly relies on understanding the size of the particles and debris that enter your system. If your particles are too big for the discs, you will increase the number of times the filters need to backwash, which will make your system inefficient. If your particles are too small for the discs, they will freely pass through the filters, and your goal of trapping debris will not be met.
The following video shows the concept of disc filters and how they work.
2. Sediment separators:
Sediment separators utilize centrifugal force and friction to remove dense particles from liquids, and the concentrated solids are removed through the purge outlet. They have been found to be effective in removing particles as low as 5 microns.
However, in this case, the debris must be heavier than the water. Otherwise, the centrifugal technology will not be as efficient as it could. It is a piece of equipment with no moving parts, requiring very little maintenance.
PEP Filters explains the concept using the image below:Image: PEP Filters
The liquid enters into an inlet port where the velocity is increased by an internal acceleration ring. As the fluid spins through the separation chamber, the centrifugal force caused by the acceleration ring forces the suspended particles to the outer wall of it.
The separated particles then slide down the outer wall to the collection chamber, while the carrier fluid undergoes a 270-degree change of direction causing further and more efficient separation. The vortex breaker allows the suspended particles to settle in the collection chamber. To remove the solids, a continuous purge can be utilized.
An automatic timed purge can be used as well based on two parameters intervals between purges and purge length.
3. Sand filters:
Sand filters consist of a large tank, containing a bed of special-grade sand. During filtering operation, fluid enters the upper body in a rotational direction, enabling the dirt/contaminants to continuously rotate with the water. This prevents the solids from blanketing the top of the media, keeping it constantly fluidized.
Gravity pulls the water down through the sand, while the sand particles trap dirt and debris. Over time, the dirt and debris collected in the sand slow down the water flow. The unit then will automatically backwash to self-clean the media.
Image: MPT (Modern Pumping Today)
According to our tech team, sand filters are probably the most effective side-stream filtration system available today, effective for particles as small as 0.5 micron depending on the application and media pack specified. They deliver superior performance and long-term reliability.
However, sand filters require a large footprint to be installed, and the sand needs to be changed regularly. Therefore, even though they are the finest solution, good maintenance is crucial for them to work properly. Also, energy savings cannot be considered a strength, because sand filters require more backwashing than the previous options. Proper backwashing is a key consideration in successfully applying a sand filter.
Regardless of the option you choose, a side-stream filtration system will treat the water but it still has limitations when it comes to keeping your cooling tower clean.
This happens for three main reasons:
- Only a portion of the water will be treated at a time (10% as a rule of thumb), which means that your cooling tower will collect the debris present in 90% of the water that circulates in it.
- The makeup water will constantly bring more solids to your equipment, increasing the chance of solids accumulation.
- Some of the airborne debris will be trapped in the cooling tower itself, not even making their way into the pipes.
To solve this problem, our tech team pointed out a very effective solution: the installation of sweepers in the cooling tower collection basin as a means to eliminate sediments that are collected in the cold water basin of the unit.
Sweeper pipe systems are composed of a separator, a circulating pump, an array of PVC piping, and several nozzles, which are positioned in pre-established areas of the cold water basin. The pump circulates cold water basin water through the PVC array and out the nozzles, sweeping the solids towards the pump suction. They then travel through the pump suction pipe to the separator, where they are separated from the water.The filtered water is sent back to the cold water basin by means of the circulation pump and nozzles.Image: SPX Cooling Technologies
To install a sweeper piping system is an inexpensive solution with a payback time usually less than 1.5 years, which along with a proper side-stream water filtration solution will keep your water clean, protect your entire hydronic system and equipment, and save you thousands of dollars in maintenance and energy consumption.