The efficiency of cooling towers and their related processes can be improved in many ways. One important way is to filter the cooling water, ensuring that any particulate capable of contaminating the process is removed. Two often used filtration technologies are bag filters and walnut-shell filters. Bag filters are a simple filtration option that can remove particulate in many different industries. Walnut-shell filters can remove solids and oils from cooling processes.
Bag Filters Clarify Noncontact Fluids in Industrial Applications
Bag filters are particularly effective when used to clean noncontact cooling water. This is usually water — sometimes with the addition of glycol — that does not come into direct contact with the process to be cooled. Despite the lack of direct process contamination, filtration is still necessary. Contaminants can enter the water system through the air in cases where the cooling pond is open to the elements. Pipe scale and a lack of clean makeup water also can introduce unwanted solid material into the system.
A filtration system must be capable of removing particles larger than 10 µm so fouling of the cooling element does not occur. Bag filters frequently are used because they can achieve this level of filtration easily, and the lack of constant addition of solids to the system means that there is not a large maintenance requirement. Bag filters are sized so that the bags are protected against an upset event that significantly changes the water quality of the system. Under normal conditions, bags will be changed infrequently.
For as many different types of cooling systems as exist, there are even more reasons that filtration is a necessary addition to a system. The standardized nature of the bag filter housing means that bags can be easily changed for a different type — whether for a new material or a different filtration rating — as the system performance fluctuates.
Older systems that have never had filtration may require a step-down approach. When the bag filter system is installed, 100-µm bags are used to clean up only larger particles. Over time, the bags plug less frequently. Eventually, bags with smaller pores are used until the proper 10-µm bags can be maintained.
In other processes, the unforeseen addition of certain types of particulate can necessitate the use of specialty bags.
Oil-Absorption Bags. In certain cooling systems, oil can be introduced to the cooling water supply. To prevent the buildup of oil, oil-absorption bag filters can be used. These bags consist of a lofted polypropylene material that attracts oil particulate. When oil comes into contact with polypropylene, it causes the material to swell. Over time, the filtration efficiency of the bags increases as more oil comes into contact with the lofted media. These bags are best when the oil concentration of the cooling water is between 25 and 500 ppm.
Magnetic Inserts. If ferrous material is consistently introduced to a cooling system, upgrading to a bag filter with magnetic inserts can minimize maintenance and maximize the lifespan of the disposable media. A frame is placed into standard filter bags, and magnetic rods are inserted into the frame. This frame is designed so that all of the liquid that passes through the filter bag is exposed to the magnetic field of the rods. These magnetic rods can remove many times more mass than a bag filter would normally be able to, resulting in much longer runtimes. When the bag filter plugs, the magnets are cleaned by removing them from the frame and using a rag to wipe the magnetic material into a waste container. The rods can be reinserted into a clean bag for unlimited uses.
Pleated Cartridges. To increase the time between bag changeouts, pleated cartridges sometimes are used. These filters have five to 10 times the surface area of standard bags, greatly increasing the amount of solid material captured. The same filter media is used for these as is used in the construction of standard bags, but the material is folded in on itself (pleated), allowing more material to be used in the same space. The media is welded to a plastic cap with a hole, allowing water to flow from the inside of the filter out through the media. This inside-out flow prevents contaminants from falling out of the filter upon removal from the housing, preventing system contamination.
Walnut-Shell Filters Handle Oily Media in Industrial Processes
To keep cooling systems efficient in processes where particulate is constantly being added, a walnut-shell filter may be used. The walnut-shell filter is a bed filter that backwashes contaminants out of its filter media automatically, so frequent operator attention is not required.
Walnut-shell filters are most frequently used to filter contact cooling water, preventing the water from becoming prohibitively dirty. Contact cooling water is water that directly touches the process for which water is providing cooling. For example, water is frequently used to cool or quench metal, and it picks up oil, grease, solids and powders that were originally on the metal. This is taken back into the process water system, and the contaminants can cause problems such as fouled heat exchangers and plugged spray nozzles.
The media bed of a water-shell filter consists of crushed black-walnut shells. This naturally occurring material is a byproduct of the food processing industry. It is especially effective in filtration applications because it removes solid material 5 µm and larger, and it allows oil droplets to coalesce onto the surface of the shells. Despite the fact the oil coalesces onto the walnut shells, they are still preferentially water wetted, and the oil is easily removed during a backwash cycle. Because other backwashing filters tend to “mudball” when exposed to oil, walnut-shell filters typically are chosen when oil exposure is expected.
Another advantage that black-walnut-shell media has over other types is that it has a high modulus of elasticity. This means that the media does not breakdown during a backwash cycle when the bed becomes disturbed. Beds that consist of other types of media typically must be replaced every three to five years; by contrast, black-walnut-shell media can last 25 to 30 years.
Turning Over the Cooling System
Contact cooling water can have a high concentration of contaminants — as high as 80 ppm. The concentration can change quickly and include oil; this makes walnut-shell filters the best available technology. Contaminant load and how much the load varies are the most important data points when sizing the filter. Typically, walnut-shell filters are sized at four to eight turnovers per day, depending on the rate at which contaminants are added. One turnover is defined as the ratio of daily flow rate/total system volume.
Walnut-Shell Filters vs. Bag Filters
Walnut-shell filters should be used in place of bag filters when there is solid material being constantly added to the cooling system. One example is contact cooling water processes. They are also used when oil is in the system to reduce the cost associated with the disposal of oil-saturated bags. Because they are treating systems with more material in them, the rate of turnover is typically higher. Disadvantages of walnut-shell filters compared to bag filters include the need for a container into which to backwash contaminants as well as the higher capital cost. Advantages include a reduction of costs associated with waste disposal, less operator attention required and more efficient removal of oils and greases. PC