How to choose the right cooling tower filtration system for your specific product (service).

Steelmaking cooling water

In general, owners and operators will ignore the impact of evaporative cooling equipment efficiency on profits. Even if the efficiency of evaporative cooling equipment, heat exchanger and cooler is slightly improved, it can save a lot of money for the owner in the whole life of the cooling system. Improving the water quality in the cooling circuit is a simple, economic and efficient way to improve efficiency.

In evaporative cooling equipment, debris in the air, such as sediment, is entrained in the fluid flow. Dirty make-up water can also lead to the accumulation of pollutants. Other problems may also be caused by scale build-up and scaling in the tower, treatment of chemical residues and algae that may build up and contaminate the circulating water. These are just a few sources of unwanted pollutants that accumulate over time and lead to poor water quality.

Saifilter recommends that each cooling tower be equipped with a customized mechanical filtration system and water treatment procedures to ensure high water quality. Both must be used to effectively treat the water in the cooling system. Proper treatment of water in the cooling system can save costs and improve efficiency so that the evaporative cooling equipment can operate according to the manufacturer's regulations.

Benefits of Clean Water

1)Reduce Energy Consumption

Only 1 / 16 inches of dust on the heat transfer surface, scale or biological deposit layer will cause the cooling tower to lose efficiency and increase energy cost.

2)Improve Chemical Properties

Dirty water requires more chemical treatment than clean water, because the accumulation of solid pollutants provides a buffer to reduce the impact of treatment chemicals. Therefore, other chemicals are needed.

3)Reduce Maintenance Cost

Frequent draining of towers and cleaning of sediments will increase the need for labor and increase the cost of replacement losses, adding water to the system and providing other chemicals.

4)Increase Productivity And Reduce Downtime

Because the machine can not operate efficiently, the scaling of the cooling system will slow down the production speed. The heat exchanger may scale the system down for a long time until the completion of maintenance, resulting in a reduction in daily production and loss of profits.

5)Control Biological Growth That May Cause Health Problems

Legionella is a kind of bacteria that propagates in an improperly maintained cooling tower environment, which is particularly important for bacteria control, because it will bring significant health risks.

Note: in the end, getting clean water every day when using the filtration system requires routine water analysis, effective water treatment procedures, and training procedures for maintenance personnel. Water treatment procedures are specific to specific applications. Please contact your local water treatment expert to diagnose the needs of the system.

Successful Filtering

A typical 200 -ton cooling tower operates for 1000 hours a year and can absorb more than 600 pounds of heat. Dust and make-up water in the air discharge particles into the water supply system. The basin or remote sump provides an ideal environment for unwanted particle deposition and accumulation. The humid environment of the drainage basin or remote water collecting basin will promote the growth of bacteria.

Working drawing of cooling tower
Cooler contamination

Chemical water treatment does control the role of these microorganisms, but its use alone does not eliminate habitats that promote biological reproduction. The use of mechanical filtration systems will not replace chemical treatment. However, chemicals can not reduce the accumulation of particulate pollutants and can reduce the propagation of microorganisms through appropriate mechanical filtration.

The successful filtration of cooling tower water depends on the system designed. The successful design depends on the owner and system designer's understanding of pollutant issues. The problem of pollutants is that it is necessary to understand filtering to realize system protection and understand the size and type of pollutants. Filtering methods are usually driven by cost, and there are obvious best choices in methods, but sometimes the cost is high. Once the filtration method is known, the filtration equipment that is most suitable for the filtration system can be determined according to the nature of pollutants.

Note: The mechanical filtration system cannot be used alone. Besides filtration, water treatment must be carried out to ensure high water quality.

What Are The Selection Of Cooling Tower Water Filtration Methods?

  • Pool cleaning - filter between 10% and 20% of the system flow, and "clean" the solid into the filtration system by using the nozzle installed in the cooling tower pool.
  • Full flow - 100% of the flow of the filtration system.
  • Percentage of side flow filtration system flow (usually 10% - 25%).

Do not confuse the above filtration methods with the use of pump suction strainers, which must be used on each cooling tower. The pump suction filter is the standard configuration for properly designing the cooling tower and is only the beginning of system filtration. The suction strainer is located at the outlet of the equipment to prevent large debris (such as sticks and stones) from entering the system. Saifilter recommends that all cooling tower equipment be equipped with pump suction screens, except for remote sump applications.

Pool Cleaning

Pool cleaning is a common filtering method, which can directly prevent solid particles from accumulating in the unit pool or remote pool. A method of applying pool cleaning as a filtration means includes pumping water from the unit pool / sump to the filter assembly, and then directly pumping the filtered water back to the tower pool (Fig. 1).

If there is no mechanical filtration system, it is usually necessary for maintenance personnel to clean the pool manually. This requires a high level of maintenance and is not as efficient as using mechanical filtration systems. In addition, the mechanical filter system can provide continuous maintenance, while the maintenance personnel can only carry out regular maintenance. Continuous maintenance ensures a cleaner system. In addition, if maintenance personnel are cleaning the contaminated system, they will face health risks. (Fig. 2)

Flow diagram of cooling tower

Figure 1.Basin Cleaning Protection Figure

Cooling tower sludge cleaning

Figure 1.Basin Cleaning Protection Figure

Reference Criteria For Configuring Mechanical Filters

Water DepthsFiltration Flow Rate
Less than 3 feet or 0.9 meters1 per square ft (2.44 m³/hr per m²)
Greater than 3 feet or 0.9 meters1.5 per square ft (3.66 m³/hr per m²)

This method prevents the control of feeding solid particles into the filtration system, and actually eliminates the accumulation of solid in the pool. However, pool cleaning does not directly filter the water pumped into the heat exchanger and cooler. From the point of view of maintenance, cleaning pool can shorten the maintenance period of cooling tower, but it can not solve the maintenance problem of heat exchanger or cooler. Full flow filtration system and side flow filtration system are the methods to provide direct protection for heat exchanger and cooler, but they will not prevent the accumulation of solid particles in the tower pool.

What Is The Difference Between Full Flow Filtration System And Side Flow Filtration System?

Full flow filtration utilizes filters installed after the cooling tower on the discharge side of the pump. This filter continuously filters the entire system flow, which means that the filter must be sized to handle the design flow of the system. Therefore, a filter with a flow rate of 200 USGPM needs to process 200 USGPM. Full flow filtration can significantly reduce the maintenance of heat exchanger and cooler, and improve the operation cycle of equipment.

Side flow filtration system is a cost-effective alternative to full flow filtration because it can continuously filter part of the flow rather than all the flow. Side flow filtration can reduce maintenance and improve the operation cycle of equipment in the cooling circuit. In this method, particles can be removed at a higher rate.

What Are The Characteristics Of Full Flow Filtration System?

Full flow filtration is the preferred filtration method, but it is not cost-effective for high flow rate systems. For example, a 400 ton cooling tower with a flow of 1200 usgpm would require a filter of 1200 usgpm. This requires a very large filtration system to accommodate a flow of 1200 usgpm. Such a large system would be costly. Moreover, for the system, such a large flow may not be easily detected when there is a reduced risk (Figure 3).

This reduction may result in an increase in pressure at the pump discharge and a reduction in the proper flow of fluid to the heat exchanger, resulting in a reduction in heat transfer. In addition, full flow systems cannot be operated and cleaned at the same time, which means that maintenance can lead to planned downtime. Although full flow filtration reduces the total solid concentration in the water pumped to the heat exchanger and cooler, this method cannot solve the problem of solid accumulation in the tower pool or the remote sump.

Whole flow chart of cooling tower

Figure 3.Full Stream Filtration

What Are The Characteristics Of Side Flow Filtration System?

Water is pumped from the cooling tower cold water pool to the heat exchanger and cooler through the side flow filtration system, and then returned to the cooling tower pool. When the total flow is high, this method is often used, which makes the full flow filtration economically infeasible. A major advantage over full flow filtration is that the side flow filtration system can be cleaned without a shutdown system, thus avoiding planned maintenance downtime. Like full flow filtration, this method reduces the total solids concentration, but does not solve the problem of solids settling in a tower or remote sump (Figure 4).

Side flow filtration system

Figure 4.Side Stream Filtration

Side Flow Filtration System Size Selection Guide?

It is very important to choose the right side of the side flow filtration system to achieve the best filtration performance. The rule often used is to adjust the size of the filter to handle the flow rate that doubles the volume of the system per hour. This flow rate is usually in the range of 3% to 10% and is usually determined by the turnover rate of the system volume per hour. For example, consider a 400-ton cooling tower with a flow rate of 1200 USGPM. The estimated system capacity is approximately 3500 USGPM. In order to switch the system volume hourly, a flow rate of 58 USGPM will be required, as shown below.

Approximate system volume = 3500 USGPM

To cycle the volume of the entire 3500 USGPM system over an hour, do the following: 3500 USGPM/ H* 1 H / 60 min = 58 USGPM side flow rate.

The side flow velocity of 58 USGPM is 400 ton cooling tower, 4.83% of 1200 USGPM(58 USGPM/ 1200 USGPM* 100 = 4.83%). It has been shown that the percentage of side flow filtration at or below 3% of the total circulation flow will seriously damage the HVAC system, which will aggravate the scaling of the whole cooling circuit. Therefore, the best design should avoid the use of low specification filters. For the same clarification filtration, the side flow filtration can make the water reach the same cleanliness as the full flow filtration, but the process will take longer.

Because only one percent of the water is filtered at a time, some solids do bypass the filter and remain in the fluid stream, but eventually they reach the filter again and are removed as the water circulates through the cooling circuit. Keep in mind that the volume of the whole system is recycled once an hour, and the particles escaping from the water filter for the first time will be captured in subsequent rounds of cyclic filtration.

Comparison Between Full Flow And Side Flow Filtration Systems

At first glance, it seems that full flow filtration is preferable to side flow filtration, because in contrast, full flow filtration can significantly reduce the maintenance of heat exchangers and coolers, and greatly improve the operation cycle of the evaporative cooling equipment. However, for high flow systems, full flow filtration is not reasonable economically, and it needs to be shut down in a planned way to maintain the filtration equipment, which makes side flow filtration a better choice in most applications. In any case, side flow filtration can easily raise the water quality to an acceptable level, which will ensure proper protection of the heat exchanger and cooler. Neither the full flow nor the side flow filtration method can solve the problem of solid accumulation in the tower basin or the remote collecting basin.

The Best Scheme Of Circulating Cooling Water Filtration System

The best way to filter water for industrial applications is to use mechanical pool cleaning and full flow or side flow filtration. The wash basin ensures that particles are directed to the filter inlet and that these solids do not accumulate in the cooling tower basin. Once the particles reach the mechanical filter inlet, the equipment selected for full flow or side flow filtration will remove the residual particles that are not needed, thus providing clean water for the heat exchanger and cooler. Using the pool cleaning technology with full flow or side flow filtration function can directly protect the cooling tower, heat exchanger and cooler, so as to minimize maintenance and improve the efficiency of equipment in the evaporative cooling circuit.

What Are The Types Of Industrial Filter Cleaning?

Common filtration technologies for full flow and side stream HVAC applications include screen (self-cleaning strainer), centrifugal separator, bag filter, and medium filter and disc filter. In addition to proper filtration, the best filters require minimal maintenance and energy consumption to meet cost-effectiveness.

Screen (self-cleaning) Strainer

Self-cleaning filters are often used for full flow filtration. The filter element adopts stainless steel screen, which can remove large particles. The bypass pipe needs to be equipped with a screen filter so that the screen can be removed for cleaning. In areas with poor water quality, the filter screen should be enlarged to provide a larger filter surface area, so as to minimize the maintenance frequency caused by insufficient filter screen. The movable parts of the screen filter allow backwashing circulation and self-cleaning of the filter, which requires regular maintenance of the screen filter elements.

Centrifugal Separator

Centrifugal separator, usually called separator, is usually used for full flow filtration. The separator produces a vortex that spins particulate contaminants out of the incoming fluid. The disadvantage of this turbulent spinning is that it allows the separator to operate at a pressure loss of typically about 5 to 10 psi. The separator does not need to be replaced frequently, because it will not catch any particles blocking or damaging its system, thus making the separator an economic choice for filtration. In the HVAC industry, separators are preferable to screen filters because they require less maintenance and replacement, but are equally effective in achieving the appropriate filtration level.

Bag and Sand Filter

Bag filters, usually made of polyester, are widely used in HVAC industry due to their low cost. The bag filter must also be replaced frequently. The sand filter distributes the polluted water to the sand bed which can filter the particles. The sand filter medium does not need to be replaced regularly. The sand filter uses an automatic backwash cycle to clean the filter media, reducing maintenance intervals.

Bag filter is relatively cheap, but its filter element is consumable and needs to be replaced regularly. This can be costly because the owner has to constantly change the bags and pay for labor each time. In contrast, the media of sand filter does not need to be changed frequently, so the cost of a sand filter is lower in the long run. The ruggedness and self-cleaning function of the sand filter further eliminates maintenance errors related to infrequent or correct time filter replacement, which may plague the owner of the bag filter.

Disc Filter

Another side flow filtration technology is disc filter. Polypropylene disc filters use a series of stacked discs that are compressed together and slotted to filter a specific micron size. Like screen and sand filters, disc filters have an automatic backwash cycle for self-cleaning, reducing maintenance. Another advantage of using a disc filter is that it uses much less water than other self-cleaning filters that use a backwash cycle. However, these energy savings can be offset by the relatively high pump power required for the backwash cycle through the disc filter. In addition, the filter disc is a consumable element and must be replaced frequently. Nevertheless, disc filter is still a feasible choice for side flow filtration.

Note: Mechanical filtration system cannot be used alone. In addition to filtration, water treatment must also be carried out to ensure high water quality.

Particle Removal Analysis

It is very important to know the size of particulate pollutants in the system, and it is necessary to distinguish particle size and particle number. It should be clarified that even if a large number of particles are removed, it will not be effective to design a filtration system to remove less than 1% of the total particle volume. It is obvious that an understanding of the specific location characteristics of the pumped water is essential for specifying the appropriate equipment, separator or filter to be used in the filtration system.

Therefore, when the particle size found in the analysis system is large, it is important to know the total volume of particle matter to be eliminated, rather than the total number of particles. In mechanical filtration, a smaller percentage of larger particles (10 to 75 microns) is of more concern than a larger percentage of smaller particles (5 microns or less). Even the water quality association, an American drinking water standards authority, has recognized that any pollutant less than 5 microns in size is most commonly identified as a bacterium, which cannot be filtered but removed by disinfection.

Table 1 below provides an example of comparison and assumption, which samples 1 trillion particles and shows sample parts of several sizes. It can be seen that if only 15% of the total particles are greater than 10 microns, 15% of the particles will account for more than 99% of the total volume. In the actual cooling water circuit, this number may be many times, but the relative proportion is still effective and important, considering which pollutant is the most concerned. This example shows that even a relatively small number of 10-75 micron-sized particles can represent a large total particle volume. This fact should be taken into account when determining particles that can form pores in a fouling heat exchanger, plug nozzles, or accumulate in the equipment's packing, sump, or remote sump.

Size of ParticleQuantity of ParticleTotal Volume
0.45microns212.5billion particles0.006 cubic inches
1micron212.5billion particles0.007 cubic inches
3 microns212.5billion particles0.190cubic inches
5microns212.5billion particles0.890 cubic inches
Sub-total:850 billion particles1.088 cubic inches
10 microns37.5billion particles1.3cubic inches
25microns37.5billion particles18.5cubic inches
50 microns37.5billion particles150.1 cubic inches
75microns37.5billion particles504.1cubic inches
Sub-total:150 billion particles674.0 cubic inches

Table 1.Particle Size vs.Volume for a Sample of Particles

In addition to the particle size to be removed, there are other economic and design factors related to determining the correct filtration equipment. These factors usually affect the purchase decision of equipment according to the situation. Economic factors are the cost of replacement parts, maintenance requirements, space requirements and personnel training. Design factors include the size of particles to be removed and the flow range of the filtration equipment, the allowable level of pressure loss and liquid loss. These economic and design factors vary greatly, and for any given cooling tower application, they vary greatly. Whether some factors affect the purchase decision depends on the application.

Conclusion

As mentioned before, high water quality can only be achieved through the use of properly designed mechanical filtration systems and professional water treatment procedures. It is a key part of designing an effective mechanical filtration system to determine the correct filtration equipment and method. Proper filtration can reduce energy consumption, improve chemical properties, reduce necessary maintenance, improve machine productivity and limit bacterial growth. Good water treatment procedures can improve the system and save costs. Determining the type of filter equipment to use depends on the application and economic needs of the purchaser.

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