Best ways to improve the Cooling Towers Efficiency and reduce the water

Tipo Cooling Towers

You might not think much about the smaller and mid-sized cooling towers that help run your plant, but that doesn’t mean they can be ignored. These towers are important as any piece of equipment you have on your property. You can improve water efficiency in your water-cooling towers and subsequently preserve energy and save you money.

WATER LOSS DETAILS IN COOLING TOWER

Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up volumetric flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles.]

The water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage or drift (W) and some of the water (E) to evaporate. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off or blown down (D) for disposal. Fresh water make-up (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.

Piping Change

Simple repairs like a change to the water piping can help you manage your energy consumption. Changing one pipe to discharge blowdown water to return hot water can improve the overall energy efficiency of the cooling system by up to 2%. It’s important to handle the equipment carefully, however. Consult with professionals who are knowledgeable about cooling towers and/or pipe replacement.

 

Focus on Recycling

Ensuring that your cooling tower is properly functioning and properly recycling is important for water management, energy preservation, and saving money. High quality cooling towers are manufactured and designed to recycle at least 98% of all wasted system water. Recycling that much water can result in major energy and water reductions and can end up saving you and your business money and improve water efficiency.

 

Periodically Inspect all Equipment

Inspections are an essential part of making sure cooling systems are properly working. If you ignore them — which many people often do — you could end up be wasting much more energy than needed — and end up spending more money. Regularly performing maintenance and repair checks to your cooling system is imperative for operating and maintaining a plant. Look for heat transfer areas, corrosion, biological growth, and any particle pollution deposits. Those damages and problems can reduce energy efficiency for the entire cooling system by at least 5% — and even more.

 

Increase Cooling Cycles

Being aware of the number of cycles your cooling tower uses is important as well. Increasing cooling water cycles from three to six can reduce make-up water by up to 20% and can even reduce cooling tower blowdown by up to half. Many people don’t focus on cooling towers.

HOW TO INCREASE THE EFFICIENCY OF COOLING TOWER

 

Although cooling towers are generally some of the most cost-effective cooling mechanisms as compared to other cooling technologies (such as many dry air cooling systems), it is still important to monitor your industrial cooling tower efficiency and run it as seamlessly as possible. This will ensure your facility can:

  1. Save energy
  2. Reduce the amount of water being consumed
  3. Decrease the amount of chemicals required for water treatment
  4. Extend the equipment service life
  5. Reduce operating costs, overall

But how can you make sure your cooling tower is being maintained in the most efficient way possible? This article breaks down the complex answer to this question below.

Understand what kind of cooling tower you have

The first step in making sure your cooling tower is being run efficiency is becoming familiar with the type of cooling tower you have and how it works. Different types of cooling towers are exposed to different types of contaminants and require specific treatment.

Some of the most common types are as follows:

Once-through cooling tower systems

Once-through cooling towers draw water from a source such as a river or lake and circulate the water through pipes to absorb heat from steam condensers. The warmed water is then discharged into the environment. Although these systems are generally the easiest and most cost-effective cooling tower option to set up and use, some facilities choose other options for cooling, either because of local regulations (if water is scarce in the area, there might be rules mandating how much water can be withdrawn and discharged in to the local environment) or because it can be difficult to locate a facility in a place that has enough water to draw from. Some facilities are converting their once-through cooling systems into closed-circuit systems.

Open recirculating cooling tower systems

If your facility uses an open recirculating cooling system, the same water is continually used and recirculated throughout the process, removing heat with evaporation. Reusing the water (not lost to evaporation) helps reduce the amount of water needed to run the tower in addition to chemical costs for water treatment, as the chemistry is generally retained in the system. This does, however, increase the likelihood of corrosion and deposition as the solids become more concentrated in the system after evaporation occurs. The heat in combination with open-air conditions can also facilitate bacterial growth and collect contaminants from the air.

Closed recirculating cooling towers

Also known as closed-circuit or closed-loop cooling systems, closed recirculating cooling towers operate similarly to open recirculating systems, except the rejected heat is transferred to a heat exchanger with no part of the process being exposed to the open air. This eliminates contamination once the proper water chemistry is attained and the only time there needs to be makeup water added to the system is if there is a leak.

Be aware of source-water quality and the quality of water needed to run the system

Depending on the type of cooling tower you are operating and how it works, your specific water treatment requirements may vary, but in general, the three main areas of the cooling tower process that typically require treatment are: feed water to the cooling tower, circulatory water in the tower, and cooling tower bleed to drain.

Feed water to the cooling tower

Depending on the quality of the cooling tower feed water, you may or may not need treatment here. If a water treatment system is needed at this part of the cooling tower water process, it is usually technology that removes hardness and silica or stabilizes / adjusts the pH.

At this point of the process, the proper treatment optimizes the tower evaporation cycles and minimizes the water bleed rate to drain beyond what might be done with chemicals alone.

Circulatory water in the tower

The second area of physical treatment a cooling tower will typically need is for the circulation water within the tower.

Normally some form of side stream filtration is your best bet. This helps keep your cooling tower water free of particles that can build up and foul the cooling tower. By running 10% of the circulated water through the side stream filter, it can be easier to retain a healthy balance of suspended solids that will reduce the particulate fouling of your equipment.

The chemical treatment of a cooling tower will typically occur at this phase within the tower. Proper chemical treatment minimizes scale formation, corrosion of critical equipment, and helps disperse suspended solids so settling can be minimized in low-flow areas.

Cooling tower bleed to drain

The last part of treatment required for cooling tower water is the blowdown or bleed from the tower.

Depending on how much water the cooling plant needs to circulate for proper cooling capacity, plants can choose to recycle and recover the water through some type of post treatment in the form of reverse osmosis or ion exchange, especially in places where water might be scarce.

This allows liquid and solid waste to be concentrated and removed while treated water can be efficiently returned to the tower and reused.

Know the materials your unit is made from

The second step in understanding how to efficiently run your cooling tower is to know what the cooling tower is made of. Is it constructed of galvanized or stainless steel? Does it use plastic piping? Fiberglass? Is there a protective coating applied to reduce corrosion? Each of these materials withstands deterioration and environmental factors in different ways.v

Some newer cooling towers are constructed from fiber-reinforced polyester (also known as FRP), which can be lightweight and hold up to higher chemicals usage and ranges in pH. Another common configuration is galvanized metal with a stainless steel sump or basin, which can provide durability without the extremely high cost of a cooling tower made entirely out of stainless steel.

It’s important to know what the tower is made of, what materials are used for what parts, and what the manufacturer and your water treatment specialist recommend in terms of cooling tower water quality to reduce the possibility and/or severity of corrosion and breakdown of equipment over time.

Keep track of your cooling system’s water loss

In cooling towers, water can be lost several ways. Knowing when and how much water your system loses through evaporation, blowdown, drift, and/or leaking is essential in making sure corrective measures are being taken to conserve this water if needed.

Keep in mind these water losses will fluctuate and some can be estimated by the cooling tower manufacturer, but according to the Environmental Protection Agency, these losses and calculations include:

  1. This is typically about 1% of the rate of recirculating water flow for every 10°F temperature drop that the cooling tower reaches.
  2. Blowdown or bleed-off. This calculation depends on the number of cycles, level of evaporation, and quantity of makeup water required. It can be calculated if you know the value of any two of these three factors.
  3. Water carried off by wind or mist is known as “drift” and can vary from 0.05% to 0.2% of the flow rate through the cooling tower without the use of drift eliminators, which can reduce this to negligible amounts.
  4. Leaks and overflows. Properly running units will not experience leaks or overflows, but if they do occur, the amount of water lost needs to be accounted for the makeup water calculation: Makeup water needed = Evaporation + Drift + Blowdown + Leaks and Overflows

Improve the efficiency of your chemical usage

Working with your water treatment specialist, explore options for monitoring and minimizing the use of chemicals, which can be costlyClosely monitoring cooling tower water chemistry can also help reduce deposition and corrosion, improving system efficiency and extending the service life of your equipment.

Properly pretreating the cooling tower makeup water can help with this as well. Water treatment systems such as ion exchange for softening or reverse osmosis for filtration can lessen the amount of chemicals required to keep heat exchangers and piping free from scaling ad fouling. But as with all water treatment options for cooling tower water, it’s important to consult a specialist who can make recommendations based on your facility’s individual needs.

water loss details of cooling towers

water loss details of cooling towers

 

Fan-induced draft, counter-flow cooling tower

Using these flow rates and concentration dimensional units:

M = Make-up water in m3/h
C = Circulating water in m3/h
D = Draw-off water in m3/h
E = Evaporated water in m3/h
W = Windage loss of water in m3/h
X = Concentration in ppmw (of any completely soluble salts … usually chlorides)
XM = Concentration of chlorides in make-up water (M), in ppmw
XC = Concentration of chlorides in circulating water (C), in ppmw
Cycles = Cycles of concentration = XC / XM (dimensionless)
ppmw = parts per million by weight

A water balance around the entire system is then:

M = E + D + W

Since the evaporated water (E) has no salts, a chloride balance around the system is:

{\displaystyle MX_{M}=DX_{C}+WX_{C}=X_{C}(D+W)}

and, therefore:

{\displaystyle {X_{C} \over X_{M}}={\text{Cycles of concentration}}={M \over (D+W)}={M \over (M-E)}=1+{E \over (D+W)}}

From a simplified heat balance around the cooling tower:

{\displaystyle E={C\Delta Tc_{p} \over H_{V}}}

where:
HV = latent heat of vaporization of water = 2260 kJ / kg
ΔT = water temperature difference from tower top to tower bottom, in °C
cp = specific heat of water = 4.184 kJ / (kg{\displaystyle \cdot }°C)

Windage (or drift) losses (W) is the amount of total tower water flow that is entrained in the flow of air to the atmosphere. From large-scale industrial cooling towers, in the absence of manufacturer’s data, it may be assumed to be:

W = 0.3 to 1.0 percent of C for a natural draft cooling tower without windage drift eliminators

W = 0.1 to 0.3 percent of C for an induced draft cooling tower without windage drift eliminators

W = about 0.005 percent of C (or less) if the cooling tower has windage drift eliminators

W = about 0.0005 percent of C (or less) if the cooling tower has windage drift eliminators and uses sea water as make-up water.

Cycles of concentration

Cycle of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Discharge of draw-off (or blowdown) is used principally to control the buildup of these minerals.

The chemistry of the make-up water, including the amount of dissolved minerals, can vary widely. Make-up waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Make-up waters from ground water supplies (such as wells) are usually higher in minerals, and tend to be scaling (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems.

relationship between cycles of concentration and flowrates

relationship between cycles of concentration and flowrates

 

As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. Often a professional water treatment consultant will evaluate the make-up water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening, pH adjustment, and other techniques can affect the acceptable range of cycles of concentration.

Concentration cycles in most cooling towers usually range from 3 to 7. In the United States, many water supplies use well water which has significant levels of dissolved solids. On the other hand, one of the largest water supplies, for New York City, has a surface rainwater source quite low in minerals; thus cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.

Since higher cycles of concentration represent less make-up water, water conservation efforts may focus on increasing cycles of concentration. Highly treated recycled water may be an effective means of reducing cooling tower consumption of potable water, in regions where potable water is scarce.

Maintenance

Clean visible dirt & debris from the cold-water basin and surfaces with any visible biofilm (i.e., slime).

Disinfectant and other chemical levels in cooling towers and hot tubs should be continuously maintained and regularly monitored.

Regular checks of water quality (specifically the aerobic bacteria levels) using dipslides should be taken as the presence of other organisms can support legionella by producing the organic nutrients that it needs to thrive.

Water treatment

See also: Industrial water treatment

Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered to remove particulates, and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water. Under certain conditions, a biofilm of micro-organisms such as bacteria, fungi and algae can grow very rapidly in the cooling water and can reduce the heat transfer efficiency of the cooling tower. Biofilm can be reduced or prevented by using chlorine or other chemicals. A normal industrial practice is to use two biocides, such as oxidizing and non-oxidizing types to complement each other’s strengths and weaknesses, and to ensure a broader spectrum of attack. In most cases, a continual low-level oxidizing biocide is used, then alternating to a periodic shock dose of non-oxidizing biocides.

The water consumption of the cooling tower comes from Drift, Bleed-off, Evaporation loss, The water that is immediately replenished into the cooling tower due to loss is called Make-up Water. The function of make-up water is to make machinery and equipment run safely and stably.

In conclusion 

Being aware of potential cooling tower water treatment issues and knowing how to solve them is essential to the success and efficiency of your process.  When it comes to deciding the right solutions for your cooling tower water treatment, it is extremely important to consult a water treatment specialist. The proper maintenance of your system will depend on your plant’s individual needs.