Does your building use cooling towers as part of its air conditioning system?
Cooling towers remove heat from a building’s air conditioning system by evaporating some of the re-circulated water to cool the remainder. While all cooling towers continually reuse water, they can consume a significant percentage of a facility’s total water use.
Towers that are in good condition, operated properly and are well maintained allow chillers to operate at peak efficiency.
Some cooling towers may use alternative water sources such as recycled water, storm water or grey water if the concentration ratio is maintained conservatively low. Similarly, blow down water may be suitable for reuse elsewhere on the site
- Reduce cooling water overflow on tower shutdown by using a non-return valve on the pump delivery side
- Many facilities use ‘once-through’ water to cool small, heat generating equipment. Once-through cooling is wasteful because water is used only once before being discharged to sewer. Typical equipment that uses once-through cooling include vacuum pumps, air compressors, condensers, hydraulic equipment, rectifiers, degreasers, X-ray processors, welders and occasionally air conditioners. Options to eliminate once-through cooling:
- Connect equipment to a re-circulating cooling system. Excess cooling capacity within the plant may be available for use
- Reuse the once-through cooling water for other facility water requirements, e.g. cooling tower
Current best practice for water use in cooling towers is 800 litres per square metre per annum. This equates to 22 kilolitres per day for a 10,000 square metre office. With the quality of water supplied by Yarra Valley Water, cycles of concentration of 8-15 are achievable using a conventional, well-designed, chemical water treatment program. ‘Cycles of concentration’ refers to the relationship between the quantities of blowdown water quality and make-up water quality. A water treatment program needs to address microbiological, corrosion, scaling and fouling issues. If you are operating at lower cycles, speak to your water treatment service provider.
Major outbreaks of Legionella have been associated with ineffective cooling water treatment practices. Scale, fouling deposits and corrosion increase the potential for microbial growth, and control of this is very important. These elements can also affect the efficiency of your cooling system leading to loss of comfort and increased
energy bills. Water loss or inefficient operation of cooling towers means water treatment chemicals are lost, which in turn compromises the effectiveness of the water treatment program and increases the cost of treatment. Optimizing your operation and cooling tower maintenance systems can offer significant savings in water consumption.
A cooling tower consumes water through evaporation, blowdown, drift, splash-out, and overflow. Its water efficiency depends on a number of factors including the flow rates and temperatures of air and water. Water is usually treated to maintain a clean heat transfer surface, minimise water consumption and meet discharge limits. The spray nozzles in the cooling tower need regular cleaning to ensure effective heat transfer.
How cooling towers can lose water
Evaporation is integral to cooling tower performance and cannot be reduced without an unacceptable reduction in performance. A general guideline for estimating the rate of evaporation from a cooling tower is 12 litres per minute per 352kW of cooling load.
Blowdown refers to water that is removed from the re-circulating cooling water to reduce build-up of dissolved solids in the tower water. Reducing blowdown to the minimum level consistent with good operating practice can conserve significant volumes of water. Treating the cooling water by physical or chemical means usually reduces the amount of water lost to blowdown. Water quality is dependent on the blowdown rate, water treatment and the quality of make-up water. Australian Standard AS/NZS 3666 requires that blowdown is controlled automatically.
Drift and other losses
Drift is a loss of water from the cooling tower in the form of droplets carried out of the tower by an air draft. It can be reduced by baffles or drift eliminators, which retain water treatment chemicals in the system to improve operating efficiency and reduce environmental impacts.
Splash-out, or windage, is water accidentally lost from water splashing or falling within the tower, or the effect of a strong wind blowing through it. Splash-out can be a major problem for large cross-flow or hyperbolic towers that suffer from strong winds blowing across the basin water surface. Splash-out both wastes water and affects operating efficiency. A remedy is to install a ‘wall’ in the middle of the cross-flow tower, preventing wind from blowing through.
Overflow occurs when the level of water within a cooling tower basin rises above a predetermined level. Normally this water flows down an overflow pipe into the sewer. It can be difficult to determine the occurrence of overflow unless you observe the tower for long periods of time or meter the overflow. Overflow is a common area of water wastage in cooling towers, usually due to inadequate maintenance. In some cases it accounts for up to 40 per cent of daily make-up water.
Other losses of water from cooling towers include:
- Cleaning as part of remedial or scheduled action
- System leakage, in which an open system leakage is often not discovered unless visible.
Water quality management
Effective control of scale, corrosion, microbiological growth and fouling, prevents system failure, maintains energy efficiency and minimises system maintenance.
- Cost savings from reduced cooling tower cleaning requirements
- Higher concentration cycles lead to reduced treatment chemical cost, although good management is required and cleaning frequency may need to be increased
- Savings from reduced tower blowdowns may include water, sewer and trade waste charges. Potential water saving opportunities
- Consider moving temperature set points indoors, reducing the amount of heat rejection
- Work closely with your chemical service provider to increase your water efficiency. For example, you should understand the purpose and action of each chemical used and the flow-on effect to the amount of water used in your cooling tower.
- Schematic of the water balance in a cooling tower, courtesy of Sydney Water Corporation
- If appropriate, establish performance-based service contracts with key performance indicators such as level of water use, corrosion rates, microbe levels etc.
- Ensure you use the right chemicals for the metals in your system
- Ensure your biocide program is effective and dosing equipment is appropriate
- Develop a risk management plan. Refer to the Victorian Department of Human Services’ A Guide to Developing Risk Management Plans for Cooling Tower Systems. Download from www.health.vic.gov.au/environment
- Check that your water treatment service provider has undertaken cooling tower efficiency training with AIRIAH
- Work with your water treatment service provider to ensure alternative approaches are safe and appropriate for your requirements
- Minimise the cooling load in a new or existing building to reduce water used in your cooling towers. It will also lead to a more energy-efficient building. Refer to Sustainability Victoria for assistance: www.sustainability.vic.gov.au
- Ensure diligent maintenance of side stream filters and fix leaks including float valves in CT basins Save water and reduce energy consumption
- Whenever outside air conditions are favourable use an economy air cycle so that the tower does not need to operate
- Some buildings can use a hybrid type of air-conditioning system that ventilates naturally via open windows whenever outside air conditions are favourable. Under these conditions the tower does not need to operate. Check air conditions first, as some areas may have poor air quality and require filtration
- Use heat recovery systems to minimise the amount of heat rejected through a cooling tower. The saved heat can be used to preheat hot water or even use hot water to re-heat coils
- The Co-efficient of Performance (COP) should be about 10 to 12. Modern chillers have greatly improved this measure and are better than some systems that use shower towers or cross ventilation (COP relates to chiller efficiency)
- When planning a new building, use expert hydraulic or design consultants to provide the latest water efficiency initiatives at the design stage
- When planning a new building, investigate options for cooling. Consider that some systems may use less water but are more energy intensive.
- Check the possibility of installing or using fan motors in towers with variable speed control, saving on energy and extending plant life
- Investigate reducing lighting. In many buildings you can effectively reduce power consumed by the lighting system by at least 30 per cent without any discernable loss of amenity. Reduced lighting consumes less power and produces less heat, which in turn reduces the air conditioning load
Fixing water overflows
- Ensure the ball float valve is set correctly and the overflow pipe is correctly positioned. If water flows from the drainpipe when the pump stops, the most common cause is an incorrectly set ball float valve. Setting the water level correctly can be difficult in towers with a low water volume such as those with a V-shaped basin: too high and you have an overflow problem; too low and you run the risk of emptying the basin on pump start-up. Consider using a break tank to increase the effective volume, or replace the ball float valve with a solenoid valve linked to electronic level detectors. A level controller can sometimes correct an overflow problem. Check if the ball valve is in a sheltered position within the cooling tower. If the tower is pressurized by the fan, the overflow pipe will require a trap. If it is subjected to water cascade it will require a shield
- Ensure pipe work configuration is not causing overflow. If condenser water pipes run above the height of the tower spray heads, water could flood back into the tower when the pump shuts down. This is easy to observe: just check the tower overflow when the pump stops. Fixing the problem usually requires reconfiguring the pipe work. Non-return valves are not recommended as over time dirt lodges in the seals and renders them ineffective. Consult a hydraulics engineer before making changes. Walkways in cooling towers often mean the condenser water pipe work in the plant room sits higher than the cooling tower basins, creating a need for check valves. If the pipe-work is out of reach of walkways, maintenance is difficult. Incorrect water balance may be an issue where there are two or more interconnected towers. The cause can be as simple as ball float valves set at different heights, in which case the floats need to be adjusted. The cause may, however, be more complex, with faulty pipe work design or inconsistent tower basin heights. In these cases an engineering review is required.
- If the area around the cooling tower is wet on a regular basis, water may be splashing out. This may be a design issue or it could be due to high winds and steps should be taken to eliminate the water loss. Anti-splash louvres or splash mats can be effective. Anti-splash louvres have the added advantage of shading sunlight from the tower basin, reducing algae growth. If wind is an issue, you may also require suitable windbreaks. Leakage from pipes, joints and seals
- Joints may need to be adjusted or sealed if water is leaking from the tower casing or basin. Replace packed gland pump seals with mechanical seals to help prevent water wastage. If water is leaking from any pump seal, ensure your maintenance personnel attend to it promptly, even if it is minor; leaks can result in significant water wastage.
Minimizing drift losses
- Ensure correct placement of the drift eliminator to help minimize the amount of water and chemicals lost to the atmosphere. Controlling blowdown
- Most cooling towers are bled off automatically when the conductivity of the water reaches a certain level. Aim to operate the bleed off on a more continual basis, optimising the conductivity of the tower and eliminating wide fluctuations of TDS. Use a conductivity controller to continuously bleed and refill water in the system
- Blowdown is minimised when the concentration ratio increases. Typical concentration ratios have been found to be as low as two to three and generally can be increased up to six or more. Increasing the concentration ratio from two to six will save 40 per cent of the initial make-up water volume. The maximum concentration ratio at which a cooling tower can still properly operate will depend on the feedwater quality, including pH, TDS, alkalinity, conductivity, hardness and micro-organism levels. Most cooling towers in Melbourne can have concentrations of 10 to 12 without detrimental consequences. The extent of use and the sensitivity of a cooling system will also affect how much blowdown can be reduced. Minimum blowdown rates must be determined in conjunction with the optimum water treatment program for cooling water.
- Consider installing check-meters on the make-up water feed line and the blowdown line to better control the blowdown and concentration ratio. Check-meters should, at a minimum, be capable of recording the total flow. Some check-meters also display instantaneous flow. It is important to read and record check-meter data regularly, establish water use and set best practice targets.
- If the condenser water system has a low heat load, the flow of condenser water through the cooling tower can be reduced via a tower by-pass valve. This valve enables the condenser water from the chiller to bypass the cooling tower and return directly to the chiller, thus heating it to a point where maximum cooling can occur across the cooling tower. Minimising the number of times the condenser water flows through the cooling tower minimizes water losses from evaporation, splash and drift. Care must be exercised when using a by-pass valve: it can cause rotating sparge cooling towers to stop rotating, and it can compromise spray nozzle patterns if flow through them falls below intended levels. In both cases, the tower can stop working.