Since time immemorial, water has been used to cool spaces by using the evaporative properties of the medium. The humble “desert cooler” used in most Indian homes in the northern part of the country showcases how effectively and at relatively low cost, water can be used to provide the occupants a better environment to work or live. Water, thus, has an important part to play in a majority of HVAC systems used in offices, malls, hospitals, industries etc. Cooling towers are essential and critical components of water cooled chiller systems. The functioning of the plant depends on all the components working as close to the design values as possible but the correct functioning of the cooling tower has a major contribution to the overall plant operating to the expected parameters.
While the importance of the cooling tower is known to the designer and a considerable amount of time, effort and money is spent on selecting the most appropriate cooling tower for the plant, the same amount of attention is not given to operating and maintain the cooling tower during its life. The main reason for this state of affairs is the lack of awareness of the importance of the cooling tower in the HVAC system of a building. Since the cooling towers are usually placed on terraces and are not in front of the operators, their maintenance takes a back seat, with an approach of “run to breakdown”. Considerable savings can be achieved if the cooling tower is operated and maintained with as much care as the main chiller.
Measuring to Manage
A very famous management phrase that is equally applicable to the topic of cooling tower performance is “What is not measured is not managed”. Since cooling towers are not in the plant room where the maintenance team spends most of their time, the focus on the cooling tower maintenance is usually less than what it is on other equipment of the chiller system – pumps, main chiller, panels etc. Modern chillers have state-of-the-art control systems that measure hundreds of parameters of the chiller and associated systems, giving data on gas pressure, water temperatures, electrical current etc. There are, however, rarely any systems that track the functioning of the cooling tower parameters. It is left to the operators to “see” how the cooling towers are operating, and occasionally log basic parameters of the tower.
A 1 degree rise in cooling water temperature going to the condenser of a chiller can lead to an increase of energy consumption by 2.5 – 3%. That’s how sensitive the system is to a change in cooling water temperature. Considering that HVAC systems consume 40 – 50% of the energy in a building, such a variation in the cooling water temperature can have considerable impact on operating and running costs for the building owners. Thus, there is a business case for measuring the performance of the cooling tower in a more in depth manner than is currently practiced in the building services industry.
Figure 1: Range and approach linkage
Performance Management of Cooling Towers
The main function of a cooling tower is to provide the water to come into contact with the ambient air for as much time as possible over as much surface area as possible so that the maximum amount of evaporation can take place. The components of the cooling tower are all designed to achieve the optimized heat transfer and hence, to manage the cooling tower to deliver the desired results, the performance of these components and systems have to be measured. Key parameters that are used to assess the performance of the cooling tower are as follows:
Range
The “range” of a cooling tower is the difference between the warm water entering the cooling tower and the temperature of the cold water leaving the tower.
Approach
This is the most important aspect of the cooling tower design and is the difference between the cold water temp and the wet bulb temperature of the ambient air. The approach of a Cooling Tower depends on the process and it determines the size and cost of the cooling tower. The smaller the approach, the larger the size and hence the cost. Figure 1 shows how range and approach are linked
Cooling tower Efficiency: The range and approach can then be used to derive the efficiency of the cooling tower which is a very good tool for the O&M team to quickly assess how the equipment is functioning. The efficiency of a cooling tower is given by
Efficiency = [Range/ (Range + Approach)] x 100
Cycles of Concentration
Since water is evaporating and there are impurities in the water, the concentration of dissolved solids in the circulating water will keep on increasing as the water passes through the cooling tower. Higher levels of salts will lead to fouling and other harmful effects that will reduce heat transfer and hence, the level of solids has to be managed and monitored. This is done by adding make up water to dilute the dissolved solids. The CoC is calculated by the formula
Hardness of the circulating water/Hardness of the makeup water
Water Quality
The hygiene level of the water used in the system is also an important factor that can be used to assess the health of the cooling tower and subsequently it’s performance. A cooling tower with poor water quality will typically show lower heat transfers or conversely, higher cold water temperatures which impact chiller performance ultimately. The key parameters of water that are assessed are the conductivity, hardness and microbiological content
Factors Effecting Cooling Tower Performance
To be able to operate the cooling tower at the optimum level, it is critical to understand the factors that impact the cooling tower’s performance. The psychometric processes that occur in a cooling tower, where the cooling water transfers heat to the draft of air are complex and there are many variables such as the surface area, flow rate, etc. which add to the complexity. The key factors that impact the cooling tower performance are:
Wet Bulb Temperature
The “Approach” of the cooling tower is a function of the cold water temperature and the wet bulb temperature of the ambient air. The required cold water temperature is arrived at from the heat load calculations of the chiller design, and the wet bulb temperature is taken after studying the site conditions and available weather data. The Approach of the cooling tower is, thus, a system function and not of the cooling tower independently. If there is a change in the wet build temperature due to external conditions such as higher humidity, higher moisture content of the ambient air due to water bodies in the vicinity etc.
Heat Load
The size of the cooling tower will depend on the heat load of the system that is being cooled. The tower is sized for a planned occupancy level of the building and during the initial ramp up phase of a building, there are chances that the cooling tower would not be operating at the designed heat load. Thus, the performance would not be optimum.
Flow rate
The heat load determines the flow rate that is needed. Heat load will vary during the operation of the tower due to fluctuations in the occupancy of the building (lower load in the day time, peak at mid-day and tapering off again in the evening).
Other Factors
Interference of the air flow and recirculation of air also impact performance> while the design will aim to avoid these issues, changes if layout of the building and surrounding developments can result in change in these factors and therefore impact performance.
Water Quality
The heat transfer efficiency reduces drastically if the water quality is poor. Nozzles get choked due to higher salt content, higher fouling of the fins leads to poor heat exchange etc. and hence, water quality has a direct impact on performance.
Monitoring Performance of Cooling Tower
The operations and maintenance team needs to take a two pronged approach to managing and monitoring the performance of the cooling tower.
Parameter Monitoring and Analyses
Cooling towers are typically not fitted with any major instrumentation, and whatever, pressure and temperature gauges are fitted, usually, do not work due to poor maintenance. Thus, the first step in performance management of the cooling towers is to get all the basic instruments in a working condition. The key instruments that should be available on a cooling towers are the cold and hot water temperature gauges (of sufficient level of accuracy), flow meters and pressure gauges for the cooling water. Energy meters on the fan motors will aid in identifying high load conditions that are indicators of potential motor problems. Level sensors for the basin are an added safety feature to prevent air entering the chilled water line in case the cooling water basin get drained out. Another important but not often installed instrument is the online water quality meter.
The O&M team needs to log the cooling tower operating parameters on a two hourly frequency if no automated system is available. The data is then used to undertake trending studies and predict any possible operating issues or failures.
A monthly assessment of the cooling tower efficiency using the range. Approach formula to give a quick health status of the cooling tower is helpful and provides a high level check to senior management of the functioning of the equipment.
Annual performance testing
The Cooling Tower Institute (CTI) has developed a testing standard to assess the thermal performance of a cooling tower. This is a comprehensive test that relies of a large no of data points and instrumentation, as well as an understanding of the design conditions and current operating conditions. The test assess the cooling water circulation rate as compared to the rate calculated by the manufacturers performance curves. The key parameters that are measured in this type of performance test are
– Flow rate
– Water temperature
– Air temperature
– Brake horse power of fans
– Head of the water being pumped
The annual performance test will give the O&M team an assessment of how close or far from the design point is the tower operating based on which corrective actions can be undertaken. Similarly, ongoing performance monitoring activities will help the operators run the plant closer to the design point so that energy costs are low and system operates at the best efficiency possible.
Conclusion
Cooling towers are not a focus for the O&M team in most building operations due to either limited awareness of their importance of lack of focus of the team. This can have serious impact on the performance of the overall chiller plant and consequently, impact operating budgets, many a times, without the teams knowing why. The cooling tower is a relatively easy to maintain device and is relatively simple to operate and maintain. Thus the incentive to operate the cooling towers at their peak design level are very high, and creating a performance management plan for the cooling towers will go a long way in getting the best out of the HVAC system of the building.
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