Before getting into the details of improvement in energy efficiency, one must properly understand about what exactly a cooling tower is, its function and key components involved in its functioning. It is also necessary to know certain fundamental terminologies related with functionalities.
The water then gets distributed by cooling tower nozzles to the wet deck. At the same time, air is being drawn through the air-inlet louvers forcing water to evaporate. Evaporation causes the heat to be removed from the make-up water.
• A cooling tower is an enclosed tower like structure through which atmospheric air circulates to cool large quantities of warm water by direct contact.
• Cooling towers are a very important part of many plants. • Cooling tower is to reject heat into the atmosphere.
• Removing low-grade heat from cooling water.
• The make-up water source is used. The principle of operation of a cooling tower fill is to put as much water surface area in contact with as much air as possible for the longest amount of time possible. Film fills allow the water to form thin flowing sheets to expose as much water surface area as possible to the interacting flow.
Drift eliminators are designed to capture large water droplets caught in the cooling tower air stream. The eliminators prevent the water droplets and mist from escaping the cooling tower.
‘Approach’ is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance.
Tower fans oscillate on a base stand and distribute air circulation at a 90-degree angle. This angle combined with the height of the unit gives the tower fan its wider area of cool air circulation. The impeller blades move air through the cylindrical column and then out of the vents of the tower fan. A closed-circuit cooling tower or dry cooling tower involves no contact between the air and the fluid being cooled. This tower has two separate fluid circuits, one in which the fluid is recirculated on the outside of the second circuit, which is a bundle of tubes through which the hot water is flowing.
Air intake louvers also prevent sunlight from entering into the cooling tower. Preventing sunlight from reaching the basin is critical to prevent algae growth. They also lower the amount of splash out of a cooling tower which preserves water.
Cooling tower bleed-off or blowdown is the flushing of a portion of high mineral concentration cooling tower system water down the drain while simultaneously replacing it with fresh water. This process dilutes the system water mineral concentrations that steadily increase due to water evaporation.
There are mainly two types of cooling towers:
Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature.
A nominal cooling tower ton is defined as the capability to cool 3 GPM (0.19 lps) of water from a 95ºF (35.0ºC) entering water temperature to an 85ºF (29.4ºC) leaving water temperature at a 78ºF (25.6ºC) entering wet-bulb temperature.
Cooling towers are rated in terms of approach and range, where the approach is the difference in temperature between the cooled water temperature and the entering air wet bulb temperature – twb – temperature. The range is the temperature difference between the water inlet and water exit. A key parameter used to evaluate cooling tower operation is ‘cycle of concentration’ (sometimes referred to as cycle or concentration ratio). This is determined by calculating the ratio of the concentration of dissolved solids in the blowdown water compared to the make-up water. Now, that the cooling season is fully underway, cooling towers are hard at work. Is it necessary to show a cooling tower the TLC it needs to be running at its peak performance efficiently? The Cooling Technology Institute made up a great Powerpoint presentation that is the basis for this post.
Maintenance and water treatment are the most important factors affecting the life and energy efficient operation of evaporative cooling equipment. They are also the most neglected regimens of cooling tower operation with cooling towers generally the most neglected component in the mechanical system. Why are these important steps often the most ignored?
• Most cooling equipment is remotely located and difficult to access. Limited maintenance resources such as staff, training, or budgets make it a problem. Staff do not realise its importance. Here are some basic maintenance practices for the varying types of cooling systems: If an operator is using a circulating water system, cleaning and flushing the system monthly is key. This prevents build-up of any solids in the system which can clog lines or make a breeding hot spot for bio growth. It is essential to inspect and repair corrosion because the longer wait can become a losing battle. It is necessary to check all the seals, because a leak can lead to huge losses of water and chemicals, racking up the bill.
• For fan driven systems, it is essential to make sure to keep the fan system in top operating condition. This means someone should be checking daily for any unusual vibrations or sounds, as this could mean there is a problem. The fan should be cleaned quarterly of heavy debris such as trash, bird droppings, or scale. there is a need to check for loose tighteners, missing balance washers, structural integrity and repair or replace any corroded hardware. Making sure that the motor is operating smoothly is also the key. Check to see that the motor is lubricated properly and is being cooled so it does not overheat, which can cause serious damage. Routine maintenance of the motor will help to ensure it has a long and useful life.
Finally, it is important to make sure to keep up with a water treatment plan. Water treatment plans can be the difference in having to replace something or not. When air and water contents enter the tower, they can build up, creating possible problems. Microbes can form, producing bio-growth that is unwanted, or corrosion could occur, which means replacing the affected parts. Both can add up to an expensive fix compared to the cost to treat the water. Following factors can seriously affect the efficiency of the cooling tower:
Selecting a Cooling Tower
i) Capacity
• Heat dissipation (kCal/hour)
• Circulated flow rate (m3/hr)
• Other factors.
ii) Range
• Range determined by process and not by system approach
• Closer to the wet bulb temperature Bigger size cooling tower
• More expensive.
iii) Heat Load
• Determined by process
• Required cooling is controlled by the desired operating temperature
• High heat load = large size and cost of cooling tower.
iv) Wet bulb temperature – considerations
• Water is cooled to temp higher than wet bulb temp
• Conditions at tower site
• Not to exceed 5 per cent of design wet bulb temp
• Is wet bulb temp specified as ambient (preferred) or inlet
• Can tower deal with increased wet bulb temp
• Cold water to exchange heat.
v) Relationship range, flow and heat load
• Range increases with increased
• Amount circulated water (flow)
• Heat load.
vi) Causes of range increase
• Inlet water temperature increases
• Exit water temperature decreases
• Consequence = larger tower.
vii) Relationship Approach and Wet bulb temperature
• If approach stays the same (e.g. 4.45°C)
• Higher wet bulb temperature (26.67°C) more heat picked up (15.5 kCal/kg air) smaller tower needed
• Lower wet bulb temperature (21.11 °C) less heat picked up (12.1 kCal/ kg air) larger tower needed.
Fill media
• Hot water distributed over fill media and cools down through evaporation • Fill media impacts electricity use
• Efficiently designed fill media reduces pumping costs
• Fill media influences heat exchange: surface area, duration of contact, turbulence.
Pumps and Water Distribution
Optimise cooling water treatment
1. Increase cycles of concentration (COC) by cooling water treatment helps reduce make up water
2. Indirect electricity savings
3. Install drift eliminators
4. Reduce drift loss from 0.02 per cent to only 0.003 – 0.001 per cent.
Fans and motors
• Fans must overcome system resistance, pressure loss: impacts electricity use
• Fan efficiency depends on blade profile
• Replace metallic fans with FBR blades (20-30 per cent savings)
• Use blades with aerodynamic profile (85-92 per cent fan efficiency).
Controls
• Wherever possible use of VFD will add to efficiency.
• Use of PLC synchronised with RTC and Load demand will lead to drastic improvement is performance, operational life, reduced maintenance and huge energy saving. The most important thing to take away from this is that it is better to be proactive than reactive. Hence, it is necessary to adopt not only proactive maintenance but also condition monitoring approach.
