Energy Efficiency Opportunities in Cooling Tower
A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. 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.
Common applications include cooling the circulating water used in oil refineries, petrochemical, and other chemical plants, thermal power stations, nuclear power stations, and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers.
Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 meters (660 ft) tall and 100 meters (330 ft) in diameter, or rectangular structures that can be over 40 meters (130 ft) tall and 80 meters (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used in some coal-fired plants and to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning.
Types of cooling towers:
Cooling towers are generally classified either by build, heat transfer methods, and airflow generation methods. We will take a look at each of the types of cooling towers.1. Cooling tower by build:
Package type: Package type cooling towers are pre-fabricated. The shell is usually made of corrosion-free, heat resistant, and durable material like fiberglass-reinforced polyester. Since they are pre-assembled, they can be easily transported to a facility of choice. Since they are compact, they are preferred in facilities with low heat rejection requirements like hospitals, malls, and office buildings.
Field erection type: These are large units that are generally used in power plants, huge manufacturing facilities such as steel processing plants or oil refineries. They are large structures compared to the package type. They can be manufactured according to custom specifications.
2. Heat Transfer method:
- Dry cooling towers: dry cooling towers operate by transferring heat through a surface that separates the working fluid from ambient air. This operates on the principle of heat transfer by a heat exchanger with extended fins. The fan is driven by an electric motor. Hence, dry cooling towers don’t consume any water.
- wet cooling towers or open circuit cooling towers: these are the most popular cooling towers because they are cost-effective and renewable. They use water to cool the facility and the heat transfer is measured by the decrease in the process temperature and a corresponding increase in both the moisture content and the wet-bulb temperature of the air passing through the cooling tower.
- wet cooling towers typically produce drift emissions. Although they are not hazardous to the environment, multiple drift eliminators are used to minimize the cooling tower drift. The evaporation of water in a wet-mechanical draft is inherently more energy efficient when compared to the other types of cooling towers like dry or fluid.
- fluid cooling towers or closed circuit cooling towers: in closed circuit cooling towers, often water is mixed with glycol to form a fluid. This fluid circulates in a coil throughout the tower and is not directly exposed to the air. They are typically used where the surface needs to be clean and free of contaminants. The advantage is that there is no scale formation and hence makes for better productivity and lesser downtime.
3. Airflow Generation methods:
The types of cooling towers based on the airflow generation are Natural draft, Mechanical draft, Cross-flow, and Counter-flow towers.
Natural Draft cooling towers use the design and shape of the tower itself to move up the air naturally using fans. They use the law of different densities between ambient air and the warm air in the tower. Hence, these towers are tall to induce the airflow and are shaped like a “hyperbole”. They are typically located outside the buildings to allow for airflow.
Mechanical Draft towers tend to use a fan to force the air. A propeller or centrifugal fan is used to circulate air inside the tower. These are much smaller in structure than natural draft towers. Capacity control is easy in these types of towers since the speed of the fan can be controlled. Unlike natural draft towers, these can be located anywhere inside the building.
Cross Flow cooling towers are structured to allow air to flow horizontally while the water flows down vertically. This is done through open trough systems in the fan deck, fitted with nozzles. Since the airflow contact time is lesser, more air is required for heat transfer to occur. This type of cooling tower has many disadvantages such as higher power consumption due to the airflow required; maintenance is time-consuming and is susceptible to scaling and clogging of openings.
Counter flow uses hot water that enters at the top, while the air is introduced at the bottom and exits at the top. Both forced and induced draft fans are used. The distribution is done through a channel with lateral pipes, fitted with splash spray nozzles. The growth of algae is highly restricted, as the lateral pipes are a closed unit and not located in direct sunlight. Their power consumption is lower than crossflow units and offers the advantage of easy maintenance.
Cooling towers are the least maintained utility in industries. Maybe the power consumption of a cooling tower system is relatively less compared to other utilities but the indirect impact on energy consumption of systems connected to the cooling tower will be high.
Below is the list of some energy efficiency opportunities for cooling towers:
- Control cooling tower fan based on leaving water
temperatures.
- Control to the optimum water temperature as determined from
cooling tower and chiller performance data to reduce cooling tower energy
consumption.
- Install 2 speed or variable-frequency drives for cooling tower
fan control if the number of fans is low else use on-off control if the number
of fans is many.
- Turn off unnecessary cooling tower fans when loads are reduced /
process is running under partial load
- Replace cooling tower aluminium fan blades with high
efficient fibber reinforced plastic (FRP) blades.
- Replace belt driven or gear driven cooling tower fan with direct
driven fan or EC Fan.
- Clean clogged cooling tower water distribution nozzles at regular
planned intervals.
- Install new nozzles to obtain a more-uniform water pattern which
enhances heat transfer.
- Replace splash bars with self-extinguishing PVC cellular-film
fill.
- On old counter-flow cooling towers, replace old spray-type
nozzles with new square-spray ABS practically-non-clogging nozzles.
- Replace slat-type drift eliminators with high-efficiency, low-pressure-drop, self-extinguishing, PVC cellular units.
- If possible, follow the cooling tower manufacturer's recommended
clearances around cooling towers and relocate or modify structures, signs,
fences, dumpsters, etc. that interfere with air intake or exhaust.
- Optimize the cooling tower fan blade angle on a seasonal and/or
load basis.
- Correct excessive and/or uneven fan blade tip clearance and poor
fan balance.
- Use a velocity pressure recovery fan ring.
- Divert clean air-conditioned building exhaust to the cooling
tower during hot weather.
- Re-line leaking cooling tower cold water basins.
- Check water overflow pipes for proper operating level.
- Optimize chemical use.
- Consider side stream water treatment.
- Restrict flows through large loads to design values.
- Shut offloads that are not in service.
- Take blowdown water from the return water header.
- Optimize the blowdown flow rate.
- Automate blowdown to minimize it.
- Send blowdown to other uses (Remember, the blowdown does not have
to be removed at the cooling tower. It can be removed anywhere in the piping
system.)
- Implement a cooling tower winterization plan to minimize
ice build-up.
- Install interlocks to prevent fan operation when there is
no water flow.
- Establish a cooling tower efficiency-maintenance program. Start
with an energy audit and follow-up, then make a cooling tower
efficiency-maintenance.
- Program a part of your continuous energy management program.
Few Important Concepts of a Cooling Tower:
Cooling Tower Blowdown:
Since the cooling tower system works on the principle of evaporation, the concentration of salts in the water rises at frequent intervals of time. This increase in concentration level will decrease the heat transfer rate in the cooling tower. Hence it is recommended to blowdown or remove water from the cooling tower sump frequently. Even very frequent blowdown causes wastage of water which is also not recommended. And the quantity of blowdown water can be estimated with the help of evaporation loss and cycles of concentrationEvaporation Loss:
Water quantity (m3/hr) evaporated from the cooling tower for cooling duty= theoretically, 1.8 m3 for every 10,000,000 kCal heat rejected
= 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2)
T1-T2 = Temp. difference between inlet and outlet water
Cycles of Concentration (COC):
The ratio of dissolved solids in circulating water to the dissolved solids in make up waterBlowdown Quantity = Evaporation Loss / (C.O.C. – 1)
Cooling Tower Efficiency:
The efficiency of a cooling tower is expressed in terms of effectiveness. The effectiveness of a cooling tower is defined as the ratio of Range to that of Range + Approach.THANK YOU !!!