COOLING TOWERS
The use of cooling towers is widespread on modern mine sites. Their application ranges from the cooling of process liquids in mineral processing to cooling ventilation air and everything in between. In a Niagara Wet Surface Air Cooler, unwanted heat is removed by passing the hot medium through a bundle of tubes, over which flows a constant stream of air and water vapour.
Cooling is achieved by the evaporative effect of the water absorbing heat from the tube’s surface as it changes state from a liquid to a gas.
While this might sound as simple as a home evaporative air conditioner, in an industrial application, there are many other factors to consider such as maintaining separation between the medium being cooled and the water/air mix of the cooling circuit, the most efficient design to optimise the cooling effect using minimal energy and fresh water inputs and having the flexibility to create a custom solution to achieve all this for a specific site, with a specific cooling requirement and a limited range of input options (such as water supply).
The Australian Mining Review recently spoke with Alfa Laval’s business development manager, Neil Wissemann, about new technologies being introduced in Australia to dramatically improve water, sustainability and cost outcomes in cooling tower requirements for heavy-duty industries such as mining. These technologies have been used internationally and locally over many decades with overwhelming success and are well suited to the Australian environment.
Niagara WSAC®
Established in Australia since 1926, Alfa Laval specialises on three key technologies: heat transfer, separation and fluid handling. All three have great significance for industrial companies and Alfa Laval holds leading global market positions within its fields of technical expertise.
The company acquired New York-based manufacturer Niagara in 2014 and incorporated the company’s products in its market leading range of heat recovery and heat transfer technologies.
The Niagara Wet Surface Air Coolers (WSAC®) are efficient closed-loop, evaporative cooling systems designed for heavy-duty industries such as power, mineral processing, refineries, wastewater facilities, oil and gas and petrochemical industries.
Originally designed in the USA, the Niagara system has the ability to cool a broad range of fluids and vapours, including wastewater, effluents, oils, glycols, hydrocarbons, drilling mud, supercritical carbon dioxide, nitrogen, vacuum steam, ammonia, hydrocarbons (propane, ethane, propylene) and refrigerants.
Perhaps one of the Niagara’s most environmentally and economically significant benefits is its capacity to utilise low grade and even wastewater as a cooling medium.
Rather than requiring fresh or scheme water for cooling, the WSAC® allows the cooling tower to be designed and engineered to operate with the by-products of RO systems, treated sewage water and even salt water if required.
This represents a significant saving to mining operations who frequently have a shortage of clean water on site and who may even have to pay to have wastewater carted off site for proper disposal.
Early Engagement
Alfa Laval emphasises the benefits of its early engagement with proposed cooling tower projects because the Niagara platform allows it to tailor a design that works for a very specific requirement.
The engineers can visit a site, engage with the client, assess the cooling requirements for a given fluid and then engineer a solution that uses the available water on site – however harsh or unconventional it may be – and match that to an energy efficient tower to create a solution that not only works well but offers a very high ROI thanks to associated potential cost savings on site.
Degrees of Separation
Neil says that Alfa Laval’s Niagara cooling towers incorporate a physical barrier between the evaporative air and coolant and the process medium being cooled.
“With a standard cooling tower, the large, constructed pieces of equipment that spray water down over the fill or structure, using the evaporative cooling of the surrounding air, however this water and air can contain solids, dust, grime and highly saline fluids,” he said.
“Designers will typically design heat exchangers in between the cooling tower and the rest of the plant processes.
“We service a niche application in the closed loop cooling tower or evaporative tower and we integrate heat exchangers with cooling tower structures into one unit, rather than separate units at mine sites.”
Available as either small packaged, modular or large field-erected WSAC® systems, the team of project engineers provide custom solutions – from design to commissioning, from tube bundles to electrical controls as well as ancillary components – to maximise performance of a site or plant’s cooling requirements throughout the lifecycle of the equipment.
How it works
Hot process medium flows through prime surface tube bundles and this medium is then cooled by the co-current flow of air and spray water on the outside of the tubes.
Recirculating recycled water is pumped up to the spray system which is mounted above the tube bundles and distributes water over the tubes at a high drenching rate. The high drenching rate is important as it helps to keep the heat transfer surface cleaner for longer.
The unique Alfa Laval wet surface technology used in the Niagara cooler system is based on evaporative cooling, where cool air is drawn down over the tubes by fans.
Spray water flows downward in a co-current direction with the air which sets up a continuous film of water over the tubes.
The heat from the process fluid inside the tubes causes some of the water to evaporate.
The air and water vapour stream makes a 180° turn, removing free water droplets by gravity separation. The vapour is removed by fans.
Unevaporated water is collected in the basin and pumped back to the top of the unit to begin the process again.
As a closed loop system, there is no risk of contaminants entering the process stream. The system also entails no fill, meaning the tube bundles are 100% prime heat transfer surface.
The WSAC® can reach much lower outlet temperatures due to its direct evaporative cooling, compared to a cooling tower-plusheat exchanger combination.
Its water-consumption is several times lower as the same spray water can be reused up to 10 times. The use of bare tubes, a large spacing between the tubes and the absence of tower fills allow for dirty wastewater to be used as spray water.
The WSAC® can also guarantee significantly lower outlet temperatures the whole year round, even during the hottest summer months, compared to a dry air cooler, while using a much smaller plot-space and having a much lower power consumption.
Benefits
The Alfa Laval Niagara WSAC® combines high efficiency maximum performance, low operating costs, compact size and reliable operation for sustainable alternatives to cooling tower systems and air coolers.
The Australian mining industry now has access to this unprecedented cooling capacity for increased production and processing output in their heavy-duty industrial applications.
FlexWater
Renowned for their robust design and minimum life cycle and maintenance costs, the benefits of the Alfa Laval Niagara WSAC® in liquid and gas cooling as well as condensing are:
• the coldest possible process outlet temperature
• increased water conservation due to higher concentration cycles
• low quality water can be used as makeup
• compact footprint
• lower parasitic energy (HP)
• competitive installed cost
• minimal maintenance required
• less drift
One of the innovations Alfa Laval developed to keep life cycle cost to a minimum while maximising cooling performance is FlexWater.
This feature allows a WSAC® system to operate on recycled water of low quality such as blowdown water, wastewater or effluent – quality that is much lower than what is required in a standard cooling tower.
The high drenching spray water flow, nozzle design, bare tubes, wide spacing between the tubes and absence of cooling tower fill prevent debris from getting caught, falling through the heat exchanger and collected in a pump screen at the bottom.
This makes it possible to re-use the spray water up to 10 times, sometimes more.
The result is much lower freshwater consumption and lower maintenance costs compared to a cooling tower.
Examples of FlexWater used in the cooling water recirculation include fresh water, sea water, treated effluent, reverse osmosis reject water, brackish water and wastewater. “One big advantage of interest to Australian mine sites is that you can use fairly low quality water sources as the evaporative medium,” Neil said.
“Australian mining customers use about 4% of total water consumption in the country, which is relatively significant as most of mine sites are in areas where water availability is very poor.
“Mine sites utilise bore water facilities or
reconstituted water and, very frequently, large reverse osmosis style systems to produce freshwater which they can either use for cooling or offsite usage as well.
“The difference with our system is that you can utilise very poor quality water sources, such as bore water directly into the Niagara system, even reverse osmosis concentrate (the by-product) as the cooling medium.
“We’ve produced systems that can use a variety of cooling mediums, even sea water, and all we did was determine the construction materials to use on the Niagara system itself.”
Efficient cooling – water on tube and no tower fill
Alfa Laval’s efficient and effective cooling in its systems is due to its unique WetSurface technology, which provides maximum cooling efficiency and the lowest possible outlet temperature.
As hot process medium flows through tubes that are sprayed on the outside with water, the heat is rejected through the evaporation of the water on the tube surfaces.
This evaporative cooling process is much more efficient than convective heat transfer taking place in a traditional dry air cooler.
The Alfa Laval Niagara WSAC® can cool the process medium to lower temperatures, has smaller footprint and less power consumption.
Unlike traditional air-cooled heat exchangers, the tube bundles in a WSAC® are sprayed with water resulting in cooling lower than the ambient air temperature.
The system is significantly smaller, has a lower power consumption and lower output temperature than an air cooler.
“The reason why the WSAC® system is different is that it doesn’t have a fill or structure within the system,” Neil said.
“It’s a blank open basin and the heat transfer happens through the tube bundle which is set on top of this structure.
“Water is sprayed over the tube bundle and into a blank basin, which is why the WSAC® system can work with poor quality water as it doesn’t have a fill or timber structure to power up.”
HybridCool
The feature HybridCool in the Alfa Laval Niagara WSAC® combines wet and dry bulb cooling for minimised water consumption.
The system is fully-customisable to requirements and these hybrid systems allow operators to choose operating mode depending on ambient conditions.
Dry mode can be engaged during cooler weather to save water while wet mode can be activated during hotter months to ensure cooling performance.
The WSAC® system can produce the coldest possible process outlet temperature compared to other technologies.
It operates with a single temperature approach to the wet bulb temperature, making it possible to cool the process medium to a lower temperature than what a traditional cooling tower plus heat exchanger combination could do, which has two temperature approaches.
Design air temperature is at 37.7°C for dry bulb and 26.1°C deg for wet bulb.
“You can achieve the lowest possible temperatures back to your plant without requiring any refrigeration just by using the dry/wet bulb temperature of the air in order to achieve the cooling,” Neil said.
Case Study 1: Darling Downs Power Station
Australia’s largest power station, based in Kogan, Western Downs region in Queensland, is capable of generating 630MW of electricity.
The challenge the Darling Downs power station had was to reduce the volume of wastewater transported off-site by truck for treatment or disposal.
Alfa Laval installed the Niagara wet surface air cooler (WSAC®) wastewater evaporator, powered by scavenger heat to significantly reduce water volume with high efficiency.
The scavenger heat source also provided auxiliary cooling.
The creation of this low emissions facility using efficient, eco-friendly technologies and durable construction materials could also be integrated as part of a multi-stage waster concentration and filtration process at the power plant.
Designed for use with high TDS water, the facility also uses the heat available on-site, created by an existing generation process, to power the evaporator and drive further efficiency.
The outcome was that the Darling Downs power station reduced water usage by 97% and emitted 50% less greenhouse gas, compared to a coal-fired plant of similar capacity.
It also reduced the requirement for wastewater trucking by 70%, reducing costs and carbon emissions related to truck transportation.
Mr Wissemann says it was cost exorbitant for the Darling Downs power station to truck their waste fluid to Brisbane.
“The installation of the Niagara system allowed the power station to take their effluent waste from site and use it as a cooling medium for some of their auxiliary systems in the power station,” he said.
“The power station therefore reduced their trucking from site down by about 70% which is a really short payback on the WSAC® system.”
The savings from transportation paid for the evaporator in just one year of operation.
Case Study 2: Salt water and extreme environment applications
Industries along the Red Sea in the Middle East such as oil refineries and cement factories use water from this sea for cooling purposes.
There is extensive demand for desalinated water, with at least 18 desalination plants along the Red Sea coast of Saudi Arabia.
A refinery in Saudi Arabia that required water from the Red Sea for its plant cooling needs found the large salt content of the sea water challenging to work with.
Along with limited water resources in the region, the sea water used caused high fouling and corrosion, making it extremely difficult for cooling.
Cooling towers were traditionally used to directly cool the sea water, which was then circulated in an open loop to plant heat exchangers.
Due to the high amounts of salt, the exchangers would need to be constructed with corrosion-resistant materials, however the issue of fouling would remain an issue, leading to further inefficiencies and additional maintenance.
The Alfa Laval Niagara WSAC® with FlexWater feature, which allows the WSAC® to operate on recycled water of low quality such as blowdown water, helped the refinery achieve a heat rejection of 169,553,000 BTUH and cooling of 24,000 GPM potable water.
Further outcomes were low fouling, low maintenance, all plant heat exchangers able to be low-cost standard material and no thermal pollution back to the sea.
In this application, potable water was cooled inside the tube, providing closed loop cooling to the plant heat exchangers.
The outside of the tubes was drenched with an open sea water loop providing the evaporative latent heat cooling.
Titanium tubes with all prime surface (no fins) minimised fouling, with fiberglass reinforce plastic (FRP) used for tube bundle casings. The sea water was used at 2.2 cycles of concentration.
About Alfa Laval
Swedish headquartered Alfa Laval is a recognised market leader in the technology areas of solid-liquid separation, heat transfer and fluid handling. Alfa Laval has a presence in 100 countries, 16,700 employees and holds 3,700+ patents worldwide.
The company prides itself of a strong local heritage of 95 years in Australia, with a significant local service and support presence including Service Centres in Sydney and Perth and field service engineers based around Australia to service even the most remote mine sites.
Alfa Laval imagines a world in which less is needed to produce even more. Alfa Laval’s innovative technologies are dedicated to improved energy efficiency and heat recovery, better water treatment and reduced emissions. Alfa Laval doesn’t just imagine the world – it builds the world together with its customers and partners.