Using Seawater for heat rejection in air conditioning systems offers several benefits, but also poses some unique challenges. One of the essential components of the system is the centrifugal pumps that circulate the sea water through the equipment.
With over 20 years of experience providing pumps and HVAC system components to customers throughout Australia and New Zealand, we know how important it is to choose the right pump for the application and setting. Here you can get a general overview of seawater systems and learn how to navigate the challenges when selecting and maintaining the centrifugal pumps for these applications.
What is a Seawater system?
A seawater system utilises the sea for heat rejection. Typically, the seawater is pumped from the harbour or bay to the heat exchanger, and then flows back to the harbour or bay. Heat is exchanged via the Plate Heat Exchanger to the closed water system, which is cooled or heated to suit the air conditioning plant connected.
For example, if there was a chiller, that closed loop would be a condenser water system. Alternatively, if there was a heat pump on heating cycle, that source system would be making cool water, and if the heat pump was on cooling cycle, that source system would be making hot water.
When should you choose a seawater system?
The decision to incorporate a sea water system is generally determined by proximity to a harbour or bay, and hence is often the preferred option by design engineers for buildings in those areas.
The circulation of harbour or bay water rather than drawing from town mains water supply is often considered a sustainable approach. For this reason, building owners may choose this method if they are concerned about incorporating Environmentally Sustainable Development (ESD) elements in their property envelope.
The circulation of harbour or bay water rather than drawing from town mains water supply is often considered a sustainable approach.
In most settings, seawater is already naturally cool (15 to 22 degrees Celsius), which eliminates the energy required to lower the temperature. There’s no need for cooling towers, which is a significant cost saving.
The system will take up less space since there are no cooling towers (a plate heat exchanger is still necessary, but far more compact).
Overall, seawater systems mean increased energy efficiency, decreased running costs and decreased plantroom space needed, eventuating in a smaller physical and carbon footprint.
One challenge is the material of construction employed in the pumps and other equipment on a seawater circuit. Cast Iron with a ceramic coating has been widely used on pumps and valves for this purpose.
However, this equipment’s life span is limited because of an increased susceptibility to intermittent cavitation and therefore erosion that will occur during the operating cycle of the pump. If the inlet is subject to varying suction lifts due to tidal conditions, then this challenge will be prominent.
316 stainless will suffer a similar fate for the same reasons, and therefore is not recommended.
The material traditionally used has been the marine grade aluminium bronze. Over the last 10 years, this has been superseded by Duplex 2205 Cast Stainless due to the fact bronze has a negative environmental impact in the manufacturing process that is not associated with Duplex.
Another challenge is the design of the inlet. Some sites around Sydney Harbour have opted for a suction lift with foot valve strainer. The collection of shells, barnacles and other marine life residue means pressure washing of the strainer must be incorporated as part of the routine maintenance. This can be an expensive procedure if access is difficult, and the services of professional divers is necessary.
Another method employed is the creation of a sump in which the sea water enters, and the long shaft column type pumps are used. This means the motors are raised above the floor level with the shaft extending down into the sump where the pump end is connected.
This removes the problems encountered in the above-mentioned suction lift method, but poses other challenges in that the exposed equipment is subject to sea water spray and is also deemed unsightly. Shaft deflection can also be a problem and thus repairs and maintenance can become costly.
Our Seawater Pumps
Our range of BakerFlow Seawater Pumps are ideal for systems using seawater and other corrosive acidic and alkaline liquids.
These quality-built sea water pumps have cast duplex 2205 casings and impeller. They include hard-faced mechanical seals fitted over a duplex 2205 machined stainless sleeve to protect the shaft from pitting over its lifetime.
Discharge sizes range from 32 to 300mm, and they are capable of flows of up to 500 litres per second.
Why We Use 2205 Duplex Stainless Steel for Our Seawater Pumps
This material gets the name ‘duplex’ from having a micro structure that combines the benefits of ferrite and austenite stainless steel; specifically, a high resistance to erosion, corrosion, and cracking, all of which are of paramount importance when choosing a pump that will be in contact with seawater.
2205 Duplex is also not susceptible to intergranular pitting, or crevice corrosion.
Not only does this material boast unrivalled efficacy when it comes to corrosion resistance, it is also twice as strong as regular ferritic and austenitic steel. The combination of low nickel content and unmatched strength lowers the costs of both manufacturing and shipping.
Other Products to Consider When Designing Seawater Pump Systems
Choosing the right pump is crucial, but every element of your system needs to be considered in whole to offer the greatest efficiency and longevity. For example, heat exchangers need to be made of titanium rather than stainless steel and for pipework, poly pipe is chosen over steel or copper.
We’ll help you when it comes to selecting your materials and components, generating performance curves with full technical data such as electrical loadings, spatial requirements, and pipeline details.