AIR CONDITIONING


close to that of OFCs at a lower capital cost. For a medium-term investment, an inverter screw can often make a sensible choice, providing a shorter payback period. Often key for consultants is an element


of redundancy, meaning that twin circuit DX systems are required as a minimum. In applications <500kW it’s often cost prohibitive to apply multiple screws / OFCs due to unattractive £/kW. This is generally why scrolls dominate in this area. Scroll compressors can be noisy unless mounted with additional enclosures. Likewise an inverter screw compressor can often emit a tonal noise and more vibration when compared to equivalent OFC compressor, which benefits from near silent operation, regardless of load. In new build applications like hospitals, universities and large retail outlets I have seen a growing trend towards consolidating plant space in a single energy centre, feeding numerous buildings. This allows larger operating plant to be installed, lending itself to high performing water-cooled chillers with capacities often >3MW.


I’ll be honest, I’m not seeing this on the top


of every client’s list of requirements yet…but its bubbling in the background and will definitely gather more traction this coming year. I think we’ll continue to see more energy efficient products in new builds and it’s likely I’ll be asked by consultants for more comparisons on air versus water-cooled chillers. Water cooled installation costs are likely higher but have lower energy impact, coupled with low GWP refrigerant.


Martyn Jackson, lead mechanical engineer The chiller compressor is critical to the overall efficiency of the unit and a key factor in any de- sign. Let’s consider the main types: The oil-free centrifugal (OFC) compressor uses centrifugal force to compress refrigerant vapour. The benefits of being oil-free are that the heat exchanger performance isn't compromised, there is no degradation of performance due to frictional wear and refrigerant lines can be sized independently of oil circulation, resulting in lower pressure drops. It also eliminates expensive oil management systems for high temperature applications and the requirement for oil filter servicing. OFC compressor chillers have a low starting


current, accurate capacity and supply water temperature control and their part-load efficiencies are typically the highest of all compressor technologies. Aligned with flooded/ spray technology, they have closer approaches


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than a typical DX system and are often compatible with economiser technology for further capacity and efficiency enhancements. Small and compact for their capacity, with low vibration and low noise emission, oil-free centrifugal compressor chillers often meet the environmental and efficiency brief. So why wouldn’t everyone chose an OFC


compressor? The most obvious answer would be that you don’t need a top-of-the-range sports car to do a five-minute shopping trip. OFC compressors and their associated technologies are typically more expensive than alternatives. Whilst this may be recouped on life-cycle operating costs, some smaller heat loads wouldn’t justify the investment. The controls logic is also typically more complex and may require more specialist knowledge and there are head pressure and ambient temperature limitations. Typically, OFC chillers are ideal for complex medium to large sized critical cooling applications, such as data centres, hospitals and universities. For smaller, fixed heat loads that require less


temperature precision, the scroll compressor is an effective solution. A scroll compressor uses one fixed and one orbiting scroll to compress the refrigerant. Robust in design and hermetically sealed, scroll chillers are suitable for high pressure refrigerants and can be applied in multiples to improve efficiency. Sometimes noisy and with the need for careful oil management,


they are a lower cost option for smaller loads, such as office buildings, schools and retail. Growing in popularity, the inverter screw is seen as the less elegant, but less expensive option. Efficiency levels have advanced to nearly match standards of an OFC and compatible with low GWP hydrocarbon refrigerants and economiser applications, the inverter screw is improving its carbon footprint. With a smaller footprint and lower power input than most OFC chillers, the inverter screw compressor can start and reach full cooling capacity quickly and typically has integrated check valves to prevent the back-flow of high pressure gas on shutdown. Available in multiple capacities, it is suitable for high ambient locations. Typical applications are medium to large cooling loads, with critical cooling requirements such as data centres, laboratories, hospitals and commercial applications.


Conclusion Manufacturers tend to promote the compressor technologies that they offer. Airedale tries to offer a balanced view with a comprehensive offer and advanced energy performance tools. Visibility of the true cost of ownership and a balanced view of the available technologies versus the application is the key to project success.


X www.airedale.com


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