Compiled by Benjamin Brits
Most processes today involving mechanical elements generate heat as a by-product, and if not removed from the system will result in unfavourable conditions.
No matter if you are looking at a processing plant, manufacturing facility, laboratory, or the cooling in a building, a version of the chiller would be a suitable solution in managing the various ways heat is generated that causes, amongst others, a negative effect on efficiency, comfort in the case of occupied spaces, and can ultimately cause varying degrees of system failure.
Chiller technology itself is by no means anything new and the first patented centrifugal water chiller dates back to 1921. With the development, trials and errors involved in getting a new system to a patent stage, it means that the product has stood the test of time; having already remained relevant for over a century.
Chiller types and respective applications (related to HVACR)
“Chillers are classified by the way in which they reject heat. Heat rejection is necessary in order to satisfy the laws of ‘give and take’ (or energy conservation). Since heat is ‘taken’ from the building, space, or process that is being cooled, it must be ‘given’ to the outside air. The emphasis here is on heat, not on air. We don’t take the air from the building and dump it outside. Although that could provide some cooling, that is not the principle behind a refrigeration/cooling unit. All refrigeration units work on the same basic principle (refrigeration cycle), from a water chiller to basic air conditioner or even a fridge. Where and how the heat is dumped and how the refrigeration circuit is driven is where the differences lie,” says Russell Hattingh, Pr.Eng. at BBE Consulting.
A hybrid chiller installed at a filling station in Kwazulu-Natal. Photo by Trane/Service First
- Air-cooled chillers
Air-cooled chillers simply reject transferred heat directly to ambient air via air-cooled condensers.
Today’s air-cooled chillers use mechanical compressors to drive the refrigeration circuit (they are vapour compression type units). (Note the distinction made with reference to absorption chillers further along in the article.)
Hattingh notes, “An air-cooled chiller uses fans to directly cool their condensers, or to reject heat. Hot refrigerant flows through a radiator type coil, which could be microchannel or conventional tube-and-fin. Fans ensure the removal of the heat from the coils. Air-cooled chillers are self-contained units and make for a relatively simple installation. These chillers will typically use screw or scroll compressors and HFC (R134a, R410), HFO (R32, R1234ze, R454b) or HC (propane) refrigerants.
- Air cooled chillers typically support cooling capacities ranging between 5kW to 2MW and in addition to screw and scroll compressors, Ian Roelofse, industrial segment manager at Daikin South Africa adds swing compressors to the list. These compressors types being available with or without inverter capacity control.
Roelofse further says, “The evaporators in chillers are generally brazed plate heat exchanger or shell and tube (direct expansion). Efficiency can vary between standard, high or premium options – similar to the model of car you buy – for the extras you pay extra, and these could include things like reduced sound levels, heat recovery options and heat pump compatibility versions.”
- Water-cooled units
The main types of water-cooled units use screw and centrifugal compressors, which would also be classified as vapour compression. “Water-cooled chillers reject the heat transferred from the cooled medium (usually water) to a condenser water circuit which is then in turn cooled by a cooling tower,” says Deo du Plessis, associate at Spoormaker and Partners.
“While you might think that an air-cooled chiller should offer better savings as heat rejection is direct, a water-cooled chiller is in fact more efficient. There are various reasons for this but the two most important are (i) water is more efficient at taking away heat and (ii) a cooling tower approaches the wetbulb temperature while an air-cooled chiller approaches the drybulb temperature,” adds Hattingh.
Water-cooled plants are generally more complex, since they require cooling towers, water treatment, separate real-estate locations for the chillers and cooling towers, but are popular for large cooling applications (big buildings, process cooling, mine air cooling) due to their superior efficiency. These chillers use HFC (R134a), HFO (R1233zd, R514b) and Ammonia.
Roelofse’s comment concurs with Hattingh on large applications. “Water cooled chillers are suitable in cooling capacities ranging from 12kW to 9MW, and then heating capacities ranging from 118kW to 914kW. R32 refrigerant is also commonly used adding to the aforementioned.”
- Adiabatic systems for chillers
Adiabatic systems are employed on air-cooled chillers, by evaporatively pre-cooling the air entering the condenser, either by water sprays or wetted pads.
“This chiller type offers a pretty neat way of both saving water and electricity during peak load demands,” Hattingh notes.
While adiabatic systems work very well on cooling towers they have some draw backs when it comes to chillers, as most systems are retrofitted. Good water quality and careful design is required to ensure these systems continue to perform as intended.
- Absorption chillers
“The free lunch! While these units are often perceived to provide ‘free’ cooling there are a couple of provisos that apply. Cooling can be obtained cheaply only if there is a waste source of heat and if cooling water is abundant,” says Hattingh.
An absorption chiller doesn’t use a compressor to drive the refrigerant circuit but rather uses a chemical process. Heat and cooling are used by various heat exchangers to achieve the same end result as any other chiller (cold water).
But there is a small catch. The process is somewhat inefficient; meaning a large amount of heat is required to provide a certain amount of cooling. This heat has to be dispensed along with the building’s heat, in the cooling towers. What this means is that an absorption chiller generally requires a cooling tower that is twice as big as a conventional water-cooled chiller’s cooling tower (and therefore uses about double the amount of water).
An R32 air cooled chiller with nominal cooling capacity 700kW at +7 degC supply / 35 degC ambient, high efficiency and low noise. Photo by Daikin South Africa
Some pros and cons of each chiller type:
- Air cooled chillers
“Air-cooled systems are increasingly being used. Originally this was primarily because water is not consumed, but efficiencies of air-cooled systems have improved significantly over the past few years with new technologies such as variable speed compressors, frictionless bearings, smart controls, and so on. The main drawback in some applications is reduced life expectancy such as in coastal or aggresive atmospheric environments”, says du Plessis.
Hattingh adds, “air-cooled chillers make for simple installations but they also have some practical limitations. For medium-sized buildings or applications, air-cooled chillers can be ideal. They are also well suited for areas where there isn’t an abundance of water. For large facilities air-cooled chillers require significant real estate and the number of units becomes impractical.”
“On the pro-side, air cooled chillers don’t typically require an internal plant room, have simplified water piping runs, only chilled water circulating pumps are needed, they offer a lower first cost, and on the operational aspect have reduced maintenance needs. On the cons side, as Russell mentioned previously, they are less efficient and have a higher running costs,” adds Roelofse.
- Water cooled chillers
Water-cooled chillers will, in general, consume less energy than their air-cooled counterparts. Water is generally inexpensive and the overall cost of ownership of a water-cooled plant is generally less.
“Exactly how much cheaper depends on facility type, and the size and manner in which it is operated. Water chillers are more versatile in that they can be customised to suit special applications (such as special cooling mediums, voltages, tubes, water boxes). A perceived downside of water-cooled chillers is that their cooling towers require some love and care to ensure efficient operation (water treatment),” says Hattingh.
Water-cooled chillers find applications in buildings, process plants, factories, mines and food and beverage facilities. Engineers shooting for ‘green’ points often try to steer clear of water-cooled chillers. The choice of water-cooled over air-cooled is really an owner decision as they need to decide what is important for them. First cost versus ownership cost. Saving water versus saving electricity.
“A water-cooled chiller system’s primary advantage is the improved energy efficiency as mentioned – obtained by water cooling. One also has the advantage of positioning the plant (excluding cooling towers) in basement plant rooms, which is often an architectural preference. However, its main drawback is that it consumes water. This is a limiting factor in many applications nowadays, particularly in our water-scarce country. For the same reason, as well as legionella risks, water-consuming systems are also discouraged in green star buildings,” du Plessis continues.
Roelofse adds, “On the contrary to air cooled chiller’s simple pipe runs, water cooled chiller pipe runs tend to involve a more complicated installation with both chilled water and condenser water piping runs, they carry a higher first cost and increased maintenance through the water treatment of the condenser water circuit.”
- Hybrid chillers
Hybrid chillers offer the simplicity of an air-cooled installation with the added bonus of reduced running costs. Water quality can seriously reduce the reliability of the system and in some cases lead to equipment damage (high mineral content tends to clog spray nozzles and deposit on condenser coil surfaces). These systems are retrofitted in most cases, and few chiller OEMs supply these as standard.
- Absorption chillers
Absorption chillers can provide large amounts of cooling at a greatly reduced running cost (despite higher water consumption). They have few running parts and are fairly robust. They are however not as responsive to changes in load as their vapour compression counterparts – meaning if tight temperature control is required these chillers would be less ideal.
“Absorption chiller systems have very high efficiencies due to the absorption cycle being fundamentally different to the vapour-compression cycle. However, these types of chillers usually only become feasible at very large cooling capacities due to their size, complexity and cost. It is therefore more frequently applied in industrial settings, and in most cases found not to be feasible for typical commercial applications. Furthermore, perceived safety risks such as ammonia leakages adds to the potential disadvantages of these chillers,” notes du Plessis.
Chillers and hybrid-type installations
Hybrid installations, comprising air-cooled and water-cooled chillers, are sometimes considered in order to minimise operational risk.
“In South Africa, electrical outages are common, but can be overcome with generators. Water supply issues, although less common, are an occurrence some end users have to factor in to ensure 24-hour operability. Water cannot be generated on site, so air-cooled chillers may be supplied as back ups for critical areas (such as data centres and broadcasting facilities). Redundancy, however, comes at a price – the cost of downtime has to be weighed up against the additional capital-spend required,” Hattingh says.
Du Plessis adds, “There are naturally various methods of hybridisation that can be considered to increase efficiency, but these must always be viewed within context. An example includes heat recovery from chillers used as heat source for domestic hot water to minimise energy, which will typically only be found feasible in applications where there is a constant hot water demand, such as hotels.”
“Heating of water is achieved through the recovery of heat from the compression cycle (partial and total recovery percentages), and a mix of cooling only chillers, heat pumps and/or multipurpose units are other ways hybrid type systems can be created,”, says Roelofse.
Changes to the chiller landscape
In recent years we have seen two big drivers shaping the chiller landscape. Water and refrigerants. Many end users have opted for air-cooled chillers to ensure uptime and eliminate the risk of having a variable water supply (consider the facility owners in the Western Cape during their water shortage). Uptime comes at the price of higher running costs but when having no water, this means not running your facility so the decision is more easily swayed.
“Changing environmental regulations have also driven a change in the chiller landscape. South Africa was a signee of the Kigali amendment of the Montreal Protocol, and as such has committed to phasing down the use of HFC refrigerants. The phase-down in South Africa is not particularly aggressive, but since the chiller OEMs are largely located in North America and Europe we are seeing that the product offering is changing to suit the global market needs. As a result chillers using HFO refrigerants are gaining traction and we will continue to see more and more owners opting for these chillers as the phase down continues”, Hattingh notes.
“There are many pro’s and cons with any HVAC installation type, and it all ultimately depends on the client or project’s specific requirements, priorities of cost, operations, flexibility and efficiency. In general, economy of scale applied to larger installations tends to favour chilled water systems in lieu of modular VRF or DX type systems, both in terms of cost, operation and complexity”, du Plessis adds.
Roelofse notes, “We are seeing that there is a definite swing towards central plant systems again, combining the benefits of cooling only, heat recovery, heat pumps and multipurpose units within an installation. 10 to 15 years ago, central plants were the answer, but then something like VRV/VRF and certain other systems came into play and became the ‘in-technology’. This resulted in less central plants being chosen in favour of a different type of solution. Now, as chillers have advanced where you can utilise heat recovery and implement or include all sorts of other technology mixes, it’s swinging back to central plants again, specifically in the bigger applications – medium and large buildings. The changes to the chiller landscape as Russell says is primarly pushed by the development of lower GWP refrigerant (and pressed by government legislation), but also there is a much higher efficiency of all components directly linked to chillers (like compressors and electric motors) and this is driven in turn by energy-related product (ErP) regulations. This is an international standard relating to how all energy consuming products perform.”
Jaco Smal, sales, service and marketing director at AHI Carrier says, ”technology advancements in this area also must take into account the services and products that suppliers create to enhance the customer’s experience. These are things like leveraging the power of wireless technologies, data analytics, cloud computing, artificial intelligence, and the internet of things (IoT). These are becoming more important differentiators which give some suppliers a competitive advantage. Utilising remote diagnostics and troubleshooting, our team of experts can for example, prevent breakdowns and ensure optimal performance of the clients HVAC systems. With ‘smart’ technology and IoT connectivity, clients can now benefit from the advanced analytics that offer improved visibility of their operations, reduce incidents of failure, increase uptime, better their equipment performance and efficiency, and reduce operating and maintenance costs. All of these being the biggest portions of technology advancement and efficiency.”
Europe has also played a significant role in promoting chiller efficiency as their regulations mandate minimum equipment efficiencies. OEMs (either based in Europe or wanting to punt their wares there) are constantly being pushed to improve efficiency levels and these changes come about in various forms.
“Microchannel condensers, falling film evaporators and magnetic and ceramic bearing compressors have certainly made a significant difference to the efficiency of today’s chillers. Developments in controls (both on-board and for plant management) allows designers significant flexibility when considering the optimum configuration of chillers for a particular application. The data centre industry is certainly challenging chiller OEMs to find ways to reduce operating costs and many OEMs have risen to the challenge making equipment that can run with much higher-leaving chilled water temperatures”, says Hattingh. “We’re going through an exciting time at the moment. The chiller industry is in some flux as the refrigerant landscape is changing. The refrigerant changes bring about significant changes to the way in which companies then make their equipment. A refrigerant is not simply changed without significantly altering the hardware. Many OEMs are therefore experimenting with lower GWP HFCs while they work out which HFO they want to use, and they adapt their equipment to suit. Change is however something that the chiller industry is accustomed to so expect to see equipment brochures with new refrigerants and new installation procedures and requirements.”
An R134a refrigerant air cooled chiller with inverter screw compressor offers premium efficiency with low noise. This model has two integrated circulating pumps with a nominal cooling capacity of 285 kW at +7 degC supply / 35 degC ambient. Photo by Daikin South Africa
“There are again various technolgies that can be considered to increase efficiency. However, this must be considered in conjunction with the context of project-specific details such as cost, complexity and size requirements. For example, an air-cooled chiller can utilise dry-cooling coil sections to use low ambient conditions directly and minimise the energy required from the compressor for cooling. However, the degree to which this is effective will depend on operating chilled water temperature and the ambient conditions. It might only be a feasible option if relatively high chilled water temperatures are acceptable and if ambient conditions are suitable at a large percentage of running hours,” adds du Plessis.
“Some of the technology where we see development related to chiller are the EC-technology on condenser fan motors, inverter technology on compressors, connectivity of units through improved controls and the cloud based monitoring of equipment. Also worth noting is that, historically, natural refrigerants like CO₂, ammonia and propane have been limited to big installations and specific process cooling, but natural refrigerants are now becoming more a part of the commercial side of the HVAC sector”, notes Roelofse.