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PUMP UP THE VOLUME
Variable refrigerant volume air conditioning systems – or variable refrigerant flow, as it is more widely known – have grown in popularity since they were first introduced in the early 1980s. As Sean McGowan reports, there are a few reasons why VRF is on the rise.
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DON’T BE FOOLED
VRV (variable refrigerant volume) and VRF (variable refrigerant flow) are not two different systems. They are one and the same, and have been around a lot longer than you might have expected, too.
The world’s first VRV system was introduced by Daikin in 1982. The company promptly trademarked the term. This meant that when other manufacturers later developed the technology it was described as VRF rather than the proprietary term.
Eventually, VRF became the industry-standard term, with the exception of Daikin, which continues to use VRV today.
Although 40 years have since passed, VRF air conditioning systems are still considered to be relatively sophisticated. They have benefited from continuous development over the decades, with improvements including increased capacity load and the incorporation of heat-recovery technologies.
IN THE BEGINNING
Before the development of Daikin’s VRV technology in 1982, achieving individual zone control for air conditioning in commercial buildings was considered a challenge. But the arrival of VRV changed that concept.
Why? Because VRV/VRF varies the refrigerant volume in a system to match the building’s requirements.
It does this by allowing the outdoor unit (compressor) to supply varying amounts of refrigerant to multiple indoor units (fan coils) located throughout the building. As each indoor unit experiences different heat loads in the space it serves, the outdoor unit is able to supply the appropriate amount of refrigerant to those indoor units.
“Refrigerant is pumped from the outdoor unit to each indoor unit,” says Daikin Australia Engineering Support Manager Blake Mortimer, M.AIRAH.
“Each indoor unit has a valve to control the volume flow rate of refrigerant to ensure the precise amount of refrigerant is fed to the indoor heat exchanger.”
Heat pump VRF systems will only allow all indoor units to operate in either heating or cooling mode. Heat recovery VRF systems allow each unit to operate in either mode. The addition of a branch selector box allows the indoor unit to switch between these modes.
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Common applications for VRF systems include small and medium commercial, and high-rise residential.
“The logic will always look at the temperature difference of the set-point and space temperature of the indoor unit(s) that are in operation, while checking the ambient conditions,” explains Mitsubishi Electric national product manager Atesh Mani.
“Based on these inputs, it will vary the speed of the compressor, condenser fan and utilise the condenser coil as full or in sections. As demand for cooling or heating increases, it will ramp up operation to meet the demand – and vice versa.”
Controls are typically pre-installed, and while proprietary for each manufacturer, can be integrated into building management systems (BMS) as required.
The introduction of heat recovery – as well as variable speed compressors and inverter-driven variable speed condenser fans – have made VRF technology even more adaptable and efficient. Now only the minimum amount of energy is required for a system to maintain set temperatures.
JUST IN TIME
Over the decades, VRF systems have evolved into a mass-produced technology by major OEMs (original equipment manufacturers).
According to Mitsubishi Electric National Product Manager Atesh Mani, this has resulted in a “just-in-time” approach to ordering and delivery.
“Because of this approach to manufacturing, the supply chain of components is robust and offers increased reliability due to modern manufacturing methods,” he says.
“Mass production accompanied by comprehensive quality control ensures a consistent and better build quality. A reliable product and strong supply chain results in minimal downtime in the case of failure – a benefit for installers as well as their customers.” ■
APPLICATIONS
Talk to manufacturers like Daikin and Mitsubishi Electric, and they’ll tell you the flexibility afforded by VRF systems makes them suitable for a wide range of applications, from small and medium commercial buildings to multi-residential including apartments and aged care.
Smaller, mini-VRF systems are even finding their way into residential homes.
“We are seeing them used more in residential and apartments to allow temperature control of each space,” says Mortimer, “but the typical office space under 2,000–3,000m² is still one of the main uses of VRV systems.”
This, Mortimer says, is because the technology is thought to be more cost-effective and efficient than alternative solutions.
As well as energy-efficient operation, VRF offers the advantage of being highly adaptable, with indoor units available in different types and sizes to make them suitable for most applications. This includes ducted, wall mounted, cassette and under ceiling (suspended) indoor units. Outdoor units are also available in a variety of types, including heat pump or heat recovery, air-cooled or water-cooled, and top or side discharge design.
“VRF systems are scalable and modular, allowing more capacity to be added at any stage as the demand increases,” says Mani.
“Typically, they require no installed redundancy, so there is a saving on the capital cost. VRF systems also offer limited impact in the event of a module failure. If one unit breaks down, it affects only a small part of the occupied space, not the whole building.”
And because of the lightweight nature of the system components, modules can be more easily installed onto rooftops than traditional chillers that require large crane lifts. They also take up less space.
But Mortimer says the choice between VRF and alternatives – such as a central plant – should come down to the type of building and its use.
“A conference centre, for example, might have large and small meeting spaces, office areas and others,” he says. “And having a large central plant may be problematic when operating just one small meeting room.
“On the other hand, a very large office tower is more suited to central plant.”
Like other systems, the most commonly found refrigerant in VRF technology is R410A, but that’s sure to change as HFCs are phased down under the Montreal Protocol’s Kigali Amendment.
Already, OEMs are testing some markets with R32 for smaller capacity VRF systems.
“We have also seen some hybrid technologies such as Hybrid VRF (HVRF) – which offer reduced gas charge as standard – become popular as we go through HFC phase-down,” says Mitsubishi Electric National Product Manager Atesh Mani.
Mitsubishi Electric’s Hybrid VRF (HVRF) is a simultaneous heating and cooling twopipe system that uses refrigerant between the outdoor unit and the hybrid branch controller (HBS), and water between the HBC and the indoor units. ■
KNOW YOUR LIMITATIONS
Like any technology, VRF systems are not without their limitations.
These usually relate to the refrigerant pipework runs – including vertical runs – and overall pipework length between the outdoor unit and indoor units.
Each manufacturer places their own limitations on overall pipe length, vertical pipe runs and the maximum refrigerant charge per system, but Mani says there is flexibility when it comes to designing with VRF systems.
“For example, in some cases the vertical limit can be increased to up to 90m [but] there are some conditions that must be met to avail that vertical distance,” he says. “Similarly, when using other features such as maximising distances from the branch box or first joint to the farther fan coil, certain conditions must be met.”
Other factors include issues around total refrigerant charge.
“When dealing with small rooms, you do have to consider the refrigerant charge of the entire system to make sure you don’t exceed the allowances for refrigerant concentration as outlined in AS/NZS 5149,” adds Mortimer.
TO VRF, OR NOT VRF
Froster Engineering team leader Jeff Froster, M.AIRAH, says VRF can be suitable in any application. Yet he says consideration must be given to refrigerant leak detection, access for maintenance and flexibility in the event of a building’s change of use.
“There are limitations with regard to refrigerant leak-detection compliance with AS/NZS 1677, and flexibility of the system is limited once it is piped and charged,” Froster says.
“Great effort may be required to modify an existing VRF system.”
It’s why he says VRF systems should be carefully designed by qualified mechanical engineers.
This design should ensure the piping length is within the system’s limits, and spaces served by a common heat pump-type VRF system have similar cooling and heating characteristics.
Other considerations when selecting VRF over alternative systems include the application, building thermal characteristics, type of outdoor and indoor units to suit the building profile, indoor air quality, plant area and risers.
“Careful consideration and a psychrometric review need to be taken up when completing a system design,” says Froster. “Pre-conditioner systems in high wet-bulb, tropical regions are likely required.” While Froster expects VRF to continue to be used in light commercial and residential applications, others are less convinced about their application in apartment complexes.
According to Full Circle Design Services Director Graham Agar, M.AIRAH, VRF may not be the best option for apartment buildings unless they are very large or very tall.
“Sharing condensers between [apartment] owners is not a good idea, in my opinion, noting that there are a few projects that have done just that,” says Agar.
“Houses are a possibility, but again you need a big house to take advantage of the system’s benefits and absorb the additional costs.”
Agar says because of issues related to cost for smaller projects, lack of control for complex projects and energy efficiency in large projects, VRF sits in between chilled water and split systems.
To this end, Agar looks to use VRF for buildings that fit a particular profile.
He says VRF is best suited to loads between 50kW and 500kW, low complexity building use, where there is limited benefit from economy cycle, and where building users have limited technical expertise and simple controls requirements.
“It is best suited in applications with moderate outside air requirements – although education is a reasonable application – and where there are lots of small, independent spaces,” he says.
GENERATIONAL DEVELOPMENT
Considered the fourth generation of VRF/VRV technology, variable refrigerant temperature (VRT) was launched by Daikin in 2014. This technology allows the refrigerant temperature to be adjusted as the system nears the required set-point to achieve greater efficiency while minimising the thermostat cycling of the indoor unit. Daikin Australia Engineering Support Manager Blake Mortimer, M.AIRAH, says the launch of fifth-generation VRV technology allows the indoor fan speed of the individual indoor units to reduce as the indoor unit nears its set temperature. “This allows for better temperature control,” he says, “and even greater efficiencies compared to previous generations of VRV.” ■
MAKING THE RIGHT CHOICE
Other considerations designers should make at an early stage include whether the application is suited to a heat pump or heat recovery-type VRF system.
According to Mani, the former are more suitable for open-plan spaces while the latter are designed to allow each indoor unit supply heat or cooling simultaneously to multiple, separate spaces. “When it comes to condenser selection, you can choose from air-cooled or water-cooled condensers,” he says. “Air-cooled condensers usually come in two different line ups – standard efficiency or high efficiency.”
VRF COMPONENTS
There are several components in modern VRF systems, but generally they can be described as having the following:
• Outdoor unit (condenser) • Indoor units (fan coils) • Refrigerant pipework • Branch controller for heat recovery units • Communications wiring • Controls.
A VARIABLE REFRIGERANT FUTURE
As you would expect from an established technology, manufacturers are constantly seeking improvements in VRF systems. These include fourth- and fifth-generation systems (see sidebars), with manufacturers eyeing opportunities to adapt the technology to suit refrigeration, data centres and medical facility applications among others.
Closer to home, there are also opportunities for these systems to provide domestic hot water and heating – including underfloor heating – in residential applications.
“As it becomes more accessible to the larger residential market, we’ll see more of it in everyday homes,” says Mani. If the past is any indicator of the future, expect VRF technology to be around for a long time yet. ■
VRF systems allow the flexibility of incorporating a variety of indoor units, including wall-mounted, cassette and suspended.
