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Variable Refrigerant Flow Systems:Commercial HVAC Energy Saving

By Nate Jabs

Introduction

According to William Goetzler, a member of the American Society of Heating, Refrigeration and Air Conditioning Engineers (Goetzler), variable refrigerant flow (VRF) systems, which were introduced more than 20 years ago, have become popular in many countries, yet they are relatively unknown in the United States (Goetzler). VRF systems are a great way for large commercial buildings to save up to 40% less energy than conventional heating, ventilation, and air conditioning (HVAC) systems, depending on the climates the systems are located in. The VRF system offers commercial buildings ductless heating and air conditioning that can permit a large number of indoor units to operate off one outdoor unit. This is beneficial, because most large commercial buildings have multiple tenants or rooms that require different climate control needs. In this report I will discuss current HVAC technologies that are being used in commercial buildings, how variable refrigerant flow systems work, and the benefits of variable refrigerant flow systems.

Current HVAC Technologies

There are three main HVAC technologies that most United States commercial buildings use to provide multiple tenants with the option of controlling their own temperatures in their space. The first HVAC technology is a constant-volume (CV) variable-temperature system. In CV variable-temperature systems, there is one central air-handling unit (AHU) that delivers a constant volume of air to the space through air ducts. The air temperature is varied by controlling the capacity of the central cooling coil (VAV Systems). The downfall of this system is that it can respond to the demands of only one thermostat, and if a building has many spaces with diverse cooling needs, each must be delivered by its own system. A variation of the CV single zone system is the CV terminal reheat system. The CV terminal reheat system is capable of producing supply temperatures for various space cooling loads. This is done by either reheating or mixing the

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cool primary air to produce supply temperatures needed to balance the various space cooling loads. In order to do this, the terminal reheat systems must have heating coils at each space to temper the air to satisfy the space load. But, this method is not very energy efficient and there is nearly a constant refrigeration load (VAV Systems). A variation and more energy efficient method of the CV system is a variable-air-volume (VAV) system. A VAV system one AHU supplies primary air to the space through air ducts at a constant temperature, but it also varies the airflow to maintain the required space at all load conditions. This is done by the use of a VAV terminal unit that is controlled by multiple thermostats by each individually controlled space as seen in Figure 1. The VAV system provides energy savings because the air volume reduction creates an opportunity to reduce fan energy required to move the air, while also reducing refrigeration energy (VAV Systems).

How Variable Refrigerant Flow Systems Work

According to Ammi Amarnath from the Electric Power Research Institute and Morton Blatt, an ASHRAE fellow, VRF technology uses smart integrated controls, VSDs, refrigerant piping, and heat recovery to provide products with attributes that include energy efficiency, flexible operation, ease of installation, and zone control (Amarnath and Blatt). Unlike the common systems CV and VAV systems previously mentioned, the VRF systems are enhanced ductless multi-split systems. Ductless multi-split systems work by circulating refrigerant to indoor units rather than circulating air like ducted systems. The heat is transferred by the refrigerant to the indoor units, which are either evaporators or condensers. When the indoor units are being used to cool they act as evaporators, and when they are heating they act as condensers (Amarnath and Blatt). Although these multi-split systems are advertised as ductless systems, there are still needs for ventilation ducts. Ventilation ducts are much smaller than supply and return air ducts, and can be ran in the same space the refrigerant piping.

The VRF systems are known to be enhanced ductless multi-split systems, allowing more indoor units to be connected to each outdoor unit and providing simultaneous heating and cooling and heat recovery (Amarnath and Blatt). Simultaneous heating and cooling allows for each of the individual units to either be heating or cooling at the same time by using heat recovery. The heat recovery also reduces energy use during the heating season. Unlike traditional multi-split system, VRF systems use

one set of piping for each outdoor unit and supplied to all of the indoor units. This reduces the amount of piping needed for the system, while also allowing for more indoor units to operate off of one outdoor unit.

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The term VRF refers to the ability of the system to control the amount of refrigerant flowing to each of the evaporators, allowing for many evaporators of different capacities (Amarnath and Blatt). The flow of the refrigerant to the evaporators is controlled by variable frequency drives (VFD), which is a key advantage to VRF systems. A two-pipe or a three-pipe system can be used for the VRF systems. Two-pipe systems consist of a cooling pipe and a heating pipe and are used when all zones in the facilities will require heating or all zones will require cooling at the same time. A three-pipe system, which adds a return pipe, works best when some spaces need to be cooled and some need to be heated during the same time (Amarnath and Blatt). In order for VRF systems to provide heat recovery, heat exchangers are used to extract the superheat from the units in the cooling mode and route it into refrigerant entering a heated zone (Amarnath and Blatt).

Benefits of Variable Refrigerant Flow Systems

There are many benefits to variable refrigerant flow systems, including: energy efficiency, flexible operation, ease of installation, and zone control. Unlike ducted systems, which are very bulky and require a lot of heavy lifting, VRF systems are much easier to install. VRF systems are lightweight and modular, allowing for each module to be transported easily and can fit in elevators. The lightweight of the system may reduce requirements for structural reinforcement on roofs, and since on ventilation ductwork is needed building heights and costs will be reduced (Goetzler). These units come in modular designs, allowing up to 20 indoor units to be supplied by a single condensing unit. This modularity makes it easy to adapt the HVAC systems to expansion or reconfiguration of the space, which may require addition capacity or different terminal units. With the VRF system being able to control many different zones, everybody will be comfortable in the environment that they set each individual indoor to.

The energy efficiency of the VRF system derives from several factors. According to Goetzler, “The VRF essentially eliminates duct losses, which are often estimated to be between 10% to 20% of total airflow in a ducted system. VRF systems typically include two to three compressors, one of which is variable speed, in each condensing unit, enabling wide capacity modulation. This approach yields high part-load efficiency, which translates into high seasonal energy efficiency, because HVAC systems typically spend most of their operating hours in the range of 40% to 80% of maximum capacity.” There have been multiple comparisons between VRF systems and typical HVAC systems used, showing that the VRF system is more energy efficient. A rooftop VAV and a VRF system were both installed on different sides of the same building, with the VRF system’s energy consumption being approximately 38% (Goetzler). VRF system’s can save up to 30% to 40% of the energy used by a chiller-based system for a 200-ton cooling system, and can even save up to 60% during winter in Brazil (Amarnath and Blatt). These savings do depend a lot on the climate that the VRF systems are installed in. Savings are estimated up to 15% in the United States (Amarnath and Blatt). The energy saving in the US may not be as high as other places around the world, but with more buildings installing VRF systems and the technology advancing every day, the VRF efficiency will begin to show high efficiency results very soon in the United States.

Conclusion

Variable refrigerant flow systems have been widely used around the world for commercial buildings, but they are very uncommon in the United States. VRF systems is a ductless system that provides heat transfer by circulating refrigerants to multiple indoor units. This technology allows each individual indoor unit to be controlled using its own thermostat, enabling the comfort for many different spaces in commercial buildings. VRF systems. The VRF systems provide many benefits, including: energy efficiency, flexible operation, ease of installation, and zone control. The energy efficiency for VRF systems has been recorded as high as 60% less energy consumption than conventional HVAC systems. Due to the climate in the Unites States, VRF systems are estimated to reduce energy consumption by 15%. With the popularity of the systems growing, the energy efficiency of VRF systems in the United States will begin to rise.

References

Amarnath A, and Morton Blatt. “Variable Refriferant Flow: Where, Why, and How.”Amarnath and Blatt 25.2 (2008): 54-60. Academic Search Complete. Web. 2 Dec. 2013.

Goetzler, William. "Variable Refrigerant Flow Systems." Goetzler 49.4(2007): 24-31. Academic Search Complete. Web. 20 Nov. 2013.

“VAV Systems: A Trane Air Conditioning Clinic.” NJATC. American Standards Inc.(2001). Web. 2 Dec. 2013.