The conception of the air conditioning has gradually developed from one unit for one house to independent units for separate zones in the same house. Variable refrigerant flow (VRF) systems vary the flow of refrigerant to indoor units based on demand. This ability to control the amount of refrigerant that is provided to fan coil units located throughout a building makes the VRF technology ideal for applications with varying loads or where zoning is required. VRF system works on the principle of simple vapour compression cycle but it has the flexibility to continuously change the flow of refrigerant to different internal units depending on the heating and the cooling load required. VRF systems are available either as heat pump systems or as heat recovery systems for those applications where simultaneous heating and cooling is required. In addition to providing superior comfort, VRF systems off er design flexibility, energy savings, and cost effective installation.
VRF Technology
In a VRF system, multiple indoor fan coil units may be connected to one outdoor unit. The outdoor unit has one or more compressors that are inverter driven, so their speed can be varied by changing the frequency of the power supply to the compressor. As the compressor speed changes, so does the amount of refrigerant delivered by the compressor System.
Each indoor fan coil unit has its own metering device that is controlled by the indoor unit itself, or by the outdoor unit. As each indoor unit sends a demand to the outdoor unit, the outdoor unit delivers the amount of refrigerant needed to meet the individual requirements of each indoor unit (Fig. 1). These features make the VRF system ideally suited for all applications that have part load requirements based on usage or building orientation, as well as applications that require zoning.
History & Development in VRF Systems
Variable refrigerant flow (VRF) systems, which was introduced in Japan more than 20 years ago, have become popular in many countries, yet they are relatively unknown in the United States. The technology has gradually expanded its market presence, reaching European markets in 1987, and steadily gaining market share throughout the world. In Japan, VRF systems are used in approximately 50% of medium-sized commercial buildings (up to 70,000 ft2 (6500 m2)) and one-third of large commercial buildings (more than 70,000 ft2 (6500 m2)).
Although vigorous marketing of VRF systems in the U.S. began only two to three years ago, several thousand systems are likely to be sold in the U.S. this year, amounting to tens of thousands of tons of capacity. Of course, the market is still very small compared to the chiller market, but VRF systems are marketed in the U.S. by at least five manufacturers.
The success of the VRF in other countries, and its historically limited market presence in the U.S., has several sources, including:
- Differences in construction practices;
- The long history and large installed base of ducted direct exchange (DX) systems and chillers in the U.S. compared, for example, to Europe, where many buildings did not have air conditioning until recent decades;
- Differences in regulatory environment (e.g., regulations that discourage electric chiller installations in Japan); and
- VRF technology has been developed and promoted by Asian companies, which had limited market presence in the U.S. until recently. Also, building owners are wary of HVAC suppliers whose parts availability, service and technical support infrastructure is uncertain.
Typical Range of Available VRF Systems
Air Cooled
6 – 24 Ton (Simultaneous)
6 – 30 Ton (Change Over)
208/230, 460 Volt – 3 Phase
62 dB(A) Operating Sound Level
Water Cooled
6 – 20 Ton (Simultaneous)
6 – 30 Ton (Change Over)
208/230, 460 Volt – 3 Phase
Single Phase
3, 4 Ton (Change Over)
208/230, 1 – Phase
Features of VRF System
- Zoning System: The main advantage of the VRF system is its ability to respond individually to fluctuations in space load conditions. The user can set the temperature of his/her room according to the requirement.
- Simultaneous Cooling & Heating: VRF systems are capable of simultaneous cooling and heating (Fig. 3).Each individual indoor unit can be handled separately by the user using a programmable thermostat. Most VRF manufacturers offer a centralized control option, which enables the user to monitor and control the entire system from a single location.
- Variable Speed Compressor: VRF systems use variable speed compressors (inverter technology) with 10 to 100% capacity range that provides unmatched fl exibility for zoning to save energy. Use of inverter technology can maintain precise temperature control, generally within ±1OF.
Simultaneous System Components
The system components are:
- An outdoor temperature
- Indoor units
- Branch controller (may be used sometimes)
- Integrated controls
VRF for commercial buildings
- Up to 50 indoor units per outdoor
- 72,000 – 360,000 BTUH
- Simultaneous heating and cooling
- Energy reclaim
- IEER up to 23.9
- COP up to 4.87
Improvements in VRF System
Over the past 21 years the technology has advanced in a number of areas:
- Standard compressors to variable speed and capacity modulated scroll compressors
- Direct driven outdoor fans to variable frequency drive, inverter-driven fans
- Direct driven indoor coil motors to direct current or ECM-type motors
- Variable capacity indoor units
- Better heat exchanger surfaces with multi-segmented coils
- Improved controls and diagnostics
- R-22 to R-410A
- Better refrigerant charge and oil management.
Recent market Sales of VRF system
With the growing increase in the technology the VRF air conditioning market is growing at a fast pace. VRF system with scroll compressor have gained wide acceptance in India. The popular applications for VRF systems have been corporate offi ces, hotels, IT offices, high-end residential apartments and villas, retail stores, supermarkets and hospitals. AHRI now has a standard for VRF product performance (ANSI/AHRI 2011) and a Product Directory of certified equipment (AHRI 2012).
Part Load Efficiency of VRF (VRV) and Water-Cooled Chiller and Air-Handler System
The system part load efficiency is shown in the figure. The efficiency decreases with the increase in load and reaches a minimum value at 50 percent and it almost remain constant and then again increases. Variable speed operation of the VRF system also contributes to energy savings by reducing cycling losses at part-load operation.
VRF system: Life cycle cost
The initial installation cost of the VRF system is 5 to 20% higher than that of the air or water cooled chilled water system. This is mainly due to the long pipelines which are used for the refrigerant to flow & multiple evaporator heat exchangers with associated controls. Building owners often have no incentive to accept the initial high cost even of the claimed payback period is short, as the energy savings are quite high in the long run.
Challenges faced by the VRF industry
VRF system are not suitable for all commercial buildings.
- Refrigerant Piping: Long pipelines are used depending on the compressor’s ability to maintain pressure drop. All split systems therefore have a maximum allowable vertical & total refrigeration pipework length. 135 feet is the length of the longest pipeline that can be used.
- Compliance with ANSI/ASHRAE Standard 15-2001: VRF systems must comply with ASHRAE Standard 15-2011 – Safety Standard for Refrigeration Systems (ANSI approved). VRF system increase the chances of refrigerant leakage which are difficult to find and repair. The refrigerant leak, especially if the system serves small rooms, can cause oxygen depletion.
- Fresh air requirements (Compliance to ANSI/ASHRAE Standard 62.1): Ventilation is required to maintain the indoor air quality and building codes which recommends Typically 15 to 20 CFM of fresh air per person. Like all other split system, VRF system also requires a separate ventilation.
- Environmental Concerns: Ozone depletion issue have become a global concern and long refrigerant lines is a strong negative for the system which requires more refrigerant. HFC refrigerants, typically R-410-A and R-407-C are commonly used.
- Particulate Matter Removal: ASHRAE Standard 62.1-2004 specifically discusses the particulate matter removal issues and how VRF indoor units can or cannot uphold the requirements. High MERV (Minimum Efficiency Reporting Value) filters can be used but it has a high cost and high pressure drop and are often suitable for some ductless units.
Conclusion
VRF system are gaining popularity in the American HVAC market due to its flexibility in operation and high efficiency over a wide range of part load operation. In India its market is increasing, but at a slower rate due to the lack of knowledge and unskilled labour in this sector. Although suppliers claim that VRF system are quite reliable but VRF system with a large no. of compressor is less reliable than a chiller system which has a lesser no. of compressors. The system capabilities and limitations must be fully understood and evaluated carefully to determine its suitability.