Variable Refrigerant Flow

(VRF)
Systems

RAHUL PRAJAPATI
15SA18
BUILDING SERVICES-7

INTRODUCTION
  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 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 offer design flexibility, energy savings,
and cost effective installation.

 This paper will outline the benefits of a typical VRF system, describe the advantages offered by
the most advanced outdoor units available, and provide general guidelines for selecting a heat
pump system versus a heat recovery system.

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.

 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

ADVANTAGES OF A VRF SYSTEM

 Control Means Comfort The key to providing comfort is to supply heating or cooling when and
where it is required without swings in room temperature.

 In conventional systems, the compressor is either on or off, so even spaces that have individual
controls experience fluctuations in room temperature as the compressor stops and then starts
again to maintain the thermostat setting (Fig. 2).
  In a VRF system, since the speed of the compressor can be varied, the compressor does not
cycle on and off, but operates continuously for longer periods (Fig. 3).

 The required refrigerant flow is supplied to the indoor fan coil and once the set point is reached,
the refrigerant flow is adjusted to maintain the room temperature smoothly without fluctuation.

 In addition to having distinct set points, the indoor unit fan speeds and louver positions can be
changed to provide additional comfort in the space. Figure 1 – Typical VRF Heat Pump System
Figure 2 – Traditional Fixed-Speed Compressor Operation Figure 3 – VRF System Inverter-Driven
Compressor Operation

Design Flexibility

 One of the major advantages of a VRF system is the flexibility provided by the diversity of the
product offering.

 Multiple types and sizes of fan coils are available to fit any application. Figure 4 shows a sample
zoning layout for a VRF system, combining outdoor units, 4-way cassette type fan coils, and hi-
wall type fan coils to create comfortable conditions for varying uses of 15 different spaces within
the same building.

When selecting a VRF system, keep in mind that not all systems have the same piping capabilities.
Systems that offer expanded piping capabilities will maximize the application flexibility provided by the
VRF technology. Important considerations when reviewing piping capabilities are:
1) the maximum elevation difference allowed between the highest and lowest indoor units on a single
system
2) the distance allowed from the outdoor unit to the farthest fan coil on the system. Cost Effective
Installation Depending on the application, the installation of a VRF system can be a cost effective
alternative to traditional systems that require ductwork or large pipe sizes, and pumps and boilers in the
case of chilled water systems. Outdoor units are light in weight and have a small footprint.
This means that they will fit in a service elevator, so no crane is required for lifting to a rooftop
installation.
In some cases, savings on the total construction cost can be achieved since the lightweight unit means
that additional support structure in the roof is not required.
Energy Savings All VRF systems provide energy savings by varying compressor speed and matching the
output of the system as closely as possible to the load.
In addition, VRF systems do not experience the same energy losses as systems that move conditioned air
through ductwork. However, differences in design in the available outdoor units will influence the
efficiency

COMPARISON OF VRF SYSTEMS

 The VRF systems available on the market today differ according to the number and type of
compressor.
The 3 types of units that will be compared here are:
• Single Variable Speed Compressor
• Variable Speed Compressor Plus FixedSpeed Compressor
• Multiple Variable Speed Compressors
 Single Variable Speed Compressor
In this system with a single, large-capacity scroll compressor, the same compressor starts and
runs when there is demand and no redundancy is available if the compressor fails.

 Variable Speed Compressor Plus Fixed-Speed Compressor

In this two-compressor system, the inverter-driven compressor always starts and ramps up until
it reaches its maximum capacity at which time the fixed-speed compressor starts and the
inverter driven compressor ramps down. This system provides back-up capability. Multiple
Variable Speed Compressors Outdoor units with multiple inverter-driven twin rotary scroll
compressors, as shown in Fig. 5, offer the most complete set of advantages achievable with a
VRF system. The system with 3 inverter-driven compressors also provides greater back-up
capability. If one of the compressors fails, the system will continue to operate at 67% of its
original capacity, and comfort will be maintained in the conditioned space until the faulty
compressor can be replaced.
The starting sequence of the compressors is rotated, equalizing their operating time and
thereby minimizing excess operation of an individual compressor.
 Multiple inverter-driven compressors allow

the unit to provide better part load performance without the need to use hot gas bypass.
Under low-load conditions, the system has the advantage of running only as many compressors
at whatever speed is required to achieve the capacity necessary to satisfy the load and maintain
comfort within the conditioned space

 DESIGN CONSIDERATIONS
The design of a VRF system begins with understanding the space layout. The orientation of the
building and the seasons during which peak loads occur must be considered. The type of load
(heating or cooling) and the distribution of loads into zones will depend on the intended use of
the space. In turn, these factors will determine whether a heat pump system or heat recovery
system will be the most efficient choice.
View shows a typical space layout, with zones specified as requiring heating or cooling and the
load reflected in the size and type of the indoor units shown.
 Type of System —
 Heat Recovery or Heat Pump?
 Heat pump and heat recovery systems both provide heating and cooling.

 A heat pump system provides either heating or cooling as required.

 A heat recovery system is ideal when simultaneous heating and cooling is required.

 The greatest efficiency will be realized when the heating and cooling loads are equal, by
maximizing the amount of energy that can be transferred from one zone to another using the
refrigerant, as shown in Fig
 Figure 11 shows a building layout with 6 zones. One way to meet the design requirements of
this building would be to install 3 heat pump systems as shown below:
 System 1 – Zones A, C, and D, which have similar profiles for heating and cooling requirements
System 2 – Zones E and F, which have similar profiles for heating and cooling requirements
System 3 – Zone B, which requires cooling only Alternatively, one larger heat recovery system
could be installed.
To determine the best design choice, an efficiency analysis and cost comparison of the two
options should be completed. In addition, the consequence of a greater amount of refrigerant
circulating through the larger system should be considered,

Size of Units
 The size of the units selected must be considered for impact on the design of the system;
smaller units will provide flexibility of zoning and require less piping and less refrigerant per
system.
 Piping Configuration Flexibility of the piping options available should be considered.
 A system that provides more options for combining Y-shape joints and headers could minimize
the amount of piping and refrigerant used, thus reducing the total cost of the job.