Vrf System

Variable Refrigerant Flow (VRF) Systems

Variable refrigerant flow (VRF) is an air - condition system configuration where there is one outdoor condensing unit and multiple indoor units. The term variable refrigerant flow refers to the ability of the system to control the amount of refrigerant flowing to the multiple evaporators (indoor units), enabling the use of many evaporators of differing capacities and configurations connected to a single condensing unit.The arrangement provides an individualized comfort control, and simultaneous heating and cooling in different zones .
Currently widely applied in large buildings especially in Japan and Europe , these systems are just starting to be introduced in the U.S. the VRF technology / system was developed and Japan who named and protected the term variable refrigerant volume (VRV) system so other manufactures use the term VRF(variable refrigerant flow) in essence both are same with a higher efficiency and increased control ability .the VRF system can help achieve a sustainable design . unfortunately, the design of VRF systems is more complicated and requires additional work compared to designing a conventional direct expansion ( DX) system.

This course provides an overview of (VRF) system technology .

Overview of VRF systems

The primary function of all air- conditioning systems is to provide thermal comfort for building occupants .There are a wide range of air conditioning systems available, starting from the basic window - fitted units to the small split systems , to the medium scale package units, to the large chilled water systems , and currently to the variable refrigerant flow ( VRF) systems .
The term VRF refers to the ability of the system to control the amount of refrigerant flowing to each of the evaporators , enabling the use of many evaporators of differing capacities and configurations, individualized comfort control, simultaneous heating and cooling in different zones, and heat recovery from one zone to another . VRF systems operate on the direct expansion (DX) principle meaning that heat is transferred to or from the space directly by circulating refrigerant to evaporators located near or within the conditioned space .Refrigerant flow control is the key to many advantages as well as the major technical challenge of (VRF) systems .
Not the term (VRF) systems should not be confused with the centralized (VAV) (variable air volume) systems , which work by varying the air flow to the conditioned space based on variation in room loads.

Split Air - conditioning Systems

Split type air conditioning systems are one -to- one systems consisting of one evaporator (fan coil) unit connected to an external condensing unit.Both the indoor and outdoor units are connected through copper tubing and electrical cabling. 
The indoor part (evaporator) pulls heat out from the surrounding air while the outdoor condensing unit transfers the heat into the environment . 
Split Air-conditioning System

Advantages of using Split Air conditioners
  • Low initial cost, less noise and ease of installation.
  • Good alternative to ducted systems.
  • Each system is totally independent and has its own control .
  • There is limitation on the distance between the indoor and outdoor unit i.e. refrigerant piping can't exceed the limits stipulated by the manufacturer (usually 100 to 150 ft) otherwise the performance will suffer .
  • Maintenance ( cleaning / change of filters ) is within the occupied space.
  • Limited air throw which can lead to possible hot / cold sports .
  • Impact on building aesthetics of large building because too many outdoor units will spoil the appearance of the building .
Multi-Split Systems
A multi - type air conditioning system operates on the same principles as a split type air- conditioning system however in this case there are multiple evaporator units connected to one external condensing unit . These simple systems were designed mainly for small to medium commercial applications where the installation of duct work was either too expensive, or aesthetically unacceptable . The small - bore refrigerant piping , which connects the indoor and outdoor units requires much lower space and is easier to install than the metal ducting . Each indoor unit has its own set of refrigerant pipe work connecting it to the outdoor unit .

Advantages of Multi-splits
  • The fact that one large condenser can be connected to multiple evaporators within the building reduces and / or eliminates the need for duct work installation completely .
  • Multi - splits are suitable for single thermal zone ( defined below ) applications with very similar heat gains / losses .
  • Inability to provide individual control
  • Multi-split systems turn OFF or ON completely in response to a single thermostat / control station which operates the whole system . These systems are therefore not suitable for areas / rooms with variable heat gain / loss characteristics .
Thermal zone : A thermal zone is referred to a space or group of spaces within a building with similar heating and cooling requirements . Each 
thermal zone must be' separately controlled ' if conditions conducive to comfort are to be provided by an HVAC system .
Any area that requires different temperature, humidity and filtration needs shall be categorized as an independent zone and shall be controlled as an independent zone and shall be controlled by dedicated control or HVAC system . Few examples below illustrate and clarify the zone concept :
  • A conference room designed for 50 people occupancy shall experience lower temperatures when it is half or quarterly occupied . Thus, the design should include a provision for a dedicated temperature controller for this zone .
  • A smoking lounge of airport has different filtration , ventilation ( air changes ) and pressure , requirement compared to other areas, therefore , it is considered a separate zone .
  • A hotel lobby area is different from the guest rooms or the restaurant area because of occupancy variations .
  • In a commercial building , the space containing data processing equipment such as servers, photocopiers, fax machines and printers, are exposed to much larger heat load than the other areas , and hence , it is a different thermal zone .
  • A hospital testing laboratory , isolation rooms and operation theatr demand different indoor conditions / pressure relationships than the rest of areas , and thus , shall be treated as a separate zones . 
  • A control room or processing facility in industrial set up may require a high degree of cleanliness / positive pressure to prevent ingress of ducst / hazardous elements , and therefore , it may be treated as separate zone .

Variable Refrigerant Flow or VRF Systems 
VRF systems are similar to the multi - split systems which connect one outdoor section to several evaporators . However , multi - split system turn OFF or ON completely in response to one master controller , whereas VRF systems continually adjust the flow of refrigerant to each indoor evaporator . the control is achieved by continually varying the flow of refrigerant through a pulse modulating valve (PMV) whose opening is determined by the microprocessor receiving information from the thermistor sensors in each indoor unit. The indoor units are linked by a control wire to the outdoor unit which responds to the demand from the indoor units by varying its compressor speed to match the total cooling and / or heating requirements .
VRF systems promise a more energy - efficient strategy (estimates range from 11 % to 17 % less energy compared to conventional unit ) at a somewhat higher cost .

The modern VRF technology uses an inverter-driven scroll compressor and permits as many as 48 or more indoor units to operate from one outdoor unit ( varies from manufacture to manufacturer ) .The inverter scroll compressors are capable of changing the speed to follow the variations in the total cooling / heating load as determined by the suction gas pressure measured on the condensing unit . The capacity control range can be as low as 6% to 100 % .
Refrigerant piping runs of more than 200 ft are possible and outdoor units are availavle in sizes up to 240000 Btuh .
A schematic VRF arrangement is indicated below .

VRF system are engineered systems and use complex refrigerant and oil control circuitry .The refrigerant pipe- work uses a number of separation tubes and / or headers (refer schematic figure above ). A separation tube has 2 branches whereas a header has more than 2 branches . Either of the separation tube or header or both can be used for branches However the separation tube is NEVER provided after the header because of balancing issues .

Compared to multi - split systems VRF systems minimize the refrigerant path and use less copper tubing . Minimizing the refrigerant path allows for maximizing the efficiency of refrigerant work .

Type of VRF

VRV / VRF systems can be used for cooling only , heat pumping or heat recovery . On heat pump models there are two basic types of VRF system 
  • Heat pump systems .
  • Energy recovery . 
VRF heat pump systems 
VRF heat pump system permit heating or cooling in all of the indoor units but not simultaneous heating and cooling . When the indoor units are in the cooling mode , they act as evaporators , when they are in the heating mode , they act as condensers , these are also known as two - pipe systems .

VRF heat pump systems are effectively applied in open plane areas , retail stores, cellular offices and any other area that require cooling or heating during the same operational periods .

Heat Recovery VRF system ( VRF-HR)

Variable refrigerant flow systems with heat recovery ( VRF-HR) capability can operate simultaneously in heating and / or cooling mode , enabling heat to be used rather than rejected as it would be in traditional heat pump systems. VRF- HR systems are equipped with enhanced features like inverter drives , pulse modulating electronic expansion valves and distributed controls that allow systems to operate in net heating or net cooling mode , as demanded by the space.
Each manufacturer has its own proprietary design (2-pipe or 3-pipe system ), but most uses a three - pipe system (liquid line , a hot gas line and a suction line ) and special valving arrangements . Each indoor unit is branched off from the 3 pipes using solenoid valves . An indoor unit requiring cooling will open its hot gas and liquid line valves and will act as a condenser .
Typically , extra heat exchangers in distribution boxes are used to transfer some reject heat from the superheated refrigerant exiting the zone being cooled to the refrigerant that is going to the zone heated . This balancing act has the potential to produce significant energy savings .

VRF -HR mixed mode operation leads to energy savings as both ends of the thermodynamic cycle are delivering useful heat exchange . if a system has a cooling COP (Coefficient Of Performance ) of 3 , and a heating COP of 4 , than heat recovery operation could yield a COP as high as 7 . It should be noted that this perfect balance of heating and cooling demand is unlikely to occur for many hours each year , but whenever mixed mode is used energy is saved . Unit are now available to deliver the heat removed from space cooling into hot water for space heating , domestic hot water or leisure applications , so that mixed mode is utilized for more of the year .
VRF-HR systems work best when there is a need for some of the spaces to be cooled and some of them to be heated during the same period . This often occurs in the winter in medium - sized to large sized buildings with a substantial core or in the areas on the north and south sides of a building .
The Coefficient of performance (COP) is the performance rating used primarily in heat pumps . it is defined as the ratio of heat output to the amount of energy input of a heat pump . it compares the heat produced by the heat pump to the heat obtained from resistance heat . COP is vary with the outside temperature , as the temperature falls the COP falls also since the heat pump is less efficient at lower temperatures . ARI standards compare equipment at two temperatures 47 F and 17 F to give you an idea of the COP in both mild and colder temperatures .
Refrigerant Modulation in a VRF system VRV /VRF technology is based on the simple vapor compression cycle (same as conventional split air conditioning systems) but gives you the ability to continuously control and adjust the flow of refrigerant to different internal units ,depending on the heating and cooling needs of each evaporator is adjusted precisely through a pulse wave electronic expansion valve in conjunction with an inverter and multiple compressors of varying capacity , in response to changes in the cooling or heating requirement within the air conditioned space .
We will discuss this further but before that let's refresh the basic refrigeration cycle .

Thank You 

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