Variable Refrigerant Flow (VRF)
Variable Refrigerant Flow (VRF) also known as variable refrigerant volume (VRV) is a flexible method of air conditioning. The flow of refrigerant can be varied to match the cooling load as heat gains in a room fluctuate. Also, if reversible heat pumps are used, the heating output can be varied to match the varying heat loss in a room. An expansion valve or control valve can reduce or stop the flow of refrigerant to each indoor unit, thus controlling its output to the room.
This type of system consists of a number of indoor units (up to 40) connected to one or more external condensing units. The overall refrigerant flow is varied using either an inverter controlled variable speed compressor, or multiple compressors of varying capacity in response to changes in the cooling or heating requirement within the air conditioned space. A control system enables switching between the heating and cooling modes if necessary. In more sophisticated versions, the indoor units may operate in heating or cooling mode independently of others.
An electrical inverter is used to vary the frequency of the power supply from a normal 50Hz enabling fine step speed control of motors (compressors) this in turn varies the flow rate of refrigerant delivered and hence varies the cooling or heating capacity of the indoor units.
A Variable Refrigerant Flow (VRF) system has the advantage that it is energy efficient and flexible when compared to constant refrigeration flow systems.
· Refrigerant flow rate is constantly adjusted by an electronic expansion valve in response to load variations as rooms require more or less cooling.
· The electronic expansion valves are situated in the rooms often above the ceiling or in a secure box at low level.
· The units can be either; cooling only, cooling and heating or cooling and heating with heat recovery.
· Scroll compressors are often used since they are efficient and quiet, these are usually hermetically sealed.
Small to medium size units may have 2 compressors.
Some larger units have 9 compressors.
· Manufacturers produce systems in which 16 indoor units can be connected to a single outdoor air cooled condenser.
· Refrigeration pipework up to1000 metres long is feasible.
· Refrigeration pipework level differences between indoor and outdoor units up to110 metres is possible.
· HFC refrigerants are used - typically R-410-A
· Some VRV units can use hot water as the heat source instead of a heat pump and these require a water and refrigerant pipework system to be installed.
· Typical small outdoor air cooled condensers units have 5 to 10 H.P. motors to drive the compressor. Large units up to 50 H.P. are available.
· Typical cooling capacity of small indoor units is from 14 kW to 28 kW.
Large units have 130 kW cooling capacity and similar heating capacity.
· COP’s (Coefficient of Performance) may be as high as 8.
· Refrigerant liquid lines tend to be about 5mm diameter and gas lines about 16mm to 22mm diameter.
· Central control of a VRV system can be achieved by centralised remote controllers.
· Some systems use a 3 –pipe system.
Some systems can operate is dual mode offering both cooling and heating simultaneously to the rooms. This generally results in higher overall efficiencies. A three pipe system is required for this.
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