Variable refrigerant flow (VRF) is a term used to describe fully modulating multi-split systems. VRF-systems provide cooling and heating with the use of refrigerant R407C or R410A as the working fluid. Heating is made by reversing the cycle (see Fig. 27.1). This type of system cooling or heating of the premises may be - but not both.
More advanced systems can provide cooling and heating simultaneously for different zones. Heat can be transferred from those spaces requiring cooling in areas requiring heating, resulting in extremely high efficiency and low maintenance costs. Outdoor units are available with different cooling and heating capacities in accordance with the application. Many control options are available, ranging from simple wireless controllers for the powerful PC software packages. All units are linked by a cable transfer, providing a fully networked system of air-conditioning.
Simplified scheme for three-pipe system is shown in Fig. 25.11. Outdoor unit contains a regulation of the compressor and heat exchangers, which can serve as either evaporators or capacitors depending on the position of the reversing valve.
Operation mode of each of the inner block is controlled branching controller. Detail branching controller is shown in Fig. 25.12.
On Fig. 25.11 system, first of all, providing cooling, three internal blocks B, C and D are the cooling unit is heating. Compressor the hot high-pressure gas for heat exchangers, acting as capacitors. Liquid from the condensers flows through the receiver for liquid line and, therefore, the indoor units B, C and D, where individual electronic expansion valves regulating evaporation and cooling. By suction gas flows through the suction pipe back to the outdoor unit. Block but gets the compressor discharge gas and condensate (reduction capacitor outdoor unit load). Condensation rate was determined by the speed with which the fluid is allowed to feed back into the liquid line (managed individual electronic valve block (which acts as an expansion valve in cooling mode).
When the system is, first of all, providing heating, solenoid valves SV1 in most branching controllers are open and indoor units are supplied with hot gas injection instead of the liquid. Fluid flow pipes is restored and the heat exchangers in the outdoor unit as evaporators, is governed by their electronic expansion valves. Four-way valves switch so that the liquid flows into the drive, where it is joined by liquid coming from any internal blocks, which are cooling. There may be " balanced " a condition in which the cooling corresponds to the heating and outdoor heat exchangers are not used.
Additional features include defrosting and oil return. Assistance with the return of oil is provided by periodically opens the valve SV2 (Fig. 25.12) in branching controller in order to increase the speed in the suction line. Individual flow restrictors placed in the circuit where it is shown. These systems are fully developed and the manufacturers mentioned, and proved within a specified maximum length of the string. The length of the line benefits were increased in connection with the transition to R410A as a working liquid pressure drop effect is smaller, because if the pressure is higher.
Variable refrigerant flow systems are extremely popular and are suitable for various types of buildings. There is a flexibility of mixing range of indoor units (e.g. floor, walls, and ceiling ducted units) with a properly chosen for the outdoor unit. The end user can select the controller that best suits them. The modular format suitable for phased installations, and compared to traditional air conditioning systems, VRF easier to design and install.