VRF Outdoor Unit Product

Overview

A VRF (variable refrigerant flow) outdoor unit is the central plant of a ductless or duct-light building HVAC system. Instead of moving heat with chilled water, it pipes refrigerant directly to dozens of indoor units, modulating the total flow with inverter-driven compressors so that capacity continuously matches the sum of every zone's demand. A single circuit can serve up to 64 indoor units through one pair (or, in heat-recovery systems, one triple) of small-bore copper lines up to 165 m long, with the outdoor unit as much as 90 m above or below the farthest indoor unit. The architecture, introduced by Daikin in 1982 as VRV, now dominates mid-size commercial buildings in Asia and Europe.

The unit packages two Inverter Scroll Compressor modules, a wrap-around Outdoor Heat Exchanger, top-discharge Axial Fan Assembly assemblies, the Refrigerant Circuit Components valve set, and a Electrical Control Box, all inside a weatherproof Cabinet & Base.

Compression and modulation

Capacity control is the defining feature. Each Inverter Scroll Compressor is a hermetic scroll machine — a fixed and orbiting Scroll Set driven through a Crankshaft by a permanent-magnet Compressor BLDC Motor — whose speed the inverter sweeps from about 15 Hz to 120 Hz. Output therefore modulates continuously from a few percent to full load instead of staging on and off. The Inverter Power Board for each compressor rectifies the 400 V supply, smooths it through a DC Reactor, and synthesizes variable-frequency three-phase power through an IGBT Power Module, with Hall Sensor current feedback enabling sensorless field-oriented control. A Noise Filter Board keeps the switching noise out of the mains, and a Compressor Sound Blanket keeps compressor noise out of the neighborhood.

Refrigerant management

The Refrigerant Circuit Components is considerably more complex than a split-system condenser. A Four-Way Reversing Valve reverses the cycle between cooling, heating, and defrost. Stepper-driven Electronic Expansion Valve valves with roughly 480 positions meter flow with ±0.5 K superheat precision — and every indoor unit carries its own EEV, which is what lets one circuit feed many zones at different loads. A Oil Separator on the discharge line captures lubricant before it can migrate down the long pipe runs, returning it to the sump; the controller still schedules periodic high-speed oil-recovery runs to sweep back what escapes. The Suction Accumulator protects the compressors from liquid slugging during transients, and a brazed-plate Subcooling Plate Heat Exchanger subcools the liquid line, which is what makes the long pipe lengths workable without flashing. Field pipes braze onto Service Stop Valve connections, and an array of Check Valve elements steers flow as the cycle reverses.

Air side

The Outdoor Heat Exchanger wraps three or four faces of the cabinet: internally grooved 7 mm Heat Exchanger Tubes in a louvered, hydrophilic-coated Heat Exchanger Fins pack, fed by Coil Header manifolds in condenser mode and through a capillary Refrigerant Distributor in evaporator mode. Above it, each Axial Fan Assembly — a swept Axial Propeller in a contoured Fan Bellmouth, driven by a variable-speed Fan BLDC Motor — draws up to 13,000 m³/h through the coil. Fan speed is itself a control variable: it trims condensing pressure in mild weather and drops to night-mode speeds when low sound matters. In heating mode below about 5 °C ambient, frost grows on the coil; Coil Defrost Sensor thermistors detect it and the unit briefly reverses to defrost, with the Base Pan draining the melt water.

Control architecture

The Main Control Board runs the whole circuit: compressor staging and speed, the position of every outdoor EEV, defrost scheduling, and oil recovery. It talks to all indoor units over a polarity-free two-wire bus terminated at the Communication Terminal Block — each indoor unit reports its zone demand, and the outdoor unit resolves the aggregate into a target compressor frequency and refrigerant distribution. Two Pressure Sensor transducers on the high and low sides close the primary control loops. Heat-recovery variants add branch-selector boxes in the piping network so one zone can heat while another cools, moving heat between them instead of rejecting it; in that mode the system's effective COP can exceed either single mode. Diagnostics, addressing, and commissioning all run over the same bus, which is why a VRF system is field-configured largely from the outdoor unit's service interface behind the Service Access Panel.