لا فرق عمليا ولكن نظريا الانظمه الصغيره منها تعرف بmulti system وتختلف عدد الوحدات الداخليه لها حسب المنتج او المصنّع فمنها 4 ومنا 8
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VARIABLE-REFRIGERANT-FLOW HEAT PUMPS
A variable-refrigerant-flow (VRF) system typically consists of a
condensing section housing compressor(s) and condenser heat
exchanger interconnected by a single set of refrigerant piping to
multiple indoor direct-expansion (DX) evaporator fan-coil units.
Thirty or more DX fan coil units can be connected to a single condensing
section, depending on system design, and with capacity
ranging from 0.5 to 8 tons.
The DX fan coils are constant air volume, but use variable refrigerant
flow through an electronic expansion valve. The electronic
expansion valve reacts to several temperature-sensing devices such
as return air, inlet and outlet refrigerant temperatures, or suction
pressure. The electronic expansion valve modulates to maintain the
desired set point.
VRF systems are most commonly air-to-air, but are also available
in a water-source (water-to-refrigerant) configuration. They
can be configured for simultaneous heating and cooling operation
(some indoor fan coil units operating in heating and some in cooling,
depending on requirements of each building zone).
Indoor units are typically direct-expansion evaporators using
individual electronic expansion devices and dedicated microprocessor
controls for individual control. Each indoor unit can be controlled
by individual thermostat. The outdoor unit may connect
several indoor evaporator units with capacities 130% or more than
the outdoor condensing unit capacity.
VRF equipment is divided into three general categories: residential,
light commercial, and applied. Residential equipment is singlephase
unitary equipment with a cooling capacity of 65,000 Btu/h or
less. Light commercial equipment is generally three-phase, with
cooling capacity greater than 65,000 Btu/h, and is designed for
small businesses and commercial properties. Applied equipment
has cooling capacity higher than 135,000 Btu/h and is designed for
large commercial buildings.
Refrigerant Circuit and Components
VRF heat pump systems use a two-pipe (liquid and suction gas)
system; simultaneous heat and cool systems use the same system, as
well as a gas flow device that determines the proper routing of
refrigerant gas to a particular indoor unit.
VRF systems use a sophisticated refrigerant circuit that monitors
mass flow, oil flow, and balance to ensure optimum performance.
This is accomplished in unison with variable-speed compressors
and condenser fan motors. Both of these components adjust their
frequency in reaction to changing mass flow conditions and refrigerant
operating pressures and temperatures. A dedicated microprocessor
continuously monitors and controls these key components to
ensure proper refrigerant is delivered to each indoor unit in cooling
Heating and Defrost Operation
In heating mode, VRF systems typically must defrost like any
mechanical heat pump, using reverse cycle valves to temporarily
operate the outdoor coil in cooling mode. Oil return and balance
with the refrigerant circuit is managed by the microprocessor to
ensure that any oil entrained in the low side of the system is brought
back to the high side by increasing the refrigerant velocity using a
high-frequency operation performed automatically based on hours