INSIGHT
THE ART OF AIR HANDLING
IS TO HAVE MORE COOLING
WITH LESS REFRIGERANT
By Michael Young, sales engineer, Trox
All HVAC systems contain some medium that allows for the transportation of
heat from one area to the next.
www.hvacronline.co.za
cooled to a greater degree while the super cooled refrigerant is
now a saturated or slight sub-cooled liquid.
All bypassed refrigerant is then fed into the suction line
which mixes with all hot gas that leaves the evaporator. All
refrigerant then enters the liquid accumulator and is then
sucked into the compressor and compressed to the desired
condensing pressure.
By introducing an additional heat interchange circuit, less
refrigerant is circulated through the system. This additional
feature allows the VRF system to comply with the requirements
as specified by EN 378 as well as reducing the effect of global
warming should there be a leak within the system.
I know how you feel right now. You have read this article and
you may feel overwhelmed or wish someone could explain this
concept with the use of the Mollier diagram. The good news is
that we are here to support and help you accomplish your goals.
So, we would like to invite you to attend a FREE one-hour
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are limited to 20 people, so call +27 073 171 2311 now to reserve
your spot. RACA
F
or a variable refrigerant flow (VRF) system, the refrigerant will
most often be R410A refrigerant with R32 being considered as
a future refrigerant replacement.
Use of specific refrigerants do however have limitations as
specified by European Standard EN 378 which provides guidelines
on the maximum amount of refrigerant that is allowed to be
pushed into an occupied space.
This standard has caused VRF manufacturers to relook at
their designs. Manufacturers are now required to manufacture
VRF systems that contain less refrigerant without compromising
cooling capacity.
The first step in accomplishing this goal is to understand the
workings of a refrigeration system with reference to the Mollier
diagram (also Enthalpy-entropy chart). Evaporator cooling capacity
is rated as the enthalpy difference between the outlet of the
expansion valve and the inlet of the compressor for an ideal cycle.
If we are able to sub-cool the liquid refrigerant to a greater
degree, we ultimately increase the enthalpy difference on the
evaporator side of the system. Since the cooling effect is increased,
the mass flow of refrigerant can be decreased for a desired amount
of cooling.
So, how do we increase sub-cooling within a condenser? The
easiest method is to introduce a heat interchanger circuit within
the VRF condenser design. In a standard VRF condenser circuit,
the amount of sub cooling is limited by the extended sub-cooling
section of the condenser coil.
By introducing a heat interchange circuit, a portion of the liquid
refrigerant is bypassed from the liquid line and throttled through
an expansion valve. Reducing the pressure further reduces the
refrigerant temperature to a lower sub-cooled refrigerant which we
will call a super cooled refrigerant.
The super cooled refrigerant is then passed through a super
cooled heat exchanger. One side of the super cooled heat
exchanger contains the primary liquid refrigerant that left the
condenser while the other side of the heat exchanger contains the
super cooled liquid refrigerant.
Heat is now exchanged from the liquid refrigerant line to the
super cooled liquid refrigerant line. The liquid refrigerant is sub-
The first step in accomplishing the goal of manufacturing VRF
systems that contain less refrigerant without compromising cooling
capacity is to understand the workings of a refrigeration system.
RACA Journal I May 2020
31