INSIGHT
THE ART OF AIR HANDLING
IS TO BRING ALL THE VRF
PIECES TOGETHER
By Michael Young, sales engineer, Trox
Variable Refrigerant Flow (VRF) was introduced in Japan more than
20 years ago.
www.hvacronline.co.za
S
ince then, VRF systems have become very popular due
to high energy savings, flexibility to couple the system
with different types of indoor units and the ability to perform
heating and cooling simultaneously.
The most common type of arrangement is to have multiple
types of indoor units to one central VRF condensing system.
The need for higher cooling requirements and higher energy
efficiency has resulted in the coupling of a VRF system to an air
handling unit (AHU).
VRF systems have gone from cooling commercial office
buildings to large data centres and even shopping malls. The
past few years has seen a number of AHU manufacturers
partnering up with VRF manufacturers to meet the new market
trends and demands.
Now before you approach the next AHU manufacturer
and request them to couple their AHU to your preferred VRF
system, just read this article to the end as there are a few
things you need to know.
If you review the VRF design guidelines, you will notice
that the maximum amount of cooling capacity that the LEV
controller is not identical for each of the various suppliers.
You will also notice that the allowable refrigerant pressure
loss through the DX coil is also not consistent between each
supplier.
So why are these two factors so important when coupling
a VRF system to an AHU? To successfully couple a VRF to an
AHU, the entire system needs to be balanced.
LEV kits control the amount of refrigerant that enters the
evaporator by superheats. Should the cooling load through
the DX coil exceed the LEV kit capabilities, the sensors will
read high superheats, the expansion valve will open, and the
compressor will increase in speed.
The problem is that the expansion valve will open to its
maximum capacity but because it is only sized for a certain
maximum cooling capacity, insufficient refrigerant will be fed
to the evaporator and there will be insufficient cooling.
If the coil is undersized, the sensors will read lower
superheats, the compressor will ramp down and insufficient
refrigerant will be fed to the evaporator. So not matching the
evaporator’s capacity with the LEV kit’s cooling capacity can
result in malfunction of the VRF system.
The next thing to note is the refrigerant pressure loss
through the DX coil within the AHU. Each VRF manufacturer
specifies a maximum pressure loss and erratic operations will
occur should this limit not be adhered to.
Exceeding the specified pressure loss through the DX coil
causes the refrigerant liquid to boil off at a very early stage
within the evaporator. This causes the overall mass flow of
refrigerant through the DX coil to decrease which reduces
heat transfer.
The air temperature does not reach the desired set point
which then increases the compressor speed and causes
inefficient operations.
So, coupling a VRF to an AHU is more complex than
just adding a condenser to the back of an AHU and laying
interconnecting refrigerant piping. To successfully get the
system work, the heat transfer aspects and control aspect
must be balanced and work in unison.
So next time you require a VRF system to be integrated
with an AHU, ask the AHU manufacturer which VRF system
they are familiar with and check they have received the
correct design training. Should you wish to learn more on VRF
systems, email me at [email protected]. RACA
RACA Journal I March 2020
Coupling a VRF to
an AHU is more
complex feat than
many may know.
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