
System Engineering
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SYSTEM ENGINEERING
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The outdoor unit selected should be large enough to offset the total
cooling block load for all spaces served by the VRF system during
the peak cooling load hour on the cooling design day (account for the
ventilation air's cooling total load if ventilation air is not pretreated to
room neutral conditions). Therefore, the corrected cooling capacity
ratio (CCR%
(clg)
) should never exceed 100%. If the corrected cooling
capacity ratio exceeds 100% (plus building diversity if considered),
increase the size of the outdoor unit or change the system design
by moving some of the building load and associated indoor unit(s) to
another Multi V system.
The outdoor unit should also be large enough to offset the sum of the
building’s space heating loads without considering building diver-
sity. In the heating season, it is typical that all spaces served by the
system will peak simultaneously in the early morning, thus building
diversity should never be considered. If the corrected heating capac-
ity ratio (CCR%
(htg)
) exceeds 100%, increase the size of the outdoor
unit , enclose the outdoor unit and provide control of the surrounding
air conditions, or change the system design by moving some of the
building load to another Multi V system.
Understanding the Combination Ratio (CR)
When first introduced to Variable Refrigerant Flow (VRF) technology,
the designer often compares the system components with those of
a traditional split-system. The VRF outdoor unit(s) is compared to a
commercial split-system condensing units. Indoor units are com-
pared with traditional fan coils and other forms of commercial and/
or residential air moving products. The significant difference being
a traditional heat pump system is comprised of a single air handler
with a single condensing unit in a “one to one” relationship. A VRF
system typically is comprised of a single frame unit (or multi-frame
outdoor unit operating as a single unit) piped to numerous indoor
units forming a “one to many” relationship between the outdoor and
indoor units. With the introduction of the one to many relationship
between VRF system ODU and IDU’s, VRF manufacturer’s intro-
duced a new component relationship concept known as Combination
Ratio (CR) to the industry.
Calculating the operating saturated suction temperature of a tradi-
tional split-system serves the same purpose as calculating the CR.
Both calculations prevent mismatching system components.
Properly Matching Traditional Commercial Split-System Compo-
nents
To properly match a traditional split-system (or split-system heat
pump) condensing unit with an air handler equipped with a direct
expansion coil, the designer would generate a refrigeration system
cross plot diagram to reveal the system’s balance point.
To generate a refrigeration system cross plot diagram, the manufac-
turer’s computerized evaporator coil selection program is used to
generate a graphical representation of the evaporator performance
at indoor design conditions. To generate the graph, the physical
characteristics of the evaporator coil design are entered into the
selection program and frozen. The designer will typically run the pro-
gram a minimum of three times, each time making a selection with a
different specified evaporator coil capacity. One selection condition
will be done using the engineer’s outdoor unit scheduled capacity,
one at a higher and finally one at a lower capacity value.
The program will return the corresponding evaporator saturated
suction temperature (SST) for each capacity specification. Each
evaporator performance data point (MBh vs. SST) is subsequently
plotted on the outdoor unit’s capacity performance chart. A line
is drawn between the plotted points. The drawn line depicts the
evaporator coils capacity at various saturated suction temperatures.
The condensing unit’s capacity performance lines are provided by
the outdoor unit manufacturer and are graphical representations of
the unit’s performance at various ambient air temperatures. The
system’s balance point and operating saturated suction temperature
is where the evaporator capacity performance line crosses the ap-
propriate condensing unit capacity performance line. The designer
then identifies the system capacity and operating saturated suction
temperatures at the balance point.
If the evaporator coil is too large or the condensing unit is too small,
the system’s operating saturated suction temperature will be too high
to adequately cool the compressor. If the evaporator coil is too small
or the condensing unit is too large, the system’s operating saturated
suction temperature will be too low and the possibility of slugging the
compressor with liquid refrigerant exists. Therefore, if a split system
is designed using mismatched components where there is an exces-
sive difference in the heat transfer surface area of the evaporator coil
relative to the condenser coil, the longevity of the system’s compres-
sor will be at risk.
Properly Matching VRF System Components
The same refrigeration theory and component matching verification
applies to VRF systems. However, instead of the designer calculat-
ing the system’s saturated suction operating temperature to verify a
proper size match between the evaporator and condenser coils, VRF
system designers must calculate and check the system ’s CR. If the
system’s CR is greater than 130%, the combined heat transfer sur-
face area of all connected indoor units will be greater than the heat
transfer surface of the outdoor unit(s), which under certain operating
conditions, may raise the saturated suction operating temperature to
an unacceptable level and result in a VRF system malfunction. If the
system’s CR is less than 50%, the combined heat transfer surface
area of all connected indoor units is much less than the heat transfer
surface of the outdoor unit(s). Under certain operating conditions,
this may lower the saturated suction operating temperature to an
unacceptable level, and result in a VRF system malfunction.
Compressor technology, system operational limitations, and the
physical characteristics of R-410a refrigerant are very similar for
EQUIPMENT SELECTION PROCEDURE
ARUN 036, 047, 053
Summary of Contents for Multi V mini
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