LG Multi V mini Engineering Manual page 53

all VRF systems. Manufacturers of VRF equipment set acceptable
system design CR parameters. LG limits the CR of a Multi V system
to be between 50% and 130%. Even though a VRF manufacturer
designs a system with what could be perceived as an excessive CR,
the system's operating saturated suction temperature must be main-
tained in a similar operating temperature range as a VRF system de-
signed with a CR between 50% and 130%. In general, compressor
failures caused by excessive component mismatch are typically not
revealed until the VRF system has operated for a substantial period
of time. If a failure occurs, it will likely occur at peak load conditions.
How to Determine the System CR
The system's CR is determined by comparing the nominal capacity
of all connected indoor units with the nominal capacity of the outdoor
unit serving them. Locate nominal capacity information for indoor
and outdoor units in the General Data tables of their respective
engineering manuals.
For example,
If a VRF system has an outdoor unit with a nominal capacity of C
and four indoor units having nominal capacity ratings of W, X, Y, and
Z respectively, the CR would be determined as follows:
(
W + X + Y + Z
CR% =
Multi V systems will not start, operate, nor can they
Note:
be commissioned if the CR is outside the allowable 50% to
130% range.
If the CR is over 100%, the designer is under-sizing the outdoor unit
relative to the combined nominal capacity of the connected indoor
units. In some applications, under-sizing of the outdoor unit is pru-
dent as it reduces the initial equipment investment and will properly
perform as long as the designer:
1. Knows the indoor unit(s) is oversized relative to the actual load(s) in
the spaces served.
2. Knows the space loads will peak at different times of the day (i.e.
building has "load diversity").
In some designs, over-sized indoor units may be unavoidable in
cases where the smallest size indoor unit available from LG is larger
than what is necessary to satisfy the space load. This scenario may
occur when an indoor unit selection one size down from the selected
unit is slightly short of fulfilling the design load requirements and the
designer must choose the next largest size unit.
If the outdoor unit is properly sized to offset the build-
Note:
ing's total cooling block load and the system's CR is above
130%, indoor units are likely oversized. In applications where
all indoor units are "right-sized" and there is no building diver-
sity, the system's CR will likely be ≤100%.
If the CR is above 130%, review the indoor unit choices and down-
size indoor units, or select a larger outdoor unit. Consider moving
indoor units to another Multi V, Flex-Multi, or single-split system if the
EQUIPMENT SELECTION PROCEDURE
x 100
C
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outdoor unit size cannot be increased.
If the CR falls below 50%, select a smaller outdoor unit or consider
adding more or larger indoor unit(s) to the system. This situation
is common on multi-phase projects where the design calls for the
majority of indoor units to be added to the system at a later date. To
raise the CR above the minimum 50% requirement:
1. Consider adding additional indoor units on the first phase.
2. Design two smaller systems in lieu of a single larger system.
Connect all "first phase" indoor units to the outdoor unit being
installed on the first phase and delay the installation of the ad-
ditional outdoor unit until a later date.
To avoid the potential of designing a system that may operate with
an excessively high (or low) saturated suction temperature leading to
premature compressor failure, do the following:
1. Design conservatively.
2. Verify the system's CR is between 50% and 130%.
ARUN 036, 047, 053
55
SYSTEM ENGINEERING
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