AAON CN-140 - Manual (Page 18)

18
Mounting Isolation
For roof mounted applications or anytime
vibration transmission is a factor, full
perimeter vibration isolators may be used.
Access Doors
Lockable access doors are provided to the
compressor and control compartment.
Low Ambient Operation
The AAON low ambient (condenser flood-
back) system is used to operate a refrigerant
system below 35°F outside air temperature.
As the ambient temperature drops, the
condenser becomes more effective therefore
lowering the head pressure. When the head
pressure gets too low, there will be
insufficient
pressure to operate the
expansion valve properly. During low
ambient temperatures, it is difficult to start a
system because the refrigerant will migrate
to the cold part of the system (condenser)
and make it difficult for refrigerant to flow.
The low ambient system maintains normal
head pressure during periods of low ambient
operation by restricting liquid flow from the
condenser to the receiver, and at the same
time bypassing hot gas around the condenser
to the inlet of the receiver. This backs liquid
refrigerant up into the condenser reducing its
capacity
that in turn increases the
condensing pressure. At the same time the
bypassed hot gas raises liquid pressure in the
receiver, allowing the system to operate
properly.
There are different types of low ambient
control used. The following figure shows the
type of system available on the CN Series.
Expansion Valve Operation
AAON recommends the use of electronic
expansion valves in matching air handling
units
when using variable speed
compressors in CN Series condensing units.
Thermostatic expansion valves do not have
enough turn down capacity and cannot
properly manage suction superheat during
low load conditions. Electronic expansion
valves must be observed during startup
through the entire operating range of the
variable speed compressor to ensure that
suction superheat never falls below 7°F.
Insufficient suction superheat can introduce
liquid refrigerant to the compressor and
significantly reduce compressor life.
Determining Refrigerant Line Size
The piping between the condenser and low
side must ensure:
1. Minimum pressure drop, and
2. Continuous oil return, and
3. Prevention of liquid refrigerant slugging,
or carryover
Minimizing the refrigerant line size is
favorable from an economic perspective,
PVC (Polyvinyl Chloride) and CPVC
(Chlorinated Polyvinyl Chloride) are
vulnerable to attack by certain
chemicals. Polyolester (POE) oils
used
with R-410A and other
refrigerants, even in trace amounts,
in a PVC or CPVC piping system will
result in stress cracking of the piping
and fittings and complete piping
system failure.
CAUTION
Line sizes must be selected to meet
actual installation conditions, not
simply based on the connection sizes
at the condensing unit or air handling
unit.
CAUTION