AAON CB-060 User Manual
CB SERIES
Condensing Units
Installation, Operation
FOR YOUR SAFETY
Do not store or use gasoline or other
flammable vapors and liquids in the
vicinity of this or any other appliance.
WARNING
If the information in this manual is not
followed exactly, a fire or explosion
may result causing property damage,
personal injury or loss of life.
WARNING
QUALIFIED INSTALLER
Improper installation, adjustment,
alteration, service or maintenance
can cause property damage,
personal injury or loss of life. Startup
and service must be performed by a
Factory Trained Service Technician.
A copy of this IOM should be kept
with the unit.
WARNING
& Maintenance
3
Table of Contents
Safety .............................................................................................................................................. 5
CB Base Model and Features Description ...................................................................................... 8
General Description ........................................................................................................................ 9
Codes and Ordinances................................................................................................................. 9
Receiving Unit ............................................................................................................................ 9
Storage ........................................................................................................................................ 9
Direct Expansion (DX) Condensing Units ............................................................................... 10
Wiring Diagrams ....................................................................................................................... 10
Installation..................................................................................................................................... 11
Locating Unit ............................................................................................................................ 11
Lifting the Unit ......................................................................................................................... 12
Electrical ................................................................................................................................... 12
Thermostat ................................................................................................................................ 14
Refrigerant Piping ..................................................................................................................... 14
General .................................................................................................................................. 14
Determining Refrigerant Line Size ....................................................................................... 15
Liquid Line Sizing ................................................................................................................ 15
Suction Line Sizing ............................................................................................................... 16
Hot Gas Bypass Line ............................................................................................................ 17
Hot Gas Reheat ..................................................................................................................... 18
Predetermined Line Sizes ..................................................................................................... 18
Startup ........................................................................................................................................... 21
Compressors .............................................................................................................................. 21
Charging Refrigerant & Adjusting Refrigerant Charge ............................................................ 21
Before Charging .................................................................................................................... 21
Checking Liquid Sub-cooling ............................................................................................... 22
Checking Evaporator Superheat ........................................................................................... 22
Adjusting Sub-cooling and Superheat Temperatures ........................................................... 22
Air Flow .................................................................................................................................... 23
Operation....................................................................................................................................... 25
Thermostat Operation ............................................................................................................... 25
Compressor ............................................................................................................................... 25
Variable Capacity Compressor Controller .................................................................................... 25
Low Ambient Options................................................................................................................... 28
Maintenance .................................................................................................................................. 28
Coils .......................................................................................................................................... 28
E-Coated Coil Cleaning ............................................................................................................ 28
DX Cooling ............................................................................................................................... 30
Condenser Fan Motor ............................................................................................................... 30
Replacement Parts ..................................................................................................................... 30
Warranty, Service, and Parts Department ................................................................................. 30
Refrigerant Piping Diagrams ........................................................................................................ 31
CB Series Startup Form ................................................................................................................ 43
Literature Change History............................................................................................................. 45
4
Index of Tables and Figures
Tables:
Table 1 - Recommended Elevation Minimums ............................................................................ 12
Table 2 - Predetermined Line sizes for CB units with two step compressors and R-410A .......... 20
Table 3 - Sub-cooling and Superheat ............................................................................................ 22
Table 4 - Performance Testing Air Flow Setpoints ...................................................................... 23
Table 5 - R-410A Refrigerant Temperature-Pressure Chart ......................................................... 24
Table 6 - Demand Signal vs. Compressor Capacity Modulation .................................................. 26
Table 7 - Thermistor Temperature vs. Resistance Values ............................................................ 27
Figures:
Figure 1 – Condensing Unit Clearances ....................................................................................... 11
Figure 2 - Riser height versus total equivalent line length ........................................................... 20
Figure 3 - Variable Capacity Compressor Controller ................................................................... 26
Figure 4 - Compressor Controller Flash Code Details.................................................................. 27
Figure 5– A/C only piping, AHU above CU ................................................................................ 31
Figure 6 – A/C only piping, AHU below CU ............................................................................... 32
Figure 7 – Modulating hot gas reheat piping, AHU above CU .................................................... 33
Figure 8 – Modulating hot gas reheat piping, AHU below CU .................................................... 34
Figure 9 – Hot gas bypass piping, AHU above CU ...................................................................... 35
Figure 10 – Hot gas bypass piping, AHU below CU .................................................................... 36
Figure 11 – Modulating hot gas reheat with hot gas bypass piping, AHU above CU .................. 37
Figure 12 – Modulating hot gas reheat with hot gas bypass piping, AHU below CU .................. 38
Figure 13 – Heat pump piping, AHU above CU .......................................................................... 39
Figure 14 – Heat pump piping, AHU below CU .......................................................................... 40
Figure 15 – Heat pump with modulating hot gas reheat piping, AHU above CU ........................ 41
Figure 16 – Heat pump with modulating hot gas reheat, AHU below CU ................................... 42
R57611 · Rev. - · 130603
(ACP J00187)
5
Safety
Attention should be paid to the following statements:
NOTE - Notes are intended to clarify the unit installation, operation and maintenance.
CAUTION - Caution statements are given to prevent actions that may result in
equipment damage, property damage, or personal injury.
WARNING - Warning statements are given to prevent actions that could result in
equipment damage, property damage, personal injury or death.
DANGER - Danger statements are given to prevent actions that will result in equipment
damage, property damage, severe personal injury or death.
ELECTRIC SHOCK, FIRE OR
EXPLOSION HAZARD
Failure to follow safety warnings
exactly could result in dangerous
operation, serious injury, death or
property damage.
Improper servicing could result in
dangerous operation, serious injury,
death or property damage.
Before servicing, disconnect all
electrical power to the unit. More
than one disconnect may be
provided.
When servicing controls, label all
wires prior to disconnecting.
Reconnect wires correctly.
Verify proper operation after
servicing. Secure all doors with
key-lock or nut and bolt.
WARNING
Electric shock hazard. Before
servicing, disconnect all electrical
power to the unit, including remote
disconnects, to avoid shock hazard
or injury from rotating parts. Follow
proper Lockout-Tagout procedures.
WARNING
FIRE, EXPLOSION OR CARBON
MONOXIDE POISONING HAZARD
Failure to replace proper controls
could result in fire, explosion or
carbon monoxide poisoning. Failure
to follow safety warnings exactly
could result in serious injury, death or
property damage. Do not store or use
gasoline or other flammable vapors
and liquids in the vicinity of this
appliance.
WARNING
6
GROUNDING REQUIRED
All field installed wiring must be
completed by qualified personnel.
Field installed wiring must comply
with NEC/CEC, local and state
electrical code requirements. Failure
to follow code requirements could
result in serious injury or death.
Provide proper unit ground in
accordance with these code
requirements.
WARNING
During installation, testing, servicing
and troubleshooting of the equipment
it may be necessary to work with live
electrical components. Only a
qualified licensed electrician or
individual properly trained in handling
live electrical components shall
perform these tasks.
Standard NFPA-70E, an OSHA
regulation requiring an Arc Flash
Boundary to be field established and
marked for identification of where
appropriate Personal Protective
Equipment (PPE) be worn, should be
followed.
WARNING
UNIT HANDLING
To prevent injury or death lifting
equipment capacity shall exceed unit
weight by an adequate safety factor.
Always test-lift unit not more than 24
inches high to verify proper center of
gravity lift point to avoid unit damage,
injury or death.
WARNING
ROTATING COMPONENTS
Unit contains fans with moving parts
that can cause serious injury. Do not
open door containing fans until the
power to the unit has been
disconnected and fan has stopped
rotating.
WARNING
Compartments containing hazardous
voltage or rotating parts are equipped
with a panel requiring tooled access.
Always re-install screws on the panel
after installation or service is
completed.
CAUTION
Rotation must be checked on all
MOTORS AND COMPRESSORS of
3 phase units at startup by a qualified
service technician. Scroll
compressors are directional and can
be damaged if rotated in the wrong
direction. Compressor rotation must
be checked using suction and
discharge gauges. Fan motor rotation
should be checked for proper
operation. Alterations should only be
made at the unit power connection
CAUTION
7
Do not clean DX refrigerant coils with
hot water or steam. The use of hot
water or steam on refrigerant coils
will cause high pressure inside the
coil tubing and damage to the coil.
Some chemical coil cleaning
compounds are caustic or toxic. Use
these substances only in accordance
with the manufacturer’s usage
instructions. Failure to follow
instructions may result in equipment
damage, injury or death.
WARNING
To prevent damage to the unit, do not
use acidic chemical coil cleaners. Do
not use alkaline chemical coil
cleaners with a pH value greater than
8.5, after mixing, without first using
an aluminum corrosion inhibitor in the
cleaning solution.
CAUTION
CAUTION
Do not use oxygen, acetylene or air
in place of refrigerant and dry
nitrogen for leak testing. A violent
explosion may result causing injury or
death.
WARNING
Do not work in a closed area where
refrigerant or nitrogen gases may be
leaking. A sufficient quantity of
vapors may be present and cause
injury or death.
WARNING
COMPRESSOR CYCLING
5 MINUTE MINIMUM OFF TIME
To prevent motor overheating
compressors must cycle off for a
minimum of 5 minutes.
5 MINUTE MINIMUM ON TIME
To maintain the proper oil level
compressors must cycle on for a
minimum of 5 minutes.
The cycle rate must not exceed 6
starts per hour.
WARNING
1. Startup and service must be performed
by a Factory Trained Service
Technician.
2. The unit is for outdoor use only. See
General Information section for more
unit information.
3. Every unit has a unique equipment
nameplate with electrical, operational
and unit clearance specifications.
Always refer to the unit nameplate for
specific ratings unique to the model you
have purchased.
4. READ THE ENTIRE INSTALLATION,
OPERATION AND MAINTENANCE
MANUAL. OTHER IMPORTANT
SAFETY PRECAUTIONS ARE
PROVIDED THROUGHOUT THIS
MANUAL.
5. Keep this manual and all literature
safeguarded near or on the unit.
8
CB Base Model and Features Description
CB - B - 060 - 3 - B - 1 : A D 0 0 0 A 0
Series and
Generation
Design
Sequence
Unit
Size
Voltage
Compressor
Type
Number
of
Circuits
1 2 3 4 5 6
7
BASE MODEL
SERIES AND GENERATION
CB
REVISION
B = Design Sequence
UNIT SIZE
024 = 24 MBH - 2 Ton - Vertical Discharge
036 = 36 MBH - 3 Ton - Vertical Discharge
048 = 48 MBH - 4 Ton - Vertical Discharge
060 = 60 MBH - 5 Ton - Vertical Discharge
VOLTAGE
1 = 230V/1Φ/60Hz
2 = 230V/3Φ/60Hz
3 = 460V/3Φ/60Hz
4 = 575V/3Φ/60Hz
8 = 208V/3Φ/60Hz
9 = 208V/1Φ/60Hz
COMPRESSOR TYPE
B = R-410A Two Step Scroll Compressor
D = R-410A Variable Capacity Scroll Compressor
F = R-410A Two Step Scroll Compressor with Sound
Blanket
H = R-410A Variable Capacity Scroll Compressor
with Sound Blanket
NUMBER OF CIRCUITS
1 = One Circuit
FEATURE 1: AMBIENT CONTROL
0 = Standard (55°F Ambient)
B = Adjustable Fan Cycling (35°F Ambient)
D = Modulating Fan Pressure Control (35°F
Ambient)
F = Flooded Condenser Ambient Controls +
Option B
G = Flooded Condenser Ambient Controls +
Option D
FEATURE 2: REFRIGERATION
OPTIONS
0 = Standard - Split System Air Conditioner
A = External Hot Gas Bypass
B = Split System Heat Pump
D = Modulating Hot Gas Reheat
F = Options A + D
G = Options B + D
FEATURE 3: CONTROLS
0 = Standard - Terminal Block
A = Suction Pressure Transducer (F1- Wattmaster
Reheat)
H = Control Circuit Transformer
J = Variable Capacity Compressor Integrated
Controls
S = Suction Pressure Transducer (Not F1 Wattmaster Controls)
T = Control Circuit Transformer w/ Suction Pressure
Transducer
W = Wattmaster CB/F1 Controller
FEATURE 4: COIL PROTECTION
0 = Standard
A = Polymer E-Coated Coil
FEATURE 5: CABINET OPTIONS
0 = Standard – Louvered Panels
A = Wire Grille
D = Corrosion Protection Paint
G = Options A + D
FEATURE 6: WARRANTY
0 = Standard
A = Second to Fifth Year Extended Compressor
Warranty
FEATURE 7: TYPE
0 = Standard
X = Special Pricing Authorization
9
General Description
Improper installation, adjustment,
alteration, service or maintenance
can cause property damage,
personal injury or loss of life. Startup
and service must be performed by a
Factory Trained Service Technician.
WARNING
Coils and sheet metal surfaces
present sharp edges and care must
be taken when working with
equipment.
WARNING
The Clean Air Act of 1990 bans the
intentional venting of refrigerant as of
July 1, 1992. Approved methods of
recovery, recycling, or reclaiming
must be followed.
CAUTION
Failure to observe the following
instructions will result in premature
failure of your system and possible
voiding of the warranty.
WARNING
AAON® CB Series condensing units have
been designed for outdoor installation only.
Startup and service must be performed by a
Factory Trained Service Technician.
Codes and Ordinances
System should be sized in accordance with
the American Society of Heating,
Refrigeration and Air Conditioning
Engineers Handbook.
Installation of CB Series units must conform
to the ICC standards of the International
Mechanical Code, the International Building
Code, and local building, plumbing and
waste water codes. All appliances must be
electrically grounded in accordance with
local codes, or in the absence of local codes,
the current National Electric Code,
ANSI/NFPA 70 or the current Canadian
Electrical Code CSA C22.1.
Receiving Unit
When received, the unit should be checked
for damage that might have occurred in
transit. If damage is found it should be noted
on the carrier’s Freight Bill. A request for
inspection by carrier’s agent should be made
in writing at once.
Nameplate should be checked to ensure the
correct model sizes and voltages have been
received to match the job requirements.
The warranty card must be completed in full
and returned to AAON not more than 3
months after unit is delivered.
Storage
If installation will not occur immediately
following delivery, store equipment in a dry
protected area away from construction
traffic and in the proper orientation as
marked on the packaging with all internal
packaging in place. Secure all loose-shipped
items.
10
Direct Expansion (DX) Condensing Units
CRANKCASE HEATER
OPERATION
Units are equipped with compressor
crankcase heaters, which should be
energized at least 24 hours prior to
cooling operation, to clear any liquid
refrigerant from the compressors.
CAUTION
CB Series condensing units are factory
assembled and wired, including a full charge
of R-410A refrigerant for up to 25 feet of
line set. Systems with the modulating hot
gas reheat option will require refrigerant to
be field added because of the additional
refrigerant components and piping
associated with the system.
Refrigeration system includes factory
provided liquid line filter drier and fully
hermetic scroll compressor with a positive
pressure forced lubrication system.
Never cut off the main power supply to the
unit, except for servicing, emergency, or
complete shutdown of the unit. When power
is cut off from the unit, compressors using
crankcase heaters cannot prevent refrigerant
migration. This means the compressor may
cool down and liquid refrigerant may
accumulate in the compressor. Since the
compressor is designed to pump refrigerant
gas, damage may occur when power is
restored.
If power to the unit must be off for more
than an hour, turn the thermostat system
switch to “Off”, or turn the unit off at the
control panel, and then cut off the main
power supply. Leave the unit off until the
main power supply has been turned on again
for at least 24 hours. This will give the
crankcase heater time to clear any liquid
accumulation out of the compressor before it
is required to run.
Always control the system from the
thermostat, or control panel, never at the
main power supply, except for servicing,
emergency, or complete shutdown of the
unit.
Note: Low Ambient Operation
Units without a low ambient option, such as
condenser fan cycling or the 0°F low
ambient option, will not operate in the
cooling mode of operation properly when
the outdoor temperature is below 55°F. Low
ambient and/or air handling unit economizer
options are recommended if cooling
operation below 55°F is expected.
Note: Multiple Systems with Multiple
Thermostats
When several heating and cooling split
systems are used to condition a space all
thermostat switches must be set in either
heating mode, cooling mode or off. Do not
leave part of the systems switched to the
opposite mode. Cooling only systems should
be switched off at the thermostat during the
heating season.
Wiring Diagrams
Unit specific wiring diagram is laminated in
plastic and located inside the controls
compartment door.
11
Installation
AAON equipment has been designed for
quick and easy installation.
Locating Unit
CB Series condensing units are designed for
outdoor application and placement at ground
level or on a rooftop. Units must be placed
on a level and solid foundation that can
support the unit’s weight.
When rooftop mounted, a steel frame must
be provided that will support the unit above
the roof itself for load distribution.
When installed at ground level, a one-piece
concrete slab or composite condenser pad
should be used with footings that extend
below the frost line (a substantial base that
will not settle). Slab should be surrounded
by a graveled area for proper drainage and
should not adjoin the building as sound and
vibration may be transmitted to the
structure. Care must also be taken to protect
the coils and fins from damage due to
vandalism or other hazards.
Airflow to and from the condensing unit
must not be restricted. Coils and fans must
be free of any obstructions and debris in
order to start and operate properly with a
correct amount of airflow. Obstruction to air
flow will result in decreased performance
and efficiency.
The installation position must provide at
least one foot of clearance from the wall for
proper air flow to the coils. When multiple
units are mounted adjacent to each other, the
clearance required between them is three
feet.
Service compartment must be accessible for
periodic servicing of controls, safety
devices, and refrigerant service/shutoff
valves. At least two feet of clearance on this
corner of the unit is recommended for
service.
Figure 1 – Condensing Unit Clearances
Condensing units should not be installed in
an enclosure or pit that is deeper than the
height of the unit. When recessed
installation is necessary, the clearance to
maintain proper airflow is at least three feet.
CB Series condensing units are single
circuited with vertical air discharge. There
must be no obstruction above the equipment.
Do not place the unit under an overhang.
Placement relative to the building air intakes
and other structures must be carefully
selected. Consider the effects of outdoor fan
noise on conditioned space and any adjacent
occupied space. It is recommended that the
unit be placed so that discharge does not
12
blow toward windows less than 25 feet
Design
Temperature
Suggest Minimum
Elevation
+15 F and above
3”
-5 F to +17 F
8”
Below -5 F
12”
Incorrect lifting can cause damage to
the unit.
CAUTION
Electric shock hazard. Before
attempting to perform any installation,
service, or maintenance, shut off all
electrical power to the unit at the
disconnect switches. Unit may have
multiple power supplies. Failure to
disconnect power could result in
dangerous operation, serious injury,
death or property damage.
WARNING
away.
Heat pumps require special location
consideration in areas where snow
accumulation can become an obstruction
and in areas with prolonged continuous
subfreezing temperatures. Heat pump unit
bases are cutout under the outdoor coil to
permit drainage of frost accumulation. The
unit must be situated to permit free
unobstructed drainage of the defrost water
and ice. A minimum 3 inches clearance
under the outdoor coil is required in the
milder climates. In more severe weather
locations, it is recommended that the unit be
elevated to allow unobstructed drainage and
airflow.
Table 1 - Recommended Elevation
Minimums
Lifting the Unit
CB Series condensing units have channels
underneath the base which provide lifting
access to the underside of the equipment and
allow moving and placement without
physical damage.
Dollies and/or carts are recommended to lift
and place the unit to prevent damage to the
equipment and injury to the installer.
Care should be taken if using spreader bars,
blocking, or other lifting devices to prevent
any damage to the cabinet, coil or
condensing fans.
Before lifting unit, be sure that all shipping
material has been removed from unit.
Hoist unit to a point directly above the
condenser pad, and lower unit into the
proper place. Unit may also be positioned
with a dolly. When the unit is in place,
remove the dolly or lifting device. Make
sure the unit is properly seated and level.
Electrical
Verify the unit nameplate agrees with the
power supply. Connect power and control
field wiring as shown on the unit wiring
diagram provided with the unit.
Route power and control wiring through the
utility entries on the unit. It is recommended
not to run power and control signal wires in
the same conduit. If the control wires are to
run inside the same conduit, 600 volt wires
should be used, or as required by applicable
codes.
Size supply conductors based on the unit
MCA rating. Supply conductors must be
rated a minimum of 75°C.
Protect the branch circuit in accordance with
code requirements. The unit must be
13
electrically grounded in accordance with
Scroll compressors are directional
and will be damaged by operation in
the wrong direction. Low pressure
switches on compressors have been
disconnected after factory testing.
Rotation should be checked by a
qualified service technician at startup
using suction and discharge pressure
gauges and any wiring alteration
should only be made at the unit
power connection.
Three phase voltage imbalance will
cause motor overheating and
premature failure.
CAUTION
Rotation must be checked on all
MOTORS AND COMPRESSORS of
three phase units. Condenser fan
motors should be checked by a
qualified service technician at startup
and any wiring alteration should only
be made at the unit power
connection.
CAUTION
CAUTION
local codes, or in the absence of local codes,
the current National Electric Code,
ANSI/NFPA 70 or the current Canadian
Electrical Code CSA C22.1.
Note: Units are factory wired for 208V,
230V, 460V or 575V. In some units, the
208V and 230V options may also be
provided in single or three phase
configurations. The transformer
configuration must be checked by a
qualified technician prior to startup.
Wire power leads to the unit terminal block
or compressor contactor. All wiring beyond
this point has been done by the manufacturer
and cannot be modified without effecting the
unit's agency/safety certification.
All units require field supplied electrical
overcurrent and short circuit protection.
Device must not be sized larger than the
Maximum Overcurrent Protection (MOP)
shown on the unit nameplate.
Codes may require a disconnect switch be
within sight of the unit.
It is recommended that the field installed
overcurrent protection or disconnect switch
not be installed on the unit.
Supply voltage must be within the min/max
range shown on the unit nameplate.
Available short circuit current should not
exceed the short circuit current rating
(SCCR) shown on the unit nameplate.
Three phase voltage imbalance will cause
motor overheating and premature failure.
The maximum allowable imbalance is 2.0%.
Voltage imbalance is defined as 100 times
the maximum deviation from the average
voltage divided by the average voltage.
Example:
(221V+230V+227V)/3 = 226V, then
100*(226V-221V)/226V = 2.2%, which
exceeds the allowable imbalance.
Check voltage imbalance at the unit
disconnect switch and at the compressor
terminal. Contact your local power company
for line voltage corrections.
Installing contractor must check for proper
motor rotation and check blower motor
amperage listed on the motor nameplate is
not exceeded.
14
Wire control signals to the unit’s low
This section is for information only
and is not intended to provide all
details required by the designer or
installer of the refrigerant piping
between the condenser or
condensing unit and the air handling
unit. AAON is not responsible for
interconnecting refrigerant piping.
Consult ASHRAE Handbook –
Refrigeration and ASME Standards.
CAUTION
voltage terminal block located in the
controls compartment.
If any factory installed wiring must be
replaced, use a minimum 105°C type AWM
insulated conductors.
Thermostat
If a thermostat is used for unit control,
thermostat should be located on an inside
wall 4-5 feet above the floor where it will
not be subjected to drafts, sun exposure, or
heat from electrical fixtures or appliances.
Follow thermostat manufacturer’s
instructions for general installation
procedure.
Thermostat control wiring size must be large
enough to prevent excess voltage drop and
ensure proper operation.
All external devices must be powered via a
separate external power supply.
Units with the modulating hot gas reheat
dehumidification feature must use a
humidistat or a thermostat with a normally
closed (NC) dehumidification option. The
following thermostats have been approved
for usage with the dehumidification feature,
Robertshaw® 9825i2 Thermostat
Honeywell VisionPRO® IAQ Thermostat
AAON Mini Controller
Refrigerant Piping
(See back of the manual for refrigerant
piping diagrams.)
General
Piping from the condensing unit to the air
handler is the responsibility of the installing
contractor.
Use only clean type “ACR” copper tubing
that has been joined with high temperature
brazing alloy.
The pipe or line sizes must be selected to
meet the actual installation conditions and
not simply based on the connection sizes at
the condensing unit or air handler.
All CB Series condensing units are provided
with in-line shutoff valves on both the liquid
and suction lines. These should remain
closed until the system is ready for start-up
after piping and vacuuming.
Piping should conform to generally accepted
practices and codes.
Upon completion of piping connection, the
interconnecting piping and air handler
MUST BE evacuated to 500 microns or less;
leak checked and charged with refrigerant.
15
Determining Refrigerant Line Size
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
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,
reducing installation costs, and reducing the
potential for leakage. However, as pipe
diameters narrow, pressure-reducing
frictional forces increase.
Excessive suction line pressure drop causes
loss of compressor capacity and increased
power usage resulting in reduced system
efficiency. Excessive pressure drops in the
liquid line can cause the liquid refrigerant to
flash, resulting in faulty Thermal Expansion
Valve (TXV) operation and improper
system performance. In order to operate
efficiently and cost effectively, while
avoiding malfunction, refrigeration systems
must be designed to minimize both cost and
pressure loss.
Equivalent Line Length
All line lengths discussed in this manual,
unless specifically stated otherwise, are
Equivalent Line Lengths. The frictional
pressure drop through valves, fittings, and
accessories is determined by establishing the
equivalent length of straight pipe of the
same diameter. Always use equivalent line
lengths when calculating pressure drop.
Special piping provisions must be taken
when lines are run underground, up vertical
risers, or in excessively long line runs.
Liquid Line Sizing
When sizing the liquid line, it is important to
minimize the refrigerant charge to reduce
installation costs and improve system
reliability. This can be achieved by
minimizing the liquid line diameter.
However, reducing the pipe diameter will
increase the velocity of the liquid refrigerant
which increases the frictional pressure drop
in the liquid line, and causes other
undesirable effects such as noise.
Maintaining the pressure in the liquid line is
critical to ensuring sufficient saturation
temperature, avoiding flashing upstream of
the TXV, and maintaining system
efficiency. Pressure losses through the
liquid line due to frictional contact, installed
accessories, and vertical risers are
inevitable. Maintaining adequate subcooling at the condenser to overcome these
losses is the only method to ensure that
liquid refrigerant reaches the TXV.
Liquid refrigerant traveling upwards in a
riser loses head pressure. If the evaporator is
below the condenser, and the liquid line
does not include risers, the gravitational
force will increase the pressure of the liquid
refrigerant. This will allow the refrigerant to
withstand greater frictional losses without
the occurrence of flashing prior to the TXV.
A moisture-indicating sight glass may be
field installed in the liquid line to indicate
the occurrence of premature flashing or
moisture in the line. The sight glass should
not be used to determine if the system is
properly charged. Use temperature and
pressure measurements to determine
liquid sub-cooling, not the sight glass.
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