Daikin AVS - Unit Ventilator Split Systems Guidelines

Page 30 of 60 IM 817-4
Figure 87 - Typical Piping and Wiring for Split System
2 Wires (+) Ground
To Open Sight Drain
TXV
(Thermal Expansion
Valve)
Filter Drier (Field Supplied)
Moisture Indicator Sight
Glass (Field Supplied)
Weatherproof Fused Disconnect Per NEC
2 Wires
(+) Ground
Insulated
Suction Line
Fused
Disconnect
Unit Ventilator Split Systems Guidelines
The following provides basic guidelines that will provide proper system
cooling and operation of an R-22 commercial DX/hot water system
for school applications. DX system components must be matched and
sized correctly (not oversized) for the load.
The DX system must incorporate the following, provided by others,
for proper operation:
• Size piping per ASHRAE Refrigeration Handbook (correct
refrigerantandcompressoroilow),seegure88andtable21.
• Use clean sealed refrigerant grade piping (prevent system
contamination)
• Install Liquid Line Filter Dryer (clean/dry system to prevent
damageofoperatingcomponents),seegure89.
• Install Liquid Line Sight Glass (indicates refrigerant dryness and
if liquid in liquid line - do not use the sight glass to determine
when refrigerant system is charged correctly
),seegure88.
• Install pressure taps on the unit ventilator's liquid line and suction
lines for subcooling and superheat measurements at the unit
ventilator,seegures88and89.
• Install High Pressure Switch at condensing unit wired in condenser
control system (protects compressor and refrigerant system from
excessive pressures - condenser fan failure or overcharging), see
gure90.
• Install Low Pressure Switch at condensing unit wired in the
condenser control system (low refrigerant pressure switch
protects the system under low refrigerant suction conditions), see
gure90.
• Install Low Ambient Temperature Switch at condensing unit wired
in the condenser control system (locks out mechanical cooling
below 60ºF - proper system operation and free economizer usage),
seegure90.
• Incorporate Compressor Time Delay (5 minute) in condensing
unit control system (reduces excessive compressor cycling), see
gure90.
• Single phase compressors - consider hard start kits to overcome
non-equalized pressure in refrigerant lines.
• Incorporate Low Refrigerant Temperature Sensor (T4) in
condensing unit control system (T4 protects the system under low
refrigerantsuctionconditions)seegure90.
• UV fans must continue to run upon Low Refrigerant Temperature
trip of T4 (controls by others) or S4 (MicroTech II) (evaporator
airowdissipatesresiduallowcoilsurfacetemperatures-suction
pressuresraised,coilfrostingreduced),seegure90.
• UV fans must continue to run for set time period during unoccupied
mode after satisfaction of the space sensor (dissipates residual low
evaporator coil surface temperatures - reducing coil frosting), see
gure90.
• Lock the Face and Bypass Damper (actuator spring return to full
face when de-energized) in the full face position during mechanical
cooling (full air through evaporator coil reduces low refrigerant
suction conditions, potential coil frosting)
• When Brazing bleed Nitrogen through piping (reduced oxides and
blockage in piping/TXV)
• Use Heat Sink when brazing to prevent overheating the TXV
valve body and bulb (avoid valve damage and erratic operation),
seegure86.
• Verify the TXV bulb securely attached at 2 or 10 o’clock for
7/8" and smaller diameter suction line piping (proper suction gas
sensingandreducedhunting)Seegure86.
• Insulate the TXV bulb (reacts to refrigerant temperatures and not
ambient),seegure88.
•  Insulatethesuctionlinepiping(minimumheatpickup),seegure88.
•  Evacuateandproperlychargetherefrigerantsystem,seegure89.
• Charge to subcooling at the condensing unit per the condensing
unit manufacturer’s instructions, typically 15ºF to 16ºF at the
unit ventilator, subcooling at 95ºF outdoor ambient (results in
correct refrigerant distribution at the coil to prevent low suction
temperatures)
• Adjust TXV for correct superheat to eliminate/minimize hunting,
seegure91.
• Set superheat to 5ºF to 7
o
F at the UV coil suction line when
95ºF outdoor ambient (proper system superheat for optimum
performance). Allow system to settle for 20 to 30 minutes to reach
stable steady state conditions and then recheck/adjust superheat
if necessary, see table 23.
• Compensate both subcooling and superheat for actual outdoor
ambient and indoor air temperatures
•  Inwindyareas,addwindbafestocondensingunitorbuilda
parapet (eliminate wind effect on condensing unit coil for proper
TXVrefrigerantowatlowerambient)
• For lower ambient conditions install variable speed condenser fan
head pressure control to maintain head pressures between 180psig
and280psig(forproperTXVrefrigerantowatlowerambient),
seegure90.