Unit Ventilator Split Systems Guidelines - Daikin AVS Installation Manual

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 flow), see figure 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 figure 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 figure 88.
•
Install pressure taps on the unit ventilator's liquid line and suction
lines for subcooling and superheat measurements at the unit
ventilator, see figures 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
figure 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
figure 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 figure 90.
•
Incorporate Compressor Time Delay (5 minute) in condensing
unit control system (reduces excessive compressor cycling), see
figure 90.
•
Single phase compressors - consider hard start kits to overcome
non-equalized pressure in refrigerant lines.
Figure 87 - Typical Piping and Wiring for Split System
Weatherproof Fused Disconnect Per NEC
Filter Drier (Field Supplied)
Moisture Indicator Sight
Glass (Field Supplied)
TXV
(Thermal Expansion
Valve)
To Open Sight Drain
Page 30 of 60
2 Wires (+) Ground
Insulated
Suction Line
Fused
Disconnect
2 Wires
(+) Ground
•
Incorporate Low Refrigerant Temperature Sensor (T4) in
condensing unit control system (T4 protects the system under low
refrigerant suction conditions) see figure 90.
•
UV fans must continue to run upon Low Refrigerant Temperature
trip of T4 (controls by others) or S4 (MicroTech II) (evaporator
air flow dissipates residual low coil surface temperatures - suction
pressures raised, coil frosting reduced), see figure 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
figure 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 figure 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 figure 86.
•
Insulate the TXV bulb (reacts to refrigerant temperatures and not
ambient), see figure 88.
•
Insulate the suction line piping (minimum heat pickup), see figure 88.
•
Evacuate and properly charge the refrigerant system, see figure 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 figure 91.
•
Set superheat to 5ºF to 7
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 baffles to condensing unit or build a
parapet (eliminate wind effect on condensing unit coil for proper
TXV refrigerant flow 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 flow at lower ambient),
see figure 90.
o
F at the UV coil suction line when
IM 817-4