AAF HermanNelson IM 725 Installation Manual - page 23
IM 725 (1-02)
23
Figure 75 - 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 flow), see figure 75 and table 17.
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 75.
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 75.
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 79.
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 79.
Install Low Ambient Temperature Switch at condensing unit
wired in the condenser control system (locks out mechanical
cooling below 60°F - proper system operation and free economizer
usage), see figure 79.
Incorporate Compressor Time Delay (5 minute) in condensing
unit control system (reduces excessive compressor cycling), see
figure 79.
Incorporate Low Refrigerant Temperature Sensor (T4) in
condensing unit control system (T4 protects the system under
low refrigerant suction conditions) see figure 79.
UV fans must continue to run upon Low Refrigerant Temperature
trip of T4 (evaporator air flow dissipates residual low coil surface
temperatures - suction pressures raised, coil frosting reduced),
see figure 79.
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 79.
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 74.
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 75
Insulate the TXV bulb (reacts to refrigerant temperatures and not
ambient), see figure 77.
Insulate the suction line piping (minimum heat pickup), see
figure 75 and 77.
Evacuate and properly charge the refrigerant system, see figure
78.
Charge to subcooling at the condensing unit per the condensing
unit manufacturer’s instructions, typically 9°F to 12°F subcooling
at 95°F 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 80.
Set superheat to 5°F to 7°F at the UV coil suction line when 95°F
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 19.
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 79.
Figure 76 - TXV Piping Detail & Descriptions (Left Hand Piping Shown)
TXV Superheat Adjustment
TXV (Thermal Expansion Valve)
Return (Suction Line)
Bulb
attached in the 2 or
10 o'clock position
Equalization Tube
Drain Cap
Supply (Liquid Line)