Factory Installed Components
(continued)
7. Condenser Fans
The unit is equipped with multiple direct-drive
condensing fans. The fans provide the necessary
airflow to cool the refrigerant in the condensing
coil. Depending on head pressure, some fans
may not be running, which is normal operation
for this unit and does not indicate a problem.
The motors are equipped with sling protection to
guard against water penetration, and are thermally
protected. The thermal overloads shut down the
condensing fans and will automatically reset.
8. High Limit Pressure Switch
To safely shut down the DX system in case of
an increase in refrigerant
pressure, a high limit pressure
switch is included. The switch
is located in the compressor
compartment. It trips when refrigerant pressure
increases to 575 psi for R-410A in the liquid line
and can only be manually reset when the pressure
drops below 420 psi. Typically, if the high pressure
limit switch trips, a failure in the system has
occurred and more investigation is needed to
determine the underlying problem.
9. Head Pressure Cycle Switc
h
To maintain proper refrigerant pressures, a Fan-
Cycle switch that operates on head pressure
is installed in the liquid line. When liquid line
refrigerant pressure increases to 400 psi for
R-410A, the switch closes and turns on an
additional condensing fan to aid in cooling the
refrigerant. After turning on, the fan will run until
liquid line pressure drops down to 300 psi for
R-410A and then it cycles off.
10. Liquid Line Filter Drier
The liquid line filter drier prevents
moisture and foreign matter from
entering the expansion valve.
11. Moisture Indicating Sight Glass
Moisture in a DX system can freeze in the system
or dilute the compressor oil. A moisture indicating
sight glass is provided for each refrigerant circuit.
During normal operating
conditions, the sight glass
should typically be liquid.
Some gas is acceptable,
but excessive bubbles may
indicate improper charge or a leak in the system.
A green dot means that moisture is below a safe
operation level in the refrigerant, while a yellow
dot indicates moisture has been introduced into
the system and needs to be dealt with.
For an Emerson brand sight glass, the color
purple indicates dry and the color pink indicates
wet.
12. Hot Gas Bypass Valve (optional)
On units equipped with hot gas bypass, hot gas
from the compressor is injected into the liquid line
of the evaporator coil after the TXV. This process
starts to occur when suction gas temperatures
drop below 28°F, which is 32°-34°F coil surface
temperature. Hot gas helps the evaporator coil
from freezing up and the compressor from cycling.
The valve is factory set, but can be adjusted to
exact specifications once installed in the field.
Valve Adjustment -
To adjust the valve, connect
a pressure gauge to the suction line and block the
entering air to the evaporator coil. The valve should
begin to open when the suction pressure drops to
approximately 115 PSIG for R-410A (the valve will
feel warm to the touch). Adjustments are made
by first removing the cap on the valve and then
turning the adjusting stem clockwise to increase
the setting pressure. Allow several minutes
between adjustments for the system to stabilize.
When adjustment is complete, replace the cap on
the valve.
13. Access Ports
For easy measurement and charging access,
several ports are provided through the system.
These can be used to measure system pressure
and also charge or evacuate the system. Most
ports are located in the compressor compartment
for easy access.
14. Reheat Valve (optional)
Units equipped with a reheat coil use a three-way
valve with actuator to
control the supply air
discharge temperature
of the unit during
dehumidfication
mode. The unit
controller provides a
0 - 10 VDC signal to
control the amount of
reheat to meet the supply temperature set point or
easy measurement and charging access.
15. Liquid Receiver (optional)
Acts as a reservoir for liquid refrigerant.
20
Model MPX Make-Up Air Unit
Model XMPX Make-Up Air Unit
