AAON LN-140 - Manual (Page 17)

17
Scroll compressors must be on a minimum
of 5 minutes and off for a minimum of 5
minutes. The cycle rate must be no more
than 6 starts per hour.
The chiller is furnished with a pressure
differential switch that is factory installed
between the chilled water supply and return
connections. This sensor must not be
bypassed since it provides a signal to the
unit controller that water flow is present in
the heat exchanger and the unit can operate
without the danger of freezing the liquid.
Compressor life will be seriously shortened
by reduced lubrication, and the pumping of
excessive amounts of liquid oil and liquid
refrigerant.
Wiring Diagrams
A complete set of unit specific wiring
diagrams in both ladder and point-to-point
form are laminated in plastic and located
inside the control compartment door.
General Maintenance
When the initial startup is made and on a
periodic schedule during operation, it is
necessary to perform routine service checks
on the performance of the chiller. This
includes reading and recording suction
pressures and checking for normal sub-
cooling and superheat. See the air-cooled
condenser sections in this manual for
specific details.
Chiller Primary Pumping
Primary pumping uses a single pump to
move water (or glycol) through the chiller
barrel and back to the building. This
pumping package provides a constant flow
of water to the system. The pump is
activated whenever the chiller is given a run
signal.
Water enters the unit through the return
water piping, and then travels through an air
separator to remove any air that is entrapped
in the water. Following this, the water flows
through a suction guide with strainer. The
end of the suction guide is removable for
strainer access. The strainer assembly is
composed of two parts, the operational
strainer and the startup strainer, (located
inside the operational strainer) which is to
be removed 24 hours after startup.
The pump is installed after the suction
guide, and before a combination valve (Flo-
Trex). This combination valve acts as
isolation valve, check valve, and flow
balancing valve. The shell and tube or
brazed plate evaporator, is placed after the
combination valve in the water circuit, with
a differential pressure switch installed across
its inlet and outlet. This pressure switch
closes when the differential pressure
increases above the setpoint, which should
be set 1-2 psig below the pressure drop
across the heat exchanger at design flow
rate. The closing differential pressure switch
signals the control system to indicate flow
through the heat exchanger and allow
cooling to activate as required to maintain
the setpoint. The water exiting the shell and
tube or brazed plate evaporator, leaves the
unit through the water out connection.
Automatic Air Vent
There is an automatic air vent installed at the
high point of the system inside the pumping
package compartment. The air vent valve
must be in the proper position for operation.
Ensure that the small vent cap is loosened
two turns from the closed position, allowing
air to be vented from the system. It is
advisable to leave the cap on to prevent
impurities from entering the valve. See
appendix for additional information.