AAON LL-540 - Manual (Page 21)

21
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
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 and boiler.
This includes reading and recording suction
pressures and checking for normal sub-
cooling and superheat. See the evaporative-
cooled condenser and 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 evaporator barrel 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
chiller barrel leaves the unit through the
water out connection.
Chiller Primary/Secondary Pumping
Primary/secondary pumping option provides
variable flow to the system. It consists of a
constant flow pump for the chiller heat
exchanger and a variable flow pump for the
building. The controls package senses
differential pressure across the pump with
pressure transducers installed at the suction
and discharge, and varies the speed of the
pump using a VFD in order to maintain a
given differential pressure across the pump.
The primary/secondary pumping package is
essentially composed of two piping loops