AAON LL-540 - Manual (Page 22)

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coupled together. The primary loop has a
constant flow rate in order to keep the chiller
heat exchanger from freezing, and the
secondary, variable flow loop, provides
water to the building. The two loops are
coupled via a water line that compensates
for excess flow in either loop. As the flow in
the secondary loop decreases below the flow
in the primary loop, excess flow bypasses
the building loop and circulates through the
bypass water line. On the other hand, as the
flow in the secondary loop increases above
the flow in the primary loop, excess flow
bypasses the chiller and circulates through
the bypass water line.
The secondary pump has its own suction
guide, combination valve, and isolation
valve, similar to the primary pump, with the
addition of an air separator to remove any
air that is entrapped in the water.
Boiler System
Optional boilers and pumping packages are
factory installed. The boiler system uses a
primary/secondary pumping package. There
can be 1-4 boilers in parallel and each boiler
has its own primary pump. The heating loop
must be designed to return at least 120°F
water to the boiler during normal operation.
Failure to return 120°F water to the boiler
will create condensation, which will reduce
the life of the heat exchanger and void the
boiler warranty. See unit submittal for unit
specific piping schematics. See the Thermal
Solutions Boiler “Installation, Operating,
and Service Instructions” that are included
with the unit for additional information
about the boiler.
Once the boiler is given a run signal, the
boiler secondary pump will be activated and
the controls package will stage boilers as
necessary to maintain the leaving water
temperature setpoint.
The controls package will also control the
speed of the secondary pump in the boiler
system to maintain differential pressure
across the pump similar to the chiller
secondary pump.
Boiler Primary/Secondary Pumping
Water enters the unit through the return
water piping, and then travels 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 boiler is placed after
the combination valve in the water circuit.
The primary/secondary pumping package
provides variable flow to the system. It
consists of a constant flow pump for the
boiler, 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
coupled together. The primary loop has a
constant flow rate in order to maintain water
temperature through the boiler, and the
secondary, variable flow loop, provides
water to the building. The two loops are
coupled via a water line that compensates
for excess flow in either loop. As the flow in
the secondary loop decreases below the flow
in the primary loop, excess flow bypasses