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SYS-APM001-EN Chiller System Design and Control 13
the series, or another pumping arrangement can be considered. Reducing the
flow rate affects this system type’s energy use all the time, so careful
attention to flow rates and temperature is critical (refer to “System Design
Options” on page 27).
Primary-secondary system
In this configuration (Figure 13), the distribution piping is decoupled from the
chiller piping and is known as the primary-secondary or decoupled system.
There is constant primary flow through the operating chiller(s) and variable
secondary flow through the loads. A bypass pipe between the two balances
the primary flow with the secondary flow. Because there are more pumps
and a bypass, this system costs more than a constant flow system to install.
Details on this system type are in “Primary–Secondary (Decoupled) Systems”
on page 45.
Variable-primary system
This pumping arrangement (Figure 14) was made possible in recent years by
advanced chiller controls that permit varying the flow through the chillers.
Like a constant flow system, the distribution piping is directly connected to
the chiller piping. Flow is varied through at least most of the loads and the
chillers. A smaller bypass (compared to the primary-secondary system)
ensures chiller minimum flow rates are avoided. Fewer pumps and smaller
bypass lead to lower first costs compared to the primary-secondary system.
Operation costs can also be lower, but the plant is controlled differently than
in other pumping arrangements and operator training is essential. This
system type is covered in detail in “Variable-Primary-Flow Systems” on
page 55.
Condenser-Water System
As in chilled-water distribution systems, condenser-water system piping—
most commonly steel, copper, or plastic—is sized to meet a project’s
operating pressure, pressure loss, water velocity, and construction cost
parameters. Pressure drop through piping and the chiller’s condenser, plus
the cooling tower static lift, is overcome by use of a condenser-water pump.
To ensure optimum heat transfer performance, the condenser-heat transfer
surfaces must be kept free of scale and sludge. Even a thin deposit of scale
can substantially reduce heat transfer capacity and chiller efficiency. Specifics
of cooling-tower-water treatment are not discussed in this manual. Engage
the services of a qualified water treatment specialist to determine the level of
water treatment required to remove contaminants from the cooling tower
water.
Cooling tower
To reject heat, water is passed through a cooling tower where a portion of it
evaporates, thus cooling the remaining water. A particular cooling tower’s
effectiveness at transferring heat depends on water flow rate, water
temperature, and ambient wet bulb. The temperature difference between the
~
Figure 13. Primary-secondary system
CV
Pump
CV
Pump
Bypass (Decoupler)
VV
Pump
Load
Chillers
Two-Way
Control
Valve
~
~
Figure 14. Variable-primary system
VV
Pump
VV
Pump
Chillers
Minimum Flow Bypass Valve
Load
Two-Way
Control
Valve