Chilled-Water-Plant Basics
SYS-APM001-EN
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conditions. Contact the manufacturer for these limits. Some chillers may allow
extended operation below the selected flow rates.
If water velocity through the condenser tubes is too low for significant periods
of time and the water is extremely hard, long-term fouling of the tubes may
also occur. Webb and Li
3
tested a number of internally-enhanced condenser
tubes at low velocity (3.51 ft/s [1.07 m/s]) and high water hardness. While they
found that some of the internally
-
enhanced tubes fouled, in the long term they
concluded:
Because of the high hardness and low water velocity used in these
tests, we do not believe that the fouling experienced is typical of that
expected in commercial installations. With use of good maintenance
practices and water quality control, all of the tubes tested are probably
suitable for long-term-fouling applications.
It is important to remember that a chiller selected for low flow (as discussed in
the next section Chilled-Water-System Options) does not necessarily have low
velocity through its tubes. If tube fouling is a major concern, consider the use of
smooth, rather than internally-enhanced, tubes in the condenser for ease of
cleaning.
Air-cooled condenser
Obviously, air-cooled chillers do not use condenser-water, since they reject their
heat by having ambient air passed across refrigerant-to-air heat exchangers. In
packaged air-cooled chillers, the manufacturers attempt to provide optimal
performance by staging fans in response to chiller load and ambient, dry-bulb
temperature.
Loads
In comfort-cooling applications, loads are usually satisfied by air handlers
equipped with coils to transfer heat from conditioned space air to circulating
chilled-water. Air is thus cooled and dehumidified as it passes across the finned
surface of the cooling coils. Since the psychrometric process of conditioning air
takes place at the coils, selection of the optimum coil size and type from the
wide variety available is important for proper system performance.