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SYS-APM001-EN Chiller System Design and Control 5
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 does not
necessarily have low velocity through its tubes, as discussed in the chapter
“System Design Options” on page 27. 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
Air-cooled chillers do not use condenser-water, since they reject their heat by
passing ambient air across refrigerant-to-air heat exchangers. In packaged
air-cooled chillers, the manufacturers improve performance by staging fans
in response to chiller load and ambient, dry-bulb temperature. Air-cooled
chillers can also be split apart. One technique is to use an indoor remote
evaporator with a packaged air-cooled condensing unit outdoors. Another
technique is to locate the compressor(s) and the evaporator indoors (also
known as a condenserless chiller) with an air-cooled condenser outdoors. It is
also possible to have an indoor air-cooled condenser.
Air-cooled versus water-cooled condensers
One of the most distinctive differences in chiller heat exchangers continues to
be the type of condenser selected—air-cooled versus water-cooled. When
comparing air-cooled and water-cooled chillers, available capacity is the first
distinguishing characteristic. Air-cooled condensers are typically available in
packaged chillers ranging from 7.5 to 500 tons [25 to 1,580 kW]. Packaged
water-cooled chillers are typically available from 10 to nearly 4,000 tons [35 to
14,000 kW].
Maintenance
A major advantage of using an air-cooled chiller is the elimination of the
cooling tower. This eliminates the concerns and maintenance requirements
associated with water treatment, chiller condenser-tube cleaning, tower
mechanical maintenance, freeze protection, and the availability and quality of
makeup water. This reduced maintenance requirement is particularly
attractive to building owners because it can substantially reduce operating
costs. However, see “Energy efficiency” below.
Systems that use an open cooling tower must have a water treatment
program. Lack of tower-water treatment results in contaminants such as
bacteria and algae. Fouled or corroded tubes can reduce chiller efficiency and
lead to premature equipment failure.
Packaged or Split System?
A number of different options are
available for packaging and splitting the
components of an air-cooled chiller. There
is an excellent discussion in Chilled-Water
Systems, part of the Air Conditioning
Clinic Systems Series (TRG-TRC016-EN).