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Chilled Water Temperature & Flow Most large buildings use air handlers with chilled water coils.
Historically, chilled water has been, supplied to these air handlers at ~
44°F on the warmest days of the summer and would return to the chiller at
about ~ 54 (i.e., 10°F warmer). Every ton of cooling delivered this way
requires 2.4 gpm of water flow. Design professionals today have a multitude
of alternatives available to them, including:
1. Increasing the difference between supply
and return chilled water temperatures (called the chilled water range) to
reduce chilled water pipe sizes and pumping power.
2. Increasing the coil surface area to
permit higher chilled water supply temperatures,
3. And using lower temperature chilled
water, perhaps in the 36 - 38°F range, to produce much colder supply air,
thereby reducing air handler air flows, fan power, and duct sizing (which
can even reduce building height).
These factors can impact energy use in
complex ways. For example, distributing low temperature chilled water is
often combined with increasing the chilled water range (e.g., supplying 38°F
water and returning 58°F water). The chilled water flow is now only 1.2 gpm
per ton, providing significant savings in chilled water distribution piping
and pumps. However, producing 38°F water potentially requires more power and
more expensive chiller designs. But, then again, maybe not. Certain building
designs (such as churches, theaters, and operating room suites) can be
"naturals" for ice storage. Similarly, chilled water at 40 - 42°F can
sometimes achieve similar benefits.
All of these tradeoffs are complex and
obviously fall within the domain of the design professional. Each design
requires careful analysis, consideration of current and future building use,
operating personnel qualifications, and the issues of initial investment and
operating cost. This information is simply an explanation of some of the
options available. Please refer to the specific cooling situation analyses
elsewhere in this information system for further information.
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