This same detailed equation can be generalized
to another common loop volume sizing
method: loop time. As a general guideline, a
minimum 3-minute loop time is required for
the evaporator chilled water system. In a
typical water-cooled application, the nominal
fluid flow through the evaporator is 2.4
gpm/ton.
At nominal flows, a 3-minute loop time is
equivalent to a minimum loop volume of 7.2
gallon/ton (2.4 gpm/ton x 3 min = 7.2
gallon/ton) and would have an allowable
leaving fluid temperature swing of +/- 4.2°F
At nominal flows, a 2-minute loop time is
equivalent to a minimum loop volume of 4.8
gallon/ton. (2.4 gpm/ton x 2 min = 4.8
gallon/ton) and would have an allowable
leaving fluid temperature swing of +/- 6.25°F.
Notice, in the preceding example, if this
system was selected for a 45°F leaving water
temperature, the temperature will vary
between 42°F to 48°F (recall the variation
tolerance +/- 3°F) with the cycling of the
compressors at the water loop volume of 641
gallons. The final selection should ensure the
leaving water temperature does not drop below
42°F (or 40°F if using a high-capacity
evaporator). If a leaving water temperature
below 42°F (or 40°F if using a high-capacity
evaporator) is indicated then the loop volume
should be increased or glycol should be
included with the design.
Glycol
If the fluid loop contains glycol, the required
water loop volume should be multiplied by the
correction factor in Table 3.
Table 7 Glycol Correction Factors
It may be necessary to install a storage tank in
the system to provide the necessary volume for
close temperature control. When this is done,
the tank should be installed in the loop between
the fluid leaving from the chiller and the
supply to the building. Figure A1 illustrates a
proper expansion tank usage.
Figure 14 Expansion Tank Usage
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General