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41
variable primary flow and motorized isolation
valves on each chiller module.
The FWCD Series chillers contain 2
compressors per module, and can be
configured in arrays containing up to 20
compressors. Use the following example as a
guide to determine swing in fluid temperature
tolerable.
Use the information in Table 1 that lists the
maximum step of capacity in each array and a
factor for that model.
FLUID VOLUME EXAMPLE
Problem: An array of three (3) FWCD 45-ton
units has a total rated tonnage of 42.7 x 3 =
128.1 tons at the operating conditions. The
chilled water flow is constant volume. It is
desired to have no greater than a +/- 3°F
leaving water temperature variation due to
compressor unloading. What is the minimum
water volume required in the chilled water
loop?
Solution: We use the following equation to
determine the minimum allowable water loop
volume.
Minimum Water Loop Volume = Actual
Tons x (Min. Volume Gal-°F Swing/Ton) /
Allowable °F Swing
Allowable °F Swing is specified in the problem
statement. With a tolerance of +/- 3°F, the total
allowable swing is 6°F.
We select the value of Min. Volume Gal-°F
Swing/Ton from Table 5 Minimum Volume
based on the number of modules in the array
and the type of flow.
Table 5 Minimum Volume
* 25% is the lowest capacity step allowed for constant
flow arrays.
For a constant flow system, the minimum
capacity step is 25%, therefore the Minimum
Volume = 30 (Gal - °F deg F Swing)/ton
Thus, we can compute the minimum water
loop volume with the known performance of
128.1 tons of cooling at the application
conditions:
Minimum Water Loop Volume = 128.1 tons x
(30 Gal - °F Swing)/ton / 6 °F swing = 641
gallons.
Using the minimum turndown of 25% for
constant volume systems, this equation can be
generalized to a commonly used guideline:
gallons per ton loop volume. By tabularizing
different allowable °F swings the minimum
volume on a gallon per ton basis can be
displayed.
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