ASHRAE Journal - March 2014 - 28

Technical FeaTure

To Load

Cell 3
Fan at Full Speed

Water at 50°F

Water
Entering at
60°F
Water Out
at 50°F

Figure 4 Cell with single fan off has higher leaving cold water temperature than the

other two.

Courtesy of sPX Cooling teChnologies

this cycling can lead to a potential for freezing in individual
Cell 1
Cell 2
Cell 3
Water
cells, using a 30°F wet-bulb temperature in the example.
Fan Off
Fan at Full Speed
Fan at Full Speed
Entering at
60°F
Water temperatures with fans in all three cells running
are equal as shown in Figure 3. Water temperatures with
Water Out
Water at 57°F
Water at 46.5°F
Water at 46.5°F
fans in two cells on are shown in Figure 4. Temperatures
at 50°F
leaving two of the cells are below the return temperature
back to the chiller.
Figure 5 single cell with fan running has substantially lower leaving cold water
Water temperatures with only the fan in one cell runtemperature than the other two.
ning are shown in Figure 5. Water could be freezing in
areas within Cell 3 as will be shown later even though
the average discharge from the cell is 40°F (4°C), while
the average temperature back to the chiller, and likely
Cell 1
Cell 2
Cell 3
Water
the only temperature monitored, is 50°F (10°C).
Fan Off
Fan Off
Fan at Full Speed
Entering at
60°F
While fan cycling and/or two-speed motors have been
used in the past, variable frequency drives (VFDs) can
Water Out
Water at 55°F
Water at 55°F
Water at 40°F
eliminate the cell-to-cell temperature gradients, and
at 50°F
are preferred especially when operating cooling towers
in sustained freezing conditions.
Figure 7 Counterflow; water sprays downward,
Each cell should be equipped with
Figure 6 Crossflow; water flows down through the
flows downward through the fill, and air flows in from
fill from the hot water basin on top, air flows horizona VFD, and they should all operate
sides and up through the fill. Water flow is counter
tally across the water path.
to airflow.
at the same setpoint temperature.
VFDs are the most energy-efficient
method of operation as well, as
discussed in last month's article.
Standard 90.1-2013 (Paragraph
6.5.5.2.2b) requires ramping up
and down the speed of VFDs on all
cells together for the most energyefficient operation, which is also the
best freezing resistant operation of
fans.
Types of Cooling Towers. Some differences in cold
and counterflow cooling tower designs. The two basic
weather operation characteristics exist between crossflow cooling tower configurations are shown in Figures 6 and 7.
28

ASHRAE JouRnAl

ashrae.org

March 2014

Courtesy of sPX Cooling teChnologies

From Load

Makeup Water

Cell 2
Fan at Full Speed

Courtesy of sPX Cooling teChnologies

Cooling Tower

Courtesy of sPX Cooling teChnologies

Cell 1
Fan at Full Speed

Courtesy of sPX Cooling teChnologies

Figure 3 even discharge temperatures from cell to cell with all fans running at the
same fan speed.

Courtesy of sPX Cooling teChnologies

Figure 2 Bypass directly to the cold water basin when water temperature falls below
manufacturer's stated minimum in freezing conditions and fans are already off.

ASHRAE Journal - March 2014

Table of Contents for the Digital Edition of ASHRAE Journal - March 2014