ASHRAE Journal - August 2014 - 48

criteria to meet the loads are standard selections, with
5 or 6 rows, and 12 to 14 fins per inch (fpi). These coils
were selected with a 9°F (5.0°C) approach. Note that the
approach temperature at economizer conditions is significantly lower, primarily because of the lower enthalpy
of the incoming mixed air.
Selecting a coil with much more heat transfer surface,
8 rows, 10 fins, and at a much lower face velocity, 400
fpm (4.5 m/s), results in a slight reduction in static pressure drop across the coil, but also a significant decrease
in coil approach temperature at economizer conditions.
Heat transfer area density, however, is constrained
by ASHRAE Standard 62.1, which limits dry coil pressure drop at a face velocity of 500 fpm (2.54 m/s) to
0.75 in. w.c. (187 Pa) as a surrogate measure to ensure
cleanability.3
Considering just design conditions, selection of a more
robust coil might offer the opportunity for reduction
in the chilled water flow rate, due to increased chilled
water temperature rise through the coil. The designer
should verify that the reduced maximum chilled water
flow through the coil will provide a sufficiently low
approach temperature so that the required supply air
temperature goal can be met during economizer conditions. For many buildings, meeting the energy code
economizer requirement will require a more robust
coil selection than is required to meet standard design
conditions.
Plate and frame heat exchanger approach is another
variable in the total approach from outdoor air wet-bulb
temperature to supply air temperature. According to
Taylor's recent column, the most cost-effective selections
for plate and frame heat exchangers is a 3°F (1.7°C).1
As will be shown later, this selection may be possible in
some climates, and may represent a cost trade-off with
the cooling tower selection. In other climates, where
further reduction in cooling tower approach is space or
cost prohibitive, a 2°F (1.1°C) selection may be required.

Cooling Tower Performance
While total interior sensible load has a great impact
on the operation of the airside system, total coil load
fraction has a great impact on cooling tower approach.
As has been reported in several recent ASHRAE Journal
articles, cooling tower approach at low wet-bulb temperatures increases significantly above that at design
conditions, depending on the heat rejection loads on the
48

ASHRAE JouRnAl

ashrae.org

Au g ust 2014

table 3 Range and cooling tower supply temperature at 45°F (7.2°C) wet bulb
for various cooling tower selections. (Based on 50°F/45°F [10°C/7.2°C] part
load fractions in different cities.)

City
Design WB
Zone

Design Range and
Approach Criteria

10°F*
7°F

14°F
7°F

10°F*
4.5°F

10°F
4.5°F

Atlanta
78°F
3A

Range (°F)
Supply Temp (°F)

5.08
50.5

5.08
52.2

5.08
48.8

5.08
49.9

Los Angeles
70°F
3C

Range (°F)
Supply Temp (°F)

6.76
50.4

6.76
52.1

6.76
48.7

6.76
49.9

Bakersfield
74°F
3B

Range (°F)
Supply Temp (°F)

4.88
49.8

4.88
51.2

4.88
52.1

4.88
49.0

New York City
78°F
4A

Range (°F)
Supply Temp (°F)

7.83
53.0

7.83
55.5

7.83
50.6

7.83
52.3

Seattle
66°F
4C

Range (°F)
Supply Temp (°F)

9.65
51.7

9.65
53.7

9.65
49.3

9.65
50.7

Chicago
78°F
5A

Range (°F)
Supply Temp (°F)

7.92
53.1

7.92
55.6

7.92
50.7

7.92
52.4

Denver
65°F
5B

Range (°F)
Supply Temp (°F)

9.52
51.7

9.52
53.6

9.52
49.2

9.52
50.7

*Water flow for 10°F (5.6°C) cooling tower selections reduced to 71% at 50°F/45°F
(10°C/7.2°C) condition to optimize heat exchanger flow.

tower.4,5 The total coil load is a function of both the interior sensible load and the latent and sensible load of the
required outdoor air fraction.
At the 50°F/45°F (10°C/7.2°C) economizer setpoint,
the coil is likely dry, because the available water from
the cooling tower is warmer than the mixed air dewpoint temperature. The reduction of coil total load
for 50°F/45°F (10°C/7.2°C) conditions compared with
design conditions is, therefore, a function of the reduction in interior sensible load caused by envelope heat
transfer at the lower outdoor air temperature and by
the reduced enthalpy of the mixed air onto the cooling
coil.
Cooling tower selection at design conditions for range
and approach temperature will have a significant impact
on the temperature of the water off the cooling tower at
the 50°F/45°F (10°C/7.2°C) condition. Table 3 shows the
performance of some selections that were made with
Cooling Technology Institute compliant software at the
50°F/45°F (10°C/7.2°C) condition.
ASHRAE Journal - August 2014

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