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P a g e 8 • D R I - S T E E M D e s i g n G u i d e

D R I - S T E E M D e s i g n G u i d e • P a g e 9

Sample Problem 3

Determine the outside air quantity in an economizer cycle system

where:

• The outside air is 20 °F.

• The return (room) air is 70 °F.

• The mixed air is 55 °F.

• The total air is 12,000 cfm.

Solution to Sample Problem 3 using

the economizer cycle method

1. A = 55 °F – 20 °F = 35 °F

2. B = 70 °F – 55 °F = 15 °F

3. V

= 12,000 cfm

4. V

= V

/ [(A/B) + 1]

= 12,000 cfm / [(35 °F/15 °F) + 1]

= 3,600 cfm = outside air

To document outside air quantity at various points (typically 10 °F

intervals) over a temperature range of outside air, create a table

using the above formula. Table 9-1 shows percentages of outside air

at a consistent 70 °F room temperature and three different mixed

air temperatures.

Calculating maximum humidification

load in an economizer system

Determining maximum humidification load involves the use of

one of the three mixed air temperatures in Table 9-1 (or a similar

table developed from different mixed and return air temperature

conditions).

Also needed is the difference (lbs/hr/100 cfm) between the desired

moisture content of the air in the space and that contained in the

outside air. This difference is made up by the humidifier.

When calculating maximum humidification load in an economizer

system for noncritical applications, calculate load using the daily

minimum % RH for year by location. However, when calculating

maximum humidification load in an economizer system for critical

applications, to ensure that humidity set point can always be

met, calculate maximum humidification load using the extreme

minimum daily RH for year by location. Table 11-1 shows these

values for select cities.

Table 9-1:

Outside air percentages with

70 °F return air and various mixed

air temperatures

Outside

temp (°F)

Outside air percentage

of 50 °F

mixed air

of 55 °F

mixed air

of 60 °F

mixed air

-20 22 17 11

-10 25 19 13

0 29 21 15

10 33 25 17

20 40 30 20

30 50 38 25

40 67 50 34

50 100 75 50

55 — 100 67

60 — — 100

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 83 84

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