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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

Economizer cycle method

Many year-round air conditioning systems use economizer

cycle control. Economizer cycles use cool outside air instead of

mechanical cooling to maintain building temperature when the

outside temperature is moderate (typically spring and fall).

Figure 8-1 shows a typical application where a mixed-air controller

positions a modulating damper motor, which adjusts the outside

air intake and return dampers. Note that the dampers are opposing

— as one moves toward open, the other moves toward closed, and

vice versa — to maintain a mixed air temperature of 55 °F as the

outside air temperature rises and falls.

When the outside air temperature rises to 55 °F (100% outside air),

the outside air damper returns to a minimum setting (usually about

10%) and mechanical cooling takes over. Table 9-1 shows how

outside air percentages change with varying outside and mixed air

temperatures.

As the outside temperature rises, the ratio of outside air to return

air increases. This works toward increasing the humidification load.

For example, to cool return air to 55 ˚F, you can admit 50%

outdoor air at 40 ˚F, but only 17% outdoor air at -20 ˚F (see Table

9-1). The warmer the outdoor air temperature, the more air volume

that can be admitted. More air volume means more air that needs

to be humidified, thereby increasing the humidification load

required to maintain RH set point.

To better understand how varying temperatures and air volumes

affect humidification load calculations, review the following

formulas, sample problems, and tables.

The formula for determining the quantity of outside air in an

economizer cycle is:

= V

/ [(A/B) + 1]

Where:

AH =

1 +

= Volume (cfm) of return air

= Volume (cfm) of outside air

A = Temperature difference between mixed air

and outside air

B = Temperature difference between return

air and mixed air

Outside air

Return air

Mixed air

Mixed air controller

set at 55 ˚F

Damper

arms

Damper motor

Damper

Figure 8-1:

Typical economizer control system

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