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P a g e 5 2 • 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 5 3

C o n t r o l l i n g D R I - S T E E M h u m i d i f i e r s

Application determines acceptable RH control range

Controlling relative humidity (RH) in commercial and industrial

environments can be easy or challenging, depending on the level

of control required. RH fluctuations of 5% to 7% are common in

commercial or office building environments where the purpose

of providing humidification is primarily to improve occupant

comfort and health. Humans are quite forgiving when it comes to

RH fluctuations, and most would not notice a 5% change in RH.

Materials used in industrial processes, however, are much more

particular about humidity fluctuations. Since most materials are

hygroscopic in nature — they absorb and release moisture — many

processes, such as printing or food processing, require humidity to

be within a set range and to not fluctuate more than ±3%. ASHRAE

publishes tables listing the ideal RH for materials used in industrial

applications ranging from 80% RH for grinding optical lenses to

50% RH for manufacturing abrasives.

The closer to set point that

RH levels track, the more processing productivity improves.

Control the variables to control the humidity

Some process environments require extremely tight RH control,

and are the most challenging environments to humidify properly.

Semiconductor and pharmaceutical manufacturing facilities,

cleanrooms, laboratories, and testing facilities typically require RH

control within 1% of set point. To achieve such tight control many

variables must be managed.

The most important variable to control is dry bulb temperature,

for as dry bulb fluctuates so does RH. (A 1 °F drop in temperature

causes a 2-3% increase in RH, and a 1 °C drop in temperature

causes a 3-4% increase in RH.) Key to controlling temperature

is careful attention to air handling system design. Moisture

containment, accomplished with vapor barriers and proper

pressurization, is also important, as are the number of air changes

per hour. As the number of air changes increases, humidifier output

fluctuations become more apparent in the humidified space. Other

variables affecting RH control are controller capabilities, sensor type

and placement, dispersion assembly placement, location of duct

components, and varying duct temperatures. This section of the

Design Guide explains how water type, water replenishment, and

energy source control affect RH control.

Notes

1995 ASHRAE Handbook, HVAC

Applications,

I-P Edition, pages 11-2, 11-3.

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