Parameter's Approach to Relative Humidity Control August 25, 2011 by Clay Hile

Main Concepts  Different methods of humidity control  Air flow requirements  PGC’s two-temperature method  Advantages and limitations

Steam Injection DI Water Source Water Boiler To Room or Chamber

Desiccant Dryer Dry applications (set points with below freezing dew points)

Two Pressure System  Precise calibration applications for small volumes  NIST method  Can generate low dew points

Classic Two- Temperature Control

Accurate Precise control Uniformity The need for uniformity in larger areas requires more air flow.

Control areas from 2 cu.ft. to 45,000 cu.ft.

Uniformity Requires elevated air flows Typical Application: Dew point is constant Humidity differences across a room or chamber are generally temperature driven

Constant Dew Point Temp. goes up, RH goes down Temp. goes down, RH goes up

Walk-in Plan View Return 73°F/50%RH Supply 300 CFM 10 x 12 x 10 Room (1200 cube)

Room with Load 73°F/50%RH Supply Return 3000 BTU Heat Source 82°F

Constant Dew Point (53.3°F) / 15 Air Changes per Hour 73°F/50%RH Supply Return 3000 BTU Heat Source 82°F/37% RH

Improving Uniformity  Reduce internal load  Location of load  Increase air flow

Increased Air Flow

Parameter’s Two-Temperature Control (PGC’s Method)

Dehumidification 53° DP

Humidification

RH Compensation

Two-Temperature Control  Control dew point by controlling water spray temperature  Control relative humidity by maintaining a tight air temperature control

 Accurate – Sensor Dependent  Precise Control – Two- temperature method  Uniformity – Higher air flow and good distribution

Ability of Two-Temperature Control

Calibration  PGC provides the control and uniformity  Chilled Mirror or other reference sensor provides accuracy.

Limitations to the Parameter “two temperature”  Range – dew point must be above freezing.  Ramp Rates – larger saturator water volumes stabilize control but increase the time to ramp dew points.  Requires a humidity sensor.  Is not a primary standard.

Conclusion Two temperature method is a proven method to maintain precise humidity control in larger areas.