1. Brian Wallingford, Applications Engineer
Controlling Dehumidification Systems
Brian Wallingford, Applications Engineer

2. Dehumidification System Design Process
1. Define the purpose of the project
2. Establish control levels and tolerances
3. Calculate heat and moisture loads
4. Select and position components
5. Select and locate controls

3. Controls Misc Humidity Control Temperature Control Airflow Control
Face and Bypass
Reactivation Energy Modulation
Humidistat
Humidifier
Temperature Control
Electric (Reactivation or Process Heating)
Steam (Reactivation or Process Heating)
Direct and Indirect Gas (Reactivation or Process Heating)
Water or Glycol (Process Heating or Cooling)
Ammonia (Process Cooling)
DX (Process Cooling)
Misc
Airflow Control

4. Methods of Humidity Control
Humidistat
On / off control
Space control
+/- 5% of control level
Small volume of make up air
Face and bypass
Modulating control
End of duct or space control
+/- 2% of control level
Make up air volume not an issue
Most energy efficient
Reactivation energy modulation
Slow response time
Higher energy consumption

6. Passive Storage- Archival
The purpose of the project is to prevent the fading of organic dyes in color film, and to prevent mold.
Control level recommendations from Kodak include:
0°F, 35% rh for 500-year storage
55 to 70°F, 30 to 40% rh for prevention of major damage
At 70°F, 35% rh, the moisture load profile looks like this:

7. Typical System for Archival Storage
Independent humidity and temperature control
Thermostat and humidistat control
DX or chilled water cooling

8. Product Drying- Candy Pan Coating
Purpose: To produce coated candy as quickly during the summer as during the winter, as with as high a level of quality.
Ideal control level is unknown—trouble starts in spring
Customer establishes delivered air condition and 50F and 50% RH
At 50°F, 50% rh, the load profile looks like this:

9. Typical Product Drying Schematic
End of duct control for humidity and temperature
Face and bypass for tight humidity control
Control valves for discharge temperature control
Chilled water preferred over DX for end of duct temperature control

10. Product Drying- Other Considerations
Often, product drying requires heat—post-cooling not always needed
In this case, air returning from pans is humid and sugar-laden—purging makes sense. In another case, recirculation may save money
In this case, DX cooling is used. When chilled water is used, be sure to add freeze-protection on weather air coils
In many product drying applications, air velocity at product surface is critical. Easy in pan coating, more difficult in large rooms. Consider adding simple circulation fans in the space.

11. Industrial HVAC System Pharmaceutical Tableting Room
Purpose: To prevent powders regaining moisture, which clogs presses, slows production and makes poor tablets
Powders absorb based on rh, but to design a system, the engineer must define the humidity ratio:
10% rh is maximum based on input from R&D
Comfort tolerance allows temperature to swing between 70 and 73°F
Engineer selects lowest temperature: 70°F, 10% = 11 gr/lb
At 70°F, 11 gr/lb, the load profile looks like this:

12. 10% RH Room- Typical Schematic
Room control for humidity and temperature
Face and bypass for tight humidity control
Control valves for discharge temperature control
Differential pressure sensor controls make up air

13. 10% RH Rooms- Other Considerations
Less margin for error.
Temperature control often as critical as humidity. Sensible heat load and uniformity needs may determine system air flow volume.
The lower the humidity, the more critical the load calculations. Be sure to specify air leak rates in construction specs.
When exhausting from low humidity room, air balance is absolutely critical.

14. Controls Summary Design condition and tolerance (50°F , +/- 5%)
Dew point or humidity control
Humidistat or humidity controller
Sensor Location
Tolerance of Sensor