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Humidification

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Process gases rarely require humidification, but one example of a gas that needs to be humidified is medical gas such as the oxygen supplied to a patient (Figure 10). The oxygen causes the nasal passages to dry, and humidification adds moisture to increase patient comfort. Figure 10 - Oxygen humidifier.
The most common example of process humidification is adding moisture to the air stream in a heating, ventilating and air-conditioning (HVAC) system (Figure 11). In northern climates, dry outdoor air enters the HVAC system. Moisture can be added to the intake air to maintain comfortable inside humidity levels.

Figure 11 - Building humidification.
The air humidification process operates as follows.
1. Outdoor air (OA) enters the system (Figure 11).
2. The air passes through dampers that maintain a mixed air setpoint (SP). The SP
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The air passes through particulate filters and the heating and cooling coils. Flow through the coils is determined by TC-2 and sensed by TT-2.
4. The air passes over the humidifier and is discharged as supply air through the supply air blower.
5. Humidity controller 1 (HC-1) sets the humidity level.
6. The humidity is adjusted according to the outdoor air temperature sensed by temperature transmitter (TT-3).
7. The humidity transmitter (HT-2) senses the return air humidity, and the steam valve is set to maintain the return air humidity. The return air comes from the conditioned space, so it is an accurate indication of interior humidity. Humidity is lowered during colder periods to prevent excessive condensation.
8. Humidity limit controller 1 (HL-1) measures the supply air humidity immediately downstream from the humidifier.
9. When the humidity levels leaving the humidifier rise above the setpoint, HL-1 overrides the steam valve through the low-select relay. This is a limit control strategy that prevents excess moisture from being added to the air during a system start-up or following an adjustment to the HC-1

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