The Environics, Inc. Post

Humidification and Gas Flow

Posted by Dr. Rachel Stansel on Thu, Aug 22, 2019 @ 09:20 AM

One of the most popular options for an Environics Gas Mixing or Dilution system is humidification. The Environics humidifier option or our stand-alone humidifier allows the operator to add specific RH levels to a blend of gases to simulate actual conditions in laboratory experiments or other cases where gas samples contain humidity. But what is humidification and how is this option applied for an Environics system?

Humidification is simply the addition of water vapor to a gas flow. This is done to create a specific relative humidity or RH. RH is defined as the ratio of partial pressure of water vapor (pH2O) in the gas to the equilibrium vapor pressure at saturation at a given temperature.

 It is normally expressed as a percentage with a higher percentage being more humid. At 100% relative humidity, the air is saturated. Flow, temperature and pressure are critical to generating a humidified gas flow.

Temperature

 

Temperature must be consistent to obtain repeatable and stable RH. The accuracy, repeatability and linearity of the humidifier depend upon how stable the gas and room temperature are maintained. The warmer the gas, the higher the capacity to hold moisture (see below). In general, the RH will be reduced by a factor of 2 for each 20 °F (10 °C) increase in temperature, assuming all other variables are constant.

By ​English Wikipedia user GregBenson, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12009747

 

Pressure

The RH is dependent not only on the temperature but also on the absolute pressure of the system of interest. This is important if you are filling a canister/cylinder with humidified gas from your Environics system. The relative humidity of the gas flow will increases as the canister is pressurized because the partial pressure of water in the system increases with the volume reduction. It is important to take this into account, as the RH could exceed 100% and water may begin to condense.

It is also important with an Environics humidification system to not exceed 5psi of back pressure as this can damage the humidifier.

Gas Flow and Component Gases

When configuring your Environics Gas mixing or dilution system, the total output flow is a crucial component to generating the desired output RH. The lower the flow, the higher the potential RH you can achieve. There are various humidifiers that can be used, which would be determined based on the required flow and RH levels. Below are a few examples of recent humidifiers. Ideally, the full output gas flow will be humidified. Some gases are able to permeate the humidifier’s nafion membrane. In those cases, the system would be designed, instead, to humidify only the balance gas.

Output Flow

Temperature

Approx Max RH

600 SCCM

72deg F

95%

2 LPM

72deg F

95%

10 LPM

72deg F

80%

25 LPM

72deg F

80%

Depending on the flow rate of your system, a water circulating pump may be required with the humidifier water source. The pump is needed to keep the water temperature from getting too cold and preventing the production of high humidity levels. As an endothermic process, heat is removed from the water while the gas is being humidified. If this is required for your requirements, it would be included as part of the humidification option.

Humidification Option Specifications

Environics humidifiers use a series of tubes (Nafion, a Teflon derivative) which have the unique ability of permeating water vapor. If these tubes are surrounded with water and the process gas is passed through the center of the tubes at a known rate, never physically coming in contact with the water, then the humidification of the gas can be controlled. Furthermore, if the temperature of the water surrounding the tubes can be controlled, then the system becomes much more accurate and repeatable. This is due to the fact that the only other variable which will affect the permeation of the water through the Nafion tubes is the local temperature of the humidifier.

In general, the RH levels a unit can attain are from 5-95%, dependent on the total flow and the temperature where the humidifier will be located.  This can be a fixed or adjustable level. The RH is accurate to 5% with a constant flow and no radical changes in the gas temperature. When a more accurate humidity level is required, we recommend a hygrometer be used on the unit output to allow fine adjustments to “dial in” the RH required.  Finally, for those not using a hygrometer, the RH can be calibrated at multiple levels and at multiple flow rates, depending on your needs. As mentioned above, a circulating pump may be required, depending on your specific requirements.

Need to humidify a single cylinder or air source? Not a problem. Our humidification system can be customized to generate the required output with or without the added abilities to mix or dilute.

Want to learn more? Contact Us here or at 860-872-1111 or by email today!