Steam Jet Humidifier - Most popular humidifier

Steam Jet Humidifier 

The most popular humidifier is the steam-jet type. It consists of a pipe with nozzles partially surrounded by a steam jacket. The jacket is filled with steam; then the steam is fed through nozzles and sprayed into the air stream. The jacket minimizes condensation when the steam enters the pipe with the nozzles and ensures dry steam for humidification. The steam is sprayed into the air at a temperature of 100°C or higher. The enthalpy includes the heat needed to raise the water temperature from 0 to 100°C, or 419 kJ plus 2256 kJ to change the water into steam. This is a total of 2675 kJ per kilogram of water at 0 kPa (gauge) as it enters the air stream. The additional heat added to the air can be Plotted on Chart No. 1 (Image A) to show the complete process.

Image A

In this example, air enters the heating coil at 13°C dry-bulb temperature (Point A) and is heated to 32°C dry-bulb temperature (point B) along a constant moisture line. It then enters the humidifier where the steam adds moisture and heats the air to Point C.

        Image A also shows use of the protractor nomograph. Assume the relative humidity of the air entering the humidifier at Point B is to be raised to 50 percent. A process line can be constructed using the protractor nomograph. The total heat of the entering steam in KiloJoule per kilogram is located on the enthalpy/ humidity ratio scale (Δh / ΔW) of the nomograph. This value, 2675 kilojoules per kilogram, is connected to the reference point of the nomograph to establish the slope of the process line on the psychrometric chart. A parallel line is drawn on the chart from Point B upto the 50 percent relative humidity line (Point C). The Line B-C is the process line. The Line X-Y (bottom of the Image A) is simply a perpendicular construction line for drawing the Line B-C parallel to the line determined on the nomograph. Note that the dry-bulb temperature increased from 32 to 33°C.

Image B

Image B is the same as the chart shown in Image A, except that it graphically displays amount of heat added by the process. Enthalpy (total heat) added is determined by subtracting the enthalpy of the dry, heated air at Point B from the enthalpy of the humidified air at Point C as follows:

                        74.5kJ/kg - 49.0 kJ/kg = 25.5 kilojoules per kilogram of dry air

The steam raised the temperature of the air from 32°C dry bulb to 33°C dry bulb. To find the latent heat added by the steam humidifier to the air, determine the enthalpy at Point D (the enthalpy of the heated air without added moisture) and subtract it from the enthalpy of the humidified air at Point C.

This is as follows:

                        74.5 kJ/kg - 49.8 kJ/kg = 24.7 kilojoules per kilogram of dry air

The remaining 0.8 kJ/kg is sensible heat. The actual moisture added per kilogram of dry air is 9.5 grams. The specific volume of the entering air at Point B is 0.874 cubic meters per kilogram.

        For a 4.72 cubic meters per second system, the weight of the air passing through is :

                            4.72 cubic meter per second / 0.874 cubic meter per kg = 5.4 kilograms per second

        The weight of the moisture added is :

                            5.4 kg/s x 9.5 g/kg = 51.3 grams per second of moisture

        Recalling that the steam added 25.5 kiloJoules per kilogram of dry air, the total heat added is:

                            5.4 kg/s x 25.5 kJ/kg = 137.7 kilojoules per second

        Summarized, a steam humidifier always adds a little sensible heat to the air, and the Process Line B-C angles to the right of the 32°C starting dry-bulb line because of the added sensible heat. When the process line crosses the moisture content lines along a constant dry-bulb line, only latent heat is added. When it parallels a constant, horizontal moisture line, only sensible heat is added.

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