Humidification Basics Part 13: Pressurized Steam Distribution

By Chad Edmondson (JMP) and Norman Hall (RLD)

A few weeks ago we reviewed the types of steam humidifiers normally supplied in building HVAC systems. But creating the steam is simply the first step; we also have provide equipment to disperse the steam into the air. This post reviews some of the dispersion methods used in buildings with isothermal humidification.

The engineer may choose from a variety of dispersion methods in the HVAC system. The choice normally depends on whether the steam is being release to the room or into the duct.

Dispersing Pressurized Steam Humidification Directly into an Area

If the requirement is to send the steam directly to a large area, DriSteem offers a fan-assisted assembly that's mounted to the humidifier. This is ideally suited to spaces where there is often limited ductwork, such as warehouses and assembly areas.

Shown below is a “fan-” or “area-type” dispersion component.  The image to the right defines the humidifier's minimum rise, spread and throw non-wetting areas; DriSteem of course has charts that assign values to these dimensions for a given humidifier.

It’s important to note that the installation of a dispersion fan should be in a place where no equipment or stock is within the dispersion area. For example, if the space is 60°F with a 40% RH requirement, a 250 PPH humidifier fan will have a 16’ throw with a 9’ spread and rise. This humidifier would best be placed in an elevated position above an aisle area.  This type of fan dispersion component may be mounted directly to a gas-fired or electric humidifier generator, or used with boiler steam as the source.

Fan dispersion may work well in warehouses, but for obvious reasons would not be suitable in an office space. 

Dispersing Pressurized Steam Humidification into Ductwork

The most traditional use of pressurized boiler steam for humidification in buildings is to introduce the steam into the ducted air stream. The most basic method is the single dispersion tube as shown below.

 single tube dispersion assembly

single tube dispersion assembly

This component is a bit more complex than one might think at first glance. Although it may look like a long tube with several holes drilled into it, a cross section (see below) of the tube reveals a highly engineered design – one that assures even distribution of steam and minimizes the risk for any condensation to form on the outside of the tube and drip into the duct.

Notice the unique “tubelets” that extend into the dispersion tube center. These assure that only the hottest, driest steam is discharged into the air and that any condensing will occur only inside the tube, not the duct.

The tubelet orifices are also calibrated to assure an even distribution of steam into the air.  This is necessary because steam, like liquid, always takes the path of least resistance, so dispersion must take place in a balanced manner.  Otherwise, more steam would always come out of the first, rather than the last tubelet.  The calibrated orifice tubelet, however, provides a greater opening diameter at the end of the tube than at the beginning, thus balancing the steam flow. This assures an even distribution of steam in the air, which again reduces wetting.

Condensate Handling with Pressurized Steam: Multiple Tube Humidification Dispersion

In some cases, multiple tube assemblies are required to ensure full absorption of steam into the air.  The absorption distance, or length of straight duct before a terminal or turn, is very long. Similar to the fan distribution, the absorption distance is many feet. To reduce this issue, the engineer may look to multiple tubes.

 multiple tube dispersion assembly - can reduce absorption distance inside ducts

multiple tube dispersion assembly - can reduce absorption distance inside ducts

   The multiple tube assemblies spread the steam over many tubes and reduce the absorption distance to a few feet. Each tube operates the same way as the single tube unit, there are just more of them.

One word of caution: these units are often shipped in pieces, which requires field assembly by the contractor. The prudent engineer would make this note on the detail to avoid confusion later in the installation process.

Although most of the steam is dispersed into the air, there will be steam condensate in these humidifiers caused by warmup and maintaining the temperature of the jacket in the cooler air stream. This condensate has the steam pressure behind it and may be lifted to a condensate return main using an inverted bucket trap. Remember that the rule is:

1 PSIG OF PRESSURE WILL LIFT WATER ABOUT 2 FEET

So, if a single tube humidifier has 3 PSIG steam pressure at the valve, the maximum lift out of the trap is 6 feet. The 6 feet may be reduced by control valve pressure drop and steam trap loss, so work with your DriSteem representative for the exact lift available.