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Reducing Energy Costs With Fog-Based Humidification
Why a California museum replaced its gas-fired steam boilers with a fog-based humidifier system.
Elliot Sloane | Mee Industries
Steam humidifiers have been the norm in commercial applications for several decades. However, high energy bills and inconsistent humidity controls have caused some facilities to turn their attention to alternatives such as fog-based humidification. This article will cover the basics of steam and fog humidification and give an example of how one facility switched from steam to fog.
How Steam Humidifiers Work
Industrial steam humidifiers use electricity, gas or pressurized steam to heat water. A steam boiler is a pressurized vessel that is used to heat water and produce steam. Instead of electric heating elements, gas-fired systems use a burner and heat exchanger to boil the water.
The steam produced is used to increase relative humidity (RH) by channeling it into air handling units (AHUs) or ductwork. Humidistats are normally utilized to control RH. If RH drops below the set point, the humidifier provides more steam. Once the set point is exceeded, it shuts off. Most systems are integrated with heating, ventilation and air conditioning (HVAC) systems to facilitate uniform moisture distribution throughout a building in alignment with temperature control.
In most cases, the goal is to maintain humidity within the 40%-60% range. By avoiding the possibility of too high or too low humidity, a healthier work environment is facilitated. Dust mites, bacteria, viruses, mold and airborne contaminants are contained. Humidity control also safeguards expensive electrical and computerized equipment from damage by mitigating the formation of static electricity.
Treated water is generally needed, such as reverse osmosis (RO), softened or demineralized water. This helps avoid mineral buildup and reduces the frequency of maintenance actions.
| IMAGE 1: A fog-based humidifier system inside an air handling unit at a large museum site (Images courtesy of Mee Industries)
| IMAGE 2: Typical fogging arrangement for air handling units
Fog Humidification
Fog humidification is similar in some ways but uses an adiabatic process. Adiabatic humidification adds moisture to the air via evaporation without the need to inject steam into the airstream. As well as adding moisture, fogging cools the air as heat energy is used to evaporate the droplets. As there is no need for a boiler or heating element, energy requirements are considerably lower.
The process begins with a direct drive pump unit with a high-efficiency motor and a variable frequency drive (VFD). The VFD enables a soft start of the pump motor and maintains the lowest pump speed possible for energy savings and longer pump life. The pump is fed with prefiltered RO water. Oil-lubricated pums are used that can be turned down easily, maintain a pressure of 1,000 pounds per square inch (psi) and serve multiple AHUs.
A control panel is mounted on the pump skid or beside the staging valves (or both). Fog staging valves are used to modulate the amount of humidification output of the system. Each valve is connected to a different number of fog nozzles. By controlling these valves, many stages of humidification can be achieved. A programmable logic controller (PLC) in the valve control panel (typically mounted on the exterior of an AHU) receives a humidity demand signal and stages the motorized ball valves that turn on stages of fogging to achieve the correct output. The PLC is tied into the building management system (BMS). A PLC can also be added to the pump station in order to provide additional pressure or alarm information if desired.
| IMAGE 3: A fog-based humidifier nozzle atomizes water with an average droplet size of 10 microns.
Within the AHU, fogging nozzles are installed after the air filters and heating coils. Ideally, allow 3-6 feet as the best distance to maximize fog absorption into the air stream. At the end of this humidification section in the AHU, adequate drainage should be established on both sides of the mist eliminator pads and the cooling coils.
A fogging system should be able to atomize water into billions of ultra-fine droplets. The average droplet size should be 10 microns, or one-tenth the diameter of a single strand of human hair. The resulting fog evaporates rapidly and maximizes efficiency. This is best achieved by using impaction-pin nozzles made from high-grade 316 stainless steel. Such nozzles feature a 0.006 inches (150 micrometer) diameter orifice that produces billions of tiny droplets per second. The fog quickly evaporates, allowing it to reach the set point efficiently. An O-ring seal at the base of the nozzle ensures a tight high-pressure seal with no leaking.
As well as eliminating the need to boil water to produce steam, a fogging system within an AHU acts as a precooler to the exhaust-side recovery coil, particularly during hotter summer months, to evaporatively cool the exhaust air. For every pound of water evaporated, approximately 1,000 British thermal units (btu) of cooling is provided. The high-pressure water pump uses a nominal amount of energy to achieve this.
A typical fogging system uses one horsepower of energy for every 600 pounds of water dispersed, which is 3% of the energy usage of compressed air-type systems and about 1% of the energy required by steam systems.
Case Study: Museum Converts to Fog Humidification
A museum in California is the home to priceless paintings, drawings, books, sculptures, manuscripts and photography. These artifacts require constant humidity to prevent damage from continual shrinking and swelling as ambient humidity levels change. Dry air, for example, can result in the cracking of canvas and other damage.
The original humidification system utilized gas-fired steam boilers that had reached end-of-life. They supplied steam to scores of AHUs that humidified and cooled different spaces, each one having unique temperature and humidity specifications. The overall campus stretches across several buildings. High energy bills and frustration with inconsistent control of humidity led the owners to consider alternative methods of humidification.
They conducted a comparison. Capital and installation costs between replacement steam systems and fogging turned out to be similar. A calculation of energy costs revealed a different story. Assuming 100,000 cubic feet per minute (CFM) at 70 F and 40% relative humidity with electricity prices at $0.0685 per kilowatt hour (kWh): Annual energy costs for an electric steam humidification system would be $77,352; a gas-fired system using $0.41 per therm and the same conditions would drop the cost to $20,011; and for a fog-based system, energy costs fell to $4,025. As this facility utilizes more than 400,000 CFM of air, the owners opted for fog to replace the aging steam humidifiers.
Due to the delicate nature of the art being protected, the change from steam humidification to fogging had to be done carefully over many phases. The fogging systems receive water from large RO water treatment systems. RO provides the level of water purity needed for the protection of the air handlers and supply air. Fog pump stations supply pressurized water to dozens of humidifier grids that have been installed inside the museum’s air handlers. Each has its own staging valve panel to provide accurate and precise moisture introduction into the supply air stream and allow for independent control of the humidification in different spaces.
All RO systems and fog pump skids have built-in redundancy to eliminate the possibility of a component or device failure shutting down the system and potentially endangering the artwork. Almost 2,000 fogging nozzles have been installed across about 50 AHUs and multiple mechanical rooms.
As well as lower energy bills, facility personnel report reduced maintenance, as they only need to replace water filters and perform minor service actions on the high-pressure pumps. They also note tighter control of humidity in each individual space.
Elliot Sloane is humidification manager at Mee Industries Inc. of Irwindale, California. For more information, visit meefog.com.
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