How Fogging Works

This type of high-pressure fogging system consists of a high-pressure pump, which delivers clean water at 1,000 to 2,000 psi to a series of fog nozzles, which are usually installed in air ducts or in the air-handling unit. A typical fog nozzle has an orifice diameter of five to seven thousandths of an inch. Water jets out of the orifice and hits the impaction pin, which breaks the water stream up into the billions of superfine fog droplets.

These fogging systems are generally controlled by a solenoid valve that turns on the fog nozzles. The speed of the high-pressure pump can also be more closely controlled with a variable-frequency drive, although this tends to raise the cost a bit more. Sometimes the controls can become a little challenging, notes John Mee, marketing manager, Mee Industries Inc., Monrovia, CA.

“The standard system for us would be our own humidity sensor, which feeds back information about whether or not the system should be on. If you’re tying it into a whole building system, then the fogging system will just check with the building system to see whether or not the fog system should be on,” says Mee.

There are also fogging systems that use compressed air and water. While slightly more expensive to operate than the water-only type, these systems offer smaller particle sizes than high-pressure water-only systems.

With compressed air and water fog humidification, the compressed air collides with the water inside special stainless steel nozzles. The dual-fluid stream accelerates through the nozzle and then further collides with an impact surface to make billions of fog particles, the majority of which are 1 micron or less.

Typical air and water pressures are 100 psi for air and 65 psi for water. A typical compressed air and water fog nozzle has an orifice diameter of between 75 and 100 thousandths of an inch. Com-pressed air and water fogging systems don’t usually have clogging problems related to water supplies, and when used with good domestic water supplies require no additional water treatment.

Compressed air and water fogging systems are generally controlled by building automation systems (bas) through modulating analog signals, such as a 4-20mA-control signal. Typical turndowns are 50-100:1. Local digital controllers can also be utilized if a bas is not present.

Dan Reens, fogging systems product manager, Armstrong International, Three Rivers, MI, says the control system is something the contractor really needs to think about ahead of time. “Fogging systems provide adiabatic cooling as they place their fog into the airstream. This is a great benefit to this type of humidifier.

“However, if the control system for the duct or air handler cannot measure this evaporative cooling effect, many challenges can arise in obtaining proper performance from the fogging system.”

Cleanliness and Cost Benefits

One of the biggest advantages to using a fogging system is cost. These systems do save energy over other humidification systems. As Mee notes, “The bottom line is that a fog system costs $75 for 1,000 hours of operation while electric steam costs $16,800. With natural gas steam, it’s $5,000. Then we get down to other types of spraying systems, and it’s $1,200 for ultrasonic and compressed air is $2,500. They all use more energy.”

Another benefit is that fogging systems do not have a reserve tank to hold water, so there are few concerns about legionella and other contaminants. “Legionella have never been found in a fogging system,” says Reens. “Because a compressed air/water fogger uses an air blast atomizing method, it obliterates the water droplets in the fog-generating process. Air/water velocities within the fogger orifice reach close to the speed of sound; then, after this enormous stress, the water particles are slammed into an impact surface called a resonator.”

Additionally, when using the domestic water supply, antibacterial additives are brought in, such as chlorine, fluoride, and other agents normally found in public drinking water.

Finally, every fogging system purges itself to drain when not in use, so standing water is not left in the system to create bacterial growth environments.

When a water supply is hard or soft and contains a reasonable amount of mineral solids, the contractor must determine whether it warrants removal of the majority of the solids prior to fogging or after fogging. To remove them prior to fogging, an average-rated reverse-osmosis (RO) system is recommended, says Reens.

RO systems remove solids at the ionic level, which is required to remove minerals dissolved in a water supply. If, on the other hand, the water quality is good but some minimal dusting may be expected, it is possible to remove the dust through medium- to high-efficiency air filters located downstream of the fogging system.

(Note: These filters must be located well after the humidification chamber, and they should never be located immediately after the fogging chamber.)

Systems Can’t Be Used Everywhere

While there are many advantages to fog humidification systems, there are some drawbacks. Namely, they can’t be used everywhere. Mee says his company does not recommend that contractors try to bid a fogging job where there is less than 250 lb/hr of water, which is approximately a 300,000-Btu boiler.

“Otherwise, the first costs are just too high,” says Mee. “You have to put in pumps and stainless steel lines sometimes. The hardware of making our system get to 1,000 psi makes it inefficient to just put one nozzle out there spraying. So I’d say there’s a broad range of home-type applications and small commercial applications that wouldn’t work.”

Another point to take into consideration is the air speed in the ductwork. The system works best when air speeds are under 750 ft per min (fpm). There are two reasons:

First, the water needs to evaporate and with speeds over 750 fpm, whatever water is collecting on the mist eliminator, which will be drained away, will actually be pulled right off the pad and put back in the airstream as a huge droplet.

Second, there’s a concern for duct cleanliness. These huge droplets can form pools, which could increase the chances for legionella growth.

Reens notes that fogging systems must be applied with the proper supports, design, and controls in mind. “Installation consists of a single compressed air line and a single water line to each system. The pipe sizes an installer would expect to use would be between 1/2 and 2 inches for compressed air, and between 3/8 and 3/4 of an inch on water.”

But there are no consumables with the fogging system (plastic tanks, elements, etc.) and only seasonal calibration is required. A simple 120-V, single-phase, 10-amp circuit is required for the electrical portion. As Reens says, “Generally, they are easy to install, operate, and maintain.”

Sidebar: ‘Generic’ Software Offered for Hvacr

A self-described “generic” software system provider for the refrigeration and air conditioning industry, Mistral, is making its first foray into the U.S. market.

Established in 1984, Mistral software is said to be used by more than 65% of United Kingdom refrigeration and air conditioning contracting firms. Programs are distributed in more than 80 countries, according to the company.

One of the services is the “Coldwind” computer program to accurately calculate a cold room refrigeration load. According to the company, the program also provides a detailed presentation for customers. In general, the company claims its software helps eliminate mistakes, reduce staff costs, and meet quality assurance procedures. Further, it claims that the programs do more than just calculate figures quickly; they also help avoid non-competitive designs.

For more information, contact the U.S. distributor, Ablaze Heating & Cooling, 82074 Hwy. 395 N., Umatilla, OR 97882; 541-567-1049; www.mistral-ltd.com (website).

Publication date: 04/16/2001