The solution is to introduce the proper amount of moisture, which lowers the transmission rate of airborne pathogens and reduces annoying static electricity problems.
Humidification also allows for greater fluctuations in indoor air temperature without causing discomfort to the occupants.
Yet many building operators seem reluctant to remedy indoor air humidity problems due to the high running costs normally associated with commercial-industrial humidification systems. As a result, high-pressure fog has gained a considerable foothold as a viable alternative to traditional forms of humidification.
A typical fog system, for example, uses only 1 hp for every 500 lb of water, substantially less than traditional humidification systems (see Figure 1).
Fog system fundamentalsSimilar to a standard evaporative cooling system, high-pressure fogging reduces airstream temperature through water evaporation.
Fog systems create an extremely large evaporative surface area by atomizing the supply water into billions of super-small spherical droplets (mean diameter of 5 microns). This is accomplished without adding heat or using compressed air, making running costs relatively insignificant.
A typical installation (Figure 2) consists of the following:
- High-pressure pump units — High-pressure pumps supply water to the fog nozzles in the air-handling unit (AHU) at pressures between 1,000 and 2,000 psi.
- Control system — Control systems have either simple on-off switches, variable-speed drives, or high-pressure solenoid valves. Variable-speed drives alter fog output according to humidity demand, while solenoid valves facilitate a staged output.
- Fog nozzle manifolds — Nozzle manifolds are usually 1/2-in.-od stainless steel tubes. The nozzles are evenly spaced within the cross section of the AHU, in such a way as not to impede airflow.
Stainless steel impaction pin nozzles (see Figure 3) with an orifice diameter of 0.006 in. produce more than five billion super-fine fog droplets per sec.
- Fog droplet filter and drain pan — Most fog systems use a fog droplet filter to remove all the liquid fog droplets from the airstream, so there is no possibility of wetting the duct downstream of the nozzle manifolds.
Normally, a drain pan is used to collect the water from the droplet filter so it can be drained away. In some cases, however, existing chiller coils can be used as a fog droplet eliminator.
- Water supply — Demineralized or reverse-osmosis water is normally used in fog humidification to eliminate mineral dusting problems. Special care must be taken with the water treatment system to ensure that there is no possibility of bacterial growth in the supply water. Usually, a UV or ozone-type treatment system is used for this purpose.
In most fog humidification projects, the nozzle manifolds are positioned inside the AHU with the nozzles facing against the airflow. This creates maximum humidification efficiency. If space is limited, a variety of alternatives can be resorted to.
Nozzles can be lined up differently, air handler housings can be extended or, if necessary, fog manifolds can be installed within the ducting itself.
In the latter case, though, it is usually necessary to increase the cross-sectional area of the duct to reduce air speeds to below 750 ft/min. Otherwise, large water droplets can be stripped from the fog droplet filter and enter the ducting.
Fog in a commercial settingHalifax Developments Ltd. owns a 1.3 million-sq-ft commercial and retail facility in Halifax, N.S., Canada, comprised of five buildings within a six-city-block area. These are all high-rise towers of 16 to 20 stories.
During the harsh Northwest Atlantic winters, temperatures range from 10Þ to 30ÞF. Faced with such a cold, very dry air supply, humidification was a serious problem at the Halifax Developments properties. Tenant complaints began to mount up.
The company first turned to existing equipment. Several buildings possessed old humidification systems, but none could be utilized due to IAQ issues. A steam boiler in one building proved unsafe due to the presence of amines (used to prevent boiler corrosion).
Other buildings once used centrifugal systems with water tanks that were open to the atmosphere, posing a significant heath hazard due to the potential for bacterial growth. Humidification effectiveness was also lacking. As a result, these systems had lain dormant for many years.
With margins tight in the property market, Halifax set out to find a system that would effectively humidify the building and satisfy indoor air quality regulations while maintaining a competitive financial edge.
“Humidification is one of the costliest parts of an hvac system,” says chief engineer Pat Poirier.
Initially he investigated a compressor-based atomizing system. While first costs appeared promising, there were several drawbacks. Compressor vibration and noise posed a design problem, and projected running costs would require a significant increase in rental rates per square foot.
Next, Halifax considered an electric-canister steam system. First costs were lower than compressed air and the system produced little noise or vibration. But when Poirier calculated that the energy costs would add $48,000 a year, he changed his mind about steam.
In the end, Halifax chose fog humidification.
“Although first costs were somewhat higher, with running costs of only $700 per year, fog was the only cost-effective option that produced adequate IAQ,” says Poirier. “Each of our seven fog units has paid for itself in eight to 24 months through reduced energy costs.”
In one tower where 100% outside air is required at all times, 1100 lb of moisture per hour are needed. Here, a reverse-osmosis water treatment system was added as a safeguard against mineral dusting. Another unit has been installed in a building that contains a variable air volume (vav) system.
As the airflow cuts off when it reaches a setpoint, humidification is made more difficult. Here, the supply temperatures have been adjusted to allow a higher volume of air than normal.
Each humidification system has its own digital control system that operates via sensors located in the treated spaces. Most have solenoid valves to add staging capabilities, controlled by temperature and humidity sensors. Poirier recommends monitoring the return air as the best way of measuring space humidity.
In total, Halifax Developments’ in- stallation has 240 fog nozzles (0.006-in. orifice, impaction pin) with an operating pressure of 1,000 psi. The water flow rate is approximately 7.7 gpm, averaging 1.9 gph per nozzle. One building runs its water supply through a reverse-osmosis unit, while the rest employ chemically demineralized water-treatment systems.
The AHUs are Trane units and range from 24,000 to 60,000 cfm each. There are eight fog pump units in total, ranging from 1 to 5 hp.
IAQ-wise, tenants are more than satisfied with the results.
“We like to keep the humidity at 30% to 35% in all of our buildings, though we can get up to 55% in the dead of winter,” says Poirier.
This is in keeping with Section 5.11 of ASHRAE Standard 62-1989, which states that relative humidity should be maintained between 30% and 60% to minimize growth of allergenic and pathogenic organisms.
Although there have been no problems with moisture carryover or accumulation in the units, the humidifiers are treated with an anti-microbial product as a precaution.
Keeping costs downFog humidification may not be the right solution for every installation. For very small sites requiring less than 200 lb/hr of moisture, for instance, equipment costs can be prohibitive.
For mid-sized and large commercial and industrial applications, however, high-pressure fog humidification should always be considered. In most cases, first costs will be found to be comparable and operating costs are typically substantially lower.