Why Fog is Better
One of the major issues all hospitals face is keeping patients, visitors and staff from catching new diseases. Maintaining the proper level of indoor humidity is a crucial part of this. As the U.S. Center for Disease Control and Prevention has noted, “Low humidity may be associated with an increased susceptibility to upper respiratory infections.” If the humidification system is set too high, it leads to mold growth in the air ducts, with the spores entering the air stream. For hospitals it is critical to maintain humidity within the 30-50% range recommended by the CDC.
Grand River Hospital, located in Kitchner-Waterloo, Ontario, took this matter seriously and installed a sprayed coil system in 1988, but in order to save energy they switched to a steam system a few years later. While this did reduce their energy use, it also meant that the boiler feed water additive chemicals were entering the air stream. This prompted the hospital to look into other forms of humidity control and found that switching to a fogging system would not only improve the air quality by eliminating the additive of chemicals from entering the air stream, but it also proved to be a cost-saving initiative.
“I worked on an energy balance which compared the running costs using boiler plant steam vs. fog,” explained Bill Goodall, the now retired Director of Facility Redevelopment at GRH. “It showed a payback in less than one season based on the costs of steam.”
The hospital wanted to use fogging on nine air handlers with a combined airflow of 157,000 CFM. Each AHU had different specs with the airflow ranging from 7200 CFM to 34,000 CFM and an outside air percentage from 33% to 100%.
The system consists of one main pump unit driven by a 5 HP variable frequency drive (VFD) to provide the fogging system with 1000 psi water. Within the inlet duct for each of the AHUs are three individually controllable arrays of impaction pin nozzles that convert that 1000 psi water into a fog of minute droplets (about 10 micron diameter) which quickly evaporate to provide the required humidity. All together, the system has 20 stages of humidification control and can add up to 1704 pounds of water per hour to the air.
Utilizing the hospital’s existing Delta Controls Energy Management Control System (EMCS), the engineers were able to install sensors in each of the GRH zones being serviced, with additional sensors to measure the return air humidity. When the EMCS detects that the air in a given zone is excessively dry, it will turn on the pump and activate one or more of the solenoid valves controlling water flow to the nozzles in the appropriate AHU. This allows for the fogging levels to increase or decrease throughout the day as needed on any of the AHUs, with the VFD speeding up or slowing down to maintain the water pressure at 1000 psi.
Air Handlers at Grand River Hospital Using Fog Humidification
A Meefog system uses only about 3% of the electrical energy that a compressed-air or ultrasonic atomizer uses and only 1% of the energy consumed by a steam humidification system. In addition, in buildings with a high heat load, the cooling effect of evaporation lowers the load on the air conditioning system. Depending on relative humidity and temperature, Grand RIver Hospital starts using the fogging system in April.
“The hospital was happy that the evaporative cooling effect during spring and fall periods saved a lot of chiller consumption, especially in the spring when the 4-5 degrees of cooling allowed the delay of starting the chiller plant with it’s electrical demand penalty,” said Goodall.