There are two main categories of industrial humidifiers. The first to be developed was isothermal humidifiers that use heat to generate steam which is then put into the ventilation system. The initial systems involved taking steam from existing gas-to-steam processes or space heating boilers. However, the chemicals added to the boiler water to prevent corrosion can be unsafe for humans to breathe. Other ventilation systems then developed using the boiler steam to boil other water (steam-to-steam), as well as gas-fired or electrically heated systems. These ventilation systems are ideal for certain uses, but they require a large amount of energy to produce the steam and additional energy may also be needed to cool the air down again.
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A second category, the adiabatic industrial humidifier, has gradually evolved over time and, unlike isothermal humidification, humidifies the air without adding heat. They require a fraction of the energy of any of the isothermal methods, can provide more precise humidity control, and cool down the surrounding air through evaporative cooling thereby reducing the load on air conditioning systems. Within the adiabatic humidification category, there are a variety of different systems. The ultrasonic commercial humidifier, for instance, uses a high-frequency piezoelectric disk to cause cavitation on the surface of the water and thereby produce a fine mist. This technology is frequently used in household units and smaller-scale commercial and industrial projects. Other systems involve compressed air atomizers and centrifugal atomizers.
High-pressure fogging is another type of adiabatic method with many commercial, industrial, and agricultural applications, for instance, fog humidification systems are ideal for HVAC installations requiring over 200 lbs of water per hour. This method is gaining in popularity due to its low operating cost, low environmental impact, controllability, and ease of incorporation into existing HVAC installations. This is why HVAC systems are becoming a more popular aspect of the process.
Figure 1 “The Energy Cost Comparison Chart” shows the energy costs for various humidification systems, taking the cost of power at ten cents per KW hour for hours of operation, with lb per hour moisture output.
Fog systems inject a fine spray of purified water into the airflow. Most of the water spray evaporates, so it both cools and humidifies the airflow. There is usually not sufficient time for all the water droplets to evaporate, so a droplet filter is installed downstream of the fog nozzles. Unevaporated fog droplets collect on the filter and the captured water is drained away. The fog nozzles are installed downstream of the heating coil to ensure there is always enough heat for evaporation of the water spray.
When calculating the system needs there are several factors to consider including:
Fog system output is controlled by opening and closing staging valves that allow high-pressure water to flow to the fog nozzles. Fog systems usually have between 3 to 15 stages of output. The fog staging level is controlled by the Building Automation System. Steam humidification systems can be gas or electric boilers, installed close to the air handling unit (AHU) with a steam dispersion manifold installed inside the AHU. Steam commercial humidifiers can also use a stream from a boiler that may be located some distance from the AHU. However, boiler steam often has hazardous corrosion inhibiting chemicals in it, so a heat exchanger is used. A boiler stream is used to boil purified water and the resulting steam is fed to a dispersion manifold located inside the AHU. Steam systems are controlled by modulating a steam throttling valve or by turning an electric immersion heater on and off.
A typical fog humidification installation in an AHU consists of high-pressure pumps, a control system, fog nozzle manifolds, a fog droplet filter, a drip tray, and a drain pan (See Figure 2, Schematic of Typical Fog Installation), as well as staging valves.
High-pressure Pump Units for Industrial Humidification
Only pressurized water (1,000 to 2,000 psi) can deliver the micro-fine droplets required in humidification. High-pressure pumps, therefore, supply water to the fog nozzles in the AHU. These are generally ceramic plunger pumps with stainless steel or bronze heads, operated at low speeds to reduce noise and vibration. All units have totally enclosed fan-cooled electric motors.
Control System
Control systems are available either with simple on/off switches or variable speed drives which alter the fog output according to humidity demand. Variable speed drives are needed where the level of moisture is high. They control the capacity of fog by changing the pressure of the fog spray to prevent over-saturation of the air. Also, optional high-pressure solenoid valves allow for staged output or for the operation of separate fog zones from one pump unit.
This concept can be applied for capacity control in several AHUs in close proximity hooked up to one pump unit, making an extremely cost-effective installation. For example, one U.S. Post Office site in Washington, D.C. uses a variable speed drive for capacity control in a 150,000 CFM AHU with an air velocity of 500 feet per minute and pounds of water added per hour. This amounts to 24.5 grains of moisture per pound of air moved. Sensors in the treated space control the level of fog output. An additional high-limit sensor in the air leaving the fog chamber, usually set at 85 percent rh, ensures that there is no wetting of the ducting.
Staging Valves
Fog systems usually have multiple staged nozzle lines with each line having a set flow rate for that stage provided by multiple nozzles, output is controlled by opening and closing staging valves.
Fog Nozzles Manifolds
The usual nozzle manifold consists of half-inch OD tubes, with the nozzles evenly spaced within the cross-section of the AHU. Such an open latticework of small pipes does not impede airflow and pressure drop is negligible. For best results, stainless steel impaction pin nozzles with replaceable filters for easy maintenance should be used (See Figure 3, Fog Nozzle).
Orifice diameters of from five to seven-thousandths of an inch produce more than five billion super-fine fog droplets per second (droplets have a mean diameter of 10 microns). This provides adequate humidification while keeping energy and water waste to a minimum.
Fog Droplet Filter
In applications where a little water in the air handling system would not be problematic, a fog droplet filter may not be required. However, in most situations, the droplet filter should be positioned downstream of the nozzle manifold. It is best to tilt the fog droplet filter at a 15-degree angle to the vertical to allow for water that coalesces on the filter to flow down to the drain pan. Properly applied, fog droplet filters prevent any possibility of wetting in the ducting.
Drip Tray and Drain Pan
Both the drip tray and drain pan, which collects all unevaporated fog droplets, are usually fabricated by the HVAC contractor who then has the ability to install a great HVAC system. Drip trays should be constructed of stainless steel sheet metal of a suitable gauge and they must have a slight slope in order to carry water to the drain pan. The drip tray extends from below the fog nozzles to the drain pan. The drain pan should be installed directly under the fog droplet filter and plumbed directly to the building’s drainage system.
Water Supply
Reverse Osmosis (RO) treated water is often used in fog humidification to prevent mineral dusting of the treated areas. In some areas, however, where the local water supply is of sufficient quality, RO water is unnecessary. Contractors are advised to use the water lab facilities of a reputable fog system vendor. Often, free water testing and advice are available.
Due to its low operating cost and controllability, high-pressure fogging humidification systems are an energy efficient alternative to traditional steam humidifiers. Not only can fog humidification save you energy and money compared to steam, it is also easier to maintain.
Humidification plays a vital role in maintaining indoor air quality for the healthcare industry; however, with growing public concern over indoor air quality and a heightened awareness of the impact of humidity on industrial and commercial environments, many new projects face the problem of how best to incorporate humidification.
Fog humidification in air handling units (AHUs) is one alternative. As mentioned in the video above, MeeFog high pressure fogging humidification systems align with the ASHRAE standard requirements so that our systems can safely be used in all hospitals and healthcare facilities and environments.
As air handlers typically have air velocities of less than 750 ft/min (usually around 500 ft/min), the most common position for fog nozzle manifolds is inside the AHU itself. Occasionally, however, alternatives to a standard AHU installation must be resorted to. The simplest of these variations concerns not having enough space between the heating and cooling coil. However, nozzles can still be installed in tight spaces of less than three feet, sometimes requiring extra layers of mist elimination media at the droplet filter. Although this arrangement does increase water consumption, it provides a cost-efficient alternative to other, more energy-intensive forms of humidification.
In one Nova Scotia office building, for example, the existing electric steam industrial humidifier lacked the power to humidify the three-story building. Unfortunately, retrofitting the air handler with fog humidification also proved problematic as space was severely limited. Even with the air filters moved back 9 inches to provide the maximum available space within the air handler, the nozzles were still too close to the air filters. In this case, extra nozzles are required as more water will be eliminated at the droplet filter. If space is seriously restricted and the air handler cannot be extended by adding an extra section, another option is to place the fog manifolds in the ducting. As air speeds within the ducts are normally between and ft/min, although, it is necessary to increase the cross-sectional area of the duct to reduce airspeeds to below 750 ft/min (air velocities in excess of 750 ft/min can strip water off the back of the fog droplet filter and propel it into the ducting).
This was the case at one New York newspaper where the existing AHU was too small for the addition of fog nozzle manifolds and not easily modified. The solution was to enlarge a section of ducting to create a 500 ft/min air velocity area in which to set up the humidification hardware.
Fog systems are more efficient than a steam industrial humidifier in part because they use excess heat in the return air to provide the energy needed to vaporize the water. A steam industrial humidifier has to supply all the energy required to vaporize the water. Another reason that fog systems are also more efficient is that they bring in less outside air when it is dry outside, so they save the water and energy required to humidify and heat the excess air.
Halifax Developments Ltd., for instance, owns a 1.3 million square foot commercial and retail facility in Halifax, Nova Scotia, where winter temperatures range from 10 to 30 degrees C. Due to increasing numbers of tenant complaints, the company decided to add a modern humidification system. Energy costs for an electric canister steam system were calculated at $48,000 per year, compared to $700 per year for fog. According to the company’s Chief Engineer, “Each of our seven fog units has paid for itself in eight to twenty-four months through reduced energy costs.”
Fog systems also provide evaporative free cooling, so less chilling is required as compared to steam commercial humidifiers. This is particularly true in areas with hot, dry climates. Fog humidification is not a complex technology when it involves an experienced fogging vendor. First-time users may require a factory start-up, where the fogging vendor comes in to ensure the installation is correctly performed, checks all the components, and conducts any final adjustments.
Did you know that in cold climates, when outdoor air is brought into a building and heated, the relative humidity can be as low as 10 percent — about the same as the Sahara Desert?
AHR Expo Educational Presentations
Dry air isn’t just uncomfortable. It can cause a variety of unwanted conditions, including:
Proper humidification in homes and workplaces helps minimize the amount of unhealthy airborne pollutants and improves comfort by reducing the evaporative effect that dry air can have on the body — something merely turning up the thermostat can’t accomplish.
Humidifiers operate by creating water vapor, either by moving air over a water-soaked media or by generating steam. This water vapor is distributed directly into the home or workplace open spaces or through duct systems where water vapor is absorbed by dry air.
Read on for more on humidification technology, product basics, installation and maintenance considerations, and standards and certification.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), along with other research, confirms that controlling indoor humidity to levels between 40 and 60 percent fosters healthy indoor living while preventing mold growth. Outside of this range, dust mites, bacteria, viruses, and fungi thrive. According to the American Society of Otolaryngology, very dry air in a home or building can make people more susceptible to infections.
ImageAlthough the more common concern is providing enough humidity during heating, having too much humidity also can be problematic. That’s why it’s important to consider the following with any humidifier application:
There are two basic categories of humidifiers: residential, and commercial/industrial. Commercial and industrial humidifiers can be divided into three types: adiabatic, direct injection steam, and heated tank. Click here to view a chart noting each type’s advantages, limitations, and potential ancillary equipment.
Atomizing and evaporative types
Adiabatic humidifiers provide direct evaporation of water into the air without adding external energy. Because no heat is added to evaporate the water, there is a cooling effect that is directly proportional to the amount of water evaporated. This is sometimes referred to as sensible cooling. Sensible BTUs are taken from the surrounding air, reducing air temperature.
Adiabatic humidifiers include both atomizing and evaporative humidifiers. View a detailed chart on adiabatic humidifiers here.
In general, atomizing humidifiers reduce water particles to a mist or a fog. When the fog is sprayed into a warm, dry space, the water evaporates without the addition of heat. Atomizing humidifiers typically require purified water to prevent plugged orifices or nozzles and to eliminate the distribution of mineral dust into the indoor environment. Please click on each adiabatic humidifier below to access a brief description of that particular type.
Centrifugal type atomizing humidifiers utilize a high speed disk to propel water onto plates or a comb to produce a fine mist. This mist evaporates as it enters the air.
ImageCompressed air and water atomizing systems combine the two fluids inside a nozzle. The rapidly expanding air shears the water into atomized water particles. The mixture of atomized water and expanding air is discharged through an orifice and deflected off a resonator tip, further reducing the water particle size and resulting in a fog.
High pressure atomizing systems use a high pressure pump to force water through one or more small orifice nozzles to produce a fine mist. Humidity demand is interpreted by control logic on a Programmable Logic Controller (PLC) or circuit board. The PLC communicates with a variable frequency drive (VFD), and the VFD drives the pump, typically operating at approximately 1,000 psi water pressure. The PLC controls a series of solenoid valves, which feed the high pressure water to a series of nozzles. High pressure water passing through the nozzles leads to evaporation.
Compressed air and water atomizing systems use compressed air to produce a fine mist at the atomizer nozzle.
High pressure and compressed air and water atomizers can provide energy benefits in many applications. In air handling systems, atomizers are best located prior to a cooling coil (with drain pan) in the air handling unit. These systems also can be used for direct area discharge.
ImageUltrasonic atomizing systems use a piezoelectric transducer immersed in a reservoir of purified water. The transducer creates a high frequency mechanical oscillation directed at the water’s surface, which results in creation of a fine mist.
ImageWetted media evaporative humidifiers circulate water over a porous core media while air is blown through the openings. The rate of evaporation varies with air temperature, humidity, and velocity. Wetted media humidifiers can be designed with or without water recirculation.
ImageSteam cup-type humidifiers receive steam in a vessel mounted beneath an air duct. Condensate drains from the cup and steam is released into the airstream.
ImageSteam separator-type humidifiers are comprised of a steam separator, integral control valve, steam jacketed dispersion tube(s), and a manifold when there is more than one dispersion tube. A steam trap and wye-type pipeline strainer are typically included in the package. Supply steam flows through the strainer and pre-heats the steam jackets before entering the separator. Liquid separates and falls with gravity through the separator drain connection to the steam trap.
High quality steam vapor travels to the control valve and, with demand for humidity, passes through the dispersion tube(s) into the airstream.
ImageSteam panel-type humidifiers typically include a wye strainer, and a steam control valve. The panel consists of a steam supply header and multiple steam dispersing tubes. Steam flows through the control valve to the panel, passes through the dispersion tubes, and is introduced to the airstream. Condensate is discharged through the drain to one or two mechanical steam or P-traps.
ImageSome steam panel-type humidifiers also have a heat exchanger that vaporizes dispersion-generated condensate and returns pressurized condensate to the boiler without additional pumps, valves, vents, or controls.
Electric (electrode type) humidifiers include microprocessor-based electronic controls, automatic fill and drain valves, and a steam generator cylinder complete with a set of electrodes inside. Electric current passes through ordinary tap or softened water, bringing it to a boil, and creating steam. Output is modulated by varying the water level in contact with electrodes. The drain valve initiates a periodic blowdown of the tank to adjust water conductivity and steam output. When scale buildup occurs within the steam-generating cylinder, the cylinder can be cleaned or, more commonly, replaced, significantly reducing maintenance requirements. Steam discharges out the top of the steam generator for dispersion into duct systems using dispersion tubes or into open spaces using fans.
ImageElectric (resistive type) humidifiers use one or more resistive heating elements, submersed in a cleanable stainless-steel tank, to heat and boil water. Steam discharges out the top of the tank. A drain valve initiates a periodic blowdown of the tank to reduce scale build up. Electric resistive-type steam humidifiers can use any type of fill water (tap, softened, RO/DI). They disperse humidification steam into duct systems using dispersion tubes or into open spaces using fans.
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ImageInfrared-tye steam humidifiers use one or more radiant heat bulbs to create heat energy to produce steam.
ImageGas-fired steam humidifiers use natural gas or propane, a burner/blower assembly, and a submersed heat exchanger to create atmospheric steam. A drain valve provides periodic tank blowdown to reduce scale buildup. Gas-fired steam humidifiers can use any type of fill water (tap, softened, RO/DI). They disperse humidification steam into duct systems using dispersion tubes or into open spaces using fans.
ImageSteam-to-steam humidifiers use treated steam from a boiler at pressures up to 15 psig as a heat source in heat exchangers submersed in clean tap, softened, or RO/DI-treated fill water. Steam control valves control the steam pressure to the heat exchanger based on humidity demand, and the clean fill water is converted to atmospheric steam, exiting from the top of the tank. Condensate that forms is drained from a steam trap. A wye strainer is typically included and installed prior to the control valve. They disperse humidification steam into duct systems using dispersion tubes or into open spaces using fans.
ImageHot water type humidifiers utilize the heat energy from an existing hot water source passing through a heat exchanger submersed in water to produce steam. These humidifiers are very similar to steam-to-steam humidifiers.
Residential humidifiers are classified by the way they add moisture to the air. There are three main classifications of residential central system humidifiers: atomizing humidifiers, steam humidifiers, and evaporative humidifiers. The capacity associated with a central system humidifier’s ability to add moisture to air expressed in gal/day of continuous operation. Humidification capacity, power input (in watts), and the energy input of any supplementary heat (in Btu/h or watts), if applicable, will be published for all humidifiers rated using AHRI Standard 610.
A central system or whole home humidifier is a device designed to add moisture to the home, and is installed directly into the central heating and cooling system. It is controlled by a humidistat, just like the temperature of the air is controlled by a thermostat. In order to add moisture to the air, humidifiers require heat, water, and air movement. Central system humidifiers can be mounted to the supply duct, return duct, or in a by-pass configuration, and should be installed by an HVAC professional. Most central system humidifiers require a hard-piped water supply. To maintain proper function, humidifiers should be maintained per the manufacturer’s instructions, on a regular basis, approximately once every six months. Whole-house humidifiers generally require much less maintenance than portable devices and have much longer lifetimes.
Residential self-contained humidifiers are used in homes with radiators or other non-ducted heating systems. Self-contained humidifiers are installed in the wall or ceiling of a house, and also have a humidistat to sense/control the home’s humidity level. They are connected to the water lines and hard-wired to the home’s electrical system. Self-contained humidifiers are alternatives to portable humidifiers.
Atomizing, or spray, humidifiers are similar to evaporative humidifiers in that they do not heat the water, but unlike their evaporative counterparts, atomizing humidifiers increase the moisture in the air by spraying small water droplets into the central system’s airstream. In nozzle-type units, a fine mist is injected into the air when water is forced through restrictive nozzles. Impeller humidifiers add moisture to the air through a spinning disk that flings water against a diffuser. Ultrasonic humidifiers, also known as piezoelectric humidifiers, atomize the water using a vibrating element that operates at a very high frequency of about 100 kHz (100,000 cycles per second).
Steam humidifiers heat water to the boiling point, producing steam. Because steam humidifiers contain their own heat source, electric heating elements, they are able to add moisture to cooler airstreams than atomizing or evaporative type humidifiers. Using electrode technology, electrical current flows between submerged electrodes, and the water resistance vaporizes the water. The submerged electrodes are stored in a canister, which is replaced at the end of a humidifier season.
Residential steam units are used when humidity is needed but other products cannot be installed or do not offer enough capacity. Most are wired to operate without warm air from the furnace, but instead utilize the furnace blower to distribute humidified air when the humidity level drops below a preselected level. Steam units can be mounted with a blower pack, if a ducting system is not available.
Evaporative humidifiers do not heat the water, but use hot air flow through a water panel and the HVAC system’s heat draw to turn the water to vapor. They are generally designed in either the bypass or fan powered style, depending upon how the air is delivered to the humidifier. In the bypass design, air is passed from the supply to the return. The fan powered version uses a fan to blow air through the unit. Dry, hot air from the HVAC system is moved through a water panel, and natural evaporation takes place. The now humidified air carrying moisture in vapor form is circulated throughout the home.
Flow Through-type
Flow through units maintain indoor relative humidity by flowing fresh water across a porous evaporator pad, where it is exposed to warm, dry air forced through the humidifier by the furnace blower or an internal fan. As the water evaporates, the heating system circulates humidified air throughout the home. Flow-thru humidifiers are designed to accommodate narrower ductwork, lower furnace temperatures, and shorter blower cycles.
Drum-type
Drum humidifiers rely on forced air, and are well-suited for warm-air heating systems. A rotating evaporator pad, or drum, lifts water from the water pan into the air stream, which is warmed and circulated by the furnace blower. The pad’s multiple holes and connecting strands increase the area from which water can evaporate.
Spray-type
Spray humidifiers are designed to be mounted on the warm air side of a furnace system and should only be used with naturally soft water. Spray humidifiers release a fine mist into the system’s ductwork, where it is distributed evenly throughout the home by the furnace blower.
When installing commercial and industrial humidifiers, several factors must be taken into consideration to ensure proper system operation and installation.
PLEASE CONSIDER THE FOLLOWING:
Humidifier noise levels vary by technology, but in general, humidifiers should be installed away from quiet or occupied areas.
Direct steam injection systems can be noisy at the control valve, where boiler steam passes through on its way to a dispersion assembly.
Non-pressurized steam generators produce noise when filling and draining. Some electric steam humidifiers produce noise when contactors cycle on and off. For very quiet areas, consider an electric unit with quiet solid state relay (SSR) control rather than contactors. When not dispersing into a duct or air handling unit (AHU), locate dispersion fan units in quiet areas away from the steam generator.
Noise from adiabatic (unheated water) humidifiers varies by type. Residential bypass humidifiers generate minimal noise. Atomizing systems have water pumps and sometimes also air compressors that generate a fair amount of noise. With these systems it is usually possible to place the dispersion nozzles remotely from the pumping station, isolating noise to a mechanical room.
Create access to installed humidification equipment to allow convenient visual inspection and maintenance. Boiler-steam humidifiers contain devices (such as a control valve, a steam trap, and a strainer) that require periodic maintenance. While these devices can operate for long periods of time without attention, they should be accessible for inspection.
Non-pressurized steam humidifiers are either cleanable or have disposable cylinders. Location should allow for easy access with ample space for servicing.
Adiabatic humidifiers can require adjacent water treatment, so allow enough space for all system components. Locate residential bypass humidifiers where it’s easy for homeowners to replace evaporative pads, and place controls so they are easily accessed by the homeowner.
Protect areas surrounding humidifiers from water damage. The potential for water damage is always present with any steam or water appliance, but a properly planned installation virtually eliminates this risk. Be aware of valuable equipment or stored materials that are either adjacent or on a lower level. Install drip pans under humidifiers to contain water leaks when necessary.
For runs over 10 feet, use stainless steel or copper tubing to connect non-pressurized steam generators to dispersion assemblies, rather than using a steam hose. Though initially more expensive, there are several advantages to using rigid interconnecting pipe, including:
Non-pressurized steam systems generate condensate in dispersion tubes and in the piping that connects the steam generator to the dispersion assembly. This water must be drained to prevent it from being discharged into the duct. Drain condensate back to the steam generator whenever possible.
There are two ways to return condensate to a non-pressurized steam generator. One method is to pitch the dispersion assembly and steam piping to return condensate to the generator. When this is done, condensate flows counter to steam flow. This method works satisfactorily only when steam piping is adequately sized for humidifier capacity and when a given minimum recommended pitch can be maintained. Otherwise, the velocity of the steam will carry condensate into the duct. Usually, humidifier manufacturers recommend a maximum steam flow rate and a minimum pitch.
The other method, used with higher capacity systems, involves returning condensate through a separate line. With this method, the dispersion assembly has a drain connected to a condensate return line, and both are pitched so that condensate returns to the humidifier. This installation requires an air vent in the return line.
When draining condensate to an open drain, a P-trap is required to prevent steam from escaping into the room. The trap is usually constructed of metal tubing. Water column height in the P-trap must be sufficient to withstand the operating pressure inside the humidifier. Consult the manufacturer’s recommendations for P-trap heights.
Many non-pressurized steam humidifiers have periodic drain and flush cycles or automated skimming where a portion of water in the tank drains to remove precipitated minerals. This water is typically 212 °F (100 °C) which, according to many municipal codes, should not be discharged to a sanitary system. In this situation, add a water tempering device to cool discharged water. Water tempering may be required for condensate to drain, as well.
This device is a must for steam and misting type humidifier installations and is mounted in the duct downstream of the dispersion assembly. It is usually set at 90 percent relative humidity (RH). If duct RH rises above that setting, it will shut down the humidifier. Wetted media humidifiers cannot over saturate airstreams and so a duct high-limit humidistat is not a requirement.
To ensure proper operation, the high-limit sensor must be located far enough downstream of the disperser to allow for full absorption and for humidity to equalize in the airstream. Otherwise, short cycling of the humidifier can occur, resulting in an unsatisfied humidification demand.
Most residential humidifiers can use an inexpensive on/off-type high-limit humidistat, except in variable air volume (VAV) systems. In this case, a modulating high-limit humidistat provides more satisfactory control.
If duct air flow stops for any reason, all humidifiers (except for wetted-media types) must be shut down immediately to prevent water damage. This can be accomplished by using a duct-mounted airflow proving switch. Use a switch that activates by the movement of the air or pressure in the duct. A sail switch or pressure differential switch may also be used.
Another type is a diaphragm-operated switch, in which the static pressure inside the duct actuates the switch. The latter type isn’t a good choice for VAV systems, because when the VAV box modulates the airflow, the duct static pressure remains high and allows the humidifier to continue to operate without sufficient volume of steam-absorbing airflow. A sail-switch is the best choice for VAV applications.
The location of the humidity sensor or transmitter has a significant impact on humidifier performance. Placing a sensor in the center of a room or inside a return air duct will give the best all-around control.
Do not place a sensor near a supply duct, on a perimeter wall, in sunlight or near a heat-producing object. The humidity sensor should be located where it can measure an average space temperature and humidity.
Residential in-duct humidifiers
Residential in-duct humidifiers should be specified and installed by licensed, qualified heating and air conditioning professionals. The installer is responsible for choosing the correct humidifier based on many factors, including the home’s humidity demand, type of heating system, floor plan, accessibility, and water characteristics.
When properly designed, specified, installed, and maintained, humidification systems operate trouble-free for years.
When operational issues occur, review the humidifier’s troubleshooting guide, but also consider that other HVAC issues, such as temperature or airflow, affect humidifier performance.
CONSIDER THE FOLLOWING HUMIDIFIER MAINTENANCE GUIDELINES:
The best prevention against field issues is regular inspection and maintenance. Maintenance requirements vary depending on humidifier type (e.g., cleanable boiling chambers vs. disposable) and water type (hard, naturally soft, softened, or demineralized water). All systems should be inspected after the first three months of duty to determine future inspection and maintenance requirements.
Operating a humidification system within the parameters for which it was designed will keep systems running smoothly. For example, if a system was designed to meet a 35 percent relative humidity (RH) requirement in a humidified space, but building occupants are operating systems at 55 percent, available absorption distance in a duct can become inadequate, resulting in duct wetness. It is important to communicate system expectations to building occupants.
All humidifiers convert water to vapor, and supply water type has the most impact on maintenance requirements.
Untreated tap water includes minerals — such as magnesium, calcium, and iron — that make water “hard.” These minerals clog boiling chambers, wetted media pads, and atomizing nozzles. Adiabatic (unheated water) systems can disperse minerals into the air causing white dust, which can contaminate furnishings and processes as well as cause inhalation hazards. Using softened water can mitigate many of these issues. Systems using properly processed demineralized water, such as deionized or reverse-osmosis filtered water have virtually no supply water maintenance issues.
Equipment using treated water should be tested regularly to ensure proper quality. Equipment using deionized water requires changing deionizing beds in a timely manner to ensure that chlorides, which corrode stainless steel, are not introduced into the system.
Start with an easy maintenance task and check and tighten electrical connections such as contactors, fuses, and sensors.
Verify signal compatibility and properly placed sensors. For example, a signal going to a humidifier from a transmitter, humidistat, or building automation system must be compatible with a signal a humidifier is set to receive. A controller will not understand a modulating humidistat sending a 4- to 20-mA signal if it is not wired or programmed to receive that signal.
Signal inputs typically can be changed at a controller board and/or through a controller’s software. A control manual will have specific instructions. All control components should be wired per an appropriate wiring diagram.
A high-limit humidistat normally should be installed in a supply duct about 12 to 15 feet downstream of where a dispersion assembly delivers steam. If it is installed too close to the dispersion assembly it either will quit operating because it is wet or shut off the humidifier because it will accurately read that duct RH is very high (high-limit humidistats are adjustable and usually are set to switch open between 80 and 90 percent RH).
Steam dispersion assemblies operate trouble-free if designed and installed correctly. They disperse distilled water so there are no issues with clogging, there are no moving parts, and they are constructed mostly of welded metal, often stainless steel. Adiabatic (unheated water) humidifiers can clog if water is untreated. Ultrasonic humidifiers have disks that need replacing periodically.
Steam generators with cleanable stainless-steel tanks operate at peak performance when clean. Mineral accumulation insulates heaters, reducing output and potentially causing heater failure. Use a manufacturer-recommended cleaning solution for stainless steel tanks. Use softened or demineralized water to reduce mineral accumulation. Or, consider using a steam generator that has a replaceable plastic cylinder, requiring no cleaning. When mineral accumulation reaches a predefined level, the humidifier’s controller will typically present a “change cylinder” message.
Residential Humidifiers usually have the capability of distributing up to 10 pounds of water per hour (1.2 Gal. per hour or 28 .8 Gal. per day) into the home. Their performance is usually tested by the manufacturer or an independent laboratory to one of the following standards:
Commercial and Industrial humidifiers usually have a capability of distributing higher capacities of water into the space than the above residential humidifiers. Their performance is usually tested by the manufacturer or an independent testing laboratory to ANSI/AHRI Standard 640, Performance Ratings for Commercial — Industrial Humidifiers.
These standards define the specific testing method and design of the test facilities, temperatures, airflow, water pressure, and weighing method that is to be used so all humidifiers are tested to the same conditions.
Residential and commercial and industrial humidifiers are usually tested for electrical and other safety considerations using one or more of the following standards:
They specify construction requirements that have been proven over the years to provide safe operation of the equipment.
Humidifiers tested and approved by independent testing agencies to the above electrical standards will have the UL or CSA logo listed on the electrical data plate of the humidifier.
Table top and portable type humidifiers are usually tested using standard ANSI/AHAM HU-1- Household Humidifiers (Tabletop and Portable Type). These humidifiers also are usually tested and approved using one of the above UL and/ or CSA standard.
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