STEP 1: REVIEW BUILDING CONSTRUCTION
When air and heat escape, so does humidity
Similar to protecting against unwanted heat loss in a building, there are a few things you can do to ensure accurate humidity control and to protect against unwanted condensation or loss of humidity. Whether working with new construction or in retrofit situations:
• Evaluate the vapor barrier qualities of the building you plan to humidify.
• Examine all areas of conditioned air loss, such as exhaust fans, open windows, and doors.
• Examine roof construction, especially steel supports, for poorly insulated areas that could result in condensation.
STEP 2: DETERMINE HUMIDIFICATION LOAD
Load is based on quantity of entering air
Properly calculating the load ensures correct and consistent humidity levels. Undersizing will result in a system that will not maintain desired relative humidity (RH); and oversizing may cause erratic humidity levels or wet ducts, filters or fans. As a general rule, the humidification load is based on the amount of air entering a building or space and the desired RH level.
How to calculate humidification load:
1. Natural ventilation method, used for buildings without a ventilation system. The load is usually calculated using the air change method.
2. Mechanical ventilation method, used for buildings with mechanical ventilation. The load is based only on the amount of air entering a building or space.
3. Economizer cycle method, used for buildings with year-round air conditioning equipped with economizer control. The load is calculated using temperatures of outside, inside, return and mixed air.
STEP 3: SELECT THE BEST BEST ENERGY SOURCE
Energy use is an important consideration
A pound of water requires approximately 1000 BTUs to vaporize. When selecting a humidification system, remember that two to three pounds of water are typically required for every 100 cfm of outside air introduced.
Two major types of humidifiers
• Isothermal systems use heat from an external source to create humidity. Electricity, natural gas, hot water and boiler steam are isothermal heat sources used to boil water into steam for humidification.
• Adiabatic systems use heat from the surrounding air to change water into vapor for humidification (evaporation). Wetted media, high pressure, foggers and ultrasonic technologies are typical adiabatic systems.
Why choose isothermal?
• If there is an on-site boiler or hot water source, direct steam injection or a heat-exchanger-type isothermal system may be most appropriate. Consider using a steam-to-steam system to avoid direct injection of boiler chemicals (see box, right).
• Electric hot element systems easily integrate. Because of their relatively small size and ease of integration into existing systems, electric element humidification systems can be used for virtually any application.
• The economic benefits of natural gas. Gas-fired humidification systems can offer dramatic energy savings over electric systems.
When is adiabatic appropriate?
• Is the supply air warm and dry? If so, your humidification needs may be met by an adiabatic system such as a fogger, which uses sensible heat in the air for its energy source. In the right environment, these systems can be very economical due to the cooling effect they provide. Exercise caution, however, when applying to standard commercial applications with short absorption distances and low discharge air temperatures.
STEP 4: CHOOSE THE APPROPRIATE WATER SOURCE
Types of water vary
Humidification systems operate using a variety of water types:
• Potable (tap) water, directly from a city or well water source.
• Softened water, either heated or unheated.
• Reverse osmosis (RO) water, which has gone through a filtering process to remove most of the minerals, as well as some contaminants.
• Deionized (DI) water, which is generally considered to be the purest water. DI water is used where the steam must be of high quality and free of minerals and contaminants, and where uninterruptable service is required.
• Direct injection of boiler water. See box below for discussion about choices when using on-site steam.
Water type affects performance
• Mineral concentration in the supply water directly relates to the amount of maintenance and cleaning your humidification system requires. The higher the mineral concentration, the more downtime required for cleaning. Environments that require a system to be continuously online should use demineralized (DI or RO) water. Systems using softened water can operate several seasons without the need for cleaning (yearly inspections are recommended).
STEP 5: DETERMINE HUMIDIFIER PLACEMENT
The humidification system generally consists of a vapor or steam generator and a dispersion assembly. The proper placement of these two components is crucial to the operating success of the humidification process. Usually, there is no single “right” placement for a humidifier. Much depends on the system design, its uses, and its applications. However, the following paragraphs and dispersion assembly placement examples are presented as guidelines for common situations.
First, check available absorption distance
Available absorption distance will affect system choice. Dispersed steam must be absorbed into the airflow before it comes in contact with any duct elbows, fans, vanes, filters, or any object that may cause condensation and dripping. Not all humidification systems guarantee absorption within a short distance, so it is important to be aware of the available absorption distance early in your design process. (See box below)
Placement of adiabatic units
Adiabatic units need to be positioned in an AHU system where sufficient heat is available to vaporize the water being added. In most systems, this is downstream of the heating coil and upstream of the cooling coil. This allows the air to be preheated as needed. Typically, adiabatic units require an AHU extension.
STEP 6: SELECT THE CONTROL SYSTEM
A wide range of relative humidity control (±1% to ± 5% RH) is achievable with today’s control technology; however, note that humidity control rests not only with the controls, but with the complete building system. Individual building dynamics, as mentioned in Step 1, can affect the accuracy and control of any humidification system.
Factors to consider when selecting humidity control
• Desired relative humidity set-point. Typical RH ranges are:
– Comfort, static control: 35% to 40% RH
– Paper storage, printing: 40% to 50% RH
– Semiconductor manufacturing: 35% to 55% RH
– Health medical facilities: 35% to 50% RH
• Acceptable RH fluctuation. What is the acceptable variance from the desired RH set-point?
• Space temperature. Stable temperatures need to be maintained to accurately control humidity.
• Component quality. The control system is only as good as the least accurate component in the system. Many system difficulties can be eliminated by selecting humidity controls that match the application.
Sensor and transmitter locations are critical (See figure)
Sensor or transmitter location has a significant impact on humidifier performance. In most cases, we recommend that you do not interchange duct and room humidity devices. Room humidity devices are calibrated with zero or little airflow, whereas duct humidity devices require air passing across them. Recommended sensor locations:
A) This is the ideal sensing location because this placement ensures the best uniform mix of dry and moist air with stable temperature control.
B) This location is acceptable, but the room environment may affect controllability, such as when the sensor is too close to air grilles, registers or heat radiation from room lighting.
C) This location is acceptable because it provides a good uniform mixture of dry and moist air, but if an extended time lag exists between moisture generation and sensing, make sure the control contractor extends the sampling time.
D) This location behind a wall or partition is acceptable for sampling the entire room if the sensor is near an air exhaust return outlet. This location is also typical of sensor placement for sampling a critical area.
E) These locations are not acceptable because they may not represent actual overall conditions in the space.
F) These locations are not acceptable. Do not place sensors near windows, door passageways or areas of stagnant airflow.
G) This is the best location for a duct high limit humidistat or humidity sensor.
