Sensor And Transmitter Locations Are Critical
Sensor and transmitter location has a significant impact on humidifier performance. Typical humidification sensors and transmitters include:
- Humidistat - A device that monitors and controls relative humidity (RH) levels. The humidistat sends a signal to the humidifier to produce or stop producing humidification vapor based on setpoint. Humidistats allow occupants of the humidified space to adjust setpoint.
- Humidity transmitter - A monitoring device that senses RH and sends an output signal based on humidity level. A separate controlling device, such as the humidifier's controller or a BMS, interprets that signal and controls the humidifier accordingly.
- High-limit humidistat - A controlling humidistat mounted in a duct to prevent duct saturation. If RH reaches a predefined level - typically 90% - the high-limit humidistat disengages the humidifier. The high-limit humidistat is a very important safety device to prevent wet ducts if, for example, duct temperature unexpectedly drops and the air cannot absorb as expected.
- Airflow proving switch - This is another important safety device. The airflow proving switch disengages the humidifier if airflow stops, preventing duct saturation.
- Temperature transmitter - Commonly installed on a north-facing window, the temperature transmitter provides an output signal based on temperature. Typically used to prevent window condensation on very cold days, a separate controlling device reduces humidifier output corresponding to temperature. This device is sometimes called a cold-snap offset transmitter.
Humidification sensors and transmitters affect humidifier output and, therefore, affect a humidification system's ability to meet design requirements. Sensor and transmitter readings must be accurate and pertinent. Figure 1 shows recommended locations for sensors and transmitters. Figures 2 through 4 also show recommended locations for high limit humidistats and airflow proving switches. Note: 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
In Figure 1, location A is the ideal sensing location for a duct humidistat or humidity transmitter because this placement ensures the best uniform mix of dry and moist air with stable temperature control.Location B 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.
Location C 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 installing contractor extends the sampling time.
Location D, 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.
Locations E are not acceptable because they may not represent actual overall conditions in the space.
Locations F are not acceptable. Do not place sensors near windows, door passageways, or areas of stagnant airflow.
Location G is the best location for a duct high limit humidistat or humidity sensor and the airflow proving switch.
Additional sensor placement considerations
A humidifier's 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.
A VAV control package is used with VAV systems, which require a higher level of control than a constant air volume system. Changing air flow quantities and the cooler supply air found in most VAV systems require the use of both space- and duct-mounted humidity controls used in conjunction with a programmable controller, which modulates the output of the humidifier.
Dispersion Unit Placement
Dispersion units are the most important humidification system component to place properly. Improper placement can cause incomplete absorption, which can cause wet ducts, dripping, mold growth, and not meeting setpoint. Not meeting RH setpoint in a humidification-for-comfort application is inefficient. However, not meeting RH setpoint in a facility that relies on stable RH to complete processes, such as a printing plant, can cause machines to go offline, costing the owner thousands of dollars.To ensure proper absorption, place dispersion units where there is enough unobstructed space downstream of the dispersion assembly for absorption to occur. Nonwetting distance is the dimension from the leaving side of the dispersion assembly to the point where wetting will not occur. Solid objects such as coils, dampers, or fans beyond this dimension will remain dry. Nonwetting distances in ducts and AHUs are measured in linear feet; area-type systems define nonwetting distance using rise, spread, and throw dimensions. Humidifier manufacturers publish nonwetting distances based on dispersion assembly type, humidification load, temperature, RH, and airflow.
Unless you have a solid understanding of psychrometrics, the easiest way to ensure you have enough available space to ensure absorption is to rely on manufacturer's data or use selection software distributed, usually free, by the humidifier manufacturer. If you have a limited amount of room for absorption, use a steam dispersion assembly with multiple tubes and steam orifices. These assemblies can achieve absorption within inches.
The most challenging type of dispersion assembly to position properly is an adiabatic unit. Adiabatic systems emit a fine mist or fog of unheated water into the air and use heat from the surrounding air to change water into vapor for humidification (evaporation).
Position adiabatic units 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. For adiabatic placement guidance, rely on manufacturer's instructions.
Placing A Steam Dispersion Assembly In An AHU
Figure 2 shows locations for placing a steam dispersion assembly, duct high-limit humidistat, and an airflow proving switch in an AHU. A high-limit humidistat ensures that RH does not exceed its setpoint up to a maximum of 90% rh. It must be located past the point of steam absorption, which will vary depending on the dispersion device chosen. An airflow proving switch disengages humidifier operation if the fan stops. These critical safety devices ensure the humidifier operates only when other HVAC components function as expected. Figure 2 shows dispersion assembly and control component placement guidelines for a typical AHU.In this application, Location A is the best location for installing a steam dispersion unit. Installing downstream of heating and cooling coils provides laminar flow through the dispersion unit; plus, the heated air provides an environment for best absorption. Use a multiple-tube dispersion unit to ensure complete absorption of steam before fan entry.
Location B is the second-best choice. However, in changeover periods, the cooling coil will eliminate some moisture for humidification.
Location C is the third-best choice. Air leaving a fan is usually very turbulent and may cause vapor to not absorb at the expected absorption distance. Allow for more absorption distance if installing downstream of a fan.
Location D is the poorest choice. The cooler air at this location requires an increased absorption distance.
Placing A Dispersion Assembly Near An Elbow
When placing a dispersion assembly near an elbow, Figure 3 shows that location A is the best choice. Better absorption occurs on the downstream side of an elbow than on the upstream side.Location B is the second best choice. Installing upstream of an elbow can cause wetting at the turning vanes. In cases where it is structurally impossible to avoid Location B, use a multiple tube dispersion unit to ensure complete absorption. Also, since more air flows along the outside of a turn, better absorption occurs if the humidifier discharges proportionately more steam in that part of the airstream.
At both locations, discharging steam against or perpendicular to the airstream gives slightly better mixing and absorption than discharging with the airstream.
