Humidification System Design Basics
Use Available Resources to Calculate Load CorrectlyA humidified building constantly loses moisture, primarily due to air changes. As a general rule, humidification load is based on the amount of outside or makeup air entering a building or space, and calculated based on the difference between entering conditions and desired conditions. Load calculation methods vary, depending on whether the building uses mechanical, natural, or economizer ventilation.
When using natural ventilation, calculate humidification load based on expected air changes per hour within the conditioned space. When using mechanical ventilation, calculate humidification load based on the amount of outside air entering the conditioned space.
A quick and accurate way to calculate load correctly is to use sizing and selection software distributed by most humidification system manufacturers. Software is especially helpful when calculating humidification load for buildings using economizer control, which require calculating humidification load based on maximum outdoor air intake at given temperature and rh values.
If calculating load is not straightforward, or if sizing humidification systems is an unfamiliar activity, work with a local humidifier manufacturer representative to guide you through the design and specification process.
Energy Source Must Fit the ApplicationWhen specifying a humidification system to meet design conditions, expect a maximum humidification load of two to three pounds per hour for every 100 cfm of outside air introduced. To convert one pound of water to vapor requires approximately 1,000 Btu. Choosing the correct energy source for this conversion not only depends on energy efficiency but on application specifics as well.
For example, if there is an existing on-site pressurized steam boiler with available capacity, a cost-effective and energy efficient humidification method is to inject pressurized boiler steam directly into a space or ducted airstream (Figure 1). However, some owners object to direct boiler steam injection humidification because boiler water anticorrosion chemicals are emitted into the airstream with the humidification steam, creating a potential health hazard. To take advantage of on-site boiler steam while avoiding the disadvantage of boiler chemicals in the airstream, consider a closed loop steam humidification system, such as a steam-to-steam evaporative steam generator (Figure 2). A steam-to-steam system routes boiler steam through a heat exchanger located in a boiling chamber filled with clean water and creates clean humidification steam that doesn't contain boiler chemicals.
Evaporative steam generators also operate using hot liquid (240°F and greater), electricity, or gas as energy sources. Historically, natural or LP gas humidification systems have provided significant energy savings over electric humidification systems. However, gas systems require flue venting and some require a sealed combustion air supply. If these requirements can be met, a gas fired evaporative steam generator is an energy-efficient choice.
For example, a humidification system operating for one year in Minneapolis - with a 100 lb/hr humidification design load humidifying 13,000 cfm with 30% outside air to meet desired conditions of 69° and 40% rh - would produce approximately 290,000 lb of humidification steam in one year. The Energy Information Agency lists average industrial utility rates for Minnesota (as of July 2005, the most current rates available at this writing) to be $0.056/kWh for electricity and $7.15/1,000 cu ft for natural gas.
Based on these rates, and adjusted for typical evaporative steam generator efficiencies, the energy cost to humidify the space described above for one year is approximately $5,600 using an electric evaporative steam generator and approximately $2,750 using a gas-fired evaporative steam generator, a significant savings. Given the fluctuating nature of energy costs, the easiest and most accurate way to compare electric and gas humidification energy costs is to use a humidifier manufacturer's energy calculation program that creates a customized report based on your local energy rates.
A significant advantage of electric resistive element or electrode evaporative steam generator systems is that they can be installed wherever power is available. Because of their relatively small size and ability to integrate into existing systems, electric humidification systems are used for many different application types.
Along with direct steam injection, electric humidification systems can provide tight rh control - to within 1% of setpoint - achieved usually with an electric resistive element steam generator operating with solid state relay (SSR) control and using deionized (DI) or reverse-osmosis (RO) treated water. This capability is critical for manufacturing processes requiring tight rh control. The many benefits of electric humidification, including lower upfront costs than gas humidification, frequently outweigh the typically higher energy costs.
Isothermal humidification systems, such as the direct steam injection, steam-to-steam, electric, and gas systems described above, are ideal for commercial and industrial applications primarily due to their high capacity, compact size, rapid response to control, uninterruptible operation, and ability to achieve steam absorption within a short distance.
An alternative to isothermal humidification is adiabatic humidification. Adiabatic systems such as foggers, ultrasonic, and piezo disk humidifiers disperse water droplets or fog into air that has enough latent heat energy to cause dispersed water to change state to vapor. As the dispersed water or fog changes state, air temperature drops, and rh increases.
Adiabatic humidification systems work best with warm, dry air and long unobstructed distances to achieve absorption. Most ducts and AHUs have supply air that is too cool to achieve adiabatic absorption without preheating, and they are too short to achieve absorption before water droplets or fog impinge on duct elbows, fans, or vanes and then drip, causing duct wetness. Area-type adiabatic systems, which disperse water droplets or fog into open spaces, are used successfully in manufacturing areas where process-generated heat rises to the location of the area-type adiabatic humidifier near the ceiling. The combination of warm air at the humidifier and distance to the floor can be enough to ensure absorption.
Choose Makeup Water Type CarefullHumidification systems operate using many water types, including:
- Potable water, directly from a city or well water source.
- Softened water, either heated or unheated.
- RO water, which has gone through a filtering process to remove most minerals, as well as some contaminants.
- DI water, which is generally considered to be the purest water. DI water is used where humidification steam must be free of minerals and contaminants, or where uninterruptible service is required.
Mineral concentration in the makeup water supply directly relates to the amount of maintenance an evaporative steam generator requires. As mineral concentration increases, so does maintenance. Environments that require an evaporative steam generator to be continuously online use demineralized (RO or DI) water and a float-based makeup water fill valve. This combination allows continuous filling and operation for several seasons without cleaning (although yearly inspections are recommended).
Equipment Location is CriticalAn isothermal humidification system includes a pressurized steam boiler or an evaporative steam generator, control components, and a dispersion assembly. While an evaporative steam generator can be placed in numerous locations, proper placement of the dispersion assembly and control components is crucial.
Available absorption distance is the key factor affecting dispersion assembly choice. Dispersed steam must be absorbed into the airflow before contacting duct elbows, fans, vanes, filters, or any solid object, or dripping will occur. For example, if there is 10 ft of available unobstructed duct or AHU length downstream of the dispersion assembly, several types of dispersion assemblies will achieve absorption within that distance.
Applications with 18 in. or less of available unobstructed duct or AHU length downstream of the dispersion assembly require a multiple tube dispersion assembly to ensure absorption. Know the available absorption distance before selecting equipment. Manufacturers list guaranteed absorption distances for dispersion assemblies based on the difference between entering and leaving rh conditions.
Be aware of operating sound characteristics when choosing system component locations. Steam-to-steam and direct steam injection systems generate sound as pressurized steam passes through control valves; the higher the steam pressure and steam volume, the louder the sound. Some electric evaporative steam generators generate sound when mechanical contactors cycle. When specifying an electric system for very quiet areas, consider a unit with SSR or silicone controlled rectifier (SCR) control rather than mechanical contactors.
Choose a location that allows access to equipment for convenient visual inspection and maintenance. Direct steam injection humidification systems include devices (such as a steam control valve, a steam trap, and a strainer) that require periodic maintenance. While these devices can operate for long periods without attention, they must be accessible for inspection. Depending on makeup water type, evaporative steam generators require periodic cleaning or cylinder replacement, so allow room for access to cleanout plates and cover removal. Electrical components are housed within the humidifier cabinet or located remotely. Allow 36 in. clearance in front of electrical panels.
If installing an adiabatic unit in an AHU, choose a location where there is sufficient heat available to ensure absorption. In most systems, this is downstream of the heating coil and upstream of the cooling coil. This location allows air to be preheated as needed.
Control Choices Affect System Choice and PerformanceA wide range of relative humidity control - from ±1% to ± 5% rh - is achievable with current humidification system control technology; however, note that humidity control rests not only with the control system, but with the complete building system. Individual building dynamics, such as temperature or air changes, can affect the accuracy and control capability of any humidification system. The factors to consider when specifying humidity control are:
- Desired relative humidity percentage (setpoint): typical rh ranges are:
- Comfort, static control: 35% to 40% rh.
- Paper storage, printing: 40% to 50% rh.
- Clean room: 35% to 55% rh.
- Medical facilities: 35% to 50% rh for general areas; 60% to 80% for critical care areas.
- Control accuracy required: How critical is control accuracy and what is an acceptable variance from the desired rh setpoint? Some manufacturing processes tolerate rh fluctuations of only ±1% from setpoint; humidification provided to improve human comfort can fluctuate up to ±5% from setpoint.
- Space temperature: The amount of moisture air can hold correlates directly to air temperature - this is why it's called "relative humidity." If air temperatures fluctuate, rh levels fluctuate. Accurate rh control requires stable temperature control.
- Component quality: The control system is only as good as the least accurate component in the system. Selecting humidity controls that match the application can eliminate many system difficulties.
- Component location: Sensor and transmitter location has a significant impact on humidifier performance. See the article titled "Location, Location, Location" in the April 2005 issue of Engineered Systems for control component placement recommendations and diagrams.
Two control components that help ensure drip-free dispersion are a duct high limit humidistat and an airflow proving switch connected to the humidifier controller.
A duct high limit humidistat ensures that rh does not exceed a typical setpoint of 85% to 90% at the humidistat location, downstream of the dispersion assembly. To ensure proper operation, locate the duct high limit humidistat far enough downstream of the dispersion assembly to allow steam to become fully absorbed and equally distributed in the airstream. Otherwise, short cycling can occur, resulting in an unsatisfied humidification demand.
Systems with constant air volume can use on/off-type high limit humidistats. Systems with VAV require a higher level of control than a constant air volume system and must use modulating high limit humidistats to track airflow changes. Changing airflow quantities require the use of both space- and duct-mounted humidity controls used in conjunction with a programmable logic controller to modulate humidifier output.
An airflow proving switch, mounted in the duct downstream of the dispersion assembly, switches open a safety circuit if airflow in the duct stops or decreases below an acceptable level, disabling the humidifier. Constant volume systems can use pressure- or sail-type airflow proving switches. VAV systems must use sail-type airflow proving switches because pressure-type switches are not accurate at low airflows.
A common control component used in cold climates is a temperature offset transmitter mounted on an inside pane of an exterior window. This device transmits window temperature to the controller, which lowers the rh setpoint when the outdoor temperature drops, preventing window condensation.
Manage Condensate to Prevent Wet DuctsAll isothermal humidification systems generate steam condensate in dispersion assemblies and in piping connecting evaporative steam generators to dispersion assemblies. This water must be drained to prevent it from being discharged into the duct. When possible, drain condensate back to the steam generator to conserve energy.
There are three ways to return condensate to an evaporative steam generator. One method is to pitch the dispersion assembly and interconnecting piping continuously back to the steam generator. This method, where condensate flows in the direction opposite of steam flow, works satisfactorily only when interconnecting piping diameter can accommodate maximum steam generator capacity and when a given minimum recommended pitch back to the steam generator can be maintained. Otherwise, the velocity of the steam will carry condensate into the duct. Typically, if the interconnecting piping diameter is at least as large as the diameter of the steam generator's steam outlet, and if piping is not longer than recommended, this method works well for returning condensate to the steam generator in lower capacity systems.
A second method, used with higher capacity systems, is to return 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 the dispersion assembly to be mounted higher than the steam generator such that condensate can drain by gravity directly to the steam generator without obstruction, and also requires an air vent in the condensate return line.
A third method is to return condensate to the steam generator using a condensate pump.
If unable to return condensate to the steam generator, waste tempered condensate to an open drain. See below for information about tempering hot drain water.
Install a metallic P-trap in all condensate lines with a water column height sufficient to force steam to exit at the dispersion assembly and not travel through the line. Consult manufacturer literature for required P-trap height, location, and piping sizes.
Temper Hot Drain WaterMany evaporative steam generators have periodic drain and flush cycles, or have automated skimming, where a portion of water in the boiling chamber overflows when filling to remove precipitated minerals. This water is typically 212°, which, according to most municipal codes, may not be discharged to a sanitary system. Some evaporative steam generators have integral water tempering, where cool water reduces hot drain water to a temperature of about 140° before draining. If water tempering is not integral to the steam generator, specify an add-on tempering device, which is typically a water mixing chamber with a temperature actuated cold water fill valve to ensure drain water cools to 140° before draining. Tempering drain water also protects PVC drain piping.
Many System Design Resources AvailableThis article touched on key issues to consider when designing or specifying an industrial or commercial humidification system. For more information about humidification system design issues, take advantage of manufacturer websites for design guides and educational newsletters, or discuss your application requirements with a manufacturer representative. A properly humidified building provides many benefits, which can be experienced only as the result of solid humidification knowledge and good system design. ES
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