How Humidifiers Work

Steam Humidification Direct Steam Injection Steam-to-Steam Humidifiers Unlike other humidification methods, Humidifiers Steam-to-steam humidifiers use a heat steam humidifiers have a minimal effect The most common form of steam exchanger and the heat of treated steam to on dry-bulb (DB) temperatures. The steam humidifier is the direct steam injection create a secondary steam for humidification humidifier discharges ready-made water type. From a maintenance point of view, from untreated water. The secondary vapor. This water vapor does not require direct steam humidification systems steam is typically at atmospheric pressure, any additional heat as it mixes with the air require very little upkeep. The steam placing increased importance on and increases relative humidity. Steam is supply itself acts as a cleaning agent to equipment location. ° ° pure water vapor existing at 212 F (100 C). keep system components free of mineral This high temperature creates a percep- deposits that can clog water spray and Maintenance of steam-to-steam humidifiers tion that steam, when discharged into the evaporative pan systems. is dependent on water quality. Impurities air, will actually increase air temperature. such as calcium, magnesium and iron This is a common misconception. In truth, Response to control and pinpoint control can deposit as scale, requiring frequent as the humidifier discharges steam into of output are two other advantages of the cleaning. Response to control is slower the air, a steam/air mixture is established. direct steam humidification method. Since than with direct steam because of the In this mixture steam temperature will steam is ready-made water vapor, it needs time required to boil the water. rapidly decrease to essentially the air only to be mixed with air to satisfy the temperature. demands of the system. In addition, direct Electronic Steam Humidifiers steam humidifiers can meter output by (Electrode) 50% means of a modulating control valve. As the system responds to control, it can Electronic steam humidifiers are used position the valve anywhere from closed when a source of steam is not available. 30% to fully open. As a result, direct steam Electricity and water create steam at humidifiers can respond more quickly and atmospheric pressure. Electrode-type units precisely to fluctuating demand. pass electrical current through water to provide proportional output. Use with pure The high temperatures inherent in steam demineralized, deionized or distilled water humidification make it virtually a sterile alone will generally not provide sufficient medium. Assuming boiler makeup water conductivity for electrode units. 70° DB is of satisfactory quality and there is no condensation, dripping or spitting in the Water quality affects the operation and The psychrometric chart helps illustrate maintenance of electrode-type humidifiers. that steam humidification is a constant DB ducts, no bacteria or odors will be disseminated with steam humidification. Use with hard water requires more frequent process. Starting from a point on any DB cleaning, and pure softened water can temperature line, steam humidification will shorten electrode life. Microprocessor- cause movement straight up along the Corrosion is rarely a concern with a properly installed steam system. Scale based diagnostics assist with constant DB line. The example illustrates troubleshooting. that 70°F DB is constant as we increase and sediment—whether formed in the unit RH from 30%-50%. This is true because or entrained in the supply steam—are steam contains the necessary heat drained from the humidifier through the (enthalpy) to add moisture without steam trap. increasing or decreasing DB temperature. Actual results Steam-to-Steam Humidification Figure 14-2. utilizing high pressure steam Direct Steam Humidification or large RH increases Figure 14-1. (more than 50%) increase DB by 1° to 2°F. As a result, no additional heating or air conditioning load occurs.

Cross Section of Manifold

Dry Steam

14 Electrode units are easily adaptable to Water Spray Response of water spray humidifiers different control signals and offer full to control is slow due to the need for The water spray process can create modulated output. However, the need to evaporation to take place before humidi- potential temperature control problems. In boil the water means control will not fied air can be circulated. On/off control order to become water vapor or humidity, compare with direct-injection units. of output means imprecise response to water requires approximately 1,000 Btu system demand and continual danger per pound to vaporize. This heat must be of saturation. Water spray systems can drawn from the air, where it will hopefully Electronic Steam Humidifiers distribute large amounts of bacteria, and vaporize. If not enough heat is available unevaporated water discharge can (Ionic Bed) quickly enough, the water remains a collect in ducts, around drains and drip Ionic bed electronic humidifiers typically liquid. This unvaporized water can result in pans, and on eliminator plates, encour- use immersed resistance heating elements overhumidification, and the water can aging the growth of algae and bacteria. to boil water. Since current does not pass “plate out” on surfaces, creating a sanita- Corrosion is another ongoing problem through water, conductivity is not a tion hazard. with water spray humidification. Scale concern. Therefore, ionic bed technology and sediment can collect on nozzles, makes the humidifier versatile enough to ductwork, eliminator plates, etc., leading accommodate various water qualities. 50% to corrosion and high maintenance costs. These units work by using ionic bed cartridges containing a fibrous media to attract solids from water as its temperature 30% Evaporative Pan rises, minimizing the buildup of solids The evaporative pan method uses steam, inside the humidifier. Water quality does ENTHALPY hot water or electricity to provide energy not affect operation, and maintenance for heating coils which in turn heat water typically consists of simply replacing the and create water vapor. This method is cartridges. most effective when installed in smaller 70° DB capacity environments either in the air Ionic bed humidifiers are adaptable to handling system or individually within different control signals and offer full the area(s) to be humidified. modulated output. Control is affected by Water spray contains virtually none of the the need to boil the water, however. heat of vaporization required to increase the RH of the air to desired conditions. For this reason, water spray humidification 50% is a virtually constant enthalpy process. However, as the psychrometric example illustrates, DB temperature changes as we 30% increase RH from 30%-50%. The result of this loss of DB temperature is an in- creased heating load to maintain 70°F. Electronic Steam Humidification Figure 15-1. 70° DB

Evaporative pan humidification can increase dry-bulb temperature as mea- Water Spray sured on the psychrometric chart. This Figure 15-2. unwanted temperature change may occur as air is forced across the warmed water Heat Taken From Air in the pan. The increase in DB can cause To Evaporate Moisture damaging results in process applications and increase the need for humidity control. The psychrometric chart helps illustrate Water Mist that evaporative pan humidification is not a constant DB process. This example shows DB temperature increasing as we move from 30%-50% RH. To maintain a constant DB of 70°F some cooling load (air conditioning) is required. Water Supply Under Pressure

15 How Humidifiers Work, Continued...

Maintenance of evaporative pan humid- Cost Comparisons Installation costs for the various types ification systems demands regular cannot be accurately formulated because To fairly evaluate the costs of selecting a cleaning of the heating coils and pan, the proximity of water, steam and electricity humidification system, you should include which are subject to “liming up.” to humidifiers varies greatly among installation, operating, and maintenance installations. Operating costs are low for costs as well as initial costs. Total humidi- The use of chemical additives added direct steam and slightly higher for steam- fication costs are typically far less than either automatically or manually to the to-steam. Water spray and evaporative pan heating or cooling system costs. water in the pan can reduce this problem operating costs are also low. Energy costs by as much as half. are higher for electronic humidifiers. Initial costs, of course, vary with the size of the units. Priced on a capacity basis, Response to control with the evaporative Direct steam humidifiers have the lowest larger capacity units are the most pan method is slow due to the time maintenance costs. Ionic bed electronic economical, regardless of the type of required for evaporation to take place humidifiers are designed specifically to humidifier, i.e.: one humidifier capable of before humidified air can be circulated. minimize maintenance while adapting to delivering 1,000 pounds of humidification Output is determined by water tempera- various water qualities. Maintenance costs per hour costs less than two 500 lbs/hr ture and surface area. for other types can vary widely, depending units of the same type. on water quality and applications. Evaporative pan humidifiers can sustain Direct steam humidifiers will provide the bacteria colonies in the reservoir and These are the principal considerations in highest capacity per first cost dollar; water distribute them throughout the humidified selecting a humidification system. Table spray and evaporative pan are the least space. High water temperatures, water 16-1 summarizes the capabilities of each economical, assuming capacity needs of treatment, and regular cleaning and humidifier type. 75 lbs/hr or more. flushing of the humidifier help to minimize the problem, however.

Evaporative Pan Figure 16-1.

Water Vapor Float Valve Steam, Hot Water Or Electricity In

Heated Water Auto Valve

Water Pan

Table 16-1. Comparison of Humidification Methods Direct Steam Steam-to-Steam Electronic Steam Ionic Bed Electronic Steam Evaporative Pan Water Spray Small Substantial Effect on temperature Virtually no change temperature rise temperature drop Unit capacity per unit size Small to very large Small Small to medium Small to medium Small Small Vapor quality Excellent Good Good Good Good Poor

Response to control Immediate Slow Fair Fair Slow Slow

Control of output Good to excellent Below average Average Average Average Average Pan subject to Subject to severe Sterile medium; Bacteria can Programmed to not Programmed to not Sanitation/corrosion corrosion; bacteria corrosion and corrosion free be present promote bacteria promote bacteria can be present bacteria problems Maintenance frequency Annual Monthly Monthly to quarterly Quarterly to semi-annually Weekly to monthly Weekly to bimonthly

Maintenance difficulty Low High Medium Low High High Costs: Price (per unit of capacity) Low High Medium Medium High Medium to high Installation Varies with availability of steam, water, electricity, etc. Operating Low Low Medium Medium Low Low Maintenance Low High High Low to medium High Very high

16 Recommended Applications The evidence supports the conclusion that With these advantages in mind, engineers Steam: Recommended for virtually all steam is the best natural medium for specify steam boilers and generators commercial, institutional and industrial humidification. It provides ready-made solely for humidification when the building applications. Where steam is not avail- vapor produced in the most efficient to be humidified does not have a steam able, small capacity needs up to 50-75 evaporator possible, the boiler. There is supply. The minimum humidification load lbs/hr can be met best using ionic bed type no mineral dust deposited, and because where this becomes economically feasible self-contained steam generating units. there is no liquid moisture present, steam falls in the range of 50-75 lbs/hr. Steam Above this capacity range, central system creates no sanitation problems, will not generator capacity is generally specified steam humidifiers are most effective support the growth of algae or bacteria, 50% greater than maximum humidification and economical. Steam should be has no odor and creates no corrosion or load, depending on the amount of piping specified with caution where humidification residual mineral scale. Unless there are and number of humidifiers and distribution is used in small, confined areas to add particular requirements to an application manifolds that must be heated. Typical large amounts of moisture to hygroscopic that can only be met with evaporative piping for boiler-humidifier installations is materials. We recommend that you pan or water spray methods, steam shown in Fig. 17-1. consult your Armstrong Representative humidification will meet system needs regarding applications where these most effectively and economically. conditions exist.

Evaporative Pan: Recommended only as an alternative to self-contained steam generating unit humidifiers for small load commercial or institutional applications. Generally not recommended where load requirements exceed 50-75 lbs/hr.

Water Spray: Recommended for industrial applications where evaporative cooling is required; typical application is summer- time humidification of textile mills in the southern U.S.

Figure 17-1. Typical Piping for Boiler-Humidifier Installation Design Guidelines— To Additional Boiler-Humidifier Combinations Humidifiers 1. Boiler gross output capacity Strainer should be at least 1.5 times the total humidification load. 2. Water softeners should be used on boiler feedwater. 3. Condensate return system is not Safety Relief Valve necessary (unless required by Level Control & Automatic circumstances). Low Water Cutoff Blowdown Steam 4. Boiler pressure should be at System Humidifier 15 psig or less.

“Packaged” 5. An automatic blowdown system Steam is desirable. Generator Trap 6. All steam supply piping should To Drain be insulated.

Auto Fill Valve 7. No limit to size or number of humidifiers from one boiler. To Drain

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