are kept at a low level then occupantswill still feel comfortable at RH levels of30 to 35 per cent.

The amount of moisture in buildingair is a function of the moisture generat-ed by the occupants and their activities,plus any moisture producing processequipment and humidification equip-ment, minus the drying effect ofmechanical ventilation and natural airinfiltration (air leakage). Moisture lossdue to mechanical ventilation is easilydetermined from a psychrometric chartor other engineering manuals.

For example, with outside conditionsof -10C and 100 per cent RH (snowing)and inside conditions of 20C and 40 percent RH, for every 100 l/s (200 CFM)of ventilation, a humidification capacityof approximately 2.8 kg/hr would berequired to maintain a moisture balance.

Moisture loss due to natural air infil-tration is more difficult to measure. It isa function of building tightness, build-ing height (stack effect), wind exposure

in general and specifically, the merits ofsteam humidifiers will be explored.

Dry indoor air conditions in winterwill often result in cool air complaintsalong with the additional commoncomplaints of dry skin, sore throats andincreased thirst and static electricity.Cool air complaints stem from the factthat up to one third of the total heat lossfrom the human body (at rest in normalindoor environments) is via evaporationfrom the skin and respiration. The lowerthe humidity level in the air, the greaterthe evaporation rate from the occupantand the less comfortable they feel.

HEALTH ISSUES

The amount of moisture in the air canaffect other aspects of air quality. Highrelative humidity (RH) levels are, ofcourse, associated with mould and otherfungal growth. Low RH levels can exac-erbate symptoms associated with somechemical contaminants. There is alsoevidence that some airborne viral andbacterial infections will remain viablelonger in dry air. Thus, proper sizingand operation of humidification equip-ment is important to both the comfortand the health of building occupants.

Proper sizing and operation ofhumidification equipment will result ina moisture balance to maintain an RHhigh enough for occupant comfort butlow enough to avoid any surface con-densation on windows, cool foundationwalls, and so on. While many designmanuals would suggest a humidityrange of 40 to 60 per cent RH, this maybe too high for older building surfacesduring the coldest winter temperatures.In addition, recent studies have shownthat if all other airborne contaminants

OPTIONS IN STEAM

humidification BY GORD COOKE

28 HPAC • JANUARY/FEBRUARY 2006

Since the thermal comfort of occupants in buildings is dependent on the amount of moisture

in the air, HVAC contractors need to understand proper sizing, installation and operational

issues associated with humidification and dehumidification systems. In this article, humidification

and building activities (operation ofdoors and windows). Contractors willhave to rely on the experience of humid-ification equipment suppliers and engi-neers for determining the load due tonatural infiltration. In general, in prop-erly ventilated commercial and institu-tional buildings, the mechanical ventila-tion load will determine the humidifica-tion design load.

SYSTEM SELECTION

In cases where supplemental humidifica-tion is required, systems must be prop-erly sized and installed to eliminatepotential issues related to the task ofadding moisture to air. That is, from anair quality perspective, humidifiers addto the complexity of maintaining warm,dry surfaces throughout the building.Standing water, damp surfaces in duct-work, controls that require adjustmentbut are inaccessible to occupants, regularmaintenance requirements and thepotential for water leaks, are of concernto indoor air quality (IAQ) control.

There are at least five types of humid-ifiers available and at least four or fivemajor brand names under which they are

Gra

phic

NO

RTEC

Air

Con

ditio

ning

Lim

ited

Strainer

Steam Valve

Dispersion Tube

Support Bracket

Cross SectionalView of Distributor

Dispersion Tube

Steam JacketSteam

Inlet Steam JacketPneumatic Actuator

Steam

Condensate

Steam Trap

CONTINUED ON PAGE 30

STEAM INJECTION HUMIDIFIER

HPAC01_028,030,031 2/3/06 9:53 AM Page 28

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Circle #20

HPAC01_029 1/30/06 3:35 PM Page 1

stream. This means there is reducedpotential for introducing water bornecontaminants into the air. (Not with-standing the issue of steam boiler chem-ical additives noted below).■ Steam systems have higher humidifica-tion capacities for a given duct size, airflow and plenum temperature. In manycommercial/institutional applications useof wet media type humidifiers would beimpractical due to capacity limitations.

■ Atomizing humidifiers that sling orspray water droplets into the air mayapproximate the capacity of steamhumidifiers but the cooling effect andproblems with mineral fallout, particularlyin hard water situations, are an issue.

marketed. For commercial and institu-tional applications and for large houseswith high humidification loads (duelargely to high air leakage rates), the sys-tem of choice should be steam humidi-fication for the following reasons:■ Steam humidification injects water invapour form into the air stream. Thisreduces the cooling effect in the airstream caused by the need for evapora-tion of water droplets inherent in other

types of humidifiers. With steam, the airtemperature in the duct is maintained.This improves comfort and reducespotential for condensation in the duct.■ By definition, steam humidifiers areinjecting boiled water into the air

30 HPAC • JANUARY/FEBRUARY 2006

humidification CONTINUED FROM PAGE 28

DIRECT STEAM HUMIDIFIERS

In buildings with steam boilers, boilersteam may be simply dispersed into theductwork of air handling equipmentusing a direct steam humidifier. Thereare a couple of methods of injecting thesteam. Sometimes a simple pot or pan isattached under the duct and steam isinjected into the side of this receptacleand the steam rises into the duct.Provisions must be made for condensateto return to the boiler and care must betaken to ensure condensate doesn’tsplash into the duct.

A more sophisticated device known asa steam injection humidifier uses singleor multiple injection tubes surroundedby a jacket of steam to ensure condensatefree steam is injected into the air stream.The biggest drawback of direct steamhumidifiers is that the neutralizing chem-icals required in steam boiler systems toeliminate acid buildup in piping can bereleased into the air stream. These chem-icals can cause a wide range of symptomsfor occupants, including skin rashes, eyeand throat irritation and nausea.

From an air quality perspective thistype of humidifier should not be usedunless the steam generation system isfree of chemical additives. An option forsteam boiler systems is to use a steam-to-steam heat exchanger, with the steam tobe used for humidification isolated fromthe boiler steam.

SELF-CONTAINED STEAM HUMIDIFIERS

Self-contained devices generate steamwith either an electric element sub-merged in a water chamber or an elec-trode-type heating element in a sealedplastic container. The steam is deliveredto the air stream of a duct system viashort piping and a dispersal tube. Bothmethods use safe potable water.

LOOKINGFOR

“In general, in properly ventilated commercial and

institutional buildings, the mechanical ventilation load will

determine the humidification design load.”

HUMIDIFIERS

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Circle #21

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HPAC01_028,030,031 2/9/06 9:45 AM Page 30

The simple electric element units canuse pretreated or softened water to min-imize scale build up. Pretreatment of the

water for sealed electrode type humidi-fiers is counterproductive as the demin-eralized water becomes less conductiveand heating performance is poor.

For electrode type humidifiers, typi-cally sealed containers and electrodes arereplaced regularly to resolve the build upof minerals in the tanks. Both of thesehumidifiers require relatively large elec-trical service inputs and this may be aninstallation issue. The actual energy con-sumption of steam humidifiers is offsetby the reduction in energy required tomaintain comfort – that is, lower airtemperature setpoints can be used whenthe RH is in a comfortable range.

In summary, steam humidifiers arethe best alternative for the high capacityrequirements of large houses and com-

mercial buildings. They offer reliableoperation with straightforward mainte-nance for HVAC contractors. In allcases humidification systems must becarefully planned and installed so as toavoid problems with leaks or wetting ofductwork or other building materials.Proper humidification can increaseoccupant comfort and indoor air qual-ity and, in some cases, reduce energyconsumption.

■ Gord Cooke, P.Eng., is president of AirSolutions Inc., Cambridge, ON. Contact him at1-800-267-6830, e-mail gcooke@airsolutions.on.caor visit www.airsolutions.on.ca for more information.

JANUARY/FEBRUARY 2006 • HPAC 31

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“The actual energy consumption of steam humidifiers is offset

by the reduction in energy required to maintain comfort.”

AN IAQ PERSPECTIVEWhen selecting humdifiers the followingdecision criteria should be considered:■ Capacity sized to the demand ■ The potential for leaks or breakdowns

that may cause wetting events■ The potential for standing water and

the introduction of water borne pollutants into the air stream

■ Scaling and overall maintenance requirements

■ Control strategies

Circle #22

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Circle #23

HPAC01_032 1/30/06 3:41 PM Page 1