Humidifiers and dehumidifiers

Humidity-Control MethodsHumidifiers, by definition, are devices for adding moisture to the air. Thus, to humidify is to increasethe density of water vapor within a given space or room. Air humidification is affected by vaporizationof water and always requires heat for its proper functioning. Thus, devices that function to add moistureto the air are termed humidifiers, whereas devices that function to remove moisture from the air aretermed dehumidifiers.

HumidifiersAs previously noted, air humidification consists of adding moisture. Following are the types ofhumidifiers used in air-conditioning systems:

Spray-type air washers Pan evaporative humidifiers Electrically operated humidifiers Air-operated humidifiers

Air-Washer MethodAn air washer essentially consists of a row of spray nozzles inside a chamber or casing. A tank at thebottom of the chamber provides for collection of water as it falls through the air and comes into intimatecontact with the wet surface of the chamber baffles. The water is generally circulated by means of apump, the warm water being passed over refrigerating coils or blocks of ice to cool it before beingpassed to the spray chamber. The water lost in evaporation is usually replaced automatically by the useof a float arrangement, which admits water from the main as required. In many locations, the water issufficiently cool to use as it is drawn from the source.In other places, the water is not cool enough and must be cooled by means of ice or with a refrigeratingmachine.The principal functions of the air washer are to cool the air passed through the spray chamber and tocontrol humidity. In many cases, the cooling coils are located in the bottom of the spray chamber so thatas the warm spray descends, it is cooled and ready to be again sprayed by the pump. In some cases, thewater is passed through a double-pipe arrangement and is cooled on the counter-current principle. Figure1 shows a sketch of an air washer. In this case, the spray pipes are mounted vertically. In someinstances, the spray pipes are horizontal so that the sprays are directed downward. As some of the finerwater particles tend to be carried along with the air current, a series of curved plates or baffles isgenerally used, which forces the cooled and humidified air to change the direction of flow, throwing outor eliminating the water particles in the process.

Figure 1 Elementary diagrams showing essential parts of air-washer unit with identification of parts.

Pan HumidifiersFigure 2 shows the essential parts of the pan-type humidifier. The main part is a tank of water heated bylow-pressure steam or forced hot water where a water temperature of 200F or higher is maintained. Theevaporative-type humidifier is fully automatic, the water level being controlled by means of a floatcontrol. In operation, when the relative humidity drops below the humidity-control setting, thehumidifier fan blows air over the surface of the heated water in the tank. The air picks up moisture. Theair is blown to the space to be humidified. When the humidity control is satisfied, the humidifier fanstops.

Figure 2 Typical evaporative pan humidifier showing operative components.

Electrically Operated HumidifiersDry-steam electrically operated humidifiers operate by means of a solenoid valve, which is energized bya humidistat. When the relative humidity drops slightly below the desired level set by the humidistat(see Figure 3), a solenoid valve actuated by the humidistat admits steam from the separating chamber tothe re-evaporating chamber.Steam passes from this chamber through the muffler directly to the atmosphere. The fan (which isenergized when the solenoid valve opens) assists in dispersing the steam into the area to be humidified.When the relative humidity reaches the desired level, the humidistat closes the solenoid valve and stopsthe fan.

Air-Operated HumidifiersAir-operated humidifying units operate in the same manner as electrical units, except that they utilize apneumatic hygrostat as a humidity controller and an air operator to open or close the steam valve (seeFigure 4). A decrease in relative humidity increases the air pressure under a spring-loaded diaphragm toopen the steam valve wider. An increase in relative humidity reduces the pressure under the diaphragmand allows the valve to restrict the steam flow. In a humidifier operation of this type, the steam supply istaken off the top of the header (see Figure 5). Any condensate formed in the supply line is knockeddown to the humidifier drain by a baffle inside the inlet of the humidifier-separating chamber.

Figure 3 Armstrong dry-steam humidifier for direct discharge of steam into the atmosphere of the area to behumidified.

Any droplets of condensation picked up by the stream as it flows through the humidifier cap when thesteam valve opens will be thrown to the bottom of the re-evaporating chamber. Pressure in this chamberis essentially atmospheric. Since it is surrounded by steam at supply pressure and temperature, any wateris re-evaporated to provide dry steam at the outlet. The humidifier outlet is also surrounded by steam atsupply pressure to ensure that there will be no condensation or drip at this point. A clamp-ontemperature switch is attached to the condensate drain line to prevent the electric or pneumatic operatorfrom opening the steam valve until the humidifier is up to steam temperature.

Figure 4 Armstrong dry-steam humidifier with steam-jacket distribution.

Figure 5 Operation of Armstrong dry-steam humidifier for area humidification.

DehumidifiersThe removal of moisture from the air is termed dehumidification. Air dehumidification is accomplishedby one of two methods: cooling and adsorption. Dehumidification can be accomplished by an airwasher, providing the temperature of the spray is lower than the dew point of the air passing through theunit. If the temperature of the spray is higher than the dew point, condensation will not take place. Airwashers having refrigerated sprays usually have their own recirculating pump.Electric DehumidificationAn electric dehumidifier operates on the refrigeration principle. It removes moisture from the air bypassing the air over a cooling coil. The moisture in the air condenses to form water, which then runs off

the coil into a collecting tray or bucket. The amount of water removed from the air varies, depending onthe relative humidity and volume of the area to be dehumidified. In locations with high temperature andhumidity conditions, 3 to 4 gallons of water per day can usually be extracted from the air in an average-size home.When the dehumidifier is first put into operation, it will remove relatively large amounts of moistureuntil the relative humidity in the area to be dried is reduced to the value where moisture damage will notoccur. After this point has been reached, the amount of moisture removed from the air will beconsiderably less. This reduction in moisture removal indicates that the dehumidifier is operatingnormally and that it has reduced the relative humidity in the room or area to a safe value. Theperformance of the dehumidifier should be judged by the elimination of dampness and accompanyingodors rather than by the amount of moisture that is removed and deposited in the bucket. A dehumidifiercannot act as an air conditioner to cool the room or area to be dehumidified. In operation, the air that isdried when passed over the coil is slightly compressed, raising the temperature of the surrounding air,which further reduces the relative humidity of the air.

ControlsAs mentioned previously, the dehumidifier (see Figure 6) operates on the principles of the conventionalhousehold refrigerator. It contains a motor-operated compressor, a condenser, and a receiver. In adehumidifier, the cooling coil takes the place of the evaporator, or chilling unit, in a refrigerator. Therefrigerant is circulated through the dehumidifier in the same manner as in a refrigerator. The refrigerantflow is controlled by a capillary tube. The moisture-laden air is drawn over the refrigerated coil bymeans of a motor-operated fan or blower.

Figure 6 Automatic electric dehumidifier.

The dehumidifier operates by means of a humidistat (see Figure 7), which starts and stops the unit tomaintain a selected humidity level. In a typical dehumidifier, the control settings range from DRY toEXTRA DRY to CONTINUOUS to OFF. For best operation, the humidistat control knob is normallyset at EXTRA DRY for initial operation over a period of 3 to 4 weeks. After this period, carefulconsideration should be given to the dampness in the area being dried. If sweating on cold surfaces hasdiscontinued and the damp odors are gone, the humidistat control should be reset to DRY. At thissetting, more economical operation is obtained, but the relative humidity probably will be higher than atthe EXTRA DRY setting.

Figure 7 Typical humidistat designed to control humidifying or dehumidifying equipment or both with oneinstrument.

After 3 or 4 weeks of operation at the DRY setting, if the moisture condition in the area being dried isstill satisfactory, the operation of the dehumidifier should be continued with the control set at thisposition. However, if at the setting the dampness condition is not completely corrected, the controlshould be returned to the EXTRA DRY setting. Minor adjustments will usually be required from time totime. Remember that the control must be set near EXTRA DRY to correct the dampness conditions butas close to DRY as possible to obtain the most economical operation.

Adsorption-Type DehumidifiersAdsorption-type dehumidifiers operate on the use of sorbent materials for adsorption of moisture fromthe air. The sorbents are substances that contain a vast amount of microscopic pores. These pores afforda great internal surface to which water adheres or is adsorbed. A typical dehumidifier based on thehoneycomb desiccant wheel principle is shown schematically in Figure 8. The wheel is formed from thincorrugated and laminated asbestos sheets rolled to form wheels of various desired diameters andthickness. The wheels are impregnated with a desiccant cured and reinforced with a heat-resistantbinder. The corrugations in the honeycomb wheel form narrow flutes perpendicular to the wheeldiameter. Approximately 75 percent of the wheel face area is available for the adsorption ordehumidifying flow circuit, and 25 percent is available for the reactivation circuit. In the smaller units,the reactivated air is heated electrically; in the larger units, it is heated by electric, steam, or gas heaters.Figure 9 shows another industrial adsorbent dehumidifier of the stationary-bed type. It has two sets ofstationary adsorbing beds arranged so that one set is dehumidifying the air while the other set is drying.With the dampers in the position shown, air to be dried flows through one set of beds and isdehumidified while the drying air is heated and circulated through the other set. After completion ofdrying, the beds are cooled by shutting off the drying air heaters and allowing unheated air to circulatethrough them. An automatic timer controller is provided to allow the dampers to rotate to the oppositeside when the beds have adsorbed moisture to a degree that begins to impair performance.

Figure 8 Assembly of components operating on the honeycomb method of dehumidification.

Figure 9 Stationary bedtype solid-absorbent dehumidifier.

Reference:Audel Air Conditioning: Home and commercial, 5th edition, Rex Miller, Mark Richard Miller, Edwin P.Anderson, Wiley Publishing Inc.