Humidifiers and dehumidifiers

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Humidifiers and dehumidifiers
Humidity-Control Methods
Humidifiers, by definition, are devices for adding moisture to the air. Thus, to humidify is to increase
the density of water vapor within a given space or room. Air humidification is affected by vaporization
of water and always requires heat for its proper functioning. Thus, devices that function to add moisture
to the air are termed humidifiers, whereas devices that function to remove moisture from the air are
termed dehumidifiers.
Humidifiers
As previously noted, air humidification consists of adding moisture. Following are the types of
humidifiers used in air-conditioning systems:
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Spray-type air washers
Pan evaporative humidifiers
Electrically operated humidifiers
Air-operated humidifiers
Air-Washer Method
An air washer essentially consists of a row of spray nozzles inside a chamber or casing. A tank at the
bottom of the chamber provides for collection of water as it falls through the air and comes into intimate
contact with the wet surface of the chamber baffles. The water is generally circulated by means of a
pump, the warm water being passed over refrigerating coils or blocks of ice to cool it before being
passed to the spray chamber. The water lost in evaporation is usually replaced automatically by the use
of a float arrangement, which admits water from the main as required. In many locations, the water is
sufficiently 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 refrigerating
machine.
The principal functions of the air washer are to cool the air passed through the spray chamber and to
control humidity. In many cases, the cooling coils are located in the bottom of the spray chamber so that
as the warm spray descends, it is cooled and ready to be again sprayed by the pump. In some cases, the
water is passed through a double-pipe arrangement and is cooled on the counter-current principle. Figure
1 shows a sketch of an air washer. In this case, the spray pipes are mounted vertically. In some
instances, the spray pipes are horizontal so that the sprays are directed downward. As some of the finer
water particles tend to be carried along with the air current, a series of curved plates or baffles is
generally used, which forces the cooled and humidified air to change the direction of flow, throwing out
or eliminating the water particles in the process.
Figure 1 Elementary diagrams showing essential parts of air-washer unit with identification of parts.
Pan Humidifiers
Figure 2 shows the essential parts of the pan-type humidifier. The main part is a tank of water heated by
low-pressure steam or forced hot water where a water temperature of 200◦F or higher is maintained. The
evaporative-type humidifier is fully automatic, the water level being controlled by means of a float
control. In operation, when the relative humidity drops below the humidity-control setting, the
humidifier fan blows air over the surface of the heated water in the tank. The air picks up moisture. The
air is blown to the space to be humidified. When the humidity control is satisfied, the humidifier fan
stops.
Figure 2 Typical evaporative pan humidifier showing operative components.
Electrically Operated Humidifiers
Dry-steam electrically operated humidifiers operate by means of a solenoid valve, which is energized by
a 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 to
the re-evaporating chamber.
Steam passes from this chamber through the muffler directly to the atmosphere. The fan (which is
energized 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 stops
the fan.
Air-Operated Humidifiers
Air-operated humidifying units operate in the same manner as electrical units, except that they utilize a
pneumatic hygrostat as a humidity controller and an air operator to open or close the steam valve (see
Figure 4). A decrease in relative humidity increases the air pressure under a spring-loaded diaphragm to
open the steam valve wider. An increase in relative humidity reduces the pressure under the diaphragm
and allows the valve to restrict the steam flow. In a humidifier operation of this type, the steam supply is
taken off the top of the header (see Figure 5). Any condensate formed in the supply line is knocked
down 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 be
humidified.
Any droplets of condensation picked up by the stream as it flows through the humidifier cap when the
steam valve opens will be thrown to the bottom of the re-evaporating chamber. Pressure in this chamber
is essentially atmospheric. Since it is surrounded by steam at supply pressure and temperature, any water
is re-evaporated to provide dry steam at the outlet. The humidifier outlet is also surrounded by steam at
supply pressure to ensure that there will be no condensation or drip at this point. A clamp-on
temperature switch is attached to the condensate drain line to prevent the electric or pneumatic operator
from 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.
Dehumidifiers
The removal of moisture from the air is termed dehumidification. Air dehumidification is accomplished
by one of two methods: cooling and adsorption. Dehumidification can be accomplished by an air
washer, providing the temperature of the spray is lower than the dew point of the air passing through the
unit. If the temperature of the spray is higher than the dew point, condensation will not take place. Air
washers having refrigerated sprays usually have their own recirculating pump.
Electric Dehumidification
An electric dehumidifier operates on the refrigeration principle. It removes moisture from the air by
passing 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 on
the relative humidity and volume of the area to be dehumidified. In locations with high temperature and
humidity conditions, 3 to 4 gallons of water per day can usually be extracted from the air in an averagesize home.
When the dehumidifier is first put into operation, it will remove relatively large amounts of moisture
until the relative humidity in the area to be dried is reduced to the value where moisture damage will not
occur. After this point has been reached, the amount of moisture removed from the air will be
considerably less. This reduction in moisture removal indicates that the dehumidifier is operating
normally and that it has reduced the relative humidity in the room or area to a safe value. The
performance of the dehumidifier should be judged by the elimination of dampness and accompanying
odors rather than by the amount of moisture that is removed and deposited in the bucket. A dehumidifier
cannot act as an air conditioner to cool the room or area to be dehumidified. In operation, the air that is
dried when passed over the coil is slightly compressed, raising the temperature of the surrounding air,
which further reduces the relative humidity of the air.
Controls
As mentioned previously, the dehumidifier (see Figure 6) operates on the principles of the conventional
household refrigerator. It contains a motor-operated compressor, a condenser, and a receiver. In a
dehumidifier, the cooling coil takes the place of the evaporator, or chilling unit, in a refrigerator. The
refrigerant is circulated through the dehumidifier in the same manner as in a refrigerator. The refrigerant
flow is controlled by a capillary tube. The moisture-laden air is drawn over the refrigerated coil by
means 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 to
maintain a selected humidity level. In a typical dehumidifier, the control settings range from DRY to
EXTRA DRY to CONTINUOUS to OFF. For best operation, the humidistat control knob is normally
set at EXTRA DRY for initial operation over a period of 3 to 4 weeks. After this period, careful
consideration should be given to the dampness in the area being dried. If sweating on cold surfaces has
discontinued and the damp odors are gone, the humidistat control should be reset to DRY. At this
setting, more economical operation is obtained, but the relative humidity probably will be higher than at
the EXTRA DRY setting.
Figure 7 Typical humidistat designed to control humidifying or dehumidifying equipment or both with one
instrument.
After 3 or 4 weeks of operation at the DRY setting, if the moisture condition in the area being dried is
still satisfactory, the operation of the dehumidifier should be continued with the control set at this
position. However, if at the setting the dampness condition is not completely corrected, the control
should be returned to the EXTRA DRY setting. Minor adjustments will usually be required from time to
time. Remember that the control must be set near EXTRA DRY to correct the dampness conditions but
as close to DRY as possible to obtain the most economical operation.
Adsorption-Type Dehumidifiers
Adsorption-type dehumidifiers operate on the use of sorbent materials for adsorption of moisture from
the air. The sorbents are substances that contain a vast amount of microscopic pores. These pores afford
a great internal surface to which water adheres or is adsorbed. A typical dehumidifier based on the
honeycomb desiccant wheel principle is shown schematically in Figure 8. The wheel is formed from thin
corrugated and laminated asbestos sheets rolled to form wheels of various desired diameters and
thickness. The wheels are impregnated with a desiccant cured and reinforced with a heat-resistant
binder. The corrugations in the honeycomb wheel form narrow flutes perpendicular to the wheel
diameter. Approximately 75 percent of the wheel face area is available for the adsorption or
dehumidifying 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 of
stationary 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 is
dehumidified while the drying air is heated and circulated through the other set. After completion of
drying, the beds are cooled by shutting off the drying air heaters and allowing unheated air to circulate
through them. An automatic timer controller is provided to allow the dampers to rotate to the opposite
side 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 bed–type solid-absorbent dehumidifier.
Reference:
Audel Air Conditioning: Home and commercial, 5th edition, Rex Miller, Mark Richard Miller, Edwin P.
Anderson, Wiley Publishing Inc.