Methods of
humidification
There are two
general methods of humidification. These are; wet humidification and steam
humidification.
Wet Humidifiers
Wet humidifiers work
by encouraging liquid water to evaporate. This creates water vapour which mixes
with the airstream to humidify it. For the water to evaporate it must absorb
heat from its surroundings. As a result wet humidifiers cause the airstream
temperature to fall during the humidification process. To overcome this problem,
in air handling units, a pre heater initially warms the incoming airstream. The
warmed air then passes through the humidifier but becomes cooled in the
humidification process. The air must then pass through a reheat coil to bring
the airstream up to the required temperature.
Air Washers are used mostly in industrial humidification. As the name implies
they provide the dual function of humidifying the
airstream and at the same time washing out some
dust and odours. The airstream is made to flow
smoothly by passing between baffle plates (as shown in figure),
it then passes through a fine mist of water droplets created by
a spray head. This provides the contact between the
liquid water and the air necessary for evaporation to
take place. Spray eliminators are placed downstream
from the humidifier to prevent the carriage of liquid
water further down the ducting.
Evaporation of the
water cools the airstream, if this is desirable, further cooling can be
obtained by using a chilled water spray. During periods of warm weather the
water in the sump may remain still for a long period. It is important that this
water is treated to avoid bacteriological growth which could lead to infection
of the building occupants. For example, Humidifier fever, an industrial disease
with flue like symptoms, is commonly associated with humidifiers with
reservoirs. However, care should be taken in the choice of biocide and that it
is not carried by the airstream into the working environment.
Capillary Washers are humidifiers with a better humidification effectiveness than
the basic spray air washer. The greater
effectiveness is obtained by directing the spray onto a
matrix of metal, glass or plastic fibre cells (as shown in figure). The spray coats these cells resulting in further
spreading out of the water due to capillary action.
The airstream passes between the gaps in the cells and
hence over the wetted surfaces. In this way close contact is
obtained between the airstream and extended water
surface. The airstream can be in the same direction
as the spray or in the opposite direction to the spray, termed
parallel and contra flow respectively.
Ultrasonic Humidifiers create an extremely fine mist of water droplets by
passing liquid water over a ceramic plate which is made
to vibrate at ultrasonic frequencies using the piezoelectric
effect (as shown in figure). The small droplet size results
in quick and effective evaporation of the water. The
consequence is a rapid adjustment in relative humidity in
response to a call for humidification from the controls.
The excellent evaporation characteristics of
the device make it very efficient in energy terms and
ultrasonic humidifiers have an extremely low energy
consumption for a given humidification load.
However, ultrasonic
humidifiers must be supplied with demineralized water.
This is to avoid scale build up which would
otherwise lead to clogging.
Atomizing Nozzle Humidifiers produce a fine spray of cold water
directly in the air or within air handling units. Droplet size is small so
evaporation and hence humidification is rapid. Water is supplied directly from the
mains, avoiding any contamination risk, and compressed air is used to create
the spray. The small size of the atomizing nozzle means that it can easily be blocked
by mineral build up. To avoid this a needle built within the nozzle head
periodically cleans the orifice automatically. When used in air handling units all
of the spray released from the nozzle evaporates removing the need for water
recirculation and chemical treatments. Atomizing nozzle humidifiers provide
close control of humidity at low running cost and with low maintenance
requirements.
