Civil Engineering Reference
In-Depth Information
Chemical drying and absorbing systems are dried-air or dehumidification
systems commonly used instead of cooling equipment. Notice that all these systems
are intended to remove water vapor and not lubricant oil in vapor state. If absolutely
oil-free air is required, non-lubricated systems are the best.
direct expansion (cold refrigerant gas expands in the air/refrigerant exchanger) or
indirect expansion by using chilled water as a medium to refrigerate air. In the latter
case, regenerative systems can conveniently be used: inlet air is partially cooled by
discharge air, thus reducing the power of the refrigerating compressor and reheating
the outlet air to the distribution line.
Advantages of reheating discharge air are the air volume is increased and thus
less free air is required to move the equipment, besides, the possibility of conden-
sation down the dew point along the distribution lines is further reduced.
Air receivers are used with reciprocating compressors. Dynamic compressors do
not require receivers because they have capacity control that should equalize line
output with demand.
Air receivers are very important additional equipment ensuring correct operation
of the compressor. An air receiver absorbs pulsations in the discharge line from the
compressor and smoothens the flow of air to the user lines. It works as a reservoir
for storing compressed air to cope with sudden peak demands exceeding the
capacity of the compressor. Compressors work between a lower and an upper
limit of discharge pressure; the dimension of the storage must be selected with
regard to technical and economic criteria, taking different factors into account, that
is, load requirement, the compressor's rated air flow, and the demand pressure. Air
receivers also allow moisture to precipitate and condense at the bottom, and prevent
it from being carried along the air distribution pipeline.
If excess condensate is produced, a water-cooled aftercooler and separator
should be added between the compressor and the receiver. This equipment dries
and cools air, which means higher efficiency and safe operation for the compressor
plant. When the water supply is scarce or too expensive, air-cooled aftercoolers are
technically and economically justified.
11.4
Technical Data on Industrial Air Compressors
and Control Systems
Table
11.2
lists the main properties of typical industrial compressors (shaft power,
shaft speed, volume of compressed air and pressure). Efficient operation is always
influenced by the following factors: intake air temperature (as cool as possible),
minimum system pressure, absence of leaks along the pipeline, heat recovery, and
selection of compressors appropriate in type and size for each application.
The intake air temperature must be kept as low as possible to reduce the work of
the compressor which varies with the volume of air linearly dependent on the
temperature. Table
11.3
shows how the power requirement varies with air
temperature.
