Chemical Engineering March 2018 - 42

TABLE 1. TYPICAL REFERENCE
CONDITIONS
Unit of
measurement
SCFM
Fixed reference/
standard conditions
14.7 psia
60°F
0% relative humidity
(RH)
OR
14.7 psia
68°F
0% RH
OR
14.7 psia
68°F
36% RH
Nm3/h
1.01325 bara
0°C
0% RH
OR
1.01325 bara
15°C
0% RH
OR
1.01325 bara
20°C
0% RH
Flow and pressure units
An critically important first step is to
decide which unit of measure will be
used to indicate flowrate, based on
the requirements of the specific process.
There are several ways to measure
the capacity of an air compressor,
as follows:
* Inlet cubic feet per minute (ICFM),
which is the inlet volume flowrate or
intake volume as measured at the intake.
For most processes, inlet flow
is not usable flow; therefore, using
the ICFM flow unit is not advised
* Cubic feet per minute (CFM), which
describes the free air delivery as
measured at the delivery point,
downstream of the aftercooler
* Standard cubic feet per minute
(SCFM) or normal cubic meter per
hour (Nm3/h), which represent the
standard or normal flow as measured
at the delivery point and established
by standard temperature
and pressure (STP) or normal temperature
and pressure (NTP) conditions.
The reference condition will
have a large impact on flow, which is
why it is good practice to specify the
reference condition you want to use
when consulting with a compressor
manufacturer. Typical reference conditions
are given in Table 1
Required pressure can also be specified
in psig, barg or kg/cm2g.
40
a combination of both technologies
work better for optimal utilization
of compressed air and decreased
energy consumption.
Receiver tank
The primary function of an air receiver
tank is to store compressed air, but it
also serves as an additional condensate
separator. Furthermore, the air receiver
ensures a steady airflow and equalizes
momentary pressure variations in the
air piping network, which could cause
frequent loading and unloading of the
compressor. Normally, the air receiver
comes with a safety valve, pressure
gage, connection to fit the test-pressure
gage, inspection cover and drain valve.
Air dryer and filtration selection
Atmospheric air contains water vapor
that must be removed to a certain degree.
Water concentration increases
at higher temperatures and decreases
at
lower
temperatures.
Therefore,
when the air is compressed, the water
concentration increases.
There are different types of dryers
available on the market, with the two
main types being refrigerated dryers
and desiccant dryers. Refrigerated
dryers can reach a 37°F pressure
dewpoint, whereas desiccant
dryers
are capable of handling negative dewpoints.
Depending on what quality of air
is needed for the application, either of
these dryer types can be chosen.
When sizing your dryer, avoid selecting
one straight from a manufacturer's
brochure. Inlet conditions can
have a major impact on the dryer's
performance, so it is recommended
to calculate sizing based on dryer
inlet conditions.
With the heatless type of desiccant
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MARCH 2018
FIGURE 4. Filtration units are
important components of compressed
air systems, and many
configurations are available,
depending on the quality of air
required for an application
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Chemical Engineering March 2018

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