Chemical Engineering January 2013 - 44

Feature Report
Pressure required at consumer end P
Element
Final filter
Air distribution piping
chemical process industries (henceforth
referred in this article as " client " )
require the vendors to adhere to global
manufacturing codes, standards, guidelines,
good recommended practices,
directives (for instance, ASME, API,
ANSI). International clients operating
multiple industrial units at times have
their own set of technical standards
and guidelines that the vendor has to
comply with. Typically, a list of such
codes and standards to be followed is
available in the design basis of a project
and needs to be conveyed to the
vendor through specification.
This section should also contain
administrative, procedural and other
temporary requirements to be followed,
including submission of experience
record proforma, complying to
equipment qualification criteria, instructions
for delivery to site, scheduling,
warranties, and spare and maintenance
agreements.
Specifying equipment data
A compressed air and drying package
contains many types of equipment,
such as air-intake filters, compressors,
inter-coolers, after-coolers, moisture
separators, receivers and so on.
The engineer who writes specifications
for the package does not necessarily
size all this equipment. Based
on rules of thumb, good engineering
practices and sound technical assumptions,
he or she can fairly estimate
the capacities and sizes of these
pieces of equipment. This may help
different engineers from disciplines
like piping, static equipment, electrical
and so on to get at least preliminary
data to proceed with their work.
For example, due to availability of
equipment sizes, the layout engineer
can assign preliminary locations for
this equipment (which will be supplied
as packages or skids) on the allotted
plot plan and fix the area for
the air unit in the basic stage.
The sizes of some of the equipment
estimated by the engineer may not
necessarily match that given by the
vendor. Though seeming correct on
paper, such equipment may or may
not give the desired result. This may
be either due to the capabilities and
limitations of the selected vendor's
manufacturing and machinery or due
Dust filter (dryer after filter)
Dryer
Flow element
Compressor after-cooler
Compressor inter-cooler
Typical pressure
drop
∆P1
∆P2
∆P3
∆P4
Coalescing filter (dryer pre filter) ∆P5
∆P6
∆P7
∆P8
∆P9
Compressor regulation range
Total pressure drop
Pressure required at the
compressor discharge flange
to the vendor's proprietary design.
There may be certain technicalities in
terms of fabrication or state-of-art development
that only the vendor may
be better aware of. For example, air
intake filters or moisture separators
are entirely a vendor-supplied proprietary
item. This will be designed by
the vendor based on the particle-size
retention and moisture data given by
the design engineer.
During the technical bid analysis
(TBA) stage based on the specification
given by the designer, different
vendors offer their proposals that have
to be evaluated technically for energy
efficiency and lifetime operating cost.
The data furnished by the vendor need
to be thoroughly checked by the engineer
to see that all of his or her technical
and operational requirements are
in line with that given in the specification.
Any other additional data furnished
as a result of proprietary design
should also be checked at least for correctness
and compliance to standards.
Air compressor selection
During compressor and drive selection,
it must be kept in mind that in
most industries it is the compressor
that utilizes more electricity than any
other equipment. Records show that in
many instances during the first year
of operation, the operating cost was almost
twice that of the initial purchase
price of the equipment.
When selecting new compressors,
industries with existing compressedair
installations have an advantage.
They monitor their current air demand
and supply trends and also the
reliability and suitability of existing
air compressors. The data thus obtained
will prove useful to them in
selecting and sizing any future compressed
air installations.
44 CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2013
∆P
P +
∆P
0.3
0.1
0.1
0.15
0.1
0.25
0.1
0.1
0.5
1.7
7.7
bar
bar
bar
bar
bar
bar
bar
bar
bar
bar
barg
The following variables, if analyzed
correctly, will provide a fair idea of
the compressor type to be selected before
consulting a compressor vendor
for details:
1. Hours of operation per month
2. Nature of demand (continuous or
intermittent)
3. Pressure and flow requirements
4. Environment (clean or dirty)
A preliminary selection of the type of
air compressor can be made from the
typical graph of inlet flow versus discharge
pressure, as given in the GPSA
handbook [2]. For example, suppose
we want to select an air compressor for
1,000 acfm and a discharge pressure of
122 psig. By using such a graph, we will
observe that for our application we will
end up selecting the following types of
compressors: reciprocating (single and
multiple stage), rotary screw and centrifugal
(single and multiple stage).
All three types of compressors can suit
the application. So how do we decide
which type of compressor is the best?
The answer is that we must not select
any compressor that simply fulfills the
flow and pressure requirements, but
the one that is best suited to the application
(see Table 4).
Suppose for the same application
given above we further know that
the nature of load will be continuous,
heavy (high flowrate) and the system
has to be lubricant free. For high flowrates
and oil-free conditions centrifugal
compressors are a common choice.
Also centrifugal compressors work
well under continuous load rather
than variable load. Due to these reasons
a centrifugal compressor will become
a first choice for our application.
Correct flowrate units
As air is compressible it will occupy
different volumes at different temTABLE
3. ESTIMATING THE WORKING PRESSURE
6
barg
http://WWW.CHE.COM
Chemical Engineering January 2013

Table of Contents for the Digital Edition of Chemical Engineering January 2013
