TABLE 3. ESTIMATING THE WORKING PRESSURE
Feature Report 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 44
Pressure required at consumer end Element
P
6 Typical pressure drop 0.3
barg
Final filter Air distribution piping
∆P1 ∆P2
0.1
Dust filter (dryer after filter)
bar
∆P3
0.1
bar
Dryer
∆P4
0.15
bar
Coalescing filter (dryer pre filter)
∆P5
0.1
bar
Flow element
∆P6
Compressor after-cooler Compressor inter-cooler
∆P7 ∆P8
0.25 0.1
bar bar
0.1
bar
Compressor regulation range
∆P9
0.5
bar
Total pressure drop
∆P
1.7
bar
Pressure required at the compressor discharge flange
P+ ∆P
7.7
barg
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.
CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2013
bar
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 tem-