Chemical Processing - April 2008 - (Page 39)

>> High efﬁciency 120.00 100.00 ◆ 80.00 Percent kW input ◆ ◆ ◆ ◆ 60.00 ◆ ◆ 40.00 ◆ ◆ ◆ 20.00 0.00 ◆ 0.00 20.00 40.00 Percent capacity 60.00 80.00 100.00 120.00 % kW input versus % capacity — with stopping Figure 3. VSD oil-injected ◆ rotary screw compressor provides more efﬁcient capacity control than possible with% kW input versus % capacity — with unloading constant speed unit. er at various capacities (Figure 3), with the comparable data for inlet throttling (Figure 2). For the oil-free rotary screw compressor, advantages include: • complete compact package; • relatively low first cost; and • no need for a special foundation. Disadvantages include: • less efficiency than water-cooled reciprocating type; and • higher long-term maintenance costs. Its operating cost at full capacity and 100-psig discharge pressure runs 18–22 kW/100 cfm two-stage. For the oil-injected unit, advantages include: • complete compact package; • relatively low first cost; • no need for special foundation; and • routine maintenance (oil, ﬁlter, separator changes). Disadvantages include: • less efficiency than water-cooled reciprocating type; and • potential problem of oil carryover. Operating cost at full capacity and 100-psig discharge pressure runs 18–19 kW/100 cfm single stage and 16–17 kW/100 cfm two-stage. 3. Reciprocating compressors. These units come in single-acting and double-acting configurations. The single-acting type uses only one side of the piston to compress the air, whereas the doublewww.chemicalprocessing.com acting uses both sides. Single-acting compressors generally are small in size, air-cooled, and limited in capacity and time of operation but can be conveniently located near the point of use. Double-acting units usually are water-cooled and can have two or more stages of compression, which increases efficiency. They also can have multiple steps of capacity reduction, providing 100/75/50/25/0% capacity within a specified pressure control band. Both lubricated and non-lubricated piston/cylinder versions are available. The advantages of double-acting compressors include: • efficient compression; • efficient multi-step capacity control; and • relatively routine maintenance. Disadvantages include: • relatively high first cost; • need for special foundations due to vibrations; and • oil carryover on lubricated versions. The operating cost at full capacity and 100-psig discharge pressure for a water-cooled double-acting unit runs 15–16 kW/100 cfm. In contrast, an air-cooled single-acting compressor runs 22–24 kW/100 cfm. Multiple compressors Many plants link together a number of compressors into a single compressed air system. In such a system, all compressors but one should be operated at full capacity and optimum efficiency. The one so-called trim comApril 2008 • 39 http://www.chemicalprocessing.com

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Chemical Processing - April 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Is It a Tragedy or Comedy for Engineers? Better Understanding Boosts Mixer Scale-up Don't Err With Air Compressors Control Performance Supervision Enhances Revamp Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - April 2008

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