10

Centrifugal

Axial (500)

10 100 1,000 10,000 100,000 1,000,000

Axial (500)

10 100 1,000 10,000 100,000 1,000,000

Inlet Capacity, acfm

Fig. 30-26 Pressure-Capacity Chart Showing the Effective Ranges of Most Compressors. Source: Compressed Air and Gas Institute

30-1 lists typical air compressor types and designs used in various capacity ranges for 100 psig (7.9 bar) air service. Under higher or lower operating pressures, selection criteria for each capacity range may differ.

Ideally, the supply side or compressed air capacity should be selected and configured after the demand side is controlled and optimized. The primary components of compressed air demand are real production requirements,

The classic trim-load unit is a reciprocating compressor. Whether operated at fixed or variable speed, reciprocating compressors offer rapid response, superior part-load efficiency, and precise modulation. Rotary screw compressors operated with variable speed drivers may be a cost-effective alternative in some cases. The electric motor-driven reciprocating compressor has one distinct advantage over prime mover-driven trim units — reliable, fast response from a dead start. While some prime mover-driven units can provide quick start, this is not desirable on a regular basis. In addition, for reciprocating engine-driven systems, the combination of the rapid decline in engine performance and reduced compressor volumetric efficiency result in inefficient operation below 40% of full load.

In many smaller facilities, particularly those with single-shift operations, only one air compressor is used. Since most facilities face varying loads, a unit with good trim characteristics is often the most attractive. Constant speed electric motor-driven compressors are most common for such applications, although variable speed electric units and packaged reciprocating engine units can also be used to enhance part-load efficiency. In some cases, packaged reciprocating engine-driven units up to 400 hp (300 kW) can be cost-effective even without heat recovery because of high-cost peak electric rates during single-shift weekday operation.

In multiple compressor systems comprising similar units, it is common for both machines to share load equally, as demand requires each successive unit to be brought on line. Efficiency is often improved by using dissimilar units that feature required trim or baseload characteristics. Energy efficiency is optimized when all but one compressor is operating at full load and a relatively small trim unit is cycled or modulated as required.

For example, given a baseload of 3,400 cfm (96 m3/m) and a peak load of 4,000 cfm (113 m3/m), the baseload units can be sized at 3,000 cfm (85 m3/m). A trim-load unit rated at 1,000 cfm (28 m3/m) would always be loaded to 400 cfm, or 40%, maintaining reasonable performance under the base condition. As the typical load on the trim unit is between 400 and 1,000 cfm (11 and 28 m3/m), the unit will operate at close to full-load efficiency all or most of the time, if operated at variable speed.

Another alternative is to split the trim-load service into two components: a bottom end and a top end. The bottom end consists of demand that is variable, but almost always present and can be met by a screw or reciprocating compressor driven by a reciprocating engine. The top end, or intermittent demand, is met by a cycling electric-

driven reciprocating unit. This strategy provides for one quick-start unit, optimizes energy operating costs, and adds redundant capacity for increased reliability.

Most large facilities with multiple-shift operations are on some type of time-of-use (TOU) differentiated electric rate. Most are fairly stratified with peak period (5 day, daytime) electricity being relatively costly and off-peak period electricity being relatively inexpensive. A disadvantage of electric motor-driven trim compressors is that electric demand charges will apply to the peak load met by the unit, whenever this coincides with the peak demand setting period of the facility. Energy charges incurred during more typical part-load operation may be only a fraction of trim compressor operating cost during the utility peak periods.

TOU or the increasingly common real-time-pricing (RTP) electric rates may present opportunities for mixed, or hybrid, system configurations, featuring both electric and non-electric drivers. During peak periods, prime mover drivers may prove economical as baseloaded units, even without using recovered heat. One potentially cost-efficient strategy is to use a prime mover-driven screw or reciprocating compressor during peak and shoulder periods and an electric motor-driven unit during off-peak periods. Unless there is a concurrent load for the thermal output, a reciprocating engine-driven unit will generally not be as economical to operate as a baseloaded electric motor-driven unit during off-peak periods. Back-pressure turbine-driven units will almost always be economical (peak or off-peak), as long as there is full use of back-pressure steam.

The operating cost advantage of a hybrid system is often significant enough to offset the incremental cost of the prime mover-driven unit. Consider the following simplified example of matching system configuration to load:

Compressed air requirement during peak electric rate period:

Compressed air requirement during off-peak electric rate period:

Assume also a back-pressure steam turbine-driven centrifugal compressor can be designed to provide 4,000 cfm (113 m3/m) with a low-pressure steam output to match the facility's minimum requirement. One useful configuration would consist of a 4,000 cfm (113 m3/m) back-pressure steam-driven centrifugal compressors and a

2,000 cfm (57 m3/m) electric motor-driven reciprocating compressor. The steam turbine-driven unit would be base-loaded, with the electric motor-driven unit serving remaining peak baseloads and meeting peak demands.

A 2,000 cfm (57 m3/m) packaged reciprocating engine-driven screw compressor could be added to further reduce operating costs and provide some redundant capacity. It would replace the electric unit during peak periods and could be expected to operate efficiently at variable speed between 50 and 100% of full load. The electric unit would continue to operate and trim load off-peak. In the event that the largest unit, the steam turbine-driven machine, was out of service, the maximum capacity shortfall would be only 2,000 cfm (57 m3/m).

If expected low-pressure steam load is reduced during the off-peak period, perhaps by half, alternative configurations would be considered. One option would be to install a 2,000 cfm (57m3/m) steam turbine-driven unit and a 2,000 cfm (57 m3/m) baseload electric motor-driven unit. With the addition of two trim units (one engine-driven and one electric motor-driven), the total system capacity would be 8,000 cfm (226 m3/m), providing 100% redundancy when any one unit is out of service.

If in this example there was no steam demand, the following configuration might be selected:

• Baseload duty by a 4,000 cfm (113 m3/m) electric motor-driven centrifugal compressor;

• Peak trim by a 2,000 cfm (57 m3/m) reciprocating engine-driven screw compressor; and

• Off-peak trim by a 2,000 cfm (57 m3/m) electric motor-driven reciprocating compressor. Alternative configurations would be considered based on maintenance and reliability concerns, required redundancy, energy prices, and available thermal loads. Because the lifecycle operating costs associated with compressed air plants are generally so much higher than initial capital costs, hybrid configurations will often show a relatively short payback when compared with least capital-cost alternatives.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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