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By Rick Stasyshan and John Kassin, Compressed Air and Gas Institute
Compressed Air Best Practices® (CABP) Magazine recently caught up with Rick Stasyshan, the Compressed Air and Gas Institute’s (CAGI) Technical Consultant, and John Kassin of Cameron to discuss variable inlet guide vanes (IGV). The following interview describes how centrifugal compressor efficiency can be improved thanks to recent developments in IGV technology.
CABP: Many of our readers own and operate centrifugal compressors, and they frequently ask about how to reduce their energy consumption. Is there anything new from CAGI’s Centrifugal Compressor Section that you can share?
CAGI: Absolutely, and energy conservation should be first on their minds. Frequently, equipment cost is the major decision factor, but energy consumption represents approximately 82 percent of the product life-cycle cost. CAGI and our members emphasize the opportunities to conserve energy while maximizing compressor system performance every chance we can. We would like to share some of the developments in centrifugal compressors utilizing variable inlet guide vanes.
LOWERING YOUR CENTRIFUGAL COMPRESSOR OPERATING COSTS
CABP: In simple terms, how can you reduce centrifugal compressor operating costs, and what are the potential results?
CAGI: For starters, you can save energy when running at less than full capacity, and you can also save energy on cooler days.
Variable inlet guide vanes are an ingenious yet simple option that, when installed on your centrifugal compressor, can deliver energy savings of up to 9 percent. By replacing a standard inlet butterfly valve with a new inlet guide vane assembly, substantial energy savings can be realized whenever the compressor operates at less than full load or when ambient air temperature is less than design temperature (usually 95°F, 35°C). Many manufacturers offer this option as a retrofit for existing compressors.
Inlet guide vanes impart a whirling motion to the inlet airflow in the same rotational direction as the impeller. This decreases the power required to deliver the rated airflow and pressure. There are opportunities for energy savings when the compressor inlet is throttled. Throttling takes place under most operational circumstances, except when plant air demand requires full 100 percent flow and when ambient air temperature is the highest expected at that location. This means your compressor inlet is likely throttled during the majority of its operation.
Inlet guide vanes in their application consist of wedge-shaped steel blades mounted around the inside circumference of a short length of inlet pipe (see Figure 1). They are turned in synchronization by a ring or yoke assembly on the outside of the pipe. Each vane is designed with an airfoil cross-section (similar to an airplane wing) to minimize air resistance when in its full open position (vanes positioned parallel to the air stream). The inlet guide vanes modulate from this fully opened position to a fully closed position, providing an infinitely variable degree of throttling and pre-rotation. When fully closed, the blades overlap to block all airflow — except that through a small center hole that is required for stable compressor operation (see Figure 2).
A standard actuator positions the yoke assembly. These are similar to the actuators used on a butterfly valve (See Figure 3). The existing control system required to operate the butterfly valve can also be used to operate the inlet guide vane (microprocessor and PLC systems provide the best results). Thus, it is typically easy to retrofit existing centrifugal compressor installations with inlet guide vanes. Only minor piping modification is required.
LOW AMBIENT TEMPERATURE MEANS THROTTLING DOWN
CABP: How does temperature come into the equation?
CAGI: Temperature indeed does play a role in centrifugal compressor performance. Centrifugal compressors are designed to produce the rated flow and pressure on the hottest day expected (hot design day) at their installation location. At lower ambient temperatures, the compressor can deliver more air at the same pressure. Since this additional air is not normally required, the compressor intake must be throttled to match the plant demand. By throttling with inlet guide vanes, a substantial power savings can be realized in comparison to a butterfly throttle valve. Compressor operators will see these savings almost every day — every day, that is, when the temperature is below the design point, and/or when the plant air system demands less air than the maximum designed volumetric flow.
CABP: Can you provide additional input?
CAGI: There are opportunities to save during "off-design" conditions. Compressors are designed to deliver pressure and flow for plant or process requirements under the most extreme anticipated atmospheric and air system demand conditions. These are often referred to as "design day" conditions. Design day conditions are highly dependent upon the ambient atmospheric temperature and the demand for compressed air from the plant. For most applications in the northeastern United States, design day conditions are considered a “worst-case scenario” of 95°F (35°C).
However, design day conditions are rarely experienced at the compressor jobsite. If we consider the case of the winter season where temperatures mainly persist below the design day maximum, significant energy savings can be realized through the unique throttling effect provided by inlet guide vanes.
Additionally, for plant air applications, it is common that the design flow rate (maximum anticipated design volumetric flow rate for peak production requirements) is not required by the plant’s compressed air system continuously.
Chart 1 considers the case where the atmospheric temperature is 30°F (-1°C) and where the demand of the compressed air system is 72 percent of the design-rated flow. At this temperature and volumetric flow demand, the inlet guide vanes throttle and impart a pre-rotation to the incoming airstream. Due to this pre-rotation, less power is required to compress the incoming airstream.
In this case, the IGV can reduce power consumption to 63 percent of the design day load. Under the same conditions, a compressor utilizing a butterfly valve (which does not impart the same favorable aerodynamic flow characteristics to the incoming airstream) would require 72 percent of design power to achieve similar performance. The difference represents a total net savings of 9 percent in power.
Conversely, by properly sizing the compressor drive motor (over-sizing) for a cold day and running it in un-throttled operation mode, the integrally geared centrifugal air compressor can deliver up to 20 percent more airflow above its design. The additional flow will require more power.
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