Author: Lukas Bijikli, Product Portfolio Manager, Integrated Gear Drives, R&D CO2 Compression and Heat Pumps, Siemens Energy.
For many years, the Integrated Gear Compressor (IGC) has been the technology of choice for air separation plants. This is mainly due to their high efficiency, which directly leads to reduced costs for oxygen, nitrogen and inert gas. However, the growing focus on decarbonization places new demands on IPCs, especially in terms of efficiency and regulatory flexibility. Capital expenditure continues to be an important factor for plant operators, especially in small and medium-sized enterprises.
Over the past few years, Siemens Energy has initiated several research and development (R&D) projects aimed at expanding IGC capabilities to meet the changing needs of the air separation market. This article highlights some specific design improvements we have made and discusses how these changes can help meet our customers’ cost and carbon reduction goals.
Most air separation units today are equipped with two compressors: a main air compressor (MAC) and a boost air compressor (BAC). The main air compressor typically compresses the entire air flow from atmospheric pressure to approximately 6 bar. A portion of this flow is then further compressed in the BAC to a pressure of up to 60 bar.
Depending on the energy source, the compressor is usually driven by a steam turbine or an electric motor. When using a steam turbine, both compressors are driven by the same turbine through twin shaft ends. In the classical scheme, an intermediate gear is installed between the steam turbine and the HAC (Fig. 1).
In both electrically driven and steam turbine driven systems, compressor efficiency is a powerful lever for decarbonization as it directly impacts the energy consumption of the unit. This is especially important for MGPs driven by steam turbines, since most of the heat for steam production is obtained in fossil fuel-fired boilers.
Although electric motors provide a greener alternative to steam turbine drives, there is often a greater need for control flexibility. Many modern air separation plants being built today are grid-connected and have a high level of renewable energy use. In Australia, for example, there are plans to build several green ammonia plants that will use air separation units (ASUs) to produce nitrogen for ammonia synthesis and are expected to receive electricity from nearby wind and solar farms. At these plants, regulatory flexibility is critical to compensate for natural fluctuations in power generation.
Siemens Energy developed the first IGC (formerly known as VK) in 1948. Today the company produces more than 2,300 units worldwide, many of which are designed for applications with flow rates in excess of 400,000 m3/h. Our modern MGPs have a flow rate of up to 1.2 million cubic meters per hour in one building. These include gearless versions of console compressors with pressure ratios up to 2.5 or higher in single-stage versions and pressure ratios up to 6 in serial versions.
In recent years, to meet increasing demands for IGC efficiency, regulatory flexibility and capital costs, we have made some notable design improvements, which are summarized below.
The variable efficiency of a number of impellers typically used in the first MAC stage is increased by varying the blade geometry. With this new impeller, variable efficiencies of up to 89% can be achieved in combination with conventional LS diffusers and over 90% in combination with the new generation of hybrid diffusers.
In addition, the impeller has a Mach number higher than 1.3, which provides the first stage with a higher power density and compression ratio. This also reduces the power that gears in three-stage MAC systems must transmit, allowing the use of smaller diameter gears and direct drive gearboxes in the first stages.
Compared to the traditional full-length LS vane diffuser, the next generation hybrid diffuser has an increased stage efficiency of 2.5% and control factor of 3%. This increase is achieved by mixing the blades (i.e. the blades are divided into full-height and partial-height sections). In this configuration
The flow output between the impeller and diffuser is reduced by a portion of the blade height that is located closer to the impeller than the blades of a conventional LS diffuser. As with a conventional LS diffuser, the leading edges of the full-length blades are equidistant from the impeller to avoid impeller-diffuser interaction that could damage the blades.
Partially increasing the height of the blades closer to the impeller also improves flow direction near the pulsation zone. Because the leading edge of the full-length vane section remains the same diameter as a conventional LS diffuser, the throttle line is unaffected, allowing for a wider range of application and tuning.
Water injection involves injecting water droplets into the air stream in the suction tube. The droplets evaporate and absorb heat from the process gas stream, thereby reducing the inlet temperature to the compression stage. This results in a reduction in isentropic power requirements and an increase in efficiency of more than 1%.
Hardening the gear shaft allows you to increase the permissible stress per unit area, which allows you to reduce the tooth width. This reduces mechanical losses in the gearbox by up to 25%, resulting in an increase in overall efficiency of up to 0.5%. In addition, main compressor costs can be reduced by up to 1% because less metal is used in the large gearbox.
This impeller can operate with a flow coefficient (φ) of up to 0.25 and provides 6% more head than 65 degree impellers. In addition, the flow coefficient reaches 0.25, and in the double-flow design of the IGC machine, the volumetric flow reaches 1.2 million m3/h or even 2.4 million m3/h.
A higher phi value allows the use of a smaller diameter impeller at the same volume flow, thereby reducing the cost of the main compressor by up to 4%. The diameter of the first stage impeller can be reduced even further.
The higher head is achieved by the 75° impeller deflection angle, which increases the circumferential velocity component at the outlet and thus provides higher head according to Euler’s equation.
Compared to high-speed and high-efficiency impellers, the impeller’s efficiency is slightly reduced due to higher losses in the volute. This can be compensated for by using a medium-sized snail. However, even without these volutes, variable efficiency of up to 87% can be achieved at a Mach number of 1.0 and a flow coefficient of 0.24.
The smaller volute allows you to avoid collisions with other volutes when the diameter of the large gear is reduced. Operators can save costs by switching from a 6-pole motor to a higher-speed 4-pole motor (1000 rpm to 1500 rpm) without exceeding the maximum allowable gear speed. Additionally, it can reduce material costs for helical and large gears.
Overall, the main compressor can save up to 2% in capital costs, plus the engine can also save 2% in capital costs. Because compact volutes are somewhat less efficient, the decision to use them largely depends on the client’s priorities (cost vs. efficiency) and must be assessed on a project-by-project basis.
To increase control capabilities, the IGV can be installed in front of multiple stages. This is in stark contrast to previous IGC projects, which only included IGVs up to the first phase.
In earlier iterations of the IGC, the vortex coefficient (i.e., the angle of the second IGV divided by the angle of the first IGV1) remained constant regardless of whether the flow was forward (angle > 0°, reducing head) or reverse vortex (angle < 0). °, the pressure increases). This is disadvantageous because the sign of the angle changes between positive and negative vortices.
The new configuration allows two different vortex ratios to be used when the machine is in forward and reverse vortex mode, thereby increasing the control range by 4% while maintaining constant efficiency.
By incorporating an LS diffuser for the impeller commonly used in BACs, the multi-stage efficiency can be increased to 89%. This, combined with other efficiency improvements, reduces the number of BAC stages while maintaining overall train efficiency. Reducing the number of stages eliminates the need for an intercooler, associated process gas piping, and rotor and stator components, resulting in cost savings of 10%. Additionally, in many cases it is possible to combine the main air compressor and the booster compressor in one machine.
As mentioned earlier, an intermediate gear is usually required between the steam turbine and the VAC. With the new IGC design from Siemens Energy, this idler gear can be integrated into the gearbox by adding an idler shaft between the pinion shaft and the big gear (4 gears). This can reduce the total line cost (main compressor plus auxiliary equipment) by up to 4%.
Additionally, 4-pinion gears are a more efficient alternative to compact scroll motors for switching from 6-pole to 4-pole motors in large main air compressors (if there is a possibility of volute collision or if the maximum permissible pinion speed will be reduced). ) past.
Their use is also becoming more common in several markets important to industrial decarbonization, including heat pumps and steam compression, as well as CO2 compression in carbon capture, utilization and storage (CCUS) developments.
Siemens Energy has a long history of designing and operating IGCs. As evidenced by the above (and other) research and development efforts, we are committed to continually innovating these machines to meet unique application needs and meet the growing market demands for lower costs, increased efficiency and increased sustainability. KT2