Conventional External Compression Air Separation Units: Economic Benefits & Selling Points

Conventional External Compression Air Separation Units: Economic Benefits & Selling Points

For decades, the external compression air separation unit has been the workhorse of industrial gas production. Its architecture is straightforward: atmospheric air is compressed, purified, and cryogenically distilled into liquid oxygen, nitrogen, and argon products at low pressure. The gaseous products are then drawn from the distillation column and compressed to user pressure by external compressors-the defining feature that gives the external compression process its name. For a broad range of applications, particularly small and medium-scale operations, this conventional design remains one of the most cost-effective gas production solutions available.

How External Compression Works

Air is drawn from the atmosphere, compressed to a moderate pressure (typically 0.5–0.7 MPa), and purified in a molecular sieve pre-purification unit to remove water, CO₂, and hydrocarbons. The clean, compressed air is cooled to cryogenic temperatures (approx -170°C to -190°C) in a main heat exchanger, then fed into a double-column distillation system. The lower tower produces a liquid oxygen-rich stream and a liquid nitrogen-rich stream; the upper tower completes the separation, delivering gaseous oxygen (GOX) and gaseous nitrogen (GAN) at low pressure (approximately 0.12 MPa or 1.2 bar). These low-pressure gases are then drawn from the distillation column and passed through external compressors-oxygen compressors and nitrogen compressors located outside the cold box-to raise the pressure to whatever level the customer requires. The process can also produce liquid products by diverting a portion of the refrigerant flow through expanders, typically yielding up to 8–10% of total output in liquid form.

 

Economic Benefits

Lower Initial Capital Investment. For small and medium-scale air separation equipment, the external compression process generally carries a lower upfront investment compared to its internal compression counterpart. The external oxygen and nitrogen compressors are mature, widely available technologies manufactured by multiple suppliers, which keeps procurement costs competitive. Where domestically manufactured turbine oxygen compressors are available, they can offer excellent performance at substantially lower prices than the air boosters required for internal compression plants. This lower capital barrier makes external compression attractive for enterprises with limited budgets or those entering the industrial gas market without a track record of high-volume consumption.

Lower Unit-Cost of Oxygen Production. When comparing levelised production costs, external compression plants produce oxygen gas at approximately 9% lower cost than internal compression plants, due primarily to their lower energy consumption. External compression is also more energy-efficient than internal compression at typical operating parameters-specific power consumption for IC plants is 7–8% higher than equivalent EC configurations.

Flexible Operation for Variable Demand. The external oxygen compressor operates independently of the main air separation unit, providing operators with flexible pressure regulation capabilities to adapt the system to changing user demands. This is particularly valuable in metallurgical applications where oxygen consumption fluctuates significantly with production cycles. For industries with variable gas requirements, the ability to adjust compression load independently reduces energy waste during periods of low demand.

Simpler Maintenance Access. By locating compression equipment outside the cold box and at grade level, external compression plants provide easier access for routine servicing and repairs. Oxygen compressors are accessible without entering the cold box or de-pressurising the distillation column, reducing the time and complexity of maintenance activities.

 

Key Selling Points

Proven, Low-Risk Technology. External compression ASUs have been in continuous operation for more than 60 years across thousands of installations worldwide. The design is well understood, component manufacturers are established, and the maintenance community is experienced. For operators who prioritise predictable performance over cutting-edge novelty, external compression offers a low-risk path to reliable gas production.

Broad Liquefaction Capability. While external compression plants are primarily designed for gaseous product delivery, they can still produce significant quantities of liquid products by adjusting expander configurations. For operators who require a modest liquid buffer against demand spikes or backup supply needs, the external compression platform provides this flexibility without requiring the more complex air booster systems of internal compression designs.

Energy Storage Potential for Load Shifting. Recent innovations have demonstrated that external compression ASUs can be integrated with liquid air energy storage (LAES) systems to reduce electricity costs through peak load shifting. An external-compression ASU with LAES functionality can reduce power consumption costs while achieving round-trip efficiencies exceeding 68% when integrated with energy storage systems. For operators facing time-of-use electricity pricing, this capability offers a pathway to further operational cost reduction.

Mature Supply Chain and Fast Spares Availability. Because external compression components are widely produced and stocked, spare parts and service are readily available from multiple sources. Lead times for replacement compressors and consumables are typically shorter than for specialised internal compression components, minimising downtime in the event of unexpected failures.

 

Where External Compression Excels

The external compression ASU is not the right solution for every application. For users requiring very high product pressures (above 3.0 MPa) or with large liquid production requirements, internal compression may be more appropriate. However, for the numerous small and medium-scale operations in the metals, glass, chemical, and general industrial sectors-where product pressures are moderate, capital budgets are constrained, and reliability is paramount-the external compression air separation unit continues to offer an unmatched combination of affordability, flexibility, and proven performance.