Project Background
A float glass manufacturing facility in Hebei Province operates compressed air as a critical utility across its production line—supplying instrument air to the tin bath atmosphere control system,process air to the annealing lehr,pneumatic actuation for the cutting and handling section,and general plant air for maintenance and auxiliary services.The compressed air station comprised four Sullair screw air compressors,two units rated at 160kW and two at 110kW,giving a total installed compressor capacity of 540kW.All four units had been running under soft starter control since the facility’s original commissioning.
The core problem was a fundamental mismatch between compressed air supply and actual plant demand.Float glass production involves distinct operational phases—cold repair shutdowns,heating-up periods,steady-state melting and forming,and reduced-output campaigns—each carrying significantly different compressed air consumption profiles.Under soft starter control,the Sullair compressors had only two operating states:running at full speed and full load,or stopped.Demand fluctuations were managed entirely through a combination of inlet valve throttling on running compressors and load-unload cycling—both inherently wasteful strategies.
Inlet valve throttling on a fixed-speed screw compressor is particularly wasteful:the motor continues consuming close to full power even as the inlet valve restricts airflow to match lower demand,because the compressor is still turning at full speed against a partially closed inlet.The energy that would have been saved by running slower is instead wasted as heat across the throttled inlet valve.Load-unload cycling—where the compressor runs fully unloaded at approximately 25–30%of full-load power consumption while waiting for system pressure to drop back to the reload setpoint—results in significant energy waste during periods of moderate demand,and generates repeated thermal cycling stress on compressor components that accelerates wear on valve plates,piston rings,and bearing assemblies.
Solution
Goalliance was contracted to design and deliver a full INOVANCE variable frequency drive retrofit across all four Sullair compressors,replacing the existing soft starters with a purpose-engineered VFD control system incorporating compressor-specific control logic,system pressure management,and multi-compressor coordination.
VFD Selection and Compressor Compatibility
INOVANCE MD500 series drives were selected for all four compressor units—the 160kW units fitted with MD500-4T160G drives and the 110kW units with MD500-4T110G drives.The MD500 series was specifically chosen for this application for several reasons:its built-in compressor-optimized motor control algorithms that account for the variable torque-speed characteristic of screw compressor loads,its flying start function that allows the drive to smoothly re-engage a coasting compressor rotor without speed search delay—important for recovering quickly from momentary power interruptions in a continuous process plant—and its advanced speed search and auto-restart capability which is essential for maintaining compressed air supply continuity in a facility where production cannot tolerate extended air pressure drops.
Pressure-Based Variable Speed Control
The central control logic implemented across all four compressors operates on a system pressure band control strategy with a tight pressure dead-band of±0.05 bar around the target system pressure setpoint of 7.5 bar(g).Each running compressor continuously modulates its speed between 40%and 100%of rated speed to maintain system pressure within this band—delivering precisely the compressed air volume the plant requires at any given moment without the pressure overshoot and undershoot cycles that characterize load-unload control.
Multi-Compressor Sequencing and Load Balancing
With four compressors available,the control system implements a demand-based sequencing strategy that determines how many compressors run and at what speed based on total system air demand.At low demand—such as during shift changeovers or maintenance windows—a single 160kW lead compressor operates at variable speed to cover the load.As demand increases and the lead compressor approaches 90%speed,a lag compressor is automatically brought online and both units share the load at reduced individual speeds.At peak demand,three or all four compressors run simultaneously,each modulating speed to share the total load equally.
Integration with Plant Air Management System
The compressor VFD control system was integrated with the facility’s plant air management system via Modbus TCP,providing the central energy management platform with real-time data on compressor speed,motor current,power consumption,discharge pressure,discharge temperature,and system airflow for each unit.An automated pressure setpoint adjustment function allows the plant’s process control team to remotely raise or lower the system pressure target from the central control room in response to changing production phase requirements—for example,reducing the pressure setpoint during the heating-up phase when instrument air demand is lower,and restoring full pressure setpoint ahead of steady-state production startup.
Results
- Compressed air station overall energy saving:43%versus pre-retrofit soft starter baseline
- Peak demand period energy saving:28%—even at high load,elimination of inlet valve throttling losses delivers significant savings versus fixed-speed operation
- System pressure stability improved from±0.3 bar under load-unload control to±0.05 bar under VFD pressure band control
- Compressor valve plate replacement interval extended from every 7 months to over 28 months following elimination of load-unload thermal cycling
- Bearing assembly service life extended—first major bearing overhaul deferred by an estimated 2.5 years beyond the pre-retrofit projected schedule
- Compressor oil consumption reduced by 22%due to elimination of excessive thermal cycling and reduced operating temperature at part-load speeds
- Full investment payback period:18 months
