VSD Compressor Energy Savings: How Does VSD Work?
Matching compressed air supply with demand can help reduce unnecessary energy use.
In many workshops, manufacturing facilities and industrial sites, compressed air demand changes throughout the day. Equipment starts and stops, production levels change and different processes require air at different times.
But your compressor still needs to respond to those changes.
This is where a variable speed drive (VSD) compressor may offer an advantage.
Why changing air demand is important
A fixed speed compressor operates its motor at a constant speed. When the system no longer requires its full output, the compressor uses its control system to respond to the reduced demand.
Depending on the compressor and control method, it may load and unload or use another form of capacity control.
The compressor can therefore continue consuming energy even when the facility doesn’t need its full compressed air output.
Where air demand changes considerably throughout the operating period, this can create an opportunity to match compressor output more closely to actual demand.
How does a VSD compressor work?
A VSD compressor measures system pressure and adjusts its motor speed as compressed air demand changes.
The variable speed drive changes the frequency supplied to the motor, allowing the compressor to increase or decrease airflow rather than continually producing its full rated capacity.
When demand increases, the compressor increases its output. When demand falls, it reduces its output and power consumption.
This ability to respond to changing demand is where the potential VSD compressor energy savings come from.
VSD compressors can also provide more consistent pressure
Modern VSD controls can maintain system pressure within a relatively narrow operating range.
Fixed speed compressors generally operate across a wider pressure band as they load and unload.
Where the system design allows, more consistent pressure control may allow the compressor to operate at a lower overall discharge pressure while still maintaining the pressure required at the point of use.
Reducing unnecessary system pressure can also help reduce energy consumption.
However, simply lowering compressor pressure isn’t always the answer. Pipework, pressure drop, air treatment, storage and actual equipment requirements all need to be considered.
Soft starting reduces starting demand
A VSD also provides a soft start by gradually increasing motor speed.
This reduces the high starting current associated with starting some fixed speed compressor motors and places less mechanical stress on the drive system during start-up.
Depending on the compressor and application, a VSD can also stop when demand falls below its effective operating range rather than continuing to run unloaded.
Does a VSD compressor always save energy?
No. A VSD compressor isn’t automatically more efficient simply because it can change speed.
If a compressor operates close to full capacity for most of its running hours, a correctly sized fixed speed compressor may be an appropriate option.
VSD technology generally offers its greatest advantage where compressed air demand regularly changes and the compressor spends significant periods operating below full capacity.
Correct sizing is also important.
An oversized VSD compressor isn’t a substitute for understanding your actual compressed air demand. Like any compressor, it needs to operate within an appropriate range for the application.
Start by understanding your compressed air demand
Before selecting a compressor, it helps to understand how your facility actually uses compressed air.
A compressed air audit can record compressor operating patterns and demand over time. This can help identify:
- Changes in compressed air demand
- Peak and average demand
- Compressor loading and unloading patterns
- System operating pressure
- Available compressed air storage
- Pressure loss
- Potential air leaks
- How multiple compressors work together
This information provides a much stronger basis for compressor selection than simply replacing an existing compressor with another unit of the same size.
The complete system also affects energy use
The compressor is only one part of the equation.
Air leaks, excessive system pressure, pressure drop, undersized pipework, insufficient storage and poorly performing air treatment can all affect how hard your compressor needs to work.
For example, installing a new VSD compressor won’t solve an underlying pressure problem caused by significant leaks or an unsuitable distribution system.
Looking at the complete compressed air system helps identify where energy is being used and where improvements may be worthwhile.
Building the business case for VSD
The potential return from a VSD compressor depends on your existing system and demand profile.
For a facility with significant variations in compressed air demand, matching compressor output more closely to that demand may reduce unnecessary power consumption.
For another facility with stable demand, the benefits may be smaller.
This is why NPS doesn’t recommend VSD based on a standard percentage saving.
We look at how the compressor will operate within the actual system.
The decision should consider:
- Current and expected compressed air demand
- Operating hours
- Required pressure and flow
- Existing compressor performance
- System storage
- Pipework and pressure loss
- Air treatment requirements
- Site conditions
- Energy use
From there, you can compare the available compressor options and determine which configuration best suits the application.
Is a VSD compressor right for your system?
Variable speed drive technology can provide a practical way to match compressor output with changing compressed air demand.
But the strongest results come from selecting the right compressor for the right application.
Nessco Pressure Systems can assess your compressed air requirements and help determine whether a VSD compressor, fixed speed compressor or combination of compressors best suits your operation.
Our team also carries out compressed air audits to provide a clearer picture of demand, compressor operation and system performance before you invest in new equipment.
To discuss your compressed air system, call our team on 1300 290 638 or contact us online.
