VDC vs. VSD: Which Technology Delivers Greater Energy Savings?

The benefits of variable speed drive (VSD) are widely recognized, however some operators conflate VSD with variable displacement compressors (VDC). While both are based on similar principles of energy efficiency, they function differently and provide vastly different benefits to operators.

VDCs are compressors with a variable displacement inlet design. This inlet design is often called a turn valve, a spiral valve or a slide valve and is more common on higher horsepower machines. While these machines yield a much better part-load efficiency than standard modulation control at the higher end of the load factor, they can become very inefficient at lighter partial loads. The inefficiency is caused by an artificially shortened rotor length, which reduces the volume available for compression. Accordingly, the farther down the rotor is set, the more the compression efficiency deteriorates.

On the other hand, VSD compressors simply measure the system pressure and maintain a constant delivery pressure within a narrow pressure band (about +/- 1.5 psi, compared to 10-15 psi in a VDC). They do so by regulating the motor speed of a compressor with frequency conversion, resulting in a varying air flow delivery. Unlike VDCs, VSD compressors can use a lower air compressor delivery pressure while still maintaining the required minimum working pressure of the system. This leads to greater energy savings, as every 2 psi reduction yields a 1% decrease in power consumption.

Many compressors can be retrofitted with a VSD component to meet air flow demand. However, these retrofit options do not always offer the complete benefits of an integrated VSD compressor. Integrated VSD packages offer benefits beyond energy savings, including less regular maintenance, lower risk of current damage and longer lasting machinery.

Want to learn more? Check out this recent article on the 8 key advantages of integrated VSD. Contact an Atlas Copco expert today to learn more.