What is a dual-speed compressor?

Perfect marriage between two technologies

What is a dual-speed compressor?

A dual-speed is a completely new type of screw compressor. As the name indicates, it has two speeds. Like a fixed-speed model, it can run at the very top of its capacity when air demand is at its peak. However, it also features a lower speed when air demand goes down. This reduced speed helps minimize energy consumption during unload.
One of the reasons why fixed-speed compressors are so inefficient is that they experience a lot of transient and unload losses. This wasted energy can total up to 40% of a fixed-speed compressor’s energy use. Dual-speed compressors significantly reduce transient and unload losses. In fact, they can cut energy use by 20% compared to the fixed-speed alternative. That makes them much more energy efficient – and less expensive to operate – than fixed-speed compressors.Other benefits of dual-speed compressors include that they can start (faster) under pressure and allow flexible pressure setting without loss of air flow or FAD. For more about dual-speed compressors, click here.

Dual-speed compressors significantly reduce transient and unload losses. In fact, they can cut energy use by 20% compared to the fixed-speed alternative.

Transient losses explained

The term “transient losses” describes a costly problem that affects fixed-speed compressors. It refers to the energy that these compressors consume without producing usable air as they cycle between operational phases.

These losses are incurred right after the machine is turned on and before it switches off.

Transient and unload losses can add up to 40% of the total energy use of a fixed-speed.

That’s because of two limitations inherent to fixed-speed compressor technology. They can only work at 100% motor speed and they cannot start under pressure. The latter means that after start-up, fixed-speed compressors first need to build up the required pressure in the air/oil separator vessel. Therefore, after being turned on, they are consuming energy already without producing usable air.

It also means that, the air/oil separator must be vented as the compressor goes into unload before shutting off. The energy wasted during this process is also referred to as “blow-off losses.” In addition, during unload, the motor runs at maximum speed without producing any air.

Again, when you are operating a fixed-speed compressor, there is no way around these costly losses. As already mentioned, transient and unload losses can add up to 40% of the total energy use of a fixed-speed.

One of the benefits of dual-speed screw compressors is that they significantly reduce transient losses. Variable Speed Drive compressors, on the other hand, suffer almost no transient losses.

This article explains transient losses in more detail.