Understanding Compressor Transient Losses and Their Importance
In the world of rotary screw compressors, there are three main types: fixed-speed, dual-speed, and Variable Speed Drive (VSD) compressors. One critical factor that sets these types apart in terms of energy efficiency is something known as compressor transient losses. This article will explain what transient losses are and why they matter in the context of compressor performance.
What Are Compressor Transient Losses?
Transient losses refer to the energy that a compressor consumes without producing any usable air while cycling between different operational phases. In simpler terms, it's wasted energy. For traditional fixed-speed compressors, these losses can account for up to 20% of total energy consumption. In contrast, dual-speed compressors experience much lower transient losses, while VSD compressors almost entirely eliminate them.
Transient Losses in Fixed-Speed vs. Dual-Speed Compressors
This article will focus on the two types of compressors where transient losses are a significant factor: fixed-speed and dual-speed compressors. We’ll explore why fixed-speed compressors are unable to meaningfully reduce transient losses and how dual-speed compressors manage to significantly minimize this wasted energy.
Transient Losses in Fixed-Speed Compressors
Transient losses are an unavoidable aspect of fixed-speed compressors because they can only operate at 100% motor speed and cannot start against a pressurized system.
Here’s a breakdown:
Start-Up: When the motor of a fixed-speed compressor starts, the air/oil separator vessel needs to fill up first. Compressed air is only supplied once the compressor reaches a pressure of 4 bar, when the minimum pressure valve opens. All the energy used during this start-up phase, before any air is produced, contributes to transient losses.
Full Load: At maximum air demand, the compressor operates at optimal efficiency, with no wasted energy.
Lower Air Demand: When demand decreases, the fixed-speed compressor enters an unload phase. During unload, the motor continues to run at full speed without producing air, leading to pure energy waste. Additionally, during the transition from load to unload, the compressor experiences further transient losses as the air/oil separator vents to 1/2 bar to prevent the compressor from restarting under pressure later on. These losses, known as blow-off losses, represent the energy used to produce the vented air.
Shut-Off and Restart: If the low demand persists, the compressor will eventually shut off. When air demand picks up again, the compressor must restart, re-initiating the cycle of transient losses.
Transient Losses in Dual-Speed Compressors
Dual-speed compressors significantly reduce transient losses compared to fixed-speed units. This is achieved by operating at a minimal speed during unload and the ability to start against a pressurized system.
Here’s how it works:
Start-Up: When starting from an empty state, a dual-speed compressor operates similarly to a fixed-speed compressor during this phase.
Full Load: At peak demand, the compressor runs at maximum speed, with no wasted energy. However, in this mode, it doesn’t offer any advantages over a fixed-speed compressor.
Lower Air Demand: When demand decreases, a dual-speed compressor also enters an unload phase, but it runs at a minimum speed during this time, significantly reducing energy waste. Additionally, because the dual-speed compressor can start under pressure, it minimizes blow-off losses.
Shut-Off and Restart: A dual-speed compressor can restart under pressure, which allows it to reboot faster and with less energy than a fixed-speed compressor.
The Impact of Compressor Transient Losses on Costs
This article has outlined what compressor transient losses are, why they occur in fixed-speed compressors, and how dual-speed compressors can reduce them. Considering that energy costs typically make up 80% of a compressor’s total cost of ownership, the ability to minimize transient losses can have a substantial impact on operational expenses.
