The science behind why screw blowers are efficient than root blowers
26 November, 2021
Till the last decade, low pressure blowers used lobe or “Roots” compression technology to produce air for use in a variety of industries. While the principle of the positive displacement blower was cutting-edge when it was discovered by the Roots brothers in 1854, there have been only minor efficiency improvements over the 150+ years since its introduction.
How does a Roots blower work?
A lobe or “Roots” blower is a valve-less displacement compressor without internal compression and it works on the principle of isochoric compression where air enters the compression chamber and the volume of the air remains constant as the identical rotors rotate in the chamber. The volume of the compression chamber decreases with continued rotation and compression occurs externally against full counter-pressure due to the incoming air from the connected pipeline.
External compression results in low efficiency and high noise levels, which relegates the use of lobe technology to very low-pressure applications and compression in a single stage, even if two- or three-stage versions were available.
Why use screw technology?
The screw blower utilizes the screw compression element, consisting of male and female rotors that rotate in opposite directions while the volume between the rotors and the housing decreases.
Each screw element has a fixed, built-in pressure ratio and has no mechanical forces that cause unbalance. This means the screw technology can work at a high shaft speed and can combine a large flow rate with small exterior dimensions.
As root blowers work on the principle of isochoric compression and screw blowers on isentropic compression. Let's look at the formulae for these processes:
For ideal gas in ideal isochoric compression;
T2 = T1 (P2/P1)
For ideal gas in ideal isentropic compression;
T2 = T1 (P2/P1) (γ-1)/y
Hence, it is clear that the temperature T2 in an isentropic compression is quite lower than what it is in isochoric compression as less work is transferred to heat compared to the Lobe element where work is radiated as heat.
In simpler words, the efficiency of the screw element is higher than the lobe element at the same pressure.
Let’s understand this concept with the help of an example:
- For Ambient Temperature of 35 deg C:
- Rated Flow: 2000 m3/hr
- Pressure: 0.7 bar(g)
Power consumed by a Roots blower is 61 kW with an air outlet temperature of 125 deg C while power consumed by the screw blower is 43 kW with an air outlet temperature of 94 deg C.
Thus, a screw blower is much more energy-efficient than a root blower.
Summary
In the screw blower, the internal airflow path is optimized to reduce pressure drops and air turbulence and the package includes a direct drive integrated gearbox instead of a belt/pulley system to reduce transmission losses. The combination of these elements plus the integrated variable speed drive (VSD) results in a machine that uses 30% less energy than the previous lobe type blowers.
Additionally, the integrated Elektronikon controller monitors your operation 24/7 to ensure maximum reliability.
All these benefits coupled with energy savings makes screw blowers a prefered choice over root blowers.
You can read more about screw blowers here. If you need help, reach out to an Atlas Copco expert to assist!