Blower Showdown: screw blower vs roots blower
Until the last decade, low pressure blowers used lobe or roots compression technology to produce air for use in a variety of industries. However, the screw blower is becoming the preferable option due to its efficiency benefits - particularly in cement plants.
The principle of the positive displacement blower was cutting-edge when the Roots brothers discovered it in 1854. That said, there have been only minor efficiency improvements over the past 150+ years.
This is why it's worth comparing the roots blower to the screw blower.
How does a roots blower work?
A lobe or "Roots" blower is a valve-less displacement compressor without internal compression. It works on the principle of isochoric compression. This is where air enters the compression chamber and the volume of the air remains constant as the identical rotors rotate.
The volume of the compression chamber decreases with continued rotation. With this, compression occurs externally against full counterpressure due to the incoming air from the connected pipeline.
External compression results in low efficiency and high noise levels. As a result, the use of lobe technology is relegated to very low-pressure applications and compression in a single stage.
Benefits of screw blower technology
The screw type blower utilizes a screw compression element. This consists of male and female rotors that rotate in opposite directions while the volume between the rotors and 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 operate at a high shaft speed and can combine a large flow rate with small exterior dimensions.
Isochoric vs isentropic compression
As pointed out above, Roots blowers work on the principle of isochoric compression. By comparison, screw blowers on isentropic compression. To better understand the difference, it's worth looking at the formulae for both processes
Ideal gas in ideal isochoric compression: T 2 = T 1 (P 2 /P 1)
Ideal gas in ideal isentropic compression: T 2 = T 1 (P 2 /P 1) (γ-1)/y
Energy consumption of screw and lobe blowers
Based on the information above, it is clear that the temperature T2 in an isentropic compression is lower than found in isochoric compression. This is because 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 °C
Rated Flow: 2000 m3/hr
Pressure: 0.7 bar(g)
Power consumed by a Roots blower is 60 kW with an air outlet temperature of 125 °C. With the screw blower, power consumed is 43 kW with an air outlet temperature of 94 °C.
Thus, a screw type blower is much more energy-efficient than a root blower.
Pressure/Volume diagram of a lobe blower
Pressure/Volume diagram of a screw blower
Conclusion
In the screw blower, the internal airflow path is optimized to reduce pressure drops and air turbulence.
The package includes a direct drive integrated gearbox instead of a belt/pulley system. This reduces transmission losses.
The combination of these elements plus the integrated variable speed drive (VSD) results in a screw blower that uses 30% less energy than lobe blowers.
Additionally, an integrated Elektronikon controller can monitor your operation 24/7 to ensure maximum reliability.
All these benefits coupled with energy savings makes screw blowers a preferred choice over root blowers.