Roots blowers have been used for decades in industries that depend on a steady and dependable air supply. These machines are known for their straightforward design and consistent performance in low-pressure applications. In this article, we look at how they work, where they are used, and what sets them apart from other blower technologies.
A roots blower is a positive displacement blower that moves a fixed volume of air from the inlet to the outlet using two counter-rotating lobes. As the lobes turn inside the housing, they trap pockets of air and carry them around the casing to the discharge side. Unlike some other air technologies, a roots blower does not compress the air internally. Instead, the pressure builds when the air reaches the outlet and meets the resistance of the system.
The working principle of a roots blower involves transporting a fixed volume of air from the inlet to the outlet port. Two symmetrical, shoe-shaped rotors rotate in opposite directions within a housing. These rotors mesh without touching, maintained by a very small clearance.
As the rotors turn, air is trapped in the space between the lobes and the casing wall. This trapped air moves around the perimeter at a constant volume, which is a process known as isochoric compression.
Unlike many compressors, a roots blower does not compress air internally. Instead, the compression happens externally at the discharge when the trapped air meets the back-pressure of the connected system. This results in a pressure increase at the outlet.
The relationship between the intake and discharge temperatures for an ideal gas in this process is defined by the formula:
In this formula:
Note: For this formula to work correctly, temperatures must be in absolute units (Kelvin or Rankine) and pressures must be in absolute units (like psia or bar absolute), not gauge pressure.
Precision is vital for the operation of these machines. A set of synchronizing timing gears ensures the rotors maintain their phase and never come into contact with each other. This non-contact design allows the rotors to operate at high speeds without the need for lubrication in the compression chamber, ensuring the delivered air remains oil-free.
Roots blowers are valued in industries that require a continuous, reliable flow of air at low-to-medium pressures.
When selecting a blower, engineers often compare roots technology to rotary screw blowers or centrifugal machines. A traditional roots blower is reliable and has a lower initial cost, but it is generally less efficient than a screw blower.
For example, to deliver approximately 1600 m³/h at 0.8 bar(g), a tri-lobe roots blower (such as the ZL3 55) has a package power consumption of 53.1 kW. In contrast, a screw blower under the same operating conditions consumes only about 39 kW. This makes screw blowers a more cost-effective choice for continuous-duty applications where energy savings are a priority.
For sensitive processes in the food, pharmaceutical, and electronics industries, air quality is non-negotiable. Roots blowers provide oil-free air because no lubricant enters the compression chamber.
Defining your air purity requirements is a critical step in system design, ensuring downstream equipment and end products remain protected from contamination. To achieve the highest air quality standards, oil-free equipment must be used.
Discover our range of reliable roots and energy-efficient screw blowers designed to meet your specific pressure and flow requirements.
A roots blower is technically a type of positive displacement compressor, but in practice it is usually used as a low-pressure blower. It moves a large volume of air, has no internal compression, and compresses against the system back-pressure at the outlet. That makes roots blowers a strong fit for aeration and pneumatic conveying, while compressors are generally chosen when the process needs higher pressure.
Roots blowers are designed for low-pressure applications. They are commonly used in the 0.3 to 1.0 bar(g) range, although the exact pressure depends on the model, operating speed, gas temperature, cooling, and whether the unit is single-stage or multi-stage. In general, roots blowers are selected when the process needs high flow at relatively low pressure.
The primary difference lies in performance stability and design: a Roots blower is a robust, positive-displacement machine that maintains a constant airflow even as pressure increases, making it the more efficient choice for demanding industrial tasks like pneumatic conveying. In contrast, a side channel blower is a compact, lower-cost alternative ideal for lighter applications below 300 mbar, offering the versatility of both vacuum and overpressure service. However, side channel blowers are less energy-efficient and suffer from a significant performance drop-off, where airflow decreases rapidly as system resistance rises.
A lobe booster usually refers to a rotary lobe, or roots-type machine, used to increase airflow, pressure, or vacuum system performance. In blower applications, the term is often used for a rotary lobe blower. In vacuum applications, it can also describe a roots-principle booster that works together with a backing pump to increase pumping speed and improve overall system performance.
A roots blower typically consists:
In packaged units, you will also usually find an inlet filter and silencer, an outlet silencer, and the drive system that transfers power from the motor to the blower element. The lobes rotate without touching each other, which helps support reliable operation.
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