Stefan Henneberger, Team leader, Engineering Low Pressure Projects
Conrad Latham, Product Manager – Low Pressure Atlas Copco Airpower n.v. Wilrijk, Belgium
Edited by Kenneth J. Korane, [email protected]
Resources: Atlas Copco, www.atlascopco.com
To improve the efficiency of equipment that uses air blowers for low-volume flows (300 to 5,000 m³/hr), engineers need to look beyond lobe-style blowers. These blowers are widely used in applications such as wastewater aeration, pneumatic conveying, and mixing, and designs have evolved from two to three-lobe designs, mainly to reduce discharge pulsations. But in regards to energy efficiency, lobe blowers have not seen significant improvements over the past 50 years.
Screw compressors, on the other hand, rely on internal compression — as opposed to external compression in lobed blowers — and this dramatically improves efficiency and can produce sizable energy savings. They also run quieter, vibrate less, and are more reliable. Here’s a look at the theoretical and practical details.
Positive-displacement, rotary-lobe blowers consist of a pair of two or three-lobed rotors that turn inside an oval-shaped casing. External power drives one rotor while synchronous gears drive the other.
As the rotors turn, air is drawn through the inlet side and forced out the outlet port against system pressure. There is no change in the volume of the air within the machine. The blower only displaces air from the suction end to the discharge end against discharge-system resistance.
Oil-free screw blowers, in contrast, are positive-displacement rotary devices with screw-shaped male and female rotors. These rotate towards each other while the volume of air trapped between them and the housing decreases. The rotors do not contact each other and are synchronized by timing gears.
Each screw blower has a fixed, built-in internal pressure ratio. This means that the outlet port is designed with a certain, fixed geometric profile. To obtain the best efficiency, the internal pressure ratio should match the required working pressure.
To better understand a screw blower’s advantages, let’s first look at a p-V (pressure-volume) diagram for a lobe-type blower. Air in the piping outside the blower exhaust port is at a higher pressure. As the rotor lobes uncover the exit port, air from the pipeline flows back into the flute space between the rotor and casing. This back flow of air equalizes pressure and compresses the entrapped air externally at constant volume (seen as line 1-2 in the accompanying diagram.) As the rotors continue to turn, air is pushed into the discharge line against full system pressure (represented by line 2-3.)
Screw blowers operate differently. At the start of the compression cycle, gas at suction pressure fills the flute spaces formed by the unmeshed rotors just under the suction flange. Gas continues to fill the flute spaces until the trailing lobe crosses the inlet port. At that point, gas is trapped inside the flute space (and equals displaced volume Vs.)
As the lobes mesh into the flute space, flute volume decreases and pressure increases as long as the gas is trapped. Gas leaves the flute space when the leading lobe crosses the discharge port. This discharge volume =
Vs/vi, where vi is the ratio of initial to compressed volumes inside the screw blower. Further rotation and meshing of the rotors forces gas into the discharge line.
The bottom line is that internal compression reduces energy consumption, as represented by the green area in the p-V diagram.
For optimal compression efficiency, the volume ratio, vi, should be sized so that the internal compression ratio matches the system compression ratio: pi = pe. If the ratios do not match, gas will be over or undercompressed.
With overcompression, as the name implies, gas is compressed more than the system requires, which takes more work. Gas compressed internally to a higher pressure must then expand down to external working pressure.
In the case of undercompression, the internal discharge pressure is less than system discharge pressure, and gas from the discharge line flows back into the flute space and equalizes pressure at constant volume, resulting in extra work from ideal compression.
The ideal compression process from p1 to pe is a reversible adiabatic (that is, isentropic) process. The isentropic work required, Wi, is:
For a lobe, the theoretical actual work done is
Vs(pe – p1). In general, the theoretical actual work done, Wa, can be written as:
The theoretically maximum achievable adiabatic efficiency, ηa, is:
ηa = Wi/Wa.
Results of the above equation are plotted graphically using various values of vi, as shown in the Theoretical adiabatic efficiency graph. These results show that the theoretical maximum efficiency for lobe-type blowers is 76.5% when the pressure ratio = 2, while a tuned rotary-screw blower could reach 100%.
Due to dynamic losses at inlet and discharge, as well as leakage and friction, the real compression work is higher and, subsequently, actual adiabatic efficiency is lower. Taking these factors into account, more-realistic adiabatic efficiencies are given in the Actual efficiencies graph.
From the user’s point of view, it’s often difficult to compare the energy efficiency of machines that use different compression technologies if data are not presented in a comparable manner. That’s certainly a problem when rating rotary lobe versus screw hardware.
Lobe-blower performance is commonly expressed in terms of the theoretical air-intake volume flow and the shaft power the compression element requires. Low-pressure compressors, on the other hand, are quoted by listing the FAD (free air delivered) at the outlet and power consumption at the power-supply terminals. This means airflow path losses, as well as electrical and mechanical transmission losses, are considered for rotary-screw devices but not lobe blowers. To better compare the compressors, let’s evaluate the efficiencies and losses of the other blower components.
The airflow path before and after the blower includes the air-inlet filter, air-inlet silencer, air-outlet silencer, and check valve. The pressure drop over these components has to be added to the performance data of the lobe blower.
Transmission losses from the power-supply terminals to the shaft of the blower consist of the losses in the electric motor and transmission (belt drive) from motor shaft to the blower.
These losses generally vary as a function of blower size and operating point. (The Blower losses table lists typical values for a small lobe-type blower (1,000 m³/hr) operating at 0.7 bar and a medium-size blower (5,000 m³/hr) operating at 0.5 bar.)
Of course, screw blowers also have air-flow and transmission losses, but they are already accounted for in data measured at the power supply and compressed-air outlet.
Neither catalog nor test data from various suppliers using different compression technologies can be easily used to analyze energy efficiency, so the best way to compare performance is in the lab. Tests can rate different units under equal operating conditions, and gauge performance using the same test and measurement equipment.
In tests performed by Atlas Copco and observed and audited by Germany’s TÜV Rheinland inspection association, technicians measured energy consumption at the blower’s power supply as well as volume flow at the outlet flange, according to ISO 1217 ed.3.
Tests were performed at different power ratings with various brands of lobe blowers. Test results were expressed in the specific energy requirement (SER, in J/l), which shows the relation of the consumed power in kilowatts divided by the free air delivery in m³/hr.
In one test, a trilobe lobe blower with a 110-kW motor and connected to a separate frequency converter was compared to a screw blower using a 75-kW motor with integrated frequency drive. The result at maximum volume flow of the lobe blower (2,145 m³/hr) shows a 32.1% higher specific energy consumption (lobe: 141.0 J/l, screw 106.7 J/l). At minimum volume flow (984 m³/hr) the difference in the specific energy requirements is 64.4 % (lobe: 191.7 J/l, screw 117.2 J/l). Similar results were seen in other tests, demonstrating that compared to traditional lobe blowers, rotary-screw devices can cut energy consumption in half.
Air outlet temperature. The extra compression work for a lobe blower, compared to a screw blower, results in more heat dissipation (and power loss) and, consequently, higher outlet temperatures. Calculate the additional power required for a lobe compared to a screw blower using:
mass flow × Cp(Tol – Tos),
where Tol = lobe outlet temperature and Tos = screw outlet temperature.
Assuming compression takes place quickly, heat transmission can be ignored and the process is approximately adiabatic.
Actual specific work Wt = Cp(To – Ti), where To and Ti = the outlet and inlet temperatures, respectively.
Based on the actual work done, the air outlet temperature can be calculated from: To = Ti + (Wt/Cp).
Efficiency during turndown. Most compressed-air blowers used in industrial and wastewater applications require that the blower adjust delivered airflow with the process. This is accomplished in various ways: cycling the blowers on and off, throttling the inlet suction, adjusting outlet diffuser vanes, or using adjustable-speed drives. In most low-flow blowers, the latter method is the preferred choice.
The change in overall efficiency related to delivered airflow is an important issue in the economics of blower operation. The Efficiency and turndown graph demonstrates how adiabatic efficiency changes with respect to turndown in capacity for various types of compressors. As can be seen, screw blowers maintain a more-stable efficiency compared to lobe blowers at partial-load flows.
Pressure pulsations. Traditionally, lobe blowers were designed with two lobed rotors. Manufacturers have put significant efforts into reducing the pressure pulsations inherent in this design.
Because pressure between the lobes is below discharge pressure when the pocket opens to the discharge line, a sudden back flow accentuates gas pulsations. Trilobe rotors partially address this as their lower pulsation levels provide smoother flow than their two-lobe cousins.
To further reduce pulsations, engineers have added helical rotors and special channels to blower casings. These advances have lowered pulsations by prefilling the reverse chamber. But the designs still induce strong pulsating forces on the rotors. These can lead to intermittent noise and vibration (rattle) in the gearing.
Screw blowers deliver more-stable flow and, thanks to a better matching of the internal and external pressures, pulsations are lower than those possible with rotary-lobe technology.
The screws’ faster rotational speed and more lobes result in a higher pulsation frequency with lower amplitudes. In general, high-frequency pulsations are easier to dampen and result in lower overall noise and pressure variations in discharge piping.
This design prolongs the life of flexible elements in aerating systems and protects conveyors against undesirable pulsations. Inside the blower, lower gas pulsation amplitudes means weaker, less-harmful vibrations are transmitted to the bearings – increasing their life.
Cp = Constant pressure specific heat for air, about 1,004 J/kg°K
pe = Working pressure (external system pressure)
p1 = Inlet pressure
vi = Built in volume ratio (vi = 1 for a lobe blower)
Vs = Displaced volume
Wt = Actual specific work, J/kg
π = External pressure ratio, pe/p1
κ = 1.4 (air)