Blowing away the 19th century
A continuous reliable regulated supply of quality air is a critical requirement for the aeration diffuser systems within municipal and industrial wastewater treatment plants in order to guarantee continuity of process operations and clean, uncontaminated discharge into the environment. Yet despite the vital role that air supply represents in the treatment process, the majority of these plants rely on basic lobe blower technology that has experienced little development since its introduction in the late 19th century.
It was to improve this scenario and provide waste water treatment plant operators with a reliable low pressure, oil-free, positive displacement blower system that combines optimum performance with protection of the process, the environment and operational energy costs, that Atlas Copco developed its range of screw technology ZS blowers. The operating principles, proven components and feature benefits of these screw blowers were specifically developed to overcome the limitations of lobe type blowers.
In making a true wire-to-air performance comparison between the old and new technologies it is necessary take into account not just comparable shaft power ratings but all of the potential power losses that can occur in a blower package − from motor and electrical system to transmission, filters, and other design features that contribute to pressure drops which are costly in energy terms. For instance, a 50 mbar/0.7psi pressure drop may appear to be small but when a blower is only providing a 500mbar/7psi discharge pressure this adds up to 10 per cent of the power consumption.
Lobe blower data is commonly offered by giving the air intake flow volume and the shaft power of the bare element whereas low pressure screw compressors are quoted by listing the FAD (free air delivered) at the unit outlet and the power consumption at the terminals of the power supply.
The air flow path before and after the blower element includes air-inlet filter, air-inlet silencer, air-outlet silencer and the check valve. The pressure drop over these components has to be added to the performance data of the lobe blower element. Such losses, as well as those incurred by electrical and mechanical transmission, are not always considered in the data of lobe blowers but they are always taken into account in the low-pressure screw compressor data.
The transmission losses, measured from the terminals of the power supply to the shaft power of the blower element, consist of the losses of the electric motor and the belt drive transmission losses from the motor shaft to the element. These figures vary generally as a function of the blower size and the operating point but typical values, for a 110kW small lobe type blower with an output of 1,000m³/h operated at 0.7bar(e) compared to a 75kW screw blower, are:
• inlet filter pressure drop: 20mbar;
• outlet silencer pressure drop:15mbar;
• check valve pressure drop 10mbar; and
• additional energy consumption: 6 per cent.
Mechanical and electrical
• transmission losses: 30kW;
• belt losses : 4 per cent;
• motor efficiency : 90 per cent; and
• total transmission losses: 14 per cent.
Screw blowers also have some degree of air flow path and transmission losses, but they are already taken into account when listing the system data, measured at the power supplies input and the compressed air outlet
As an example, the ZS screw blower features an integrated gearbox that allows better transmission of the power from the motor to the element and gains more than 3 per cent saving compared to a belt/pulley system. Moreover, the screw blower’s inlet filter and smooth discharge system are specifically designed to minimise pressure drops.
To deliver a flow of 1600m³/hr (942cfm) at a pressure of 0.8bar(e) (11.6 psig), a tri-lobe blower would consume 61 kW (82 hp) on average whereas a comparable screw blower demands only 43kW (58hp) for the same volume. In fact, the SER (Specific Energy Requirement) is on average 28 per cent lower and at a 600 millibar 50 per cent load this can increase to as much as 34 per cent.
Enter XXX
Waste water plants make cost savings
Anglian Water, one of the UK’s largest water and wastewater treatment companies, has chosen Atlas Copco’s low-pressure ZS blowers to supply air to the aeration diffusers at four of its wastewater treatment re-development sites.
Wastewater treatment plants use bacteria to break up waste which requires large quantities of air to be blown into aeration tanks. In a typical biological wastewater treatment plant, the blower system will account for up to 70 per cent of energy usage. Anglian Water approached Hydrok, an aeration diffuser and IFAS specialist, about supplying submerged, high-efficiency, bubble aeration framework systems incorporating bio textile curtains and fixed bed media. These units improve the distribution of aeration air to maintain the desired level of dissolved oxygen within the wastewater containment system.
Hydrok matched their high efficiency diffuser technology with Atlas Copco ZS low pressure screw blowers, which make energy cost savings of up to 35 per cent possible, for optimum performance.
• Rob Boughton is low pressure sales manager at Atlas Copco
