At a wastewater treatment plant, the blowers use the most electricity, so this is where the largest savings can be made. Gergely Halász, lead procurement engineer at Water4All Zrt., explains why turbo blowers are displacing older Roots blowers, what makes the shaft float in air- and magnetic-bearing machines, and how energy savings of up to 30 percent can be achieved.
When we want to make a real reduction in a wastewater treatment plant’s energy use, we usually think first of the largest consumers, the blowers. Is that the right approach?
Absolutely. The technology holds enormous potential for energy savings. For the user, of course, besides energy use, operational reliability, service costs and service life are also important factors, and the purchase cost is not negligible either.
Let’s take these one by one.
Since electricity is the largest cost in operating blowers, it matters a great deal what type of equipment we choose and how it works. In the past, we used almost exclusively Roots-type rotary lobe blowers. The technology is named after the American Roots brothers, who patented it in the 1860s. In essence, the housing contains two counter-rotating rotors, usually with two or three lobes, in a figure-of-eight shape. These carry the air from the intake side to the discharge side. There is no internal compression in the blower itself: the pressure builds up on the discharge side, against the resistance of the system. Its advantages are that it is simple, robust, inexpensive and easy to maintain, and it can be repaired in relatively basic workshop conditions. Its drawbacks are that it is noisy and less efficient than screw blowers or turbo blowers.
Technically, what explains the low efficiency?
They work with significant slip losses: a large share of the air stays inside the stage instead of being delivered to the pipe. Their energy use is particularly high when turned down. The machines are also bulky, which increases manufacturing and installation costs. In recent years, however, Roots blowers have been replaced by more energy-efficient turbo blowers. Whereas Roots machines run at 1,500–3,000 rpm, turbo blowers typically operate at 30,000–50,000 rpm or even higher.
How does that lead to lower operating costs?
Turbo blowers are mechanically more efficient. Their losses are much smaller, and their electric drive motors are also considerably more efficient. In the vast majority of turbo blowers, the motor drives the rotor directly, with no clutch or V-belt. They use permanent-magnet synchronous motors, which by their nature are more energy-efficient than conventional asynchronous industrial motors. The other half of the energy saving comes from the fact that turbo blowers can operate without friction.
How so? Do they have no bearings?
This usually means one of two designs. With air bearings, above a certain speed an air film forms between the shaft and the housing, so friction disappears completely in normal operation. The other solution is magnetic levitation, where a magnetic bearing creates the floating state. The advantage of the magnetic bearing over the air bearing is that the shaft floats continuously, even at standstill, and never touches the housing. Wear is zero with magnetic bearings, whereas with air bearings the shaft effectively settles onto the housing at every start and stop, so each start cycle causes wear. You could say these machines are worn out by their start–stop cycles.
So it’s best not to keep switching them on and off.
That’s right. With air bearings, continuous operation is ideal; where frequent shutdowns are expected, we clearly recommend the magnetic-bearing solution. What speaks for the air-bearing blower is its simplicity and its price.
Which manufacturer does Water4All prefer and recommend when it comes to an investment?
We distribute Raetts products. Raetts is a Chinese manufacturer that has been building air-bearing machines for a very long time, and over the past seven to eight years it has also been manufacturing and supplying magnetic-bearing machines; several thousand of its reference units are in operation. To our current knowledge, apart from Sulzer it has hardly any competitors on the European market, at least in the relatively small blower size range most common in Hungary. In the 200–300 kW range and above, there is naturally a wider choice of magnetic-bearing machines.
What do you mean by a relatively small blower size?
Raetts magnetic-bearing blowers are available from 15 kW motor output up to 300–350 kW. For larger requirements, we recommend multiple units instead, and there are a number of technical arguments in favour of this.
Can the output of turbo blowers be controlled?
That is standard today. They can typically be turned down from 100% of their rated output to somewhere in the 40–60% range. This is similar for air-bearing and magnetic-bearing machines; controllability has nothing to do with the bearing design.
What is the service and spare-parts support like for Raetts blowers?
We provide service for Raetts blowers in Hungary. What you need to know about operating these machines is that by far the most important task is the scheduled replacement of the intake air filters. Most manufacturers recommend doing this every three months, and we agree. Filter replacement cycles can be planned, and they are just as important as with car engines: if the machine draws in dust or other particles, there is a risk of failure. Replacing the filters is an extremely simple four-to-five-minute job that can be done immediately after the unit is shut down.
How many air filters does a blower have?
Four, eight or sixteen, depending on machine size. They are fabric or paper filters similar to car air filters, and priced accordingly, at about 40 euros each. This is a negligible share of the total operating cost. Apart from the air filters, there are no wearing parts at all, so V-belt or oil changes, for example, never come up.
We haven’t yet talked about the most important point. How much less electricity does a turbo blower use than a Roots blower with the same air delivery?
The saving is at least 20%, and at some plants it can reach 30%, depending on how close to their duty point the machines operate — in other words, how well the blower was sized for the plant it serves. Correct selection is a design task, and routine work for the specialists at Water4All Zrt.
What criteria do you use to select the ideal blower?
When we design the plant ourselves, we naturally determine the oxygen demand and the required air flow, and choose the best-matching turbo blower from the range. When we replace old Roots blowers with turbo blowers, however, we examine the plant’s operating condition and load profile in meticulous detail and select accordingly. This cannot be done without a thorough assessment. With turbo blowers, oversizing causes operating problems as serious as undersizing, if not more so, so the “leave plenty of reserve” principle that operators are used to must not be applied here, for technical reasons. Oversizing leads to stop-go operation, ruins energy efficiency and causes premature failure. Because there is no friction, the design life stated by manufacturers is over 20 years for air-bearing machines and over 25 years for magnetic-bearing machines, although operating conditions naturally affect this.