It is a necessity of time to come up with the most efficient and reliable machines, and the chiller industry is no exception. Indeed, the industry has been moving full steam ahead in considering all feasible options that would allow that extra watt of energy to be saved. And oil-free bearing is certainly one of the crucial elements.

The use of oil-free bearings for chillers is not new, and there are a few options available, such as…
1. Magnetic-bearing centrifugal chiller, in the capacity range starting from 165 RT to 1,425 RT
2. Refrigerant-lubricated-bearing centrifugal chiller, in the capacity range starting from 500 RT to 800 RT
3. Air-bearing centrifugal chiller, of 100 RT capacity

All these technologies offer compact design, lower noise and better efficiencies compared to oil-lubricated bearing chillers… and yes, they come with a small premium, in terms of first cost; but we need to bear in mind that the long-term overall cost of ownership is quite attractive and cannot be ignored.

So, then… what’s better, when it comes to oil-free centrifugal chillers — magnetic-bearing or refrigerant-lubricated-bearing? Let’s have a look at the various oil-free bearing systems in some detail.

THE ACTIVE MAGNETIC BEARING (AMB)
Magnetic-bearing centrifugal chillers employ key technologies, such as electromagnetic force driving, smart controls and advanced high-speed precision spindles. Permanent magnet AC (PMAC) motors replace the magnetic field induced in the conductors in the rotor with permanent magnets, made usually from alloys of rare-earth metals – this gives them much lower electric resistive losses than AC  induction motors, because no electric current is induced in the rotor. This allows magnetic fields generated by PMAC motors to deliver the same torque as AC induction motors with a smaller, lighter motor (Figure 2). To support the shaft without any physical contact, magnetic bearings use electromagnetic forces. Such a magnetic bearing design does not require oil or refrigerant for lubrication. The suspended rotating shaft maintains frictionless operation, thereby creating a device that features high efficiency, low noise and a long lifespan.

Having oil-free bearings makes the chiller system relatively simple and easy to maintain. Magnetic-bearing systems use considerably fewer moving parts than oil- or even refrigerant-lubricated drivelines, almost 80% less in some cases. This results in enhanced reliability, reduced maintenance and improved efficiency. (Magnetic-bearing systems have been in use in some mission-critical chillers since 1998, so they do not constitute a new technology that some of us may believe to be so.)

To provide emergency power in case of sudden loss of power, magnetic-bearing chillers are provided with integrated Uninterrupted Power Supply (UPS), as a form of backup, built into the design in order to maintain the rotation of rotary shaft and magnetic bearings to operate stably in case of mains failure. In the event of a power loss, the UPS provides power to the controls and coasts down to a slow speed to de-levitate the shaft and rest on the touchdown bearings.

Some more advantages worth mentioning are…
• Magnetic bearing doesn’t suffer from wear.
• Non-contact design, hence there is an elimination of mechanical frictional losses, resulting in high reliability
• Can often accommodate irregularities in the mass distribution automatically, allowing rotors to spin around their centre of mass
with very low vibration
• Low power losses, which allow AMB-supported machines to achieve higher running speeds.
• Low noise with virtually no structure-borne vibration, due to no mechanical contact
• Conventional oil-lubricated-bearing chillers typically cannot operate below 50 degrees F (10 degrees C), but some of the magnetic chiller machines can operate with entering condenser water temperatures as low as 36 degrees F (2.2 degrees C)

Having highlighted some of the advantages, here are some typical concerns surrounding magnetic-bearing chillers that must be pondered
upon…
• AMBs are complex mechatronic systems that have many potential power loss mechanisms that are required to be dealt with successfully in
order to have a reliable system.
• They have a high first-cost associated with them; having said that, it’s the total cost of ownership payback that makes these machines
attractive.
• Magnetic bearings typically require a back-up (touchdown) bearing in the case of power or control system failure.

REFRIGERANT-LUBRICATED-BEARING CENTRIFUGAL CHILLER
One of the leading bearing manufacturers has developed a unique bearing lubrication solution for centrifugal compressors in the form of hybrid bearing that runs in pure refrigerant and does not require oil lubrication. This hybrid bearing consists of high Nitrogen stainless steel rings, Silicon Nitride rolling elements and a glass fibre reinforced Polyetheretherketone (PEEK) cage. This proven media-lubricated design is helping to pave the way for a new generation of high-efficiency, low-maintenance compressors, used in water-cooled chillers1

It is claimed that the refrigerant-lubricated rolling bearings can improve energy efficiency up to four per cent, compared to traditional hydrodynamic bearings.

Bearings lubricated with just refrigerant rather than refrigerant/oil mix further boost energy efficiency of the chiller, not only because of the considerably reduced friction in the bearings but also through improved heat transfer in the condenser and the evaporator heat exchangers. The need for oil maintenance and oil costs is eliminated by having bearings that don’t require oil lubrication system.

In comparison to the magnetic-bearing system, the refrigerant-lubricated-bearing system is refrigerant-dependent, and its operation for any new series of refrigerant may need to be verified. Also, the refrigerant-lubrication systems require components very similar to oil-lubricated systems (Figure 5) and, therefore, need regular maintenance.

AIR-BEARING CENTRIFUGAL CHILLER
Another well-known chiller manufacturer has in its arsenal an inverter centrifugal single compressor 100 RT chiller, which uses air-bearing technology, which also allows it to operate without lubricating oil.

The air bearings use pressure from the refrigerant to create space between the shaft and the leaf of the rotor (Figure 6). When the motor is stationary, the motor shaft drops down, and the leaf does not receive pressure from the system and makes contact with the shaft. Problems can occur when the motor is stationary and the shaft and leaf make contact; in order to prevent any damage due to contact, a special coating technology is used.

All this sounds a little complicated – and it certainly is, when compared to the other systems, discussed earlier. Furthermore, it seems like a hybrid system that uses air and refrigerant. Technical details of this system are not yet available, neither in the manufacturers’ product catalogues nor in the public domain.

The chiller manufacturer claims to achieve the industry’s highest energy-efficiency rating for its centrifugal chiller, thanks to its effective three￾level inverter and two-stage compression cycle. The manufacturer further claims that the use of its oil-free inverter technology in its centrifugal chillers results in operating costs that are 29% lower than traditional screw chillers, which operate at a fixed speed. These are claims, as they have not been verified.

FOR WHAT IT’S WORTH
All oil-free-bearing systems offer considerable improvement in the efficiency and reduction in operating costs of the chillers when compared to the traditional hydrodynamic bearings, although manufacturers occasionally do get carried away with their claims. However, there is definitely a case for these energy-efficient alternatives, and we can see that more and more designers and developers are considering oil-free chillers in their projects.

As for which system may be better, we let laws of physics prevail for now – friction is loss of energy and, therefore, magnetic-bearing chillers would definitely have an edge over other technologies that are being used in parallel. In addition, magnetic-bearing technology has come of age and can now be considered as a reliable alternative. Furthermore, when compared to the other technologies, it’s definitely very low maintenance.

Undoubtedly, use of oil-free bearings and, specifically, magnetic bearings is another tangible step towards making very efficient machines with minimum internal losses. We can see that the industry is moving in the direction where more and more chiller manufacturers are considering these alternatives to make their machines more reliable and efficient while keeping the maintenance requirements low.

Rehan Shahid is Director at P&T Architects & Engineers. He may be contacted at rehan@ptdubai.ae.