In terms of instal­la­tion space, low weight, high effi­ciency, and long service life, elec­tri­cally driven, gas-bearing turbo compres­sors are one of the best solu­tions for this. The newly devel­oped oil-free CompaNamic turbo compres­sors from ebm-papst in the motor power range from 10 kWe to approx. 55 kWe enable vari­able delivery rates for different refrig­er­ants and require signif­i­cantly less refrig­erant thanks to an internal volume reduc­tion of 90% (intro image). The 10 kWe vari­ants devel­oped for R290 and R1234ze(E) are suit­able for many areas of appli­ca­tion. However, maximum gas purity through the elim­i­na­tion of oil lubri­ca­tion and a wide range of appli­ca­tions required inno­v­a­tive approaches to oil-free bearing tech­nology, aero­dy­namics, motor design, and power elec­tronics. New methods for the preci­sion machining of compres­sors and gas bear­ings were inte­grated for large-scale series produc­tion planned for the end of 2028.

The task of a compressor for heat pumps and refrig­er­a­tion tech­nology is to compress refrig­er­ants as effi­ciently as possible over long periods of time with low oper­ating noise. Ideally, it should operate oil-free, keeping the system clean, increasing heat transfer effi­ciency, and elim­i­nating the need for auxil­iary equip­ment such as oil sepa­ra­tors, oil sumps, and the design complexity of oil return systems (Figure 1). Depending on the area of appli­ca­tion, the compres­sors and, above all, their bear­ings must operate with as little main­te­nance as possible for many years, both in contin­uous oper­a­tion and in start-stop cycles. The end units should also be as small and light as possible, so that compact and dynamic PFAS-free compres­sors are in demand, which can be installed in any posi­tion without having to worry about oil return. While the drive and bear­ings can remain virtu­ally unchanged for a wide range of refrig­er­ants, the aero­dy­namics some­times must be adapted signif­i­cantly to the gas used.

In prac­tice, the devel­opers were able to achieve this by using high-speed internal rotor motors enable the construc­tion of very compact compres­sors with 10 kWe at a diam­eter of only 130 mm and a length of 300 mm, weighing only 6 kg. At the same time, an internal volume reduc­tion to just 400 cc in the ebm-papst P3 compressor – compared to conven­tional compres­sors with approx. 5000 cc – has made it possible to dras­ti­cally reduce the amount of refrig­erant required. By elim­i­nating oil, heat transfer is improved due to the lack of surface wetting with lubri­cant (oil including addi­tives) in the heat exchangers, and pres­sure losses are mini­mized. In addi­tion, there is no oil cont­a­m­i­na­tion and effi­ciency degra­da­tion of the heat exchangers due to oil over their service life. As only pure refrig­erant is in circu­la­tion, oil main­te­nance and oil handling equip­ment are no longer neces­sary. The perfectly balanced rotating parts result in vibra­tion-free, quiet oper­a­tion. The sound power level is around 60 dB(A). To achieve this, advanced tech­nical solu­tions are required in many areas.

Aero­dy­namics is the key element

The compressor size is deter­mined by the different require­ments of the heating or cooling tech­nology used. A compressor power of 10 kWe for the refrig­er­ants R290 and R1234ze(E) marks the start of the product port­folio. This is partic­u­larly suit­able for larger build­ings or indus­trial appli­ca­tions. In order to effi­ciently cover the different require­ments resulting from the enve­lope of pres­sure ratio and mass flow, the entire range of design options for the wheel and blade geom­etry must be utilized. The SCOP (Seasonal Coef­fi­cient of Perfor­mance) points from different appli­ca­tions are used for this purpose. In addi­tion, espe­cially with small impeller diam­e­ters, it is crucial to reduce the blade gap losses that occur to a tech­ni­cally feasible minimum. This ensures highly effi­cient aero­dy­namics. 

The enve­lope (Figures 2 and 3) describes the oper­ating limits. The X-axis repre­sents the evap­o­ra­tion temper­a­ture, and the Y-axis repre­sents the conden­sa­tion temper­a­ture. The MidLift and High­Lift vari­ants require higher pres­sure ratios and there­fore 2-stage solu­tions. A single-stage solu­tion is suffi­cient for the LowLift version. After simu­la­tion, the char­ac­ter­istic maps were also measured, veri­fied, and opti­mized in ebm-papst’s own new, state-of-the-art compressor test facility. The company’s core busi­ness in fans also provides access to a high-perfor­mance envi­ron­mental testing center. Here, the compres­sors are subjected to long-term testing.

Bearing tech­nology

The high perfor­mance and dynamics of the units in rela­tion to their size require a sophis­ti­cated bearing system. The table (Figure 4) shows the differ­ences between high-speed bearing tech­nolo­gies, with a focus on radial bear­ings. Inex­pen­sive plain and ball bear­ings have a short service life at high speeds and can also cont­a­m­i­nate the gas with oil or grease. Magnetic and gas bear­ings, on the other hand, operate with virtu­ally no wear. However, magnetic bear­ings are still very expen­sive, so gas bear­ings are a good compro­mise. Aero­dy­namic rigid gas bear­ings in partic­ular enable precise posi­tioning of the shaft compared to film bear­ings, allowing a minimum blade gap between the blades and volute to be set during oper­a­tion for a partic­u­larly effi­cient flow machine. During oper­a­tion, the shaft floats on a cushion of compressed working gas. Lubri­cants and coolants are not required, and possible cont­a­m­i­na­tion by addi­tional substances is avoided. The refrig­er­ants remain absolutely pure. A suit­able choice of mate­rials and hard coating of the bearing surfaces ensures virtu­ally wear-free start-up and rotor run-out. ebm-papst compres­sors are designed for a service life of approx. 150,000 hours and more than one million start-stop cycles.

Figure 2: Compressor enve­lope for R290. (Image | ebm-papst)

Figure 3: Compressor enve­lope for R1234ze(E). (Image | ebm-papst)

Effi­cient motor with compact dimen­sions

Motor power is defined as the product of torque and speed, which is why high power can be achieved at high speeds even with a compact elec­tric motor. Despite their low torque, the compact, high-speed internal rotor motors deliver 10 kWe at approx. 160,000 rpm, and larger versions will deliver up to 55 kWe at approx. 60,000 rpm. However, high-speed drives require precisely matched geom­etry of the stator and perma­nent magnet rotor, as other­wise there will be signif­i­cant iron, copper, and rotor losses, which are further increased by frequency-related effects. Higher volt­ages allow smaller copper cross-sections, but place increased demands on the winding insu­la­tion. Various cooling concepts have been devel­oped to dissi­pate the waste heat gener­ated. The same applies to the power elec­tronics.

Power elec­tronics for high-speed motors

The power elec­tronics must be adapted to the oper­ating condi­tions. In compres­sors, they can utilize the wide range of supply volt­ages and allow for good power scaling thanks to the modular design of the output stages. The usual mains volt­ages are approx. 320V to 480V 3-phase AC. The high speed of the internal rotor motors requires currents with frequen­cies of up to over 100 kHz for control in pulse width or pulse ampli­tude modu­la­tion, which the power output stages must provide reli­ably. All compo­nents must achieve the highest possible effi­ciency of more than 97% even at partial load, and the applic­able stan­dards and user spec­i­fi­ca­tions, e.g., for harmonics, must be complied with.

New processes for series manu­fac­turing

The gas bearing in the high speed CompaNamic compres­sors requires very precise manu­fac­turing toler­ances. These can only be main­tained with a special produc­tion envi­ron­ment and preci­sion measure­ment tech­nology. New processes are there­fore neces­sary for the large-scale produc­tion of preci­sion parts. The entire produc­tion process takes place at a constant temper­a­ture, i.e., in an air-condi­tioned envi­ron­ment. Instead of the conven­tional 3 to 4 µm, toler­ances of less than 1 µm can thus be main­tained. The entire manu­fac­turing process is moni­tored by in-line preci­sion measure­ments. It goes without saying that all compo­nent-related measured values are recorded in a data­base and stored with a serial number. To be able to produce large series econom­i­cally despite the effort involved, produc­tion must generate as little waste as possible and must be contin­u­ously read­justed accord­ingly.

This enables the manu­fac­ture of econom­i­cally effi­cient, oil-free turbo compres­sors that compress partic­u­larly envi­ron­men­tally friendly and natural refrig­er­ants with very low global warming poten­tial. The high effi­ciency and low oper­ating noise protect the envi­ron­ment, and the vibra­tion-free oper­a­tion prevents struc­ture-borne noise trans­mis­sion into the system. This makes the new ebm-papst CompaNamic high-speed compres­sors suit­able even for demanding appli­ca­tions with high require­ments for smooth oper­a­tion and reli­a­bility.