Achieving energy efficiency through retrofitting

Achieving energy efficiency through retrofitting
Dan Mizesko, managing partner, US Chillers.
Published: 6 March 2018 - 6:39 a.m.
By: CW Guest Columnist

With a substantial amount of water cooled chillers in this region now starting to reach maturity and not being as energy efficient as the newest chillers being sold today, retrofit solutions exist for the market that will allow you to keep your current chiller and meet existing efficiency levels as well as comply with today’s refrigerants, all without having to face costly chiller replacement and loss of capacity.

Retrofit allows people to modernise their chillers without any loss of capacity and will also make the existing chillers much more energy efficient while allowing them to move away from currently phased out refrigerants. Of course, each case will require a full site analysis and testing of the existing chiller’s tubes and shells as well as some other data analysis but I can confidently state the retrofits I am about to explain will work well, allow conversion of the chiller’s refrigerant with no capacity loss and greatly improve the efficiency of the chillers by substantially lowering the KW per tonne performance.

Multiple oil-less magnetic bearing compressor can replace the single or dual centrifugal compressors. No centrifugal chiller offered today (even with a VFD installed on the compressor) can operate as efficiently as an oil-less magnetic bearing centrifugal chiller. Magnetic bearings offer superior reliability and performance for chiller operation.

A retrofit of an existing large tonnage single compressor chiller to multiple oil-less Mag bearing compressors will operate even more efficiently than the current single oil-less Mag chillers offered today. I’m recommending installing multiple smaller compressors which allow substantial savings due to part load staging of the compressors as well as larger heat exchanger on the condenser when operating under lighter loads. This offers large opportunities for additional savings beyond the efficiencies of a single compressor oil-less magnetic chiller. Chillers using the variable speed oil free multiple centrifugal compressors with magnetic bearings offer a rated efficiency in the range of 0.33 to 0.37 kW/tonne (integrated part load value/IPLV).

History of Oil-Less Mag Compressor

Research and development on a new oil less compressor started in 1993. The design goals for developing a small centrifugal compressor included lubricant free operation and a direct drive system.

The result of these efforts is a compressor that has a capacity of 60-90 tonnes, uses refrigerant R-134a, uses magnetic bearings (no oil) and a direct drive system (no gears). Additional benefits include a light weight design (80% less than traditional compressors) and reduced noise and vibration. By 2001, beta test sites proved the compressor design was viable for market introduction.


The compressor’s rotor shaft and impellers levitate during compression and float on a magnetic cushion. Two radial and one axial magnetic bearings are employed. The compressor has an integrated variable frequency drive (VFD). VFD’s provide the best part load efficiency and operate most effectively with centrifugal compression. The speed of the compressor adjusts to changes in load and/or condenser water temperature. The minimum load on the compressor is 15%. The auto balance feature repositions the magnetic bearing 100,000 times a minute to maintain centred rotation at all times. It uses 1.6 amps to start up (elevate the shaft) and operates at 16,000 to 40,000 RPMs. The motor is a permanent magnet brushless DC motor and the motor, electronics, and VFD are refrigerant cooled. The compressor is designed to handle a power outage. The motor becomes a generator. After the compressor comes to a complete stop, the rotor de-levitates normally onto touchdown bearings. Should the computer fail, the compressor is designed to handle eight “crashes”. The oil-less design eliminates some typical operating problems associated with oil flooded compressors. Water cooled units often use flooded evaporators and any oil in the evaporator tubes will cause a decrease in heat transfer.

ASHRAE Research Study 601 determined that the vast majority of installed chillers have an excess amount of oil in the cooling system. The systems in the study had between 2.9% to 22.9% of oil in the cooling system. For the purpose of life cycle cost, it is assumed that 3.5% oil concentration occurs after two years of operation for flooded evaporator systems. 3.5% of oil in the refrigerant charge reduces system efficiencies by 8%.

Magnetic bearings and sensors keep the shaft properly centred and positioned at all times. The rotor shaft is held in position with ten separately controlled electromagnetic cushions which continually changes in strength to keep the shaft centrally positioned. The shafts position is monitored with 10 sensor coils whose signal is fed back to a digital controller. Back up carbon or roller bearings are fitted to catch the shaft and prevent damage should a control or bearing failure occur. Shaft is monitored and positioned at 100,000/sec.

The compressor’s speed adjusts automatically to match the load and current operating conditions so that optimum efficiency is gained.  Primary capacity control is done using the on board VFD and only uses the Inlet Guide Vanes to supplement VFD controls. IGV’s prevent surge conditions at low turndown. IGV’s normally operate at the 110% position.  The slower the compressors, the greater the efficiency; as speed is reduced, energy consumption is reduced.

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