Achieving energy efficiency through retrofitting

Dan Mizesko, managing partner, US Chillers, writes about retrofit solutions that can help modernise chillers

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Dan Mizesko, managing partner, US Chillers.
Dan Mizesko, managing partner, US Chillers.

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.

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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.

Operation

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.

Chillers with oil-less magnetic technology have virtually no vibration and the sound is substantially less than any other chiller on the market today. A chiller with 5 compressors operating at full speed only produces 75 DB of sound at 10 feet, about the sound level of your television. In addition, having five or six compressors allows for much better capacity control and gives the chiller redundancy in the unlikely event of a compressor not being operational.

An oil-less magnetic chiller will allow up to six compressors in one circuit and provides the ability to use the entire heat transfer surface even when using few compressors, thereby ensuring  improved efficiency.

If consideration is given to this retrofit solution, we suggest that eddy current testing and inspection is performed on the chiller’s condensers and evaporators and any defective or rejected tubes are replaced. You must recover the existing refrigerant charge, remove existing compressor and controls, clean the interior shells, and remove any oil and contaminants. Install new control system and oil-less Mag compressors with all required interconnecting piping, sensors, valves, etc. Evacuate and dehydrate the system and charge virgin refrigerant. Put chillers into beneficial operation and benchmark Tonnage and Kw per tonne operation. No loss of capacity and increased efficiency will be realised.

Oil-Less Magnetic Bearing Centrifugal Chillers

As I stated at the start of this article, 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. The chart below demonstrates the dramatic ill effects of oil in the cooling system. Magnetic bearings offer superior reliability and performance for chiller operation.

It is roughly estimated that chillers alone consume about 50%-70% electricity of an HVAC system. If looking at the chilled water plant alone, it’s more like 65%-80% of the plant’s electricity. The majority of new “energy efficient” chilled water plants in this region utilise electric driven centrifugal chillers and a small sub segment of these plants utilise VFD technology on the compressors. Around 99% of these chilled water plants utilise standard centrifugal compressors which have oil-lubricated bearings. Therefore, a more efficient chiller technology would offer significant opportunity to reduce annual energy costs as well as an opportunity to reduce annual energy consumption.

Large chillers with centrifugal compressors are known for high efficiency and reasonably high turn-down ratios. However, as the capacity of centrifugal chillers decreases, so does the advantage of centrifugal chillers. Historically, centrifugal chillers were infrequently offered below 300 tonnes. Positive displacement compressors, such as the rotary screw, tended to dominate applications in the 100 to 300 tonne range. This compressor selection strategy is changing with the impact of variable speed drives, which allow for much more efficient centrifugal applications in smaller capacities.

Chillers using the variable speed oil-free centrifugal compressor with magnetic bearings offer a significant efficiency benefit over chillers with positive displacement compressors, such as the rotary screw, in capacities as low as 60 tonnes—a capacity previously unseen in centrifugal chillers.

The chiller using the variable speed oil-free centrifugal compressor with magnetic bearings offers a rated efficiency in the range of 0.33 to 0.37 kW/tonne (integrated part load value/IPLV). The other great news is that chillers using the variable speed oil-free centrifugal compressor with magnetic bearings are now available in sizes up to 2,500 tonnes, so these chillers are now available to be utilised in large sized chilled water plants that are generally installed for large buildings and facilities as well as the district cooling industry.

In closing, whether you retrofit to oil-less magnetic bearing centrifugal technology or purchase an oil-less magnetic bearing centrifugal chiller the energy and cost savings benefits it offers is just so compelling that it is hard to understand why anyone would not embrace and insist this technology be utilised. We at US Chiller Services have been involved in the selection, installation, start-up, commissioning and ongoing service and maintenance of four chilled water plants with oil-less magnetic bearing technology and I can state without hesitation that these four chilled water plants operate at world class efficiencies.

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