The Real Cost of Oil in Centrifugal Chillers
Oil is a necessary evil in most of today's refrigeration and air-conditioning machines. Used for lubrication of moving parts and the removal of heat due to friction, it's the life-blood of most compressors. Fortunately most centrifugal chillers don't "consume" oil like a car or need to have the oil changed unless there is a major problem. Chillers shouldn't leak much oil either, unless it is an "open-drive" design where the motor is located outside of the compressor. This design utilizes a shaft seal between the two that's actually designed to leak oil and refrigerant to keep the seal from drying out. Other possible leak points are the oil pump, oil filter, oil separator, sight glass, purge unit or any of the oil management components. You'll know there's a leak by the pool of oil on the floor beside the chiller, just mop up the mess and add some oil back in. Another way oil gets added to a chiller (whether it needs it or not) is during servicing. Some technicians feel due diligence involves "toping-off" the oil charge just in case there may be a leak somewhere.
Open-Drive Centrifugal Compressor, Designed to Leak Oil and RefrigerantThe Effects of Oil:
ASHRAE Research Contract RP-751, "Effects of Oil on Boiling of Replacement Refrigerants Flowing Normal to Tube Bundle, Part I and II" studied the effect oil has on system performance. It concluded a marked decrease in heat transfer with the addition of even a small amount of oil throughout various heat loadings. Even at 1- 2% oil, the heat transfer coefficient is reduced by one-third from its no-oil baseline. At substantial oil content (5% to 15%) a 40-50% reduction is noted.
As a point of reference a new McQuay WSC 500-ton traditional centrifugal chiller would ship with a factory refrigerant charge (R-134a) of about 1402 lbs. and about 8-gallons of oil. Assuming 7-lbs. per gallon this equates to 56 lbs. of oil or about 4% oil by weight mixed with the refrigerant. Even this modest charge has a negative impact on chiller performance and efficiency.
ASHRAE study 601-TRP studied refrigerant and oil samples from ten operating chillers. These chillers had oil concentrations ranging anywhere from 3-23%. The chart above indicates the extreme loss of efficiency that would be encountered with oil in these concentrations. If 13% is considered average it would indicate a degradation in performance of somewhere between 35-40%.
Based on ASHRAE findings our original 500-ton chiller could end up with as much as 266 lbs. of excess oil mixed with its refrigerant. It isn't the cost per gallon of the oil that's expensive (the initial charge or the extra oil that find its way into the chiller) the real expense is the life-time operating cost associated with decreased capacity/efficiency and increased energy consumption due to oil's detrimental effects.
Real World Effects:
Oil mixed with refrigerant eventually finds its way into the chillers evaporator where it coats the tubes and creates a thermal barrier. This reduces heat transfer efficiency and cooling capacity. While the manufacturer's initial charge of oil may only have a minimal impact on efficiency, the more oil in the system the greater the reduction in capacity and efficiency. Any oil on nucleate boiling tubes can cause a significant degradation of performance.
Although we have known for years that oil buildup inside a chiller occurs, the significant impact on capacity, efficiency and operational costs are just now becoming a topic of real concern. When considering the 23-year projected lifespan of a modern centrifugal chiller a modest 15% decrease in IPLV efficiency could increase operational costs by over $450,000.00 and this is where oil shows its true cost.
Real World Solutions:
Some solutions to this costly problem include chiller charge-reclamation and oil-acid-moisture purging systems (such as the one shown below). A reclamation contractor will typically remove the entire charge, clean the refrigerant to remove all oil, moisture, acid, air and other contaminants then reinstall the specified refrigerant and oil charge back into the chiller adding the oil-purger in the process. The down side is the yearly maintenance and energy required to operate the purge unit, but it does help maintain the prescribed oil to refrigerant ratio in the evaporator. Typically, facility operators are amazed by the substantial improvement in performance they see.
McQuay Oil-PurgerAnother solution would be to eliminate oil entirely from the chiller. This simple concept would not only increase the chiller performance but would contribute to maintaining maximum lifetime efficiency and avoiding the service costs involved in maintaining oil and the oil management system. Until now oil-less chillers were only a pipe dream but with the introduction in 2004 of McQuay's WMC/WME Magnitude series centrifugal chillers, oil-less, frictionless- magnetic bearing technology has become a reality. With over 1500-chillers successfully installed worldwide (over 100 in Texas) the advantages of a truly "oil-free" design enables owners to drastically reduce their maintenance and utility costs while maintaining capacity and efficiency over the life of the chiller.
Magnetic Bearings are Key to Oil-less, Frictionless, Efficient Chiller OperationWhile the elimination of efficiency robbing oil definitely contributes to the superior performance of the McQuay WMC/WME chiller, the benefits of this design reach far beyond that of just lifetime efficiency alone. With only one major moving part in the compressor (the shaft and impeller levitates on a magnetic blanket while revolving) sound, vibration, friction and heat are dramatically reduced (or eliminated) as well.
The McQuay "Magnitude" frictionless, oil-less-magnetic bearing centrifugal chiller not only offers unparalleled reliability but provides industry leading part-load efficiencies as well. With part-load performance as low as 0.31 kW /ton IPLV it leads the industry in sustainability.
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