Coolers are utilized on lubricating oil reservoirs of large rotating equipment to control viscosity and reject heat. Lube oil cooler performance is impacted by a number of plant-specific variables, such as ambient temperature, plant utilities, equipment condition, and the like. This article presents design considerations, an engineered approach and plant case project for cost-effective, temporary, supplemental heat rejection from large lube oil reservoirs. The success of the approach depends, in large part, on an accurate engineering understanding of heat transfer in lubricating oil systems. Performance predictions from commercially available computer models had been found to be misleading. A spreadsheet-based model that included mass/heat transport in viscous, laminar flow was found to be quite accurate. Field data taken during operation confirmed the conclusions and validated the spreadsheet model. In a full scale application, Allegheny Energy collaborated with Aggreko to provide temporary, supplemental lube oil cooling for the No. 1 and No. 2 turbines at the Pleasants, W.Va., power station. Each of two temporary systems was designed to reject an additional increment of about 2 MMBtuH from the lube oil reservoirs to the plant cooling water system. The projects were installed and on-line within 11 days following the decision to proceed. The success of the projects confirms that temporary, supplemental cooling can be rapidly and safely implemented. This also permits conservation of capital and avoids permanent equipment that would only be used during a small portion of the year. Viscosity (and thus lubricity) control of lubricating oil in large rotating equipment service is important for bearing life, controlled maintenance and operating costs and machine reliability. The generally accepted practice for high horsepower motors and turbines is to install a reservoir of reasonable capacity and some means of continuously removing heat from the circulating lubricating oil system. This practice is found in machines performing service in the power, refining, petrochemical, pulp and paper and other industries. The design of every system is unique to the conditions of that system. Every design represents a compromise among a number of elements, including available utilities, ambient conditions, cost, anticipated needs, machine mechanical condition and others. The nature of a compromise is that it generally does not completely satisfy all of the elements. Furthermore, some of the elements change over time. One result is that the lubricating oil cooling system can become, under some conditions, limited and unable to achieve the desired oil temperature. Heat Transfer in Lube Oil Cooling Purposes of lubricating oil include reducing rolling and sliding friction in bearings, providing a seal/fluid barrier to prevent mass transport from inboard (process) environment to the outboard environment and removing unwanted heat from the bearing zone. The suitability of lubricating oil to perform its tasks depends on its thermophysical properties at point-of-use. The user can control one important property—viscosity—by adjusting lubricating oil temperature. In practice, temperature control can be challenging. Lubricating oil systems that support large rotating equipment are subjected to variable (and, generally large) heat rate inputs. Sufficient heat must be removed so that oil returned to service is within a desired temperature range. That range is rather narrow. The means for removing heat from the oil is generally limited and the oil has poor heat transfer characteristics. Heat rejection and temperature control, while related, are two distinct topics. Problems arise when they are casually treated as one topic. Lubricating oil systems supporting large machinery incorporate heat exchangers for heat rejection and temperature control. Enthalpy is extracted from the lubricating oil by heat transfer through the exchanger wall.
Posted on: Wed, 30 Jul 2014 21:44:27 +0000
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