Utilities Manager: Improving Energy Efficiency Through Optimized Lubricants
Kathy | February 1, 2008
Cradle-to-grave life-cycle costing is not just for equipment. You may be quite surprised by the value-added information these analyses can provide when it comes to lubricant selection.
Lubricants can be optimized for specific types of equipment to help achieve reduced fuel and energy consumption. Unfortunately, facilities often do not use energy-efficient lubricants even though they may lead to measurable savings. This is because the initial purchase cost of energy-efficient lubricants can be higher than for conventional products. A life-cycle cost analysis that takes into account operating costs as well as the initial purchase cost of the lubricant may bring out the true benefits of these products.
Energy-efficient lubricants can be beneficial in many types of mobile and industrial equipment. For instance, in hydraulic systems, changes in ISO viscosity grades can lead to energy savings. Companies also can benefit from optimizing their selection of gear lubricants. Moving from gear oils formulated with mineral base stocks to those formulated with synthetic base stocks often has been found to lead to both lower friction losses and lower lubricant temperatures.
When conducting a life-cycle cost analysis, not only must the purchase price of the lubricant be considered, the impact on the operating costs also must be reflected. Frequently, fuel or electricity cost savings outweigh the increased purchase cost.
Energy-efficient hydraulic fluids
Hydraulic systems are widely used throughout the world. Earthmoving equipment, diggers, etc., are examples of mobile hydraulic systems that are exposed to changes in temperature while operating in an outdoor environment. Conversely, hydraulic systems such as plastic injection molding machines operate under a consistent ambient temperature in a factory environment—but they are energyintensive processes operating 24 hours per day. In both applications, the energy used by these systems depends on the hydraulic fluid used.
In hydraulic systems, the friction is usually dominated by pipe losses, which vary linearly with viscosity. In contrast, studies have shown that in internal combustion engines, the largest contribution to engine friction arises from the piston assembly, where the friction power loss varies as the square root of the fluid viscosity. Therefore, the potential for cold-start energy savings in hydraulic systems due to optimizing fluid viscosity is greater than that for engines.
In addition to energy losses, pump performance also is critical in hydraulic systems. If the hydraulic fluid is too viscous, then pump mechanical efficiency is too low. On the other hand, if the lubricant viscosity is too low, leakage within the pump can occur. As a result, the pump’s volumetric efficiency may become too low. One way to overcome these problems is to use a hydraulic fluid with a higher viscosity index (VI). Such a fluid has a flatter viscosity-temperature response. The idea behind the use of such a fluid is illustrated in Fig. 1.
The use of a higher VI hydraulic fluid, possibly combined with a change of ISO grade, can give energy benefits under cold-start conditions, and also can give volumetric efficiency benefits under high-temperature conditions when compared to a low VI fluid. These effects influence performance.
In a hydraulically operated digger, it may take a few hours for the system to reach operating temperature. The hydraulic fluid will need time to warm before it can provide proper lubrication. Until the fluid is warm enough to flow adequately, fuel may be wasted as the hydraulic system experiences friction. At low temperatures, when the viscosity of the oil is high, more work is done to maintain the pumps’ mechanical efficiency. Thus, the engine must use more fuel for a given amount of hydraulic output.
On the other hand, as the digger operates during the day the system can heat up, reducing the lubricant viscosity, which can result in increased leakage losses and reduced volumetric efficiency of the pump. The use of a high VI hydraulic fluid could help to overcome both these issues. There are benefits from matching the correct lubricant to the demands of the application. Operators should consider lubricants specially designed to meet these challenges. That includes opting for products that are formulated to reduce variations in viscosity during change in temperature.
For a typical stationary hydraulic machine operating at a temperature of around 50 C, in-house Shell data indicates that changing from an ISO 68 hydraulic fluid to an ISO 46 hydraulic fluid would lead to electricity savings of up to 20%. In tests using oil mist lubrication, Shell data has demonstrated that moving from an ISO 68 mineral oil to an ISO 32 synthetic oil may result in energy savings of 13%. For adequate lubrication and to avoid damage to the pump, these fluid changes should only be made if the new fluid meets the minimum viscosity requirements of the pump.
Energy-efficient gear oils
If there is insufficient lubricant film to separate and support loaded parts like gears, rolling element bearings or valve trains, even modest loads can produce high pressures. In some applications, the pressures can be high enough to elastically deform the metal surface. This deformation can benefit lubrication and increase the load-carrying capability by spreading the load over a larger surface area. This is called elasto-hydrodynamic lubrication (EHD).
With elasto-hydrodynamic lubrication, a fluid film is generated due to increases in viscosity of the fluid as the pressure increases. The pressure-viscosity coefficient or alpha value of the fluid defines the increase in fluid viscosity with increasing pressure. For lubricated contacts that are in the EHD lubrication regime, the pressure-viscosity coefficient α of the oil will be important in determining frictional loss.
In contrast to engine oils and hydraulic oils, gear and transmission oils operate for the majority of their time in the EHD lubrication regime. It is well known that friction losses within gears are correlated with temperature rises of the gearbox lubricant. It also is known that moving from a mineral oil base stock to a synthetic base stock results in lower gearbox oil temperature rises and lower friction losses. The reason for this phenomenon is due to the lower pressure-viscosity coefficients α for these synthetic base stocks. Table I shows typical values of α for different base stocks.
Three separate lubricant properties must be considered for understanding and optimizing oil film thickness and friction—atmospheric pressure viscosity, pressure-viscosity coefficient α and limiting shear stress. The first two properties determine the oil film thickness profile in the contact area, while the third property determines friction in the contact area. It was found, broadly speaking, that there was a correlation between EHD friction coefficient and α. Clearly, there needs to be a check to ensure that moving to a lubricant with a lower value of α does not adversely affect the durability of the gears. Lubricants that are formulated with a special synthetic base fluid (polyalkylene glycol) to provide an optimized pressure-viscosity coefficient in worm gear applications are readily available.
Life-cycle cost analysis
Energy-efficient lubricants should help reduce operating costs since they can result in lower energy consumption. Accordingly, operators should consider the entire life-cycle cost of using the product. A life-cycle cost analysis would take into account:
1. The initial purchase price of the product;
2. The effect on operating costs, over the lifetime of the product;
3. Any changes in costs due to different service intervals (e.g. the oil drain interval may be extended); and
4. Disposal costs.
Lubricants have a role to play in helping improve the energy efficiency in industrial machinery. The technology to do this is well understood, although care is needed, both on the part of the machine designer and the lubricant formulator, so that any reduction in lubricant viscosity does not result in decreased durability.
One of the main reasons why such lubricants are not used more widely is that often lubricants are selected on the basis of their price alone, without much regard for the potential impact of the lubricant on operating costs. When a more sophisticated life-cycle cost analysis is performed, the results may reveal that energy-efficient lubricants are more cost-effective than conventional lubricants.
Felix Guerzoni is a product application specialist at the Shell Global Solutions Westhollow Technology Center in Houston, TX. Telephone: (800) 231-6950; Internet: www.shell-lubricants.com
The graphic image on the cover of this Utilities Manager supplement and within this article, as well as the other graphics here, are used courtesy of Shell Lubricants.