The Next Generation of Lubricants for Improved System Performance

By Dr Steven.J.Randles, Dr Stephen Boyde and Dr Peter Gibb
Uniquema

Introduction

Refrigeration lubricants can have a significant beneficial impact on two major performance parameters of critical importance to the refrigeration and air conditioning industry:

- Improved energy efficiency

- Noise reduction

Energy Efficiency

Energy efficiency improvements are driven by the need to reduce green house gases. On average, it has been estimated that during the life of an refrigeration system ⁽¹⁾ some 84% of green house emissions are due to the energy consumed during the life of the fridge. This can rise to as much as 97% for appliances⁽¹⁾. Governments have responded this issue with a variety of schemes

- USA:

· DoE 30% improvement by 2001, 15% by 2003

· Energy Star label (20% better than minimum for RAC)

- Europe: CEN classification system

- Japan:

· Voluntary steps towards Kyoto protocol (25-30%)

· Energy labeling scheme to be introduced 21 August 2000

§ Orange: failed to meet standard by X%

§ Green : achieved standard by X%

Recently, George Bush mentioned appliance standards as an important element of the new administration national energy plan⁽²⁾. Further, legislation or tax credit schemes could be forthcoming.

In the Industrial commercial sector, there has also been movement towards improvements in energy efficiency standards to be achieved by 2006⁽³⁾. These are:

- DoE has proposed a 20% increase in federal minimum efficiency standards

- The institute urged adoption of targeted tax incentives

- If this was adopted today 84% of central air conditioning and 66% of heat pumps would not comply with the proposed legislation

Correct selection of the compressor lubricant can have a significant impact on the energy efficiency performance of a system. Lubricant selection can impact compressor energy efficiency in a number of ways. This paper will focus on the energy losses arising from frictional losses in lubricated contacts and heat transfer processes. However, it is important to recognise that lubricant properties can also impact energy efficiency in other ways. For example, use of a lubricant with too low viscosity, or too high refrigerant gas solubility, can reduce volumetric efficiency in some compressor types.

Frictional energy losses due to viscous drag between the lubricant and bearing surfaces in an appliance compressor can contribute significantly to the overall efficiency of a refrigeration system. In order to establish the expected relationship between lubricant viscosity and energy consumption, refrigerator efficiency tests based on the conditions of ISO 5155 were carried out on standard commercial refrigerator and freezer cabinets.

Figure 1: The Effect of Viscosity on the Energy Efficiency of a Domestic Fridge and Freezer Cabinet.

The graph clearly shows that by reducing lubricant viscosity energy efficiency can be improved by at least 10%.

Friction modifiers or lubricity additives can also give marked reduction in friction. To illustrate the frictional loss behaviour of lubricated contacts across the range of lubrication regimes, the friction coefficient data was obtained using a conforming-pin-on-ring tribometer of in-house design⁽⁴⁾.

Figure 2: The Effect of Friction Modifiers/Lubricity agents on the Coefficient of Friction

Use of friction modifier additives will contribute to energy efficiency only if a significant proportion of power losses occur at boundary or mixed film lubricated contacts. (It should be noted that even if a contact operates in the hydrodynamic regime at standard operating conditions, it must pass temporarily through the boundary regime at start up and shutdown. This is particularly important at start-up, when the contacts may be lubricant-starved due to drainage occurring during while the equipment is stationary.

It has been previously supposed that lubricants generally have a negative impact on heat transfer. However, it is possible that lubricants can improve, rather than reduce, the heat transfer efficiency. Improvements in heat transfer can also lead to marked improvements in energy efficiency.

It has been recognised for nearly forty years that a lubricant-rich fluid layer exists near the tube walls in evaporators⁽⁵⁾. Recently, Kedzierski has proposed a logical link between the pool boiling enhancement/degradation associated with lubricant contamination and the accumulation of the lubricant at the boiling surface ^(6,7). The excess concentration (excess surface density) arises from the relatively low vapour pressure of the lubricant compared to refrigerant. At the heated wall, the refrigerant preferentially evaporates, leaving behind a relatively thin liquid layer enriched in lubricant. It has been hypothesised that the lubricant excess layer establishes the bubble size and site density for boiling, which are known to have profound effects on the magnitude of the heat transfer^(6,7).

In joint collaboration with Kedziersk several polyol esters have been screened using the NIST pool boiling apparatus. It has been found that:

- Larger lubricant mass fractions promote smaller bubble departure diameter, which in turn, leads to poorer heat transfer.

- Proximity of the bulk fluid temperature to the Critical Solution Temperature of the mixture benefits heat transfer by the formation of additional excess liquid films that draw superheated liquid into the bubbles sides

- High Lubricant viscosity benefits pool boiling by promoting a thick thermal boundary layer.

Heat transfer of POE lubricant gave results of 45% - 120% of pure R134a. Although very system dependent, a very crude rule of thumb is, 1% improvement in energy efficiency can be achieved for every 2% improvement in heat transfer. Further work is ongoing to optimise lubricant properties to give improved heat transfer characteristics.

In summary, major improvements in energy efficiency for appliance compressor can be achieved through reducing the lubricant viscosity. For larger industrial commercial systems, improvements in energy efficiency can be obtained through a combination of viscosity optimisation, reduction in friction coefficient and improvements in the heat transfer properties of the lubricant.

Noise Reduction

Manufacturers of domestic appliances are increasingly seeking to reduce noise as a route to competitive advantage. It is widely believed that foaming contributes to noise reduction in refrigeration lubricants⁽⁸⁾, and lubricant formulations designed to induce foaming have been reported⁽⁹⁾. The presence of foam on the surface of the lubricant undoubtedly acts to reduce noise generation due to lubricant droplet impacts. However, the mechanisms by which foams could actually absorb acoustic energy are not clear and it is likely that entrained vapour bubbles actually make a much greater contribution to acoustic attenuation in refrigeration lubricants⁽¹⁰⁾.

Vapour bubbles form readily in a refrigeration lubricant because it is saturated with dissolved refrigerant gas. The presence of such vapour bubbles at the metal – lubricant interface will significantly reduce the acoustic transmission coefficient of the interface. The presence of vapour bubbles also amplifies the effects of the relaxation attenuation processes occurring in the liquid. Because the vapour is more compressible than the bulk liquid, the bubbles contract disproportionately more than the liquid in response to the acoustic pressure wave. Consequently, the surrounding liquid responds with a greater amount of viscous flow and absorbs much more energy than in a homogeneous liquid^(11,12). Gas bubbles also refract and scatter acoustic energy.

Foam generation and vapour entrainment are related, but not identical, properties of a lubricant formulation, which are partly dependent on bulk properties such as viscosity, but more sensitive to surface properties including surface tension, Gibbs elasticity and interfacial shear and dilational viscosities ⁽¹³⁾. These properties can be modified both by control of the chemical structure of the basefluids and by use of appropriate additives in the lubricant formulation. However, this presents a particular challenge in lubricant formulation as both foaming and vapour entrainment can have detrimental effects on compressor lubrication and system energy efficiency. For example, high foam levels can enter the suction port leading to excess lubricant transport, whereas entrained vapour bubbles may reduce the hydrodynamic lubricant film thickness. Lubricant formulations for improved noise and vibration performance must maintain stable, controllable levels of foam and vapour entrainment in order to deliver improved acoustic performance without compromising reliability or efficiency.

Standard foam stability and vapour entrainment test methods^(14,15) can be used to select candidate basefluids and acoustically active additives and to optimise dose rates. Light scattering analysis can be used to compare bubble size distribution under standard conditions. In one such test, the lubricant is stirred and sparged with gas for a defined period. After the gas flow is stopped, the bubble size distribution is determined as a function of time. Figure 3 shows results illustrating how a candidate acoustically-active additive stabilises entrained vapour bubbles in a model lubricant basefluid. In the pure basefluid, the mean bubble size is initially high and rapidly decreases to zero as the largest bubbles rapidly rise to the surface and burst. In the formulated fluid, the initial bubble size is lower and the mean bubble size changes much more slowly over time, showing that the bubbles are retained within the bulk of the lubricant.

Figure 3: Effect of Acoustically Active Additives on Mean Bubble Size using Laser Diffraction

Control of the bubble size distribution is important, because this determines the frequency dependence of the excess acoustic attenuation due to the entrained bubbles⁽¹¹⁾. Acoustic attenuation is particularly high at frequencies corresponding to resonant modes of the bubble cavity. It is therefore possible to tune the absorption characteristics of a fluid to match the noise spectrum of the equipment, by control of the bubble size distribution.

Previous workers have found that the spectrum of radiated noise from a compressor is very similar to the vibration spectrum of the compressor shell, and that this is determined by the resonant frequencies of the compressor shell⁽¹⁶⁾. These resonant modes each have a characteristic distribution of intensities across the shell, so the measured vibration spectrum is highly dependent on the measurement position. It is therefore necessary to measure vibration at a number of locations in order to assess the overall potential for noise radiation.

Accelerometer attachment studs were fixed to the shell of a representative compressor at a range of locations, including one that was designated as the reference point. The compressor was then charged with a standard amount of refrigerant gas and a test lubricant and operated until steady state temperature and pressure conditions were established. Shell acceleration was then recorded sequentially at each location, over a designated time interval typically ranging from 5s to 60s, for post processing. Acceleration at the reference point was recorded simultaneously and monitored to confirm that the operating conditions were constant during the entire measurement sequence.

Acceleration data were recorded and post-processed using a commercial vibration analysis software package. For each measurement point, the raw acceleration data were integrated to give velocity as a function of time, then Fourier transformed to give the vibration velocity frequency spectrum. The A–weighting was applied to the spectrum, and the mean vibrational velocity calculated for the frequency range 0 – 8 kHz and expressed as dB(A) re 5x10^-8m/s. The process was repeated for each lubricant formulation. Acoustic performance of each formulation was ranked according to the comparative value of the A-weighted mean vibrational velocity.

Figure 4: Instrumented Noise Evaluation Test Stand

It should be emphasized that this is a screening test to assess the relative performance of different lubricants. The absolute numerical value of the mean vibratory velocity in dB(A) re 5x10^-8m/s will not necessarily be close to the sound power level radiated by the compressor in dB(A) re 1x10^-12 W, because the compressor shell surface is not flat and the radiation ratio s is not unity. However, as part of an overall screening technique the method has the advantage of being quick, cost effective, and is a useful screening tool before more in-dept test are carried out.

Results using this technique can be seen in Figure 5.

Figure 5: Noise Evaluation on various Polyol Ester Appliance Grades

Noise reducing lubricant compositions will not be effective in all compressor designs. The reasons for this behavior are complex and are dependent on a large number of variables. Obviously, compressors, which already have high levels of entrained refrigerant in the lubricant, may have excessive foaming if noise reducing compositions are used. In addition, the optimum bubble size to damp a particular frequency in a specific compressor design may have not been the correct one for an alternate compressor design.

Conclusions

Lubricants can now be specifically designed to deliver targeted performance advantages in the areas of improved energy efficiency and noise reduction. Delivering these effects via the lubricant, rather than the more expensive system design route, can results in significant cost savings. However, a detailed knowledge of the structure property relationships of the lubricant is required to achieve the optimum balance of properties

References

(1) March Consulting Group, Final Report 30/09/98

(2) Assosiation of Home Appliance Manufacturers (18^th May 2001). http://www.aham.org

(3) E.Dooley. (13^th April 2001). Good News for Consumers on Minimum Efficiency Standards for Central Air Conditioners and Heat Pumps, ARI, http://www.ari.org

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(6) M.Kedzierski. (2001). The Effect of Lubricant Concentration, Miscibility and Viscosity on R134a Pool Boiling. International Journal of Refrigeration, Vol. 24, No. 4.

(7) M.Kedzierski and M.Kaul. (1993). Horizontal Nucleate Flow Boiling Heat Transfer Coefficient Measurements and Visual Observations for R12, R134a, and R134a/Ester Lubricant Mixtures,” NISTIR 5144, U.S. Department of Commerce, Washington, D.C.

(8) ASHRAE handbook (1998). Refrigeration, 8. 7.20. American Society of Heating, Refrigeration and Air-Conditioning Engineers.

(9) K S Sanvordenker .US Patent US 3792755.

(10) S.Boyde. (2001) The role of Lubricant in Reducing Vibration and Noise in Refrigeration Compressors. To be presented at ImechE Compressors and their Systems conference to be held in November.

(11) L E Kinsler and A R Frey. (1962). Fundamentals of Acoustics, 2^nd Edition, John Wiley.

(12) J R Allegra and S A Hawley. (1972). J. Acoust. Soc. America, 51, 1545.

(13) Kirk-Othmer Encylopaedia of Chemical Technology, 4^th Edn. Vol 11, p. 783.

ASTM D892-98 Standard Test Method for Foaming Characteristics of Lubricating Oils

(14) ASTM D3427-99 Standard Test Method for Air Release Properties of Petroleum Oils

(15) T Sisson and F Simpson. (1984). Proceedings 1984 International Compressor Engineering Conference at Purdue, p273 - 284.