REQUIREMENTS AND CHARACTERISTICS
The properties of the oil must take proper account of its behaviour in the system, namely oil return from the system to the compressor, and the effect of oil on the heat transfer process in the evaporator and condenser. Part of the compressor designer’s task is to ensure lubricant fitness for both the compressor and the system.
In the working environment the lubricant is always a mixture of oil and refrigerant and therefore its composition and properties are governed by solubility characteristics that are pressure and temperature dependent. The CFC, HCFC and ammonia refrigerant/mineral oil combinations are backed by many years of experience; their properties are well known. Compressor designers have utilized the combination of viscosity and the excellent boundary lubrication (lubricity) properties of the chlorine-containing refrigerants to good effect in the design of the moving parts. With the advent of HFC refrigerants came the need to move from mineral oils to synthetic oils in order to ensure miscibility with the refrigerant and hence adequate oil return from the system. POE oils were chosen for most applications, based on their properties, cost and availability. POEs are made from organic acids and alcohols, which combine to produce esters and water. The formulation of the ester is determined by the original acid structure. As its name implies, a POE is a mixture of esters derived from a mixture of acids.
The behaviour of lubricating oil in a refrigerant circuit and its physical interaction with the refrigerant itself are dominant factors in the design of circuits in general and evaporators in particular. It should be noted that the solubility of ammonia in most lubricants is very low.
A degree of solubility of refrigerant in oil is desirable because viscosity of the mixture in the evaporator is reduced, allowing it to become more mobile, which aids transport back to the compressor. The most important property for compressor operation is the viscosity of the solution for bearing lubrication. As the low-side pressure changes with evaporator temperature the refrigerant concentration changes also and this in turn affects viscosity. A typical behaviour for halocarbon refrigerants is shown in Figure 5.2 .
At low pressure of 1 bar corresponding to evaporation at say -40°C a small amount is refrigerant is dissolved and this has negligible effect on viscosity. With a higher evaporation pressure at, say 6 bar, corresponding to 10°C the oil absorbs 10% refrigerant which effectively reduces the viscosity to half that of the base oil and the bearing load carrying capacity is reduced. These effects are studied with the aid of viscosity/temperature/pressure diagrams, the format
Pressure bar, Abs
Figure 5.2 Typical low side sump refrigerant content and mixture viscosity
Figure 5.3 Viscosity/temperature/pressure diagram: shows percentage refrigerant in solution and corresponding viscosity.
Of which is shown in Figure 5.3. This diagram is just intended to illustrate the general form of the characteristic and is based on refrigerant R134a and POE oil. Specific data may be found in the ASHRAE Refrigeration Handbook.
When liquid lubricants and liquid refrigerants are mixed together and allowed to settle out, a homogeneous mixture may be formed. In this case the pair are said to be miscible at the prevailing temperature and pressure. Alternatively, two separate phases may form; one of which is an oil-rich solution, the other being a refrigerant-rich solution. In most cases the heavier refrigerant-rich solution is at the bottom. This can cause problems with systems where the compressor is in a cold location and refrigerant condenses in the crankcase during shutdown. On start up, the oil pump will tend to draw a very low-viscosity refrigerant — rich mixture. Crankcase heaters and pump down cycles are used to avoid this problem. This is not the case with ammonia as it generally does not mix with lubricants except in small amounts and the oil tends to accumulate at the bottom of evaporators where it can be drained.
Desired properties for lubricant may be summarized as follows. Many of the characteristics are influenced by the refrigerant and so the oil property cannot be considered in isolation.
1. Adequate lubrication viscosity at the temperatures and pressures in the bearings, and adequate lubricity for sliding contacts.
2. Stability, so that chemical reactions or decomposition do not occur at the conditions to be encountered. Normally the highest temperature and pressure is at the discharge of compression. Resistance to oxidation is measured with a flash point test.
3. The lubricant must be moisture and contamination free, as far as is possible.
4. The lubricant must be compatible with the materials used in the system. Particular points are flexible non-metallic rubber and plastic components such as seals. Copper cannot be used with ammonia.
5. The viscosity of the solution on the low-temperature side should be low enough for adequate oil return.
6. Solids should not be precipitated. Mineral oils can sometimes precipitate waxes at low temperature; this is identified with a floc point test.
7. High electrical resistance is necessary for enclosed motors.
8. Foaming characteristics must be considered.
9. Availability at an acceptable cost is essential.
By way of comment on some of the above points, it should be noted that the lubrication characteristics in the actual working environment can only be proven by actual experience and/or life tests. It is a tribute to the engineering efforts of compressor builders and system installers that the changeover to HFC refrigerants and POE oils had been a very smooth and trouble-free process. Chemical stability has to be adequate in the presence of moisture and air although the aim is always to exclude these contaminants from the system. Traces are nevertheless present in practice and this is dealt with below. Excessive foaming is undesirable when it is caused by rapid refrigerant release when the compressor is started and the crankcase pressure reduces because it tends to give rise to loss of oil to the system. Some foaming during normal running can assist oil distribution in the compressor and reduce sound level.
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