Ideal Oil

Ideal Oil

International requirements for environmental protection have led to the development of new technology, therefore lubricating oils have also had to be developed. Currently this branch of the industry has needed changes in the refrigerants that were traditionally used.

Each and every one of the elements that make up the refrigeration and air conditioning systems fulfill a fundamental role for its operation, since without any of these, it would cease to be a system and therefore its usefulness would decrease because it is a complex gear whose mechanisms act harmoniously and interconnected.

In this sense, the oil used in these systems is essential because both the compressors, the evaporator, the valves, among other systems, interact with each other and require lubricant that in addition to keeping the mechanical parts of the compressor in optimal conditions, serves as a barrier to separate the gas from the discharge zone from that which comes from the suction area.

In the same way, the oil works as a cooling medium since it transfers the heat from the bushings and from all the elements of the compressor mechanism to the crankcase from where it is sent to the compressor walls. In fact, it is considered that the more viscous the lubricant, the more optimal the sealing and the lower the noise level.

When a system is hermetic or semi-hermetic, in which the electric motor is exposed to coolant gas and oil, a lubricant with dielectric properties is needed, as the coolant transports a small portion of lubricant throughout the cooling system. This oil must return to the compressor quickly and must be able to flow at low temperatures, be free of suspended particles or elements such as wax, which can obstruct flow control or deposit in the evaporator and thus affect heat transfer.

In an airtight system, the oil will only be charged once and must last the entire life of the compressor. Probably, the characteristic that differentiates lubricants in refrigeration systems from those used in other applications, is the chemical stability required in the presence of refrigerant gas, metals, varnish, compressor electric motor insulation and contaminants.

Nature of oils

Oils are classified into two groups, those of mineral and synthetic origin:

Oils of mineral origin

They are those that are extracted from the distillation of petroleum and are formed by chains of hydrocarbons.

This type of lubricants can be grouped into the following sets:

Paraffinic Oils: Consisting of a saturated linear chain.

Naphthenic Oils: Consisting of a saturated cyclic chain.

Aromatic Oils: Constituted by an unsaturated cyclic chain.

In the refrigeration sector, the use of oils of mineral origin as lubricants has been limited to Paraffinics and Naphthenics, although the latter have been the most used because in the last quarter of the twentieth century, the separation of waxes in the Paraffinics at low temperatures, represented a great problem.

Commonly used oils are not obtained from a single base and are formed by a mixture of the three types. However, the component that defines the oil is the base, since the additives deteriorate easily over time and, in addition, sometimes present incompatibilities with the materials used.

The disadvantage of mineral oils is that they are obtained by the fractional distillation of petroleum, being formed by a set of hydrocarbon chains of different lengths (different number of carbon atoms).

During the work cycle, shorter chains (more volatile hydrocarbons) vaporize more easily, which increases the viscosity of the excess fluid, which will cause poor lubrication of the machine.

It is for this reason that mineral lubricants should be specially selected for:

• Withstand various working conditions

• Be an excellent lubricant at high temperatures

• Remain stable over a wide temperature range

• Have the ability to mix properly with the refrigerant, so that the proportion of oil that travels through the system, transported by the refrigerant gas, remains attached to it and is returned to the compressor crankcase

• Have a high viscosity index, without lowering its temperature in the evaporator, increasing its viscosity and remaining stuck in it, separating from the refrigerant that returns to the compressor.

• Have a low flocculation point.

• Have hygroscopicity, defined as the ability to retain moisture by interacting forces of molecular attraction of a substance with water.

Oils of synthetic origin

They are those that are obtained only by chemical synthesis, since they do not exist in nature. A difference between oils of synthetic origin versus minerals is that the former show a defined and known molecular structure, as well as predictable properties, derived from their molecular information.

As with mineral oils, synthetics can also be grouped into the following large families:

Alkylbenzene Type Lubricants: In their structure they contain a benzene chain and thanks to their outstanding characteristics in lubricating properties and especially to their high chemical and thermal stability, as well as the absence of paraffins, they have replaced mineral oils in systems operated with CFC gases or HCFC gases. The fact of being highly hygroscopic (compounds that attract water in the form of steam or liquid from their environment) is considered by compressor manufacturers as a manageable variable, by implementing moisture control measures during the production and loading of the lubricant, and creating acceptable conditions in a system, to achieve maximum dehydration levels, which are achieved through the use of high-capacity filter dryers, taking into account an effective system dehydration procedure through a high vacuum process."

Alkylbenzene lubricating oil due to its homogeneous molecular structure is not highly hygroscopic but low moisture absorption.

Polyolester Type Lubricants: It has the characteristic of being more hygroscopic than mineral oils, even compared to synthetic alkylbenzene type. Their moisture saturation levels reach 1000 parts per million (ppm), compared to 100 ppm for mineral oils and 200 ppm for alkylbenzenes.

Therefore, the necessary precautions during loading as well as the required humidity levels are strict and carefully controlled methods must be employed during use.

Alkylglycol Type Lubricants: They are the first lubricants developed to be used with R-134a refrigerant and are currently only intended for automotive air conditioning. Their lubricating properties are better than those of polyolesters, but they are much more hygroscopic, with moisture saturation levels reaching 10,000 ppm."

Main characteristics of oils

Origin: It can be mineral or synthetic.

Viscosity: The property of fluids is resistant to change in shape.

Viscosity index: The viscosity index is a method that assigns a numerical value to the change in viscosity with temperature. If a high viscosity oil and a low viscosity oil have the same viscosity at room temperature, as the temperature increases the high-index oil will maintain a higher viscosity than the low-index oil.

Pour point: Also known as apparent freezing, it indicates the temperature at which the oil no longer flows due to gravity.

Clouding point: It corresponds to the temperature at which the precipitation of paraffin in a pure oil begins.

Flocculation point: It is determined by the temperature at which paraffin precipitation begins in an oil-cooling fluid mixture.

Acidity index: It is given by the amount of a base needed to neutralize the free acids present per unit mass of oil.

Flash point: The temperature at which the oil vapors are released in such an amount that, if their mixture with the air comes into contact with a flame, inflammation begins.

Dielectric stiffness: Maximum voltage that can be applied per unit distance between two electrodes.

Saponification index: It corresponds to the content of free fatty acids.

Ashes: Mineral residue of the oils that is obtained after filtration and subsequent calcination at temperatures between 700 and 800ºC.

Content in solid subjects: It consists of all the foreign bodies insoluble in oil and dyes.

Emulsifiability: Qualitative index that indicates to what extent the oil disintegrates with the fluid, in a stable way.

Stability: Invariability with respect to changes in the state of the properties of oils and their mixtures with refrigerated fluids.

Unctuousness: It is the ability of the lubricant to form a film of adhesion and thickness between two sliding surfaces, being suppressed the friction between them.

Solvent power: Index of the solvent effect exerted by oils on the non-metallic parts of an installation.

Aniline point: The minimum temperature at which the oil is soluble in aniline.

Miscibility: This concept indicates the ability to form a homogeneous mixture that two or more liquids have when they are contained in the same container.

Solubility: It is the ability of one fluid to dissolve in another, forming a homogeneous mixture.

Miscibility and solubility between refrigerants and oils

The difference between Miscibility and Solubility lies in the fact that two miscible fluids form a homogeneous mixture in any proportion that they join, while two soluble liquids have a limited capacity of dilution exceeded, which produces a separation of the fluids going the heaviest to the lower part and the lightest to the upper part (it is also possible the formation of what is called under the name of "lagoons" in the case of two fluids of similar density).

The concept of solubility can also be expressed as "partial miscibility", which represents that up to a certain, or from a certain, proportion there is an absolute miscibility and that from or up to that proportion the immiscibility is complete.

These concepts take on even greater relevance in the analysis of refrigeration facilities. Ideally, both fluids, refrigerant and oil, are totally miscible throughout the temperature field that can occur along the circuit, as this situation will allow the oil, starting from the compressor, to circulate throughout the circuit and return to the compressor again (implying a fluidity adequate to the desired suction temperature).

Incidence of oil in refrigeration installations

Amount of oil leaving the compressor: In all installations where lubricated compressors are used, a refrigerant migration occurs, which is produced by drag, as a result of the speed of the refrigerant in the discharge of the compressor and subsequently in the oil separator, which through the use of modern coalescing filters can be limited to quantities between 1 and 4 p.p.m. Each type of oil and for each discharge temperature corresponds to a vapor pressure, so that the oil content in the compressor discharge will respond to a minimum equivalent to this amount.

Oil return from the low sector

The oil that is dragged by the discharge gas or in the vapor phase, can constitute an obstacle to heat exchange, if it accumulates in evaporators and exchangers; it should therefore be withdrawn from those points. How this is carried out will depend on the miscibility of the oil with the coolant and its viscosity. If it is a pair that are not miscible with each other (as is the case with oils of mineral and synthetic origin with R717), the periodic manual purging of the oil accumulated in the evaporators must be carried out. If it is a miscible oil with the refrigerant and the feed of the evaporators takes place with thermostatic expansion valve, then the mixture is dragged through the evaporator and, subsequently, from the suction pipe by the speed of the refrigerant (this is the case of R717 with polyglycol oils, HCFCs with mineral and alkylbenzene oils, or HFCs with polyester oils). If it is a cooling system by pump or gravity feeding, then the oil is decanted in the low sector and must be recovered by other means.

Since, in the low sector, a mixture of refrigerant with oil enters and only the refrigerant is evaporated (the oil remaining in that sector), the concentration of oil would increase in a progressive and constant way, preventing the exchange of heat. To avoid this, a small part of the oil-rich mixture that is formed in the evaporation system is taken and passed through a heat exchanger, removing the refrigerant contained in the mixture by evaporation, for which the hot gases or the hot liquid from the condenser are used.

To determine the permissible oil content in the mixture, the miscibility curves and the percentage of oil that will affect the heat exchange are taken into account. The amount of oil to be recovered from the system is the same as that leaving the compressor.

Dilution of the oil in the refrigerant

The oil comes out together with the gas discharged by the compressor and it is necessary to proceed to its recovery from the low sector, so it will be necessary to have a viscosity low enough – at the expected evaporation temperature – to guarantee its return.

On the other hand, in the compressor it would be necessary to have a viscosity high enough to ensure optimal lubrication conditions.

Opinions of Ararat and Acemire regarding which is the Ideal Oil for a Refrigeration or Air Conditioning equipment.

In our country two of the companies that venture and offer a wide variety of lubricants are ARARAT and ACEMIRE. The first makes available to the Mexican industry high-tech products with mineral-based oils for traditional lubrication, and synthetic bases for optimal lubrication, in addition to offering advice to its customers to select the most appropriate lubricant based on their needs, economy and especially the recovery of their waste.

For its part, Acemire created, developed and patented the synthetic lubricant type alkylbenzene in Mexico as well as the most complete line of mineral lubricants, polyolester and specialties. It has become a supplier in the aftermarket as well as that of original equipment manufacturers reaching distributors in Central and South America. The most appropriate way to select a lubricating oil for refrigeration and air conditioning systems is by following the instructions of the manufacturer of the equipment to which the addition and/or filling will be performed. Generally the compressor will have a plate attached to one of its side parts in which the type of refrigerant, type of lubricant and viscosity of lubricant is specified, which can be selected in SUS (Universal Second Saybolt) or in CST (centistokes). The experience acquired by Acemire allows us to make recommendations such as; CAPACITY in HP, revolutions per minute, capacity (cooling, freezing, preservation), pressures, make, model and type of compressor (domestic, commercial, industrial, automotive).

It is of great importance to mention that the lubricant must be completely miscible and compatible with the refrigerant gas since the correct operation and the useful life of your equipment depends on it. Acemire's constant concern for the HVAC&R market has allowed it to develop a line of products for each segment and for each need, thus maintaining its leadership in quality and prestige for almost 50 years. As a generator of lubricants (considered a hazardous material), Acemire is also ecologically responsible for which it formed its own Department of Ecology and has the permits and manifests in force before SEMARNAT and SAGARPA for its handling, transportation, confinement and, where appropriate, destruction of the used product.

Why know how to choose The Ideal Oil

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