REFRIGERANTS AND LUBRICANTS

Transcription

1 Service Engineers Section Bulletin No 10 REFRIGERANTS AND 1.0 INTRODUCTION - THE NEED FOR IN REFRIGERATION SYSTEMS Lubricants (oils) are required in refrigeration systems to: Reduce friction by interposing a film between the moving parts, reducing direct solid-to-solid contact and lowering the co-efficient of friction. Seal the compressed gas between suction and discharge sides To act as a coolant by removing heat from the bearings and transferring heat from the crankcase to the compressor exterior. Attenuate noise generated by moving parts. Seal rotating shaft seals retain gas pressure and keep out contaminants. 2.0 DESIRABLE PROPERTIES OF REFRIGERATION Unlike lubricants in many other systems, refrigeration lubricants should not be considered alone as they act as a refrigerant-lubricant mixture. The salient features when considering any type of lubricant for a refrigeration application are: VISCOSITY Too low: Insufficient lubrication, gas blow-by due to insufficient sealing between high and low sides, poor noise attenuation properties. Too high: Power consumption high due to excessive fluid friction, insufficient lubrication, better sealing between high and low sides, better sound attenuation properties. CHEMICAL STABILITY Must resist chemical attack by refrigerant or other system components and not breakdown at anticipated operating conditions to cause acids, sludging, carbon deposits and copper plating. Essential for charged for life sealed hermetic systems. DIELECTRIC STRENGTH Especially important in hermetic and semi-hermetic compressors. Affected by contaminants, (e.g. moisture, dissolved metals etc.). Good lubricants should have a di-electric rating of > 25kV POUR POINT Lowest temperature that oil will still flow. Should be well below minimum temperature found in the evaporator. Very important with immiscible and partly miscible refrigerants. SES Technical Bulletin No 10 Page 1 of 5 March 2002

2 FLASH POINT Must have a high flash point (>150 C). 3.0 CFC S, HCFC, S, HFC, S AND MINERAL OILS Three major classifications of mineral oils: 1. Naphthene base 2. Paraffin base 3. Mixed base Naphthene base oils are generally preferred for refrigeration applications because: a) Naphthenes demonstrate better flow characteristics at low temperatures. b) Carbon deposits formed on hot surfaces by decomposition are soft in nature and easily removed. Paraffin oils leave hard, adhesive carbon deposits and form sludges. c) Less wax deposits at low temperatures. Until the last decade, the most widely used lubricant in refrigeration systems. Excellent performance with traditional CFC (e.g. R12) based refrigerants. Susceptible to wax separation under certain operating conditions (low temperatures in evaporators) that can cause blockages. Mineral oils with high saturate contents are less miscible with highly polarised refrigerants e.g. HCFC R22. Lack of Chlorine atom in HFC based refrigerants (e.g. R134a) renders them immiscible with mineral based lubricants. SYNTHETIC Limited solubility of R22 and R502 led to the development of synthetic oils for refrigeration applications. Immiscibility of HFC refrigerants with mineral oils expanded development of synthetic based lubricants. Generally more thermally and mechanically stable than mineral oils. The high polarity of synthetic lubricants results in better contact with metal surfaces than mineral oils giving superior lubricating properties. High polarity of synthetic oils can cause oxides and other ingrown dirt to become dislodged in retrofitted plants which become free moving impurities in the system. Not susceptible to wax formation under all operating conditions. 4.0 SYNTHETIC FOR REFIGERATION APPLICATIONS Three types synthetic lubricants are in widespread use for refrigeration applications: 1. Alkylbenzene (AB) 2. Polyol ester (POE) 3. Polyalkylene glycol (PAG) CHARACTERISTICS OF SYNTHETIC Alkylbenzene (AB) Synthesized by reacting an olefin or chlorinated paraffin with benzene in presence of a catalyst. Most widely used with highly polarised HCFC or CFC refrigerants such as R22 and R502 with which it has excellent miscibility at all operating conditions. Better high temperature and oxidation stability than mineral based lubricants. High di-electric strength makes AB s suitable for use with hermetic and semihermetic compressors. Polyol ester (POE) Synthesised by reacting an alcohol with an organic acid (carboxylic) in the presence of a catalyst. The process of esterification results in water and ester which are separated, leaving the POE. Used with all non-chlorinated HFC based refrigerants. Also suitable for use with chlorinated CFC s and partially chlorinated HCFC based refrigerants. Excellent thermal stability compared to mineral oils results in less lubricant decomposition problems at high compressor discharge temperatures. High polarity results in better lubricating qualities compared to mineral oils. SES Technical Bulletin No 10 Page 2 of 5 March 2002

3 High di-electric strength makes POE s suitable for use with hermetic and semihermetic compressors. Extremely hygroscopic. Have been known to form metallic soaps that can cause capillary blockages. Polyalkylene glycol (PAG) Derived from ethylene or propylene oxide whereby polymerisation is initialised by butyl alcohol. Used with all non-chlorinated HFC based refrigerants. Also suitable for use with chlorinated CFC s and partially chlorinated HCFC based refrigerants. Excellent lubricity, low pour point and low temperature fluidity. Compatible with most elastomers. Extremely hygroscopic. Low di-electric strength renders them unsuitable for use with hermetic/semihermetic compressors. Main application area is vehicle air conditioning systems that have open drive compressors. 5.0 REFRIGERATION AND ACID FORMATION Inorganic acid formation Inorganic acids are formed when refrigerants containing chlorine or fluorine decompose. Organic acid formation The origin of organic acid formation in refrigeration systems is always the oil. 5.1 Modes of organic acid formation in refrigeration systems MINERAL OILS The formation of carboxylic acids with mineral oils is synonymous with the oil being subjected to thermal or mechanical loads beyond its capacity in the presence of water and/or oxygen and/or catalytic activity. SYNTHETIC OILS (POE) The increased thermal and mechanical stability of POE based lubricants compared to mineral based oils means that organic acid is not formed by the lubricant being subjected to severe thermal or mechanical loads beyond its capacity. The cause of organic acid formation in POE s is due entirely to hydrolysis. POE s are formed in the following manner: Acid + Alcohol Ester + Water The double arrow indicates the reaction can go both ways, i.e. the ester oil together with any moisture in the system can hydrolyse to form carboxylic acid and alcohol. Whatever the mode of formation, acid is undesirable in refrigeration systems because free carboxylic acid can react with metal surfaces and cause corrosion and attack motor winding insulation in hermetic and semi-hermetic compressors. The handling of POE s is also more problematic because the high polarity of POE s makes them extremely hygroscopic. Therefore, system hygiene and handling procedures for POE s and PAG s are critical. 5.2 Acceptable moisture levels The refrigeration industry has generally accepted moisture levels that refrigeration systems can tolerate without invalidating equipment warranties: Compressors shipped with oil and nitrogen holding charge: <50ppm Acceptable system moisture levels: 30ppm to 75ppm System moisture levels in excess of 100ppm will almost certainly invalidate any equipment warranty. 6.0 LUBRICANT ADDITIVES Additives are sometimes added to refrigerant lubricants to: Depress the pour point. Depress the floc point (mineral oils). Improve viscosity. Improve thermal stability. Anti-wear additives. Rust inhibitors. Anti-foaming agents. Oxidation inhibitors. Care should be exercised when using lubricants with additives as: System filter driers can strip out the additives. SES Technical Bulletin No 10 Page 3 of 5 March 2002

4 The additives may not be inert to all system components. Certain additives may not be soluble with the refrigerant at low temperatures and can precipitate out causing blockages in capillaries or expansion valves etc. Certain additives may not be stable at high temperatures and may cause chemical reactions leading to harmful deposits. Although not exclusively the sole criterion for selecting a suitable lubricant for a refrigeration system, the lubricant with the lowest viscosity that gives the necessary sealing properties with the refrigerant used for the entire range of anticipated temperatures and pressures should be used. The table overleaf shows some typical recommended viscosity ranges. 7.0 LUBRICANT SELECTION Lubricant viscosity at 38 C Refrigerant Compressor type SSU mm 2 /s R717 (Ammonia) Screw R717 (Ammonia) Reciprocating R123 Centrifugal R134a Centrifugal R134a Screw R22 Centrifugal R22 Reciprocating R22 Scroll R22 Screw Viscosity system for industrial fluid lubricants (ASTM D 2422) Viscosity System Grade Midpoint viscosity mm 2 /s Kinematisc viscosity limits mm 2 /s at 40 C Approximate equivelants, SSU Indentification at 40 C Minimum Maximum units ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG SES Technical Bulletin No 10 Page 4 of 5 March 2002

5 8.0 FOR NON HALOCARBON REFRIGERANTS 8.1 R717 Ammonia (NH 3 ) R717 is immiscible with the majority of generally available refrigeration lubricants. The high isentropic exponent of compression, which results in high discharge temperatures, compounds this. Therefore, a lubricant that is extremely thermally stable is required. The immiscibility of R717 with most lubricants necessitates the use of expensive oil separation and recovery equipment 8.2 Hydrocarbon refrigerants R600a (iso-butane), R290 (propane) and R1270 (propylene) due to their environmentally benign features (zero ODP and low GWP) are seeing growing use in many areas of refrigeration and air conditioning applications despite concern over flammability issues. R600a applications are mainly charged for life domestic refrigerator applications. R290 and R1270 find applications in larger systems. Research has shown HC refrigerants are compatible with most lubricants but current favorites are mineral oils (used by Danfoss in R600a domestic hermetic compressors) or alkylbenzene as used in many R290 systems. The extreme solubility of HC refrigerants in both mineral and AB lubricants necessitates the use of higher viscosity lubricants than would ordinarily be used for halocarbon type refrigerants. The Service Engineers Section of the Institute of Refrigeration would like to thank Mr D Garcia and Harp International for their assistance in preparing this Bulletin. The information contained in the Bulletin should be seen as a guide to interpretation of relevant industry standards, legislation and statutory information which should be consulted by the relevant competent person responsible for servicing refrigeration equipment. This bulletin does not represent a recommendation to use any specific refrigerant fluid for a specific application. The Service Engineers Section and the Institute of Refrigeration accept no liability for any errors or omissions. Service Engineers Section of the Institute of Refrigeration, Kelvin House, 76 Mill Lane, Carshalton SM5 2JR SES Technical Bulletin No 10 Page 5 of 5 March 2002