00.09/ 0 040 030e - 1/4
The properties of the refrigeration oil have an influence on the operation of refrigerating plants with oil-flooded screw compressors, as it cannot be excluded that residues of the refrigeration oil get into the refrigerant circuit despite of highly effective oil separators. For this reason, when selecting the oil consider besides
• a sufficient lubricity of the oil at the bearing points of the screw compressor (minimum oil viscosity with consideration of the solubility of refrigerants in oil depending on both the pressure and temperature), • the vapour pressure of the oil for a proper separation behaviour in the oil separator,
• a sufficient fluidity of the oil at both the evaporating and suction temperature
• the requirements upon the miscibility of the liquid phases of the refrigerant and the oil (miscibility gap). The refrigerant used, the operation conditions and the specific plant design determine the required characteristics of the refrigeration oil.
At present, five different base oil brands are used: 1. Mineral oils for ammonia and R22
2. Polyalphaolefins for ammonia 3. Alkyl benzene for ammonia and R22 4. Polyglycol (PAG-oil) for ammonia
5. Ester oil for R 404A, R 134a and R 507 as well as other new refrigerant blends such as R 410A or R 407C Besides the pure base oil components there are also used blends of mineral oil and alkyl benzene or of polyalphaolefin and alkyl benzene.
The characteristics of the refrigerants with relation to the oils mentioned differ greatly.
Two basic requirements are placed upon the working media combination of refrigerant and refrigeration oil: a. Minimum oil viscosity of 7 cSt at the compressor inlet with consideration for the refrigerant solubility in oil and
b. Miscibility of both liquid phases of a certain portion of the oil (ab. 1 to 2 %) and the refrigerant. Besides the requirement of a sufficient viscosity of the lubricating oil, the discharge temperatures in the
compressor must be high enough to enable a subcooling of the refrigerant-laden oil by at least 10 K in order to prevent foaming in the compressor in case of decrease in pressure and/or increase in temperature, before the oil gets to the bearing points.
The basic requirement b) is not met by mineral oil, alkyl benzene and polyalphaolefin in connection with ammonia, as there exists a miscibility gap of 100 %, and neither a solubility of the refrigerant vapour in the oil nor a miscibility of liquid phases is given. Nevertheless these oils are preferably used in NH3-plants. Oil fine
separation stages prevent greater portions of the oil from getting into the refrigerant circuit.
The base oil versions mentioned will bring about differing oil carry-over rates as the flash points of the oils cited differ greatly from each other (lowest flash point of alkyl benzene at ab. 160 °C, highest flash point of
polyalphaolefin considerably above 200 °C).
Although the fluidity of the oil is characterized by the pour point indicated by the oil manufacturers, the base oil brands mentioned above feature different VT-characteristics so that at the same initial viscosity of say 68 cSt, there may arise differences in viscosity at low temperatures in the evaporator in the range of ab. 1500 and 20000 cSt at —20°C.
Grasso
Refrigeration DivisionSCREW COMPRESSORS
HINTS FOR SELECTION OF REFRIGERATION OIL
040 030e - 2/4 00.09/ 0
With relation to oils, the refrigerants feature the following properties: • Ammonia
Exclusive of PAG-oil, ammonia is not soluble with other lubricants. However, mechanical intermixing is very intensive so that the oil always carries along ammonia. Due to the low portion of the ammonia the lubricity of the oil will not be changed. The miscibility between the liquid phases of oil and refrigerant is not given, too. For this reason, an efficient oil separation is required.
• HFC (e.g. R134a, R404A, R507)
possess no chlorine and are not limited in their application. Ester oils are used for these refrigerants. The higher solubility of these refrigerants in ester oil has to be considered when selecting an oil as the initial viscosity of the oil might drastically change by the refrigerant solved in the oil. However, the fluidity of the oil in the evaporator is given due to proper miscibility over a wide range.
The most important properties of the main oil groups are described in the following: 1. Mineral oil
Naphthenic base mineral oils are most suitable for refrigeration systems, but paraffinic base oils are also used. Due to special treatment (dewaxing) paraffinic base oils have more or less the same properties as naphthenic base oils. Mineral oils are characterized by a relatively low miscibility with HCFC’s (e. g. R22) at low
temperatures. Mineral oils feature a relatively high viscosity index and a low vapour pressure (higher flash point) whereby oil carry-over is positively affected.
2. Alkyl benzene (known also as alkyl benzole)
Alkyl benzenes represent synthetic oils produced from natural gas. They are characterized by high miscibility with HCFC’s (e.g. R22) even at lower evaporating temperatures. Alkyl benzenes feature higher thermal stability than mineral oils (use of ammonia in piston compressors). Due to the lower flash point, however, they have a tendency to heavy foaming in the oil separator and to higher oil carry-over. After changing from mineral oil to alkyl benzene it should be considered that alkyl benzenes have a considerable cleaning effect causing in its turn the filters to clog faster than usually in the time immediately after the oil change.
3. Polyalphaolefins
Polyalphaolefins represent synthetic oils of high chemical and thermal stability. Therefore, they are preferably used in compressors operating at high discharge temperatures e.g. in heat pumps.
Polyalphaolefins are also employed in ammonia systems. The very low pour point permits very low evaporating temperatures. The high flash point of the polyalphaolefins leads to smaller oil carry-over rate. Attention: The high aniline point of polyalphaolefins causes a relatively high shrinkage of O-rings whereby leakages may occur even at static seals, when mineral oils or alkyl benzenes are replaced by
polyalphaolefins. For this reason, the use of pure PAO-oils in Grasso-compressors is not permitted at present. Shrinkage is prevented by adding of alkyl benzene. Therefore synthetic oil blends comprising polyalphaolefin and alkyl benzene may be employed.
4. Ester oils
In contrast to mineral oil, alkyl benzene and polyalphaolefin ester oils are soluble in the new non-chlorinated HFC’s (R 134a, R 404A, R 507 etc.). Therefore, the ester oils are the only lubricants at present which may be used for the HFC’s. Ester oils possess a higher flash point whereby the oil vapour portion from the oil separator and hence the oil carry-over rate are favourably affected. Ester oils are hygroscopic. They absorb water when getting in contact with the atmosphere. For this reason, ester oils have to be stored in closed tanks. Prior to oil charging, the compressor has to be thoroughly evacuated.
5. Polyglycol oil
Polyglycol oils are ammonia soluble and very hygroscopic. That’s why the same requirements have to be imposed as when handling ester oil. When selecting the oil take into consideration the reduction of viscosity caused by the refrigerant solved in the oil. Check the fluidity of the oil in the evaporator with consideration of the miscibility between the lubricating oil and refrigerant at the corresponding evaporating temperatures.
Grasso
Refrigeration DivisionSCREW COMPRESSORS
HINTS FOR SELECTION OF REFRIGERATION OIL
00.09/ 0 040 030e - 3/4
PARAMETERS USED FOR OILS: Specific gravity
The difference in specific gravity between the refrigerant liquid and the oil may be of interest for oil return. In this case it should be considered that alkyl benzene has a lower specific gravity than mineral oil and polyglykol has a higher specific gravity than mineral oil. The method for specific gravity measurement is described in DIN 51757.
Viscosity
According to ISO standard 3448 lubricants are classified in viscosity grades indicated as ISO VG numbers. The ISO-no. is a nominal guide value only, i. e. the actual viscosity may vary in certain ranges (DIN 51562). The viscosity indicated refers to 40 °C and to 100 °C.
Viscosity index
The viscosity index gives the relationship between the change in viscosity depending on the temperature (ISO 2909). A high viscosity index means smaller changes in viscosity at temperature changes as compared to a low viscosity index.
Flash point
The flash point indicates at which temperature the vapours escaping from a heated cup may be ignited over a flame. The method of measurement is described in ISO 2592. Oils with high flash point have a low oil vapour pressure. This will enhance the possibilities of oil separation from a compressed gas in the oil separator and reduce the oil carry-over rate from the compressor into the plant.
Pour point
The pour point is the temperature at which the fluidity of an oil decreases to an extent that under certain conditions it will no more leave a jar within 5 sec. According to standard the pour point temperature is by 3 % lower than the measured temperature (method of measurement according to ISO 3016). The pour point is interesting for working media combinations which are not soluble with each other. Oils having a low pour point can be returned to the suction side more easily than oils having a high pour point. The practice, however, shows that oils may be used even at evaporating temperatures lower than the pour point without any operating problems.
Floc point
The floc point indicates the temperature at which liquid R12 with the addition of 10 % oil will show flocculation by wax particles precipitating from the oil when chilled (method of measurement according to DIN 51351). The floc point is of interest when oil and refrigerant can be mixed. The floc point indicates that an oil contains but a few wax particles and that plants employing HCFC (e. g. R22) can be operated at low evaporating temperatures. When wax precipitation from the oil takes place, problems are to be expected at the expansion valve or at the regulating valves. For ester oils a critical solution temperature is given measured with a mixture of 10 % oil and 90 % of R134a. The critical solution temperature is the temperature at which the oil completely precipitates from the refrigerant (non-standardized value).
Aniline point
The aniline point indicates the temperature at which the oil concerned shows a homogenous mixture with pure aniline. The aniline point is the measurement of the quantity of unsaturated carbon which can be found in the oil. It is an indication of the compatibility of different sealing materials coming into contact with the oil (method of measurement according to ISO 3977). Most lubricating oils possess a low aniline point. Neoprene and
chloroprene will swell. For this reason, O-rings have to be replaced after dismantling. Polyalphaolefin has a high aniline point causing neoprene to shrink. (use of HNBR required).
Neutralization number
The neutralization number gives the acid content of an oil and is determined by titration with potassium hydroxide solution (KOH). The value is given in mg of KOH per g of oil (method of measurement according to DIN 51558). Fresh oils should have a low neutralization number.
Hints for oil change
Prior to changing the oil brand or the manufacturer of an oil, consult the compressor manufacturer to prevent problems in plant operation. In case of incompatible oil brands precipitations from the oil can occur which might lead to problems within the plant (oil filter, lubrication of bearings, oil return not guaranteed). If, however, it should be necessary to employ another type of oil, it is of vital importance to remove all the oil from the plant and to thoroughly clean both the compressor and oil separator (if possible, perform additional scavenging). Oil selection table
All oils permitted for the Grasso-screw compressors are given in the oil selection table. Depending on the specific conditions of a plant consider the above technical features when selecting an oil.
Grasso
Refrigeration DivisionSCREW COMPRESSORS
HINTS FOR SELECTION OF REFRIGERATION OIL
040 030e - 4/4 00.09/ 0
TABLE
REFRIGERANT — OIL COMPATIBILITY Ammonia (NH3) R22 R134a R404A R507 Mineral oil (M) x x - - - Polyalphaolefins (PAO) * Alkyl benzene (AB) x x - - - Polyglycol (PAG) * - - - - Ester oil - - * * * M + AB x x - - - PAO + AB x - - - -
x = suitable, use of CR-O-rings * = suitable, use of HNBR O-rings - = not suitable
Grasso
Refrigeration DivisionP + I DIAGRAMS
SCREW COMPRESSOR
SERIES SH AND MC
06.10.00/ 0 040 800e - 1/20I. Screw compressors for single stage
screw packages
Oil pump P+I-No A Mono-Packs: 1 screw in 1 package
for refrigeration/a.c./heat pump (cooled injection oil) intern SM11 for booster / for refrigeration and a.c. with low condending temperature
(external oil pump for functional oil supply only)
extern SM12
heat pump (uncooled injection oil) intern SM13 for ammonia chiller with PAG oil
(external oilpump for mechanical shaft seal only)
extern SM14
B Duo-Packs: 2 screws in 1 package
part load range of each 15%/100%, with position indicator)
for refrigeration/a.c./heat pump (cooled injektion oil) intern SD11 for booster / for refrigeration and a.c. with low condending temperature
(external oil pump for functional oil supply only; 2 oil stop valves each compressor)
extern SD12
for booster/ for refrigeration and a.c. with low condending temperature (external oil pump for whole oil supply; 1 oil stop valve each compressor)
extern SD13
for heat pump (uncooled injection oil) intern SD14
II Screw compressors for two stage packages Oil pump P+I-No
A as high pressure stage (external oil pump for functional oil supply only) extern SM21 B as Booster (with side load at intermediate pressure)
(external oil pump for 4 - solenoid valve block only)
extern SM22
Remarks: Oil Pump - “intern” means: oil pump is a part of the screw compressor - “extern” means: oil pump is a part of the package
• The P & I diagramms have to be used for all permitted refrigerants including ammonia and all permitted oils. For use of the combination of Ammonia and PAG-oil please contact Grasso R.T. in Berlin.
• Screw compressors for DuoPacks are suitable for use in Multi-Packages with more than 2 screw compressors • The external oil pump has to supply functional oil with a pressure of pk+0,5 bar < poil < pk+2,5 bar.
• The delivery flow of external oil pump has to be selected depending on compressor size (see compressor selection program) and depending on the number of compressors in a package. The compressor selection program calculates the oil volume flow for 1 screw compressor.
• The integrated oil pump delivers 3 l/min only and supplies it to the hydraulic system (solenoid valve block); the oil pressure of the integrated oil pump is limited by means of a integrated oil pressure control valve to 6 bar over pressure of functional oil.
• Screw compressors for heat pump application are equipped with one side acting thrust bearings. It is not allowed to use an external oil pump in screw compressors series SH and MC for heat pump application.
Grasso
Refrigeration DivisionP + I DIAGRAMS
SCREW COMPRESSOR
SERIES SH AND MC
040 800e - 2/20 06.10.00/0P+I DIAGRAM SM11
SERIES SH
MONOPACK, SINGLE STAGE, INTERNAL OIL PUMP
oil supply
injected oil
functional oil from oil filter to oil filter
C
basic plate oilmanagement block 1
K
solenoid valve block
R
X2
at discharge side of oil pump measuring oil pressure
oil flow control valve injected oil
p
X1
L
integrated oil pump, flanged housing control valve
integrated oil pressure