Servicing of refrigeration systems. Substitution of R12 for R134a

• Substitution of R12 for R134a
• Substitution of R502 for R404A or R507
• Substitution of R12 for R401A, R409A, R401B. Substitution of R500 for R401B.
• Substitution of R502 for R402A, R408A, R402B
• Substitution of R22 for R407C

General information. At substitution (retrofit) of R12 for R134a attention should be paid to possible changing of cold-productivity. On Fig. down there is shown changing of the initial cold-productivity of the facility ("L'Unite Неrmetiquc" model), having been operating on R12 and transferred for R134a, depending on boiling point. As it can be seen from Fig., with boiling point decreasing cold-productivity is reducing. Reduction of cold-productivity can be prevented in two ways:

• By increasing the capacity of compressor cylinders for compensation of cold-productivity drop;
• By increasing operational efficiency of the facility with the aim of restoration of the initial cold-productivity or maximum approximation to it.

However, it can happen that cold-productivity of the system during operation on the new refrigerant will be higher than that on the old one. In this case it is necessary to restrict its value, and there are different techniques for that.

There are a number of requirements made toward the refrigeration systems charged with R134a refrigerant.

Dependence of relative cold-productivity Q0 (compared to R12) during operation on R134a on the boiling point: 1 - low boiling pressure; 2 - average and high boiling pressure

1. In the operating compressor it is necessary to substitute mineral oil for synthetic polyester oil. Synthetic oils should have proper viscosity, which is obtained with the help of addition agents, and should be stable within a long period of time.

Selection of refrigerating oil depends on several factors, including the possibility of its return to the compressor, lubricating ability, as well as compatibility of materials. Polyester oils are produced, for example, by "Castrol", "Mobil", "Labrizol", "Khegkel" companies and others. Recommendations as to what kind of oil should be used in the refrigeration equipment should be taken from the factory-manufacturer of the refrigeration equipment. While working with polyester oils, it is necessary to be especially cautious due to their tendency to absorb moisture, which makes it a bit problematic during installation and servicing of refrigeration facilities. Moreover, they are aggressive toward copper parts and they resolve copper, which is then deposited on the other structural elements due to formation of chemical compounds. Decreasing of the level of hydrophily of oils will allow at the same time to decrease their aggressiveness toward copper.

It is necessary to reduce to zero the oil contact with the air; one should store oils in the hermetic container. Polyester oils are not mixed with mineral oils; therefore, during retrofit of the equipment (operating on R12 and mineral oil) using R134a and polyester oil with the aim of obtaining equivalent miscibility, oil residue should constitute no more than 5 % of the total lubrication introduced into the system. This requirement makes introduction of repeated wash of the system into the procedure of retrofit necessary, which is not obligatory when using average pressure service blends and alkylbenzene oil. Acceptable residue content of mineral oil considerably depends on the system structure and operation conditions. If in the refrigeration equipment there are observed symptoms of low heat transfer in the evaporator or insufficient oil return into the compressor, there can occur the necessity in further reduction of residue content of mineral oil. A number of sequence washes with the use of compound ethers can reduce, as a rule, concentration of mineral oil till low levels.

2. The obligatory requirement is hermiticity of structural elements of the refrigeration machine due to increased fluidity of R134a.

Up till present time there has not been yet solved a problem of prevention of R134a leakage through the walls of flexible hoses of pipelines. Covering of hose inner walls with the layer on the base of nylon and elastomer increases their rigidity which can worsen their ability to absorb noises and vibrations.

Since R134a is more fluid than R12, it is necessary to use adjustment equipment with solder bonds for the facilities operating on R134a. Manufacturing of hermetic refrigerating contours allows to avoid leakage and it positively effects both the environment and costs.

When soldering it is necessary to take measures in order to eliminate formation of oxides inside pipelines. For that when soldering they are blown off with nitrogen. Moreover, the ends of pipes and other openings should be closed with plugs up to the moment of assembling.

3. In heat-exchangers effect of oil on structural materials, especially copper ones, needs experimental testing.

4. Adjusters do not need any serious alterations to be introduced; however, parameter determination or adjustment should be made taking into account possible changing of consumption.

5. Pads made of the material being used for R12 should be substituted. At present, pads being adequate for use in combination with many refrigerants are manufactured of polyethylene fabric (EFDM) or chlorinated polyethylene characterized by high resistibility in the environment of polymer oils and alternative refrigerants. Material on the base of polychloroprene is also considered to be resisting enough.

6. Adsorbents used in the filters-dehydrators should correspond to the chosen refrigerant. Thus, filter-dehydrator operating with R12, cannot completely ensure elimination of moisture from R134a. Some substances, which have recently appeared on the market, have their ability to absorb moisture being 10% lower than substances used in the filters-dehydrators for R12. Due to this their mass should be approximately 20% increased or it is necessary to use in the system the filter-dehydrator with adsorbent-molecular sieve intended for the structure of R134a molecule.

7. During technical maintenance, control of charging ratio for the systems with R134a is more difficult than that for R12 system; moreover, possible leakage of R134a cannot be revealed with the help of usual means responsive to chlorine. New leak detectors should be responsive to chlorine, and in order to reach the level starting from which leakage is detected, their responsivity should be considerably higher than that of ordinary detectors.

8. Operating facilities can be charged with R134a refrigerant instead of R12 without disassembly of main aggregates (compressor, condenser, evaporator), but with substitution of thermoregulating valve whose pressure should be intended for the use of R134a. Marking of the thermoregulating valve should unambiguously indicate that it is intended for R134a.

Loss of pressure in the valve of EVR6 type, installed on pipelines for R134a (1) and R12 (2)

9. In small hermetic refrigeration facilities operating on R134a, connecting tube should be 10...15% longer that when using R12. Moreover, when using R134a it is necessary to calculate correctly the dimensions of several other systems: electric valves, return valves, pressure adjusters, taking into account new values of pressure consumption and loss. Losses of pressure in EVR6 electric valve, intended correspondingly for R134a and R12, are given on the figure. At the same time overwhelming majority of controllers being used, for example, pressostats, thermostats, as well as sight glasses, can be used also in the facilities for operation on the refrigerant R134a.

10. Before using R134a, scales of pressure gages should be calibrated for this refrigerant if the refrigeration facility was operating on some other refrigerant.

Refill capacities and things for drainage should be new and clean. One cannot use the tools, which had even minor contact with R12 or mineral oil. Flexible hoses for R134a should have increased hermiticity. When assembling and disassembling, special releasable connections of quick action ensure preservation of the refrigerant in hoses. All the tools being used during technical maintenance of the facilities operating on R134a and polyester oils are provided with a proper marking. These accessories are recommended to use only for operation with R134a.

For searching of leakage in the contour along which R134a is circulating, there exist several methods. Many introducers supply electronic leakage detectors, which apply sound signal when detecting leakage. In other leakage detectors there are used ultraviolet lamps. Into the refrigerant there is added an addition agent which is mixed with polyester oil. In case of leakage, the oil flowing out of the contour together with the addition agent becomes visible in ultraviolet rays. Ultraviolet lamps of leakage detectors of the old model are not suitable for R134a.

Though R134a is non-toxic and harmless for ozone layer, its recovery and repeated using are purposeful (due to ecological and economic considerations). At present time, there are manufactured mobile aggregates to eliminate R134a out of the contours during their vacuumization and refrigerant restoration with the aim of its repeated using. The aggregate contains built-in powerful vacuum pump ensuring deep vacuum.

Transfer of the refrigeration system operating on R12, for R134a can be performed with the use of usual service equipment and usual practice of servicing of the refrigeration equipment.

For performing retrofit the following equipment is required: workmanship instructions; safety engineering means (gloves, glasses, etc.); measuring equipment placed on pipelines; thermocouples; vacuum pump; leakage detectors; scales; assembly for the refrigerant collection; gage cylinder for the refrigeration system charging; container for oil collection; oil-substitute; new filter-dehydrator; thermostatic expansion valve; labels with indication of the oil and refrigerant having been used.

Further there are given main retrofit stages of the refrigeration systems during transfer from R12 refrigerant to R134a.

Determination of working parameters of the operating refrigeration system. They determine and put down parameters of the refrigeration system operating on R12. In particular, it is recommended to those who begin to be involved in retrofit of the equipment. Information on pressure and temperature (in evaporator, condenser, throttling device, at compressor intake and charging, etc.) under different environment temperatures and in the cooled volume can occur useful for optimization of the refrigeration system operation after transfer to R134a refrigerant.

Substitution of mineral or alkylbenzene oil for polyester oil in the refrigeration system. In the majority of refrigeration systems operating on R12, mineral or alkylbenzene oils are used. These oils are not mixed with R134a and should be substituted for polyester oils. When substituting mineral or alkylbenzene oil, R12 refrigerant remains in the system. In the systems with small hermetic compressors where there is no opening for drainage of the oil, there can be required disassembling of the compressor in order to draw the oil out of it. In such cases the oil can be drained from the compressor suction line. In the majority of small systems it is possible to eliminate up to 90...95 % of oil in such a way. If the system contains oil eliminator, then all the oil contained in it is drained.

After that they measure the quantity of collected oil (not less 50 %) and compare it with the value given in the equipment specification in order to make sure that the main portion of the oil has been drained out of the compressor. They put down how much oil has been eliminated out of the system. Then the compressor is charged with polyester oil in required quantity equal to the quantity of oil having been eliminated on the previous stage. If there are not any additional recommendations of the factory-manufacturer, they use polyester oil with the same viscosity that that of mineral or alkylbenzene oil (in the refrigeration equipment with R12, operating within the range of moderate temperatures, typical viscosity is 32*10^-6m²/s). In order to obtain miscibility being equivalent to miscibility of R12 with mineral or alkylbenzene oil, the residue of mineral or alkylbenzene oil should constitute no more than 5 % of the total quantity of oil used in the equipment (1 % according to recommendations of "Danfoss" company). Such residue level is obtained by means of repeated backwashing with polyester oil; moreover, there can be required up to three backwashings.

Backwashing of the refrigeration system presupposes:

• Drainage of the oil from the system according to the technology described above;
  selection of polyester oil whose viscosity should be equal to viscosity of mineral or alkylbenzene oil drained out of the system;
• Charging of the system with polyester oil in the quantity equal to the removed mineral or alkylbenzene oil;
• Introduction of the system into operation with R12 for thorough mixing of polyester and mineral oil. The system should operate more than 24 hours.

All the above-mentioned stages are repeated two more times. During the last backwashing, R12 is substituted for R134a. On this stage the compressor is reinstalled if it has been removed from the refrigeration aggregate for the oil drainage.

Removal of R12 refrigerant out of the refrigeration system and its utilization. R12 is removed from the system and collected into the ballon for refrigerant collection. There exist different versions of mechanisms allowing to realize this procedure and to create required vacuum in the system (34...67кPа). On this stage the removed refrigerant is weighed (especially if the quantity of the refrigerant recommended for charging into the system is unknown). Initial charging of service blend is determined proceeding from the quantity of R12, removed from the system. Residue content of R12 in the contour should not exceed 0,02 %.

Substitution of the filter-dehydrator and TEV. Substitution of the filter-dehydrator during retrofit is an ordinary procedure realized in the process of technical maintenance of the refrigeration system. They choose a filter-dehydrator with adsorbent compatible with R134a refrigerant (for example, HN-9 or HN-7 type of UOP company).

TEV should also have marking reflecting possible operation on R134a; pressure should be meant for using of R134a.

Vacuumization of the refrigeration system and checking of its hermiticity. In order to remove air and other permanent gases, the system is vacuumized up to pressure 0,14 кPа and make sure that there is no leakage in the system. The best result can be achieved using two-stage vacuum pump compatible with R134a refrigerant. It is forbidden to use pumps which have served for vacuumization of the contours with chlorinated refrigerants before or have been in turn used for operation with different refrigerants.

Charging of the refrigeration system with R134a refrigerant. The system is charged with R134a refrigerant in gaseous or liquid state from the ballon. Ballons with R134a of repeated use are provided with immersion legs. It creates conditions for drawing of liquid out of the ballon being in vertical position. Disposable ballons (13,6kg) are not provided with immersion legs.

Mass of R134a, required for charging of the refrigeration system is lower than that of R12. Optimal charging depends on operational conditions, dimensions of evaporator, condenser and receiver, as well as on the length of connection pipes in the system. For the majority of types of the equipment, the quantity of the refrigerant constitutes 75...90 % of the initial charging of R12, having been performed by the equipment producer.

Charging of the system is performed in several stages. On the first stage it is recommended to introduce R134a in the quantity of about 75 % of the initial charging of R12. First, R134a refrigerant is introduced on the saturation line (the compressor is not operating at that); after pressure equalization in the system and in the ballon, the system is charged with the remained portion of the refrigerant through the suction line of the compressor (the compressor is operating at that). Liquid refrigerant should never be introduced through the suction line of the compressor due to danger of water hammering in the compressor. In case of necessity of the refrigerant charging through the suction line of the compressor, there can be used throttling valve so that liquid would necessarily be transformed into vapor before entry into the system.

Start of the refrigeration system, regulating of the refrigerant charging dose and (or) regulator devices to ensure the assigned operating mode. The system is started. After stabilization, values of operating parameters are put down. If values of working parameters indicate that the equipment is undercharged, R134a is added in small portions (3...5 % of the initial charging), until working parameters have not reached desired values. In order to compare pressures and temperatures on the saturation line during operation on R134a and R12, one can use the table. In general, suction pressure during operation on R134a will be 7...12 кPа lower than during operation on R12. During operation on R134a (compared to R12) there will be observed high pressure and lower discharge temperature. Typical increase of discharge pressure will constitute 103...172 кPа, and typical decrease of discharge temperature will constitute 0...5,6 ^oС.

One can also control the quantity of the charged refrigerant through level meters (sight glasses), installed on the liquid line; however, at the same time, one should determine operating parameters of the system (strength of current consumed by electric motor of compressor, heat, etc.). Attempt to charge the system up to the level when the sight glass will be completely flooded with the refrigerant can cause overloading of the system.

Marking of the refrigeration system. After retrofit the system is marked so that in future there were used the same refrigerants and oils for servicing. It is necessary to do to avoid mixing of different refrigerants in future. Such blends can have unpredictable thermodynamic characteristics, which will lead to decreasing of cold-productivity and dropping of efficiency of the system operation.