2017-08-15

Today, there are VRF systems on the market from original Japanese, Korean and Chinese brands. Even more VRF systems from numerous OEM manufacturers. Outwardly, they are all very similar, and one gets the false impression that all VRF systems are the same. But “not all yoghurts are created equal,” as the popular advertisement said. We continue the series of articles aimed at studying the technologies for producing cold that are used in the modern class of air conditioners - VRF systems.

Designs of separators (oil separators)

The oil in oil separators is separated from the gaseous refrigerant as a result of a sharp change in direction and a decrease in the speed of steam movement (up to 0.7-1.0 m/s). The direction of movement of the gaseous refrigerant is changed using partitions or pipes installed in a certain way. In this case, the oil separator catches only 40-60% of the oil carried away from the compressor. That's why best results gives a centrifugal or cyclonic oil separator (Fig. 2). The gaseous refrigerant entering the pipe 1, falling on the guide vanes 3, acquires a rotational motion. Under the influence of centrifugal force, oil droplets are thrown onto the body and form a film that slowly flows down. When exiting the spiral, the gaseous refrigerant abruptly changes its direction and leaves the oil separator through pipe 2. The separated oil is separated from the gas stream by a partition 4 to prevent the secondary capture of oil by the refrigerant.

Despite the operation of the separator, a small part of the oil is still carried away with freon into the system and gradually accumulates there. To return it, a special oil return mode is used. Its essence is as follows. The outdoor unit switches on in cooling mode at maximum performance. All EEV valves in indoor units are fully open. But the fans of the indoor units are turned off, so freon in the liquid phase passes through the heat exchanger of the indoor unit without boiling away. Liquid oil located in the indoor unit is washed off with liquid freon into gas pipeline. And then returns to outdoor unit with freon gas at maximum speed.

Refrigeration oil type

Type of refrigeration oil used in refrigeration systems for lubricating compressors, depends on the type of compressor, its performance, but most importantly, on the freon used. Oils for the refrigeration cycle are classified as mineral and synthetic.

Mineral oil is primarily used with CFC (R12) and HCFC (R22) refrigerants and is based on naphthene or paraffin, or a mixture of paraffin and acrylic benzene. HFC refrigerants (R410a, R407c) are not soluble in mineral oil, so synthetic oil is used for them.

Crankcase heater

Refrigeration oil is mixed with the refrigerant and circulates with it throughout the entire refrigeration cycle. The oil in the compressor crankcase contains some dissolved refrigerant, but the liquid refrigerant in the condenser contains no large number dissolved oil. The disadvantage of using the latter is the formation of foam. If the chiller is shut down for an extended period and the compressor oil temperature is lower than the internal circuit, the refrigerant condenses and most of it dissolves in the oil. If the compressor starts in this state, the pressure in the crankcase drops and the dissolved refrigerant evaporates along with the oil, forming oil foam. This process is called “foaming”, it causes oil to escape from the compressor through the discharge pipe and deteriorate the lubrication of the compressor. To prevent foaming, a heater is installed on the compressor crankcase of VRF systems so that the temperature of the compressor crankcase is always slightly higher than the temperature environment(Fig. 3).

The influence of impurities on the operation of the refrigeration circuit

1. Process oil (machine oil, assembly oil). If process oil (such as machine oil) gets into a system using HFC refrigerant, the oil will separate, forming flocs and causing clogged capillary tubes.
2. Water. If water gets into a cooling system using HFC refrigerant, the acidity of the oil increases and destruction occurs. polymer materials, used in the compressor motor. This leads to destruction and breakdown of the electric motor insulation, clogging of capillary tubes, etc.
3. Mechanical debris and dirt. Problems that arise: clogged filters and capillary tubes. Decomposition and separation of oil. Destruction of the compressor motor insulation.
4. Air. Consequence of a large amount of air entering (for example, the system was filled without evacuation): abnormal pressure, increased acidity of the oil, breakdown of the compressor insulation.
5. Impurities of other refrigerants. If a large amount of refrigerant enters the cooling system various types, abnormal operating pressure and temperature occur. The consequence of this is damage to the system.
6. Impurities of other refrigeration oils. Many refrigeration oils do not mix with each other and precipitate in the form of flakes. The flakes clog filters and capillary tubes, reducing freon consumption in the system, which leads to overheating of the compressor.

The following situation is repeatedly encountered related to the oil return mode to the compressors of outdoor units. A VRF air conditioning system has been installed (Fig. 4). System refueling, operating parameters, pipeline configuration - everything is normal. The only caveat is that some of the indoor units are not installed, but the load factor of the outdoor unit is acceptable - 80%. However, compressors regularly fail due to jamming. What is the reason?

And the reason is simple: the fact is that branches were prepared for the installation of the missing indoor units. These branches were dead-end “appendixes” into which the oil circulating along with freon entered, but could not come back out and accumulated there. Therefore, compressors failed due to normal “oil starvation.” To prevent this from happening, it was necessary to install shut-off valves on the branches as close to the splitters as possible. Then the oil would circulate freely in the system and return in oil collection mode.

Oil lifting loops

For VRF systems from Japanese manufacturers, there are no requirements for installing oil lifting loops. Separators and oil return mode are considered to effectively return oil to the compressor. However, there are no rules without exceptions - on MDV V5 series systems, it is recommended to install oil lifting loops if the outdoor unit is higher than the indoor units and the height difference is more than 20 m (Fig. 5).

The physical meaning of the oil lifting loop comes down to the accumulation of oil before the vertical lift. Oil accumulates at the bottom of the pipe and gradually blocks the hole for freon passage. Gaseous freon increases its speed in the free section of the pipeline, while capturing the accumulated liquid oil.

When the cross section of the pipe is completely covered with oil, freon pushes this oil out like a plug to the next oil lifting loop.

Conclusion

Oil separators are the most important and mandatory element of a high-quality VRF air conditioning system. Only by returning freon oil back to the compressor is reliable and trouble-free operation of the VRF system achieved. Most best option design - when each compressor is equipped with a separate separator, since only in this case is a uniform distribution of freon oil achieved in multi-compressor systems.

Refrigerant pressure loss in pipes refrigeration circuit reduce the efficiency of the refrigeration machine, reducing its cooling and heating capacity. Therefore, we must strive to reduce pressure losses in the tubes.

Since boiling and condensation temperatures depend on pressure (almost linearly), pressure losses are often estimated by condensation or boiling point losses in °C.

• Example: for refrigerant R-22 at an evaporation temperature of +5°C, the pressure is 584 kPa. With a pressure loss of 18 kPa, the boiling point will decrease by 1°C.

Suction line losses

When there is a loss of suction line pressure, the compressor operates at a lower inlet pressure than the evaporation pressure in the refrigeration evaporator. Because of this, the flow of refrigerant passing through the compressor is reduced and the cooling capacity of the air conditioner is reduced. Pressure losses in the suction line are most critical to the operation of the refrigeration machine. With losses equivalent to 1°C, productivity decreases by as much as 4.5%!

Discharge line losses

If there is a loss of pressure in the discharge line, the compressor has to work harder high pressure than the condensation pressure. At the same time, the compressor performance also decreases. For discharge line losses equivalent to 1°C, performance is reduced by 1.5%.

Liquid Line Losses

Pressure loss in the liquid line has little effect on the cooling capacity of the air conditioner. But they cause the danger of the refrigerant boiling. This happens for the following reasons:

1. because of reducing pressure in the tube it may be that the refrigerant temperature is higher than the condensation temperature at this pressure.
2. refrigerant heats up due to friction against the walls of the pipes, since mechanical energy its movement turns into heat.

As a result, the refrigerant may begin to boil not in the evaporator, but in the tubes in front of the regulator. The regulator cannot operate stably on a mixture of liquid and vapor refrigerant, since the flow of refrigerant through it will greatly decrease. In addition, the cooling capacity will decrease, since not only the air in the room will be cooled, but also the space around the pipeline.

The following pressure losses in the tubes are permissible:

• in the discharge and suction lines - up to 1°C
• in the liquid line - 0.5 - 1°C

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Oil lifting loops are almost always necessary during installation and installation:

• household and semi-industrial air conditioners;
• window, wall, floor-ceiling, duct, cassette split systems.

We sell original oil lifting hinges directly from the manufacturer without an intermediary markup.

In our online store you can buy everything at once: not only various oil lifting hinges, but also other components. We have large selection loops of various markings.

If the section of the refrigeration unit is non-standard, the company representative will recommend installing an additional loop or, conversely, reducing the number of oil lifting loops for effective hydraulic resistance. Our company employs professionals.

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The purpose of the oil lifting loop is to provide additional hydraulic resistance based on the calculation of the length of the section of the refrigeration circuit of the freon unit.

Oil lifting loops are needed when we're talking about about installation refrigeration units with vertical sections from 3 meters long. If vertical equipment is mounted, you will need to use a loop every 3.5 meters, and at the highest point - a reverse loop.

In our online store you will find a reasonable price for oil lifting loops and other components, as well as consumables (freons, etc.). Call the phone number listed on the website and our managers will help you make the right choice.

In the process of acceptance testing, over and over again we have to deal with errors made during design and installation copper pipes wires for freon air conditioning systems. Using the accumulated experience, as well as relying on the requirements regulatory documents, we tried to combine the basic rules for organizing copper pipeline routes within the framework of this article.

We will talk specifically about the organization of routes, and not about the rules for installing copper pipelines. Issues of pipe placement, their relative position, problems of choosing the diameter of freon pipes, the need for oil lifting loops, compensators, etc. We will ignore the rules for installing a specific pipeline, the technology for making connections and other details. At the same time, issues of a larger and more general view of the design of copper routes will be raised, and some practical problems will be considered.

Mainly this material concerns freon air conditioning systems, whether they are traditional split systems, multi-zone air conditioning systems or precision air conditioners. However, we will not touch upon the installation of water pipes in chiller systems and the installation of relatively short freon pipelines inside refrigeration machines.

Regulatory documentation for the design and installation of copper pipelines

Among regulatory documentation Regarding the installation of copper pipelines, we highlight the following two standards:

• STO NOSTROY 2.23.1–2011 “Installation and commissioning of evaporative and compressor-condensing units household systems air conditioning in buildings and structures";
• SP 40–108–2004 “Design and installation internal systems water supply and heating of buildings from copper pipes."

The first document describes the features of installing copper pipes in relation to vapor compression air conditioning systems, and the second - in relation to heating and water supply systems, however, many of the requirements are also applicable to air conditioning systems.

Selection of copper pipeline diameters

The diameter of copper pipes is selected based on catalogs and calculation programs for air conditioning equipment. In split systems, the diameter of the pipes is selected according to the connecting pipes of the indoor and outdoor units. In the case of multi-zone systems, it is best to use calculation programs. IN precision air conditioners Manufacturer's recommendations are used. However, with a long freon route, non-standard situations may arise that are not indicated in the technical documentation.

In general, to ensure oil return from the circuit to the compressor crankcase and acceptable pressure losses, the flow rate in the gas line must be at least 4 meters per second for horizontal sections and at least 6 meters per second for ascending sections. To avoid the occurrence of unacceptable high level noise, the maximum permissible gas flow speed is limited to 15 meters per second.

The refrigerant flow rate in the liquid phase is much lower and is limited by the potential destruction of shut-off and control valves. Maximum speed liquid phase - no more than 1.2 meters per second.

At high elevations and long routes, the internal diameter of the liquid line should be chosen so that the pressure drop in it and the pressure of the liquid column (in the case of an ascending pipeline) do not lead to boiling of the liquid at the end of the line.

In precision air conditioning systems, where the length of the route can reach or exceed 50 meters, vertical sections of gas lines with a reduced diameter are often adopted, as a rule, by one standard size (by 1/8”).

We also note that often the calculated equivalent length of pipelines exceeds the limit specified by the manufacturer. In this case, it is recommended to coordinate the actual route with the air conditioner manufacturer. It usually turns out that excess length is permissible by up to 50% maximum length routes indicated in the catalogues. In this case, the manufacturer indicates the required pipeline diameters and the percentage of underestimation of the cooling capacity. According to experience, the underestimation does not exceed 10% and is not decisive.

Oil lifting loops

Oil lifting loops are installed in the presence of vertical sections of 3 meters or more in length. At higher elevations, loops should be installed every 3.5 meters. In this case, a return oil lifting loop is installed at the top point.

But there are exceptions here too. When agreeing on a non-standard route, the manufacturer may either recommend installing an additional oil lifting loop or refuse the extra ones. In particular, in conditions of a long route, in order to optimize hydraulic resistance, it was recommended to abandon the reverse upper loop. In another project, due to specific conditions on a rise of about 3.5 meters, it was necessary to install two loops.

The oil lifting loop is an additional hydraulic resistance and must be taken into account when calculating the equivalent route length.

When making an oil lifting loop, it should be borne in mind that its dimensions should be as small as possible. The length of the loop should not exceed 8 diameters of the copper pipeline.

Fastening copper pipelines

Rice. 1. Scheme of pipeline fastening in one of the projects,
from which the clamp is attached directly to the pipe
it is not obvious, which has become the subject of controversy

When it comes to fastening copper pipelines, the most common mistake is fastening with clamps through the insulation, supposedly to reduce the vibration impact on the fasteners. Controversial situations in this issue can also be caused by insufficiently detailed drawing of the sketch in the project (Fig. 1).

In fact, to secure the pipes, metal plumbing clamps should be used, consisting of two parts, twisted with screws and having rubber sealing inserts. They will provide the necessary vibration damping. The clamps must be attached to the pipe, and not to the insulation, must be of the appropriate size and provide rigid fastening of the route to the surface (wall, ceiling).

The choice of distances between fastenings of pipelines made of solid copper pipes is generally calculated according to the methodology presented in Appendix D of document SP 40–108–2004. TO this method should be used in case of using non-standard pipelines or in case of controversial situations. In practice, specific recommendations are more often used.

Thus, recommendations for the distance between the supports of copper pipelines are given in table. 1. The distance between the fastenings of horizontal pipelines made of semi-solid and soft pipes it is permissible to take 10 and 20% less, respectively. If necessary, more accurate values ​​of the distances between fasteners on horizontal pipelines should be determined by calculation. At least one fastening must be installed on the riser, regardless of the height of the floor.

Table 1 Distance between copper pipe supports

Note that the data from table. 1 approximately coincide with the graph shown in Fig. 1 clause 3.5.1 SP 40–108–2004. However, we have adapted the data of this standard to suit the relatively small diameter pipelines used in air conditioning systems.

Thermal expansion compensators

Rice. 2. Calculation scheme for selecting compensators
thermal expansion of various types
(a – L-shaped, b – O-shaped, c – U-shaped)
for copper pipelines

A question that often baffles engineers and installers is the need to install thermal expansion compensators and the choice of their type.

The refrigerant in air conditioning systems generally has a temperature in the range from 5 to 75 °C (more precise values ​​​​depend on which elements of the refrigeration circuit the pipeline in question is located between). The ambient temperature varies in the range from –35 to +35 °C. Specific calculated temperature differences are taken depending on where the pipeline in question is located, indoors or outdoors, and between which elements of the refrigeration circuit (for example, the temperature between the compressor and the condenser is in the range from 50 to 75 ° C, and between the expansion valve and the evaporator - in the range from 5 to 15 °C).

Traditionally, U-shaped and L-shaped expansion joints are used in construction. Calculation of the compensating capacity of U-shaped and L-shaped pipeline elements is carried out according to the formula (see diagram in Figure 2)

Where
Lk - compensator reach, m;
L is the linear deformation of the pipeline section when the air temperature changes during installation and operation, m;
A is the elasticity coefficient of copper pipes, A = 33.

Linear deformation is determined by the formula

L is the length of the deformed section of the pipeline at installation temperature, m;
t is the temperature difference between the pipeline temperature in various modes during operation, °C;
- coefficient of linear expansion of copper equal to 16.6·10 –6 1/°C.

For example, let's calculate the required free distance L to from the movable support of the pipeline d = 28 mm (0.028 m) before the turn, the so-called overhang of the L-shaped compensator at a distance to the nearest fixed support L = 10 m. The pipe section is located indoors (pipeline temperature at idle chiller 25 °C) between the refrigeration machine and the remote condenser ( operating temperature pipeline 70 °C), that is t = 70–25 = 45 °C.

Using the formula we find:

L = L t = 16.6 10 –6 10 45 = 0.0075 m.

Thus, a distance of 500 mm is quite enough to compensate for the thermal expansion of the copper pipeline. Let us emphasize once again that L is the distance to the fixed support of the pipeline, L k is the distance to the movable support of the pipeline.

In the absence of turns and the use of a U-shaped compensator, we find that for every 10 meters of a straight section a half-meter compensator is required. If the width of the corridor or other geometric characteristics of the pipeline installation site do not allow installing an expansion joint with an overhang of 500 mm, expansion joints should be installed more often. In this case, the dependence, as can be seen from the formulas, is quadratic. When the distance between expansion joints is reduced by 4 times, the extension of the expansion joint will become only 2 times shorter.

To quickly determine the offset of the compensator, it is convenient to use the table. 2.

Table 2. Compensator overhang L k (mm) depending on the diameter and extension of the pipeline

Pipeline diameter, mm	Extension L, mm
	5	10	15	20
12	256	361	443	511
15	286	404	495	572
18	313	443	542	626
22	346	489	599	692
28	390	552	676	781
35	437	617	756	873
42	478	676	828	956
54	542	767	939	1 084
64	590	835	1 022	1 181
76	643	910	1 114	1 287
89	696	984	1 206	1 392
108	767	1 084	1 328	1 534
133	851	1 203	1 474	1 702
159	930	1 316	1 612	1 861
219	1 092	1 544	1 891	2 184
267	1 206	1 705	2 088	2 411

Finally, we note that there should be only one fixed support between two expansion joints.

Potential places where expansion joints may be required are, of course, those where there is the greatest temperature difference between the operating and non-operating modes of the air conditioner. Since the hottest refrigerant flows between the compressor and condenser, and the most low temperature is typical for outdoor areas in winter, the most critical are the outdoor sections of pipelines in chiller systems with remote condensers, and in precision air conditioning systems - when using internal cabinet air conditioners and a remote condenser.

A similar situation arose at one of the facilities, where remote condensers had to be installed on a frame 8 meters from the building. At this distance, with a temperature difference exceeding 100 °C, there was only one outlet and rigid fastening of the pipeline. Over time, a pipe bend appeared in one of the fasteners, and a leak appeared six months after the system was put into operation. Three systems mounted parallel to each other had the same defect and required emergency repairs with changing the route configuration, introducing compensators, re-pressure testing and refilling the circuit.

Finally, another factor that should be taken into account when calculating and designing expansion joints, especially U-shaped ones, is a significant increase in the equivalent length of the freon circuit due to the additional length of the pipeline and four bends. If the total length of the route reaches critical values ​​(and if we are talking about the need to use compensators, the length of the route is obviously quite large), then the final diagram indicating all compensators should be agreed with the manufacturer. In some cases, through joint efforts it is possible to develop the most optimal solution.

The routes of air conditioning systems should be laid hidden in furrows, channels and shafts, trays and on suspensions, while when laying hidden, access to detachable connections and fittings by installing doors and removable panels, on the surface of which there should be no sharp protrusions. Also, when laying hidden pipelines in locations dismountable connections and fittings should be provided with service hatches or removable panels.

Vertical sections should be cemented only in exceptional cases. Basically, it is advisable to place them in channels, niches, furrows, as well as behind decorative panels.

Anyway hidden gasket copper pipelines should be made in a casing (for example, in corrugated polyethylene pipes Oh). The use of corrugated PVC pipes is not permitted. Before sealing the pipeline laying areas, it is necessary to complete the as-built installation diagram for this section and conduct hydraulic tests.

Open laying of copper pipes is allowed in places that exclude them mechanical damage. Open areas can be covered with decorative elements.

It must be said that the laying of pipelines through walls without sleeves is almost never observed. However, we recall that for passage through building structures it is necessary to provide sleeves (cases), for example, made of polyethylene pipes. The inner diameter of the sleeve should be 5–10 mm larger than the outer diameter of the pipe being laid. The gap between the pipe and the case must be sealed with a soft, waterproof material that allows the pipe to move along the longitudinal axis.

When installing copper pipes, you should use a tool specially designed for this purpose - rolling, pipe bending, press.

Quite a lot useful information Information about the installation of freon pipes can be obtained from experienced installers of air conditioning systems. It is especially important to convey this information to designers, since one of the problems of the design industry is its isolation from installation. As a result, projects include solutions that are difficult to implement in practice. As they say, paper will endure anything. Easy to draw, difficult to execute.

By the way, this is why all advanced training courses at the APIK Training and Consulting Center are taught by teachers with experience in the field of construction and installation work. Even for management and design specialties, teachers from the field of implementation are invited to provide students with a comprehensive perception of the industry.

So, one of the basic rules is to ensure at the design level a height for laying freon routes that is convenient for installation. It is recommended to maintain a distance of at least 200 mm to the ceiling and to the false ceiling. When hanging pipes on studs, the most comfortable lengths of the latter are from 200 to 600 mm. Shorter length pins are difficult to work with. Longer studs are also inconvenient to install and may wobble.

When installing pipelines in a tray, do not hang the tray closer to the ceiling than 200 mm. Moreover, it is recommended to leave about 400 mm from the tray to the ceiling for comfortable soldering of pipes.

It is most convenient to lay external routes in trays. If the slope allows, then in trays with a lid. If not, the pipes are protected in a different way.

A recurring problem for many objects is the lack of markings. One of the most common comments when working in the field of copyright or technical supervision- mark cables and pipelines of the air conditioning system. For ease of operation and subsequent maintenance of the system, it is recommended to mark cables and pipes every 5 meters in length, as well as before and after building structures. The marking should use the system number and pipeline type.

When installing different pipelines above each other on the same plane (wall), it is necessary to install lower the one that is most likely to form condensate during operation. In the case of parallel laying of two gas lines one above the other various systems, the one in which the heavier gas flows should be installed below.

Conclusion

When designing and installing large facilities with multiple air conditioning systems and long routes, special attention should be paid to the organization of freon pipeline routes. This approach to developing a general pipe laying policy will help save time both at the design and installation stages. In addition, this approach allows you to avoid a lot of mistakes that you encounter in real construction: forgotten thermal expansion compensators or expansion joints that do not fit in the corridor due to adjacent engineering systems, erroneous pipe fastening schemes, incorrect calculations of the equivalent pipeline length.

As implementation experience has shown, taking these tips and recommendations into account really has a positive effect at the stage of installing air conditioning systems, significantly reducing the number of questions during installation and the number of situations when it is urgent to find a solution to a complex problem.

Yuri Khomutsky, technical editor of Climate World magazine

Today on the market there areVRF -systems of original Japanese, Korean and Chinese brands. Even moreVRF -numerous systemsOEM manufacturers. Outwardly they are all very similar and one gets the false impression that allVRF - the systems are the same. But “not all yoghurts are created equal,” as the popular advertisement said. We are starting a series of articles aimed at studying the technologies for producing cold that are used in the modern class of air conditioners -VRF -systems. We have already examined the refrigerant subcooling system and its effect on the characteristics of the air conditioner and various compressor unit layouts. In this article we will study -oil separation system .

Why is oil needed in the refrigeration circuit? For compressor lubrication. And the oil must be in the compressor. In a conventional split system, oil circulates freely along with freon and is evenly distributed throughout the entire refrigeration circuit. U VRF systems The refrigeration circuit is too large, so the first problem faced by manufacturers of VRF systems is a decrease in the oil level in the compressors and their failure due to “oil starvation.”

There are two technologies by which refrigeration oil is returned back to the compressor. First, the device is used oil separator(oil separator) in the outdoor unit (in Figure 1). Oil separators are installed on the compressor discharge pipe between the compressor and the condenser. Oil is carried away from the compressor both in the form of small drops and in a vapor state, since at temperatures from 80C to 110C partial evaporation of the oil occurs. Most of the oil settles in the separator and is returned through a separate oil line to the compressor crankcase. This device significantly improves the lubrication of the compressor and ultimately increases the reliability of the system. From the point of view of the design of the refrigeration circuit, there are systems without oil separators at all, systems with one oil separator for all compressors, systems with an oil separator for each compressor. Ideal option uniform oil distribution is when each compressor has its own oil separator (Fig. 1).

Rice. 1. Diagram of the VRF refrigeration circuit - a system with two freon oil separators.

Designs of separators (oil separators).

The oil in oil separators is separated from the gaseous refrigerant as a result of a sharp change in direction and a decrease in the speed of steam movement (up to 0.7 - 1 m/s). The direction of movement of the gaseous refrigerant is changed using partitions or pipes installed in a certain way. In this case, the oil separator catches only 40-60% of the oil carried away from the compressor. Therefore, the best results are obtained by a centrifugal or cyclonic oil separator (Fig. 2). The gaseous refrigerant entering pipe 1, hitting the guide vanes 4, acquires a rotational motion. Under the influence of centrifugal force, oil droplets are thrown onto the body and form a film that slowly flows down. When exiting the spiral, the gaseous refrigerant abruptly changes its direction and leaves the oil separator through pipe 2. The separated oil is separated from the gas stream by a partition 5 to prevent secondary capture of the oil by the refrigerant.

Rice. 2. Design of a centrifugal oil separator.

Despite the operation of the oil separator, a small part of the oil is still carried away with freon into the system and gradually accumulates there. To return it, a special mode is used, which is called oil return mode. Its essence is as follows:

The outdoor unit switches on in cooling mode at maximum performance. All EEV valves in indoor units are fully open. BUT the fans of the indoor units are turned off, so freon in the liquid phase passes through the heat exchanger of the indoor unit without boiling away. The liquid oil located in the indoor unit is washed off with liquid freon into the gas pipeline. And then it returns to the outdoor unit with gaseous freon at maximum speed.

Refrigeration oil type used in refrigeration systems to lubricate compressors depends on the type of compressor, its performance, but most importantly the freon used. Oils for the refrigeration cycle are classified as mineral and synthetic. Mineral oil is primarily used with CFC (R 12) and HCFC (R 22) refrigerants and is based on naphthene or paraffin, or a mixture of paraffin and acrylic benzene. HFC refrigerants (R 410A, R 407C) are not soluble in mineral oil, so synthetic oil is used for them.

Crankcase heater. Refrigeration oil is mixed with the refrigerant and circulates with it throughout the entire refrigeration cycle. The oil in the compressor crankcase contains some dissolved refrigerant, and the liquid refrigerant in the condenser contains a small amount of dissolved oil. The disadvantage of using soluble oil is the formation of foam. If the chiller is shut down for an extended period and the compressor oil temperature is lower than the internal circuit, the refrigerant condenses and most of it dissolves in the oil. If the compressor starts in this state, the pressure in the crankcase drops and the dissolved refrigerant evaporates along with the oil, forming oil foam. This process is called foaming, and it causes oil to escape from the compressor through the discharge pipe and deteriorate the compressor's lubrication. To prevent foaming, a heater is installed on the compressor crankcase of VRF systems so that the compressor crankcase temperature is always slightly higher than the ambient temperature (Fig. 3).

Rice. 3. Compressor crankcase heater

The influence of impurities on the operation of the refrigeration circuit.

Process oil (machine oil, assembly oil). If process oil (such as machine oil) gets into a system using HFC refrigerant, the oil will separate, forming flocs and causing clogged capillary tubes.

Water. If water gets into a cooling system using HFC refrigerant, the acidity of the oil increases and the polymer materials used in the compressor engine are destroyed. This leads to destruction and breakdown of the electric motor insulation, clogging of capillary tubes, etc.

Mechanical debris and dirt. Problems that arise: clogged filters and capillary tubes. Decomposition and separation of oil. Destruction of the compressor motor insulation.

Air. Consequence of a large amount of air entering (for example, the system was filled without evacuation): abnormal pressure, increased acidity of the oil, breakdown of the compressor insulation.

Impurities of other refrigerants. If a large amount of different types of refrigerants enters the cooling system, abnormal operating pressure and temperature will occur. The consequence is damage to the system.

Impurities of other refrigeration oils. Many refrigeration oils do not mix with each other and precipitate in the form of flakes. The flakes clog the filter and capillary tubes, reducing freon consumption in the system, which leads to overheating of the compressor.

The following situation is repeatedly encountered related to the oil return mode to the compressors of outdoor units. A VRF air conditioning system has been installed (Fig. 4). System refueling, operating parameters, pipeline configuration - everything is normal. The only caveat is that some of the indoor units are not installed, but the load factor of the outdoor unit is acceptable - 80%. However, compressors regularly fail due to jamming. What is the reason?

Rice. 4. Scheme of partial installation of indoor units.

And the reason turned out to be simple: the fact is that branches were prepared for the installation of the missing indoor units. These branches were dead-end “appendixes” into which the oil circulating along with freon entered, but could not come back out and accumulated. Therefore, compressors failed due to normal “oil starvation.” To prevent this from happening, it was necessary to install shut-off valves on the branches MAXIMUM CLOSE TO THE BRANCHES. Then the oil would circulate freely in the system and return in oil collection mode.

Oil lifting loops.

For VRF systems from Japanese manufacturers there are no requirements for installing oil lifting loops. Separators and oil return mode are considered to effectively return oil to the compressor. However, there are no rules without exceptions - on MDV series V 5 systems, it is recommended to install oil lifting loops if the outdoor unit is higher than the indoor units and the height difference is more than 20 meters (Fig. 5).

Rice. 5. Diagram of the oil lifting loop.

For freonR 410 A It is recommended to install oil lifting loops every 10 - 20 meters of vertical sections.

For freonsR 22 andR 407C oil lifting loops are recommended to be installed every 5 meters in vertical sections.

The physical meaning of the oil lifting loop comes down to the accumulation of oil before the vertical lift. Oil accumulates at the bottom of the pipe and gradually blocks the hole for freon passage. Gaseous freon increases its speed in the free section of the pipeline, while capturing liquid oil. When the cross-section of the pipe is completely covered with oil, freon pushes the oil out like a plug to the next oil lifting loop.

	Oil	HF (domestic)	Mobile	TOTAL PLANETELF	SUNISO	Bitzer
R12	Mineral	HF 12-16			Suniso 3GS, 4GS
R22	Mineral, Synthetic	HF 12-24	Mobil Gargoyle Arctic Oil 155, 300, Mobil Gargoyle Arctic SHC 400, Mobil Gargoyle Arctic SHC 200, Mobil EAL Arctic 32,46,68,100	LUNARIA SK	Suniso 3GS, 4GS	Biltzer B 5.2, Biltzer B100
R23	Synthetic		Mobil EAL Arctic 32, 46,68,100	PLANETELF ACD 68M	Suniso SL 32, 46,68,100	Biltzer BSE 32
R134a	Synthetic		Mobil Arctic Assembly Oil 32,	PLANETELF ACD 32, 46,68,100, PLANETELF PAG	Suniso SL 32, 46,68,100	Biltzer BSE 32
R404a	Synthetic		Mobil EAL Arctic 32.46, 68.100	PLANETELF ACD 32.46, 68.100	Suniso SL 32, 46,68,100	Biltzer BSE 32
R406a	Synthetic	HF 12-16	Mobil Gargoyle Arctic Oil 155,300		Suniso 3GS, 4GS
R407c	Synthetic		Mobil EAL Arctic 32.46, 68.100	PLANETELF ACD 32.46, 68.100	Suniso SL 32, 46,68,100	Biltzer BSE 32
R410a	Synthetic		Mobil EAL Arctic 32.46, 68.100	PLANETELF ACD 32.46, 68.100	Suniso SL 32, 46,68,100	Biltzer BSE 32
R507	Synthetic		Mobil EAL Arctic 22CC, 32, 46,68,100	PLANETELF ACD 32.46, 68.100	Suniso SL 32, 46,68,100	Biltzer BSE 32
R600a	Mineral	HF 12-16	Mobil Gargoyle Arctic Oil 155, 300		Suniso 3GS, 4GS

Conclusion.

Oil separators are the most important and mandatory element of a high-quality VRF air conditioning system. Only by returning freon oil back to the compressor is reliable and trouble-free operation of the VRF system achieved. The most optimal design option is when each compressor is equipped with a SEPARATE separator, because only in this case is a uniform distribution of freon oil achieved in multi-compressor systems.

Brukh Sergey Viktorovich, MEL Company LLC