There are VRF systems of original Japanese, Korean and Chinese brands on the market today. More VRF systems from numerous OEMs. Outwardly, they are all very similar, and there is a false impression that all VRF systems are the same. But "not all yoghurts are created equal," as popular advertisements put it. We continue a series of articles aimed at studying cold production technologies that are used in the modern class of air conditioners - VRF systems.

Separator designs (oil separators)

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 by means of baffles or specially installed pipes. In this case, the oil separator only traps 40-60% of the oil entrained from the compressor. That's why top scores gives a centrifugal or cyclonic oil separator (Fig. 2). The gaseous refrigerant entering the branch pipe 1, falling on the guide vanes 3, acquires a rotational motion. Under the action of centrifugal force, the oil droplets are thrown onto the housing and form a slowly flowing down film. The gaseous refrigerant at the exit from the spiral changes its direction abruptly and leaves the oil separator through the branch pipe 2. The separated oil is baffled from the gas stream by a baffle 4 in order to prevent the secondary capture of oil by the refrigerant.

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

Refrigeration oil type

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

Mineral oil is mainly used with refrigerants CFC (R12) and HCFC (R22) and is based on naphthene or paraffin, or a mixture of paraffin and acrylbenzene. HFC refrigerants (R410a, R407c) do not dissolve in mineral oil, so they use synthetic oil.

Crankcase heater

Refrigeration oil mixes 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 does not a large number of 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 up in this state, the pressure in the crankcase drops and the dissolved refrigerant evaporates together with the oil, forming an oil foam. This process is called "foaming" and causes oil to escape from the compressor through the discharge pipe and degrade the lubrication of the compressor. 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).

The influence of impurities on the operation of the refrigeration circuit

1. Process oil (machine oil, assembly oil). If process oil (eg engine oil) gets into a system using HFC refrigerant, the oil will separate, flocculating and clogging the capillary tubes.
2. Water. If water gets into the 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 the destruction and breakdown of the insulation of the electric motor, clogging of the capillary tubes, etc.
3. Mechanical debris and dirt. Problems that arise: clogged filters, capillary tubes. Decomposition and separation of oil. Destruction of the compressor motor insulation.
4. Air. A consequence of the ingress of a large amount of air (for example, the system was charged 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 different types, abnormal operating pressure and temperature occur. This will damage the system.
6. Impurities of other refrigeration oils. Many refrigeration oils do not mix with each other and precipitate in the form of flakes. Flakes clog filters and capillary tubes, reducing the consumption of freon in the system, which leads to overheating of the compressor.

The following situation is repeatedly encountered related to the mode of oil return to the compressors of outdoor units. A VRF air conditioning system has been installed (Fig. 4). System refueling, operating parameters, piping configuration - everything is normal. The only caveat is that some of the indoor units have not been installed, but the load factor of the outdoor unit is permissible - 80%. However, compressors fail regularly due to seizure. 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 the freon got into, but could not come back out and accumulated there. Therefore, the compressors were out of order due to the usual "oil starvation". To prevent this from happening, it was necessary to put 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 the oil collection mode.

Oil lifting hinges

For VRF systems of Japanese manufacturers, there are no requirements for the installation of oil lifting loops. It is believed that separators and an oil return mode effectively return oil to the compressor. However, there are no rules without exceptions - on V5 series MDV systems, it is recommended to install oil lifting loops if the outdoor unit is higher than the internal ones and the height difference is more than 20 m (Fig. 5).

The physical meaning of the oil lifting loop is reduced to the accumulation of oil before vertical lifting. Oil accumulates in the lower part of the pipe and gradually blocks the opening for the passage of freon. 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 blocked by oil, freon pushes this oil out like a plug to the next oil lifting loop.

Output

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

There are currently on the marketVRF -systems of original Japanese, Korean and Chinese brands. Much moreVRF -systems of numerousOEM manufacturers. Outwardly, they are all very similar and there is a false impression that allVRF -systems are the same. But "not all yoghurts are created equal," as popular advertisements put it. We are starting a series of articles aimed at studying cold production technologies that are used in the modern class of air conditioners -VRF -systems. We have already examined the refrigerant subcooling system and its influence on the characteristics of the air conditioner, various layouts of the compressor unit. In this article, we will explore -oil separation system .

Why do you need oil in the refrigeration circuit? For compressor lubrication. And the oil should be in the compressor. In a conventional split system, oil circulates freely together with freon and is evenly distributed throughout the entire refrigeration circuit. VRF systems have a too large refrigeration circuit, so the first problem faced by VRF system manufacturers is a decrease in the oil level in the compressors and their failure due to "oil starvation".

There are two techniques by which the refrigerant oil is returned back to the compressor. First - the device is applied oil separator(oil separator) in the outdoor unit (in picture 1). Oil separators are installed on the compressor discharge pipe between the compressor and the condenser. The oil is carried away from the compressor both in the form of small drops and in the vapor state, since at temperatures from 80C to 110C, partial evaporation of oil occurs. Most of the oil settles in the separator and returns through a separate oil line to the compressor crankcase. This device significantly improves the lubrication regime of the compressor and ultimately increases the reliability of the system. In terms 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. Perfect option even distribution of oil is when each compressor has its "own" oil separator (Fig. 1).

Rice. 1 . Refrigeration circuit diagram VRF - systems with two freon oil separators.

Separator designs (oil separators).

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 by means of baffles or in a certain way installed pipes. In this case, the oil separator only traps 40-60% of the oil entrained from the compressor. Therefore, the best results are obtained by a centrifugal or cyclonic oil separator (Fig. 2). The gaseous refrigerant entering the branch pipe 1, falling on the guide vanes 4, acquires a rotational motion. Under the action of centrifugal force, oil droplets are thrown onto the housing and form a slowly flowing down film. The gaseous refrigerant at the exit from the spiral changes its direction abruptly and leaves the oil separator through branch pipe 2. The separated oil is fenced off from the gas stream by a baffle 5 to prevent the secondary capture of oil by the refrigerant.

Rice. 2. The design of the centrifugal oil separator.

Despite the operation of the oil separator, a small part of the oil is nevertheless 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 is turned on in cooling mode for maximum performance. All EEV valves in indoor units are fully open. BUT the fans of the indoor units are turned off, so the freon in the liquid phase passes through the heat exchanger of the indoor unit without boiling away. The liquid oil in the indoor unit is flushed 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 for lubricating compressors depends on the type of compressor, its performance, but most importantly the freon used. Refrigeration cycle oils are classified as mineral and synthetic. Mineral oil is mainly used with refrigerants CFC (R 12) and HCFC (R 22) and is based on naphthene or paraffin, or a mixture of paraffin and acrylbenzene. HFC refrigerants (R 410A, R 407C) do not dissolve in mineral oil, therefore synthetic oil is used for them.

Crankcase heater... Refrigeration oil mixes 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 foam formation. 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 up in this state, the pressure in the crankcase drops and the dissolved refrigerant evaporates together with the oil, forming an oily foam. This process is called foaming and 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 crankcase of the VRF-systems compressor so that the temperature of the compressor crankcase 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, flocculating and clogging the capillary tubes.

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

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

Air. A consequence of the ingress of a large amount of air (for example, the system was charged without evacuation): abnormal pressure, increased acidity of the oil, breakdown of the compressor insulation.

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

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

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

Rice. 4. Diagram 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 together with freon got into, but could not get back out and accumulated. Therefore, the compressors were out of order due to the usual "oil starvation". To prevent this from happening, it was necessary to install shut-off valves on the branches as CLOSE to the BRANCHES as possible. Then the oil would circulate freely in the system and return in the oil collection mode.

Oil lifting hinges.

For VRF systems of Japanese manufacturers, there are no requirements for the installation of oil lifting loops. It is believed that separators and an oil return mode effectively return oil to the compressor. However, there are no rules without exceptions - on V 5 series MDV systems, it is recommended to install oil lifting hinges if the outdoor unit is higher than the internal ones and the height difference is more than 20 meters (Fig. 5).

Rice. 5. Diagram of the oil lifting loop.

For freonR 410 A oil lifting loops are recommended to be installed every 10 - 20 meters of vertical sections.

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

The physical meaning of the oil lifting loop is reduced to the accumulation of oil before vertical lifting. Oil accumulates in the lower part of the pipe and gradually blocks the opening for the passage of freon. Gaseous freon increases its speed in the free section of the pipeline, while capturing liquid oil. With a complete overlap of the pipe cross-section with oil, freon pushes the oil out like a plug to the next oil lifting loop.

Output.

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

Brukh Sergey Viktorovich, MEL Company LLC

Freon chain oil

The oil in the freon system is needed to lubricate the compressor. It constantly leaves the compressor - circulates in the freon circuit along with freon. If, for any reason, the oil does not return to the compressor, the CM will be insufficiently lubricated. The oil dissolves in liquid freon, but does not dissolve in vapor. Moving along the pipelines:

• after the compressor - superheated steam of freon + oil mist;
• after the evaporator - superheated freon vapor + oil film on the walls and oil in drip form;
• after the condenser - liquid freon with oil dissolved in it.

Therefore, there may be a problem with oil retention on steam lines. It can be solved by observing a sufficient speed of steam movement in pipelines, the required pipe slope, and installing oil-lifting loops.

The evaporator is below.

a) Oil scraper loops should be located at intervals of every 6 meters on risers to facilitate oil return to the compressor;

b) Make a collecting pit on the suction line after the expansion valve;

The evaporator is higher.

a) At the outlet of the evaporator, install a water seal above the evaporator to prevent the liquid from draining into the compressor when the machine is parked.

b) Make a collecting pit on the suction line after the evaporator to collect liquid refrigerant that may accumulate during parking. When the compressor is switched on again, the refrigerant will quickly evaporate: it is advisable to make a sump far from the thermostatic expansion valve sensor in order to avoid this phenomenon affecting the operation of the expansion valve.

c) On the horizontal sections of the discharge pipeline, a slope of 1% in the direction of the freon movement to facilitate the movement of the oil in the right direction.

The capacitor is below.

No special precautions need to be taken in this situation.

If the condenser is lower than the KIB, then the lifting height should not exceed 5 meters. However, if the CIB and the system as a whole are not best quality, then liquid freon may have difficulty in lifting and at lower elevation differences.

a) It is advisable to install a shut-off valve on the condenser inlet to prevent liquid freon from flowing into the compressor after shutdown refrigeration machine... This can happen if the capacitor is located in environment with a temperature higher than the compressor temperature.

b) On the horizontal sections of the discharge pipeline, a slope of 1% in the direction of the freon movement to facilitate the movement of oil in the right direction

The capacitor is higher.

a) To exclude the overflow of liquid freon from the CD to the CM when the refrigeration machine stops, install a valve in front of the CD.

b) Oil lifting loops should be located at intervals of every 6 meters on risers to facilitate oil return to the compressor;

c) On horizontal sections of the discharge line, a 1% slope to facilitate oil flow in the correct direction.

Oil lifting loop operation.

When the oil level reaches the top of the tube, the oil will push further towards the compressor.

Calculation of freon pipelines.

The oil dissolves in liquid freon, so it is possible to maintain a small speed in liquid pipelines - 0.15-0.5 m / s, which will provide a low hydraulic resistance to movement. An increase in resistance results in a loss of cooling capacity.

The oil does not dissolve in the vaporous freon, therefore it is necessary to maintain the speed in the steam lines high in order for the oil to be carried by the steam. When moving, part of the oil covers the walls of the pipeline - this film is also transported by high-speed steam. Compressor discharge speed 10-18m / s. Compressor suction speed 8-15m / s.

On horizontal sections of very long pipelines, it is allowed to reduce the speed to 6m / s.

Example:

Initial data:

Refrigerant R410a.
Required cooling capacity 50kW = 50kJ / s
Evaporating temperature 5 ° С, condensation temperature 40 ° С
Overheating 10 ° С, supercooling 0 ° С

Suction piping solution:

1. The specific refrigerating capacity of the evaporator is q u = Н1-Н4 = 440-270 = 170kJ / kg

2. Mass flow rate of freon

m= 50kW / 170kJ / kg = 0.289kg / s

3. Specific volume of vaporous freon on the suction side

v sun = 1 / 33.67kg / m³ = 0.0297m³ / kg

4.Volume flow rate of vaporous freon on the suction side

Q= v sun * m

Q= 0.0297m³ / kg x 0.289kg / s = 0.00858m³ / s

5.Inner diameter of the pipeline

From the standard copper freon piping, select a pipe with an outer diameter of 41.27mm (1 5/8 "), or 34.92mm (1 3/8").

Outer pipe diameters are often selected according to the tables in the Installation Instructions. When compiling such tables, the steam velocities necessary for transferring oil were taken into account.

Calculation of the volume of refueling freon

Simplified calculation of the mass of the refrigerant charge is made according to a formula that takes into account the volume of liquid lines. This simple formula does not take into account the steam lines, since the volume occupied by the steam is very small:

Mzapr = P Ha. * (0.4 x V isp + TO g * V res + V w.m.), kg,

P Ha. - density of the saturated liquid (freon) РR410a = 1.15 kg / dm³ (at a temperature of 5 ° С);

V isp - the internal volume of the air cooler (air coolers), dm³;

V res is the internal volume of the receiver of the refrigeration unit, dm³;

V l.m. - internal volume of liquid lines, dm³;

TO g - coefficient taking into account the capacitor mounting scheme:

TO g = 0.3 for condensing units without a hydraulic condensing pressure regulator;
TO g = 0.4 when using a hydraulic condensing pressure regulator (outdoor installation or version with remote condenser).

Akaev Konstantin Evgenievich
Candidate of Technical Sciences SPb University of Food and Low Temperature Technologies