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 modern class air conditioners - VRF systems.
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 top scores gives a centrifugal or cyclonic oil separator (Fig. 2). The gaseous refrigerant entering the nozzle 1, falling on the guide vanes 3, acquires rotational movement. 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 secondary capture of the 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 found in indoor unit, is washed off with liquid freon into the gas pipeline. And then returns to outdoor unit with freon gas at maximum speed.
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.
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 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 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).
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 oil, compressor insulation breakdown.
5. Impurities of other refrigerants. If a large amount of refrigerant enters the cooling system various types, an abnormality occurs operating pressure and temperature. 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 often 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.
For VRF systems from Japanese manufacturers, there are no requirements for installing oil lifting loops. The 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.
Oil separators are the most important and mandatory element 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.
Oil in freon chain
The oil in the freon system is necessary 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 not be sufficiently lubricated. Oil dissolves in liquid freon, but does not dissolve in vapor. The pipelines move:
Therefore, oil retention problems may occur on steam lines. It can be solved by maintaining a sufficient speed of steam movement in the pipelines, the required slope of the pipes, and installing oil lifting loops.
The evaporator is below.
a) Oil scraper loops should be located at intervals of every 6 meters on the rising pipelines to facilitate the return of oil 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 fluid from draining into the compressor when the machine is parked.
b) Make a collection pit on the suction line after the evaporator to collect liquid refrigerant that may accumulate during shutdown. When the compressor turns on again, the refrigerant will evaporate quickly: it is advisable to make a pit away from the sensing element of the expansion valve to avoid this phenomenon affecting the operation of the expansion valve.
c) On horizontal sections of the discharge pipeline, there is a 1% slope along the direction of freon movement to facilitate the movement of oil in the right direction.
The capacitor is below.
No special precautions need to be taken in this situation.
If the capacitor 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 lifting even at smaller 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 condenser is located in an environment with a temperature higher than the compressor temperature.
b) On horizontal sections of the discharge pipeline, a slope of 1% along the direction of movement of freon to facilitate the movement of oil in the right direction
The capacitor is higher.
a) To prevent the flow of liquid refrigerant from the pressurizer into the compressor when the refrigeration machine is stopped, install a valve in front of the pressurizer.
b) Oil lifting loops should be located at intervals of every 6 meters on the rising pipelines to facilitate the return of oil to the compressor;
c) On horizontal sections of the discharge pipeline, a slope of 1% is required to facilitate the movement of oil in the correct direction.
Operation of the oil lifting loop.
When the oil level reaches the top wall of the tube, the oil will be pushed further towards the compressor.
Calculation of freon pipelines.
Oil dissolves in liquid freon, so you can keep the speed in liquid pipelines low - 0.15-0.5 m/s, which will ensure low hydraulic resistance movement. An increase in resistance leads to a loss of cooling capacity.
Oil does not dissolve in freon vapor, so the speed in the steam lines must be maintained high so that the oil is carried by the steam. When moving, part of the oil covers the walls of the pipeline - this film is also moved by high-speed steam. The speed on the discharge side of the compressor is 10-18m/s. The speed on the suction side of the compressor is 8-15m/s.
On horizontal sections of very long pipelines, it is allowed to reduce the speed to 6 m/s.
Example:
Initial data:
Refrigerant R410a.
Required cooling capacity 50kW=50kJ/s
Boiling point 5°C, condensation temperature 40°C
Overheating 10°C, subcooling 0°C
Suction pipe solution:
1. The specific cooling capacity of the evaporator is q u=H1-H4=440-270=170kJ/kg
Saturated liquid | Saturated steam |
||||||||
Temperature, °C | Saturation pressure, 10 5 Pa | Density, kg/m³ | Specific enthalpy, kJ/kg | Specific entropy, kJ/(kg*K) | Saturation pressure, 10 5 Pa | Density, kg/m³ | Specific enthalpy, kJ/kg | Specific entropy, kJ/(kg*K) | Specific heat of vaporization, kJ/kg |
2. Freon mass flow
m=50kW/ 170kJ/kg= 0.289kg/s
3. Specific volume of freon vapor on the suction side
v sun = 1/33.67kg/m³= 0.0297m³/kg
4.Volume flow of freon vapor on the suction side
Q= v sun* m
Q=0.0297m³/kg x 0.289kg/s =0.00858m³/s
5.Inner diameter of pipeline
From standard copper freon pipelines, we select a pipe with an outer diameter of 41.27 mm (1 5/8"), or 34.92 mm (1 3/8").
Outer The diameter of the pipelines is often selected in accordance with the tables given in the “Installation Instructions”. When compiling such tables, the steam velocities required for oil transfer are taken into account.
Calculation of the volume of freon filling
A simplified calculation of the mass of refrigerant charge is made using a formula that takes into account the volume of liquid lines. This simple formula does not take steam lines into account, since the volume occupied by steam is very small:
Mzapr = P Ha. * (0.4 x V isp + TO g* V res + V f.m.), kg,
P Ha. - density of saturated liquid (freon) PR410a = 1.15 kg/dm³ (at a temperature of 5°C);
V isp - internal volume of the air cooler (air coolers), dm³;
V res - internal volume of the receiver of the refrigeration unit, dm³;
V l.m. - internal volume of liquid lines, dm³;
TO g is a coefficient taking into account the capacitor installation scheme:
TO g=0.3 for compressor-condensing units without a hydraulic condensation pressure regulator;
TO g=0.4 when using a hydraulic condensation pressure regulator (installation of the unit outdoors or version with a remote condenser).
Akaev Konstantin Evgenievich
Candidate of Technical Sciences St. Petersburg University of Food and Low-Temperature Technologies
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. VRF systems have a refrigeration circuit that is too large, so the first problem faced by manufacturers of VRF systems is a decrease in the oil level in 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. Perfect 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 the pipe 1, falling on 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 often 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. The 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 |
Suniso SL 32, 46,68,100 |
Biltzer BSE 32 |
|
|
R410a |
Synthetic |
Mobil EAL Arctic 32.46, 68.100 |
PLANETELF |
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
Loss of refrigerant pressure in the refrigeration circuit tubes reduces 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.
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%!
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%.
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:
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.
Acceptable next losses pressure in tubes:
