There are original Japanese, Korean and Chinese VRF systems 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 yogurts are created equal," as the popular ad said. We continue a series of articles aimed at studying the technologies for obtaining cold, which are used in modern classroom air conditioners - VRF-systems.
Designs of separators (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 with the help of baffles or nozzles installed in a certain way. In this case, the oil separator captures only 40-60% of the oil carried away from the compressor. That's why top scores gives a centrifugal or cyclone oil separator (Fig. 2). The gaseous refrigerant entering the nozzle 1, falling on the guide vanes 3, acquires a rotational motion. Under the action of centrifugal force, oil drops are thrown onto the body and form a film slowly flowing down. The gaseous refrigerant, when leaving the coil, abruptly changes its direction and leaves the oil separator through pipe 2. The separated oil is separated from the gas jet by partition 4 in order to prevent the 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 turns on in cooling mode at maximum capacity. 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 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.
Refrigeration oil type
The type of refrigerant oil used in refrigeration systems to lubricate 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 CFC (R12) and HCFC (R22) refrigerants 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 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, while the liquid refrigerant in the condenser contains no 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 oil temperature in the compressor is lower than in the internal circuit, the refrigerant will condense and most of it will dissolve 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 causes oil to escape from the compressor through the discharge pipe and deteriorate compressor lubrication. 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 ambient temperature (Fig. 3).
Influence of impurities on the operation of the refrigeration circuit
1. Process oil (machine, assembly oil). If process oil (such as machine oil) enters a system using HFC refrigerant, the oil will separate, forming flakes and causing clogging of the capillary tubes.
2. Water. If water enters the cooling system using HFC refrigerant, the acidity of the oil increases, destruction occurs polymer materials used in the compressor motor. This leads to destruction and breakdown of the motor insulation, clogging of capillary tubes, etc.
3. Mechanical debris and dirt. Emerging problems: clogging of filters, capillary tubes. Decomposition and separation of oil. Compressor motor insulation failure.
4. Air. A consequence of the ingress of a large amount of air (for example, the system was charged without evacuation): abnormal pressure, hyperacidity oil, compressor insulation breakdown.
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 the filters and capillary tubes, reducing the flow of freon in the system, which leads to overheating of the compressor.
The following situation occurs repeatedly, related to the mode of oil return to the compressors of outdoor units. A VRF air conditioning system has been installed (Fig. 4). Refueling of the system, operating parameters, pipeline 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 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 "appendices", into which the oil circulating along with freon got into, but could not go back and accumulated there. Therefore, the compressors failed due to the usual "oil starvation". To prevent this from happening, it was necessary to install shut-off valves on the branches as close as possible to the splitters. The oil would then circulate freely in the system and return in oil recovery mode.
Oil lifting loops
There are no requirements for the installation of oil lifting loops for Japanese VRF systems. It is believed that the separators and the oil return mode effectively return the 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 indoor unit 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 the vertical lift. 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 pipe section is completely covered with oil, freon pushes this oil out like a plug to the next oil lifting loop.
Output
Oil separators are an essential and indispensable element of a quality VRF air conditioning system. Only thanks to the return of freon oil back to the compressor, reliable and trouble-free operation of the VRF system is achieved. Most best option design - when each compressor is equipped with a separate separator, since only in this case is achieved a uniform distribution of freon oil in multi-compressor systems.
Loss of refrigerant pressure in the refrigeration circuit pipes reduces efficiency refrigeration machine, reducing its cold and heat output. Therefore, it is necessary to strive to reduce pressure losses in the tubes.
Since the evaporating and condensing temperatures depend on pressure (almost linearly), pressure losses are often measured by condensing or evaporating temperature losses in °C.
- Example: for refrigerant R-22 at an evaporating 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 pressure in the suction line, the compressor operates at a lower inlet pressure than the evaporation pressure in the evaporator of the chiller. Because of this, the flow of refrigerant passing through the compressor is reduced, and the cooling capacity of the air conditioner is reduced. Suction line pressure loss is most critical to chiller operation. With losses equivalent to 1°C, performance is reduced by as much as 4.5%!
Discharge line losses
With a loss of pressure in the discharge line, the compressor has to work with more high pressure than the condensing pressure. At the same time, the performance of the compressor is also reduced. With losses in the discharge line equivalent to 1°C, the performance is reduced by 1.5%.
Liquid Line Loss
Pressure losses in the liquid line have little effect on the cooling capacity of the air conditioner. But they cause the danger of boiling of the refrigerant. This happens for the following reasons:
- due to pressure reduction in the tube, the refrigerant temperature may be higher than the condensing temperature at that pressure.
- the refrigerant heats up due to friction against the walls of the pipes, since the mechanical energy of its movement is converted into heat.
As a result, the refrigerant may begin to boil not in the evaporator, but in the tubes before the regulator. The regulator cannot work stably on a mixture of liquid and vapor refrigerant, since the refrigerant flow 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 pipes are allowed:
- in the discharge and suction lines - up to 1°С
- in the liquid line - 0.5 - 1°С
The online store Potok Kholoda offers to buy oil lifting loops with a quality guarantee from a reputable manufacturer and prompt courier delivery
Oil lifting loops are almost always necessary during installation and installation:
- domestic and semi-industrial air conditioners;
- window, wall, floor-ceiling, channel, cassette split-systems.
We sell original oil lifting eyes directly from the manufacturer without intermediary markup.
In our online store there is an opportunity to buy everything at once: not only various oil lifting loops, but also other components. We have big choice 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.
Oil lifting loop - price and quality from Potok Kholod
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 installation.
Oil lifting eyes are needed when it comes to 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 in top point- 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 number listed on the site and our managers will help you make the right choice.
Oil in the freon circuit
The oil in the freon system is necessary to lubricate the compressor. It constantly leaves the compressor - it 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. Moves through pipelines:
- after the compressor - superheated freon vapor + 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, oil retention problems can occur on steam lines. It can be solved by observing a sufficient speed of steam movement in pipelines, required slope pipes, installation of oil lifting loops.
Evaporator below.
a) Oil scrapers should be spaced every 6 meters on the ascending pipelines to facilitate the return of oil to the compressor;
b) Make a collecting pit on the suction line after the expansion valve;
Evaporator above.
a) At the outlet of the evaporator, install a water seal above the evaporator to prevent liquid from draining into the compressor when the machine is stopped.
b) Make a collection pit in the suction line downstream of the evaporator to collect any liquid refrigerant that may accumulate during parking. When the compressor is turned on again, the refrigerant will quickly evaporate: it is advisable to make a sump away from the sensing element of the expansion valve, in order to avoid this phenomenon affecting the operation of the expansion valve.
c) On horizontal sections of the discharge pipeline, a slope of 1% in the direction of freon movement to facilitate the movement of oil in the right direction.
Capacitor below.
No special precautions need to be taken in this situation.
If the condenser is lower than the CIB, 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 experience difficulty in lifting even at lower elevations.
a) It is desirable to install shut-off valve on the inlet pipe of the condenser to prevent the flow of liquid freon into the compressor after the refrigeration machine is turned off. This can happen if the capacitor is located in environment with a temperature higher than the compressor temperature.
b) On horizontal sections of the discharge pipeline, a slope of 1% in the direction of freon movement to facilitate the movement of oil in the right direction
capacitor above.
a) To exclude the overflow of liquid freon from the HP into the CM, when the refrigeration machine is stopped, install a valve in front of the HP.
b) Oil lifting loops should be placed at intervals of every 6 meters on the ascending pipelines to facilitate the return of oil to the compressor;
c) On horizontal sections of the discharge pipeline, a slope of 1% to facilitate the movement of oil in the correct direction.
Oil lifting loop operation.
When the oil level reaches the top wall 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 velocity in liquid pipelines - 0.15-0.5 m / s, which will provide low hydraulic resistance to movement. An increase in resistance leads to a loss in cooling capacity.
The oil does not dissolve in the vaporized freon, so it is necessary to maintain a significant speed in the steam pipelines 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
Evaporating temperature 5°C, condensing temperature 40°C
Superheat 10°C, Subcooling 0°C
Suction line solution:
1. The specific cooling capacity of the evaporator is q u=H1-H4=440-270=170kJ/kg
saturated liquid | Saturated steam |
||||||||
Temperature, ° С | 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 consumption
m\u003d 50kW / 170kJ / kg \u003d 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 rate of freon vapor on the suction side
Q= v Sun * m
Q\u003d 0.0297 m³ / kg x 0.289 kg / s \u003d 0.00858 m³ / s
5. Pipe inner diameter
From standard copper freon pipelines, we 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 in accordance with the tables given in the "Installation Instructions". When compiling such tables, the steam speeds necessary for the transfer of oil are taken into account.
Calculation of the volume of refueling freon
Simplified, the calculation of the mass of 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 steam lines, since the volume occupied by steam is very small:
Mzapr = P Ha. * (0.4 x V Spanish + TO g* V res + V l.m.), kg,
P Ha. - density of saturated liquid (freon) РR410a = 1.15 kg/dm³ (at 5°С);
V isp - internal volume of the air cooler (air coolers), dm³;
V res - the internal volume of the receiver of the refrigeration unit, dm³;
V l.m. - internal volume of liquid lines, dm³;
TO g is the coefficient taking into account the capacitor mounting scheme:
TO g=0.3 for condensing units without hydraulic condensing pressure regulator;
TO g=0.4 when using a hydraulic condensing 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
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