Recall that VRF systems (Variable Refrigerant Flow - systems with variable flow refrigerant), are today the most dynamically developing class of air conditioning systems. Global sales growth for VRF class systems is increasing by 20-25% annually, displacing competing air conditioning options from the market. What is the cause of this growth?

First, thanks to the extensive capabilities of Variable Refrigerant Flow systems: big choice outdoor units - from mini-VRF to large combinatorial systems. Huge selection of indoor units. The length of the pipelines is up to 1000 m (Fig. 1).

Secondly, due to the high energy efficiency of the systems. Inverter compressor drive, absence of intermediate heat exchangers (in contrast to water systems), individual refrigerant consumption - all this ensures minimum energy consumption.

Thirdly, the modularity of the design plays a positive role. The required system performance is recruited from separate modules, which is undoubtedly very convenient and increases overall reliability as a whole.

That is why today VRF systems occupy at least 40% of the world market for central air conditioning systems and this share is growing every year.

Refrigerant subcooling system

What is the maximum length of freon piping that a split air conditioning system can have? For household systems with a capacity of up to 7 kW of cold, it is 30 m.For semi-industrial equipment, this figure can reach 75 m (inverter outdoor unit). For split systems, this value is maximum, but for VRF class systems, the maximum pipeline length (equivalent) can be much greater - up to 190 m (total - up to 1000 m).

Obviously, VRF systems are fundamentally different from split systems in terms of the freon circuit, and this allows them to work with long pipelines. This difference lies in the presence of a special device in the outdoor unit called a refrigerant subcooler or subcooler (Fig. 2).

Before considering the peculiarities of VRF systems operation, let's pay attention to the freon circuit diagram of split systems and understand what happens to the refrigerant with long freon pipelines.

Refrigeration cycle of split systems

In fig. 3 shows the classic freon cycle in the air conditioner circuit in the "pressure-enthalpy" axes. Moreover, this is a cycle for any split systems on R410a freon, that is, the type of this diagram does not depend on the performance of the air conditioner or brand.

Let's start from point D, with the initial parameters in which (temperature 75 ° C, pressure 27.2 bar) freon enters the condenser of the outdoor unit. Freon at the moment is a superheated gas, which first cools down to saturation temperature (about 45 ° C), then begins to condense and at point A completely passes from the state of a gas to a liquid. Further, the liquid is supercooled to point A (temperature 40 ° C). It is believed that the optimal amount of subcooling is 5 ° C.

After the heat exchanger of the outdoor unit, the refrigerant enters the throttling device in the outdoor unit - a thermostatic valve or a capillary tube, and its parameters change to point B (temperature 5 ° C, pressure 9.3 bar). Note that point B is located in the zone of a mixture of liquid and gas (Fig. 3). Therefore, after throttling, it is the mixture of liquid and gas that enters the liquid pipeline. The greater the amount of freon supercooling in the condenser, the more the proportion of liquid freon enters the indoor unit, the higher the efficiency of the air conditioner.

In fig. 3, the following processes are indicated: В-С - the process of boiling freon in the indoor unit with a constant temperature of about 5 ° C; С-С - overheating of freon up to +10 ° C; С -L - the process of refrigerant suction into the compressor (pressure losses occur in gas pipeline and elements of the freon circuit from the heat exchanger of the indoor unit to the compressor); L-M - the process of compressing gaseous freon in a compressor with increasing pressure and temperature; М-D - the process of pumping gaseous refrigerant from the compressor to the condenser.

The pressure loss in the system depends on the freon speed V and the hydraulic characteristics of the network:

What will happen to the air conditioner when the hydraulic performance of the network increases (due to the increased length or a large number local resistance)? Increased pressure losses in the gas line will lead to a drop in pressure at the compressor inlet. The compressor will start capturing refrigerant with a lower pressure and therefore a lower density. The refrigerant consumption will drop. At the outlet, the compressor will deliver less pressure and, accordingly, the condensing temperature will drop. A lower condensing temperature will result in a lower evaporation temperature and freezing of the gas line.

If increased pressure losses occur in the liquid pipeline, then the process is even more interesting: since we found out that in the liquid pipeline, freon is in a saturated state, or rather, in the form of a mixture of liquid and gas bubbles, then any pressure loss will lead to a small boiling of the refrigerant and an increase in the proportion of gas.

The latter will entail a sharp increase in volume steam-gas mixture and increasing the speed of movement in the liquid line. The increased speed of movement will again cause an additional loss of pressure, the process will become "an avalanche".

In fig. 4 shows a conventional graph of specific pressure losses depending on the speed of movement of the refrigerant in the pipeline.

If, for example, the pressure loss with a pipe length of 15 m is 400 Pa, then with an increase in the length of the pipelines twice (up to 30 m), the losses increase not twice (up to 800 Pa), but seven times - up to 2800 Pa.

Therefore, simply doubling the length of pipelines in relation to standard lengths for a split system with an On-Off compressor is fatal. The refrigerant consumption will drop several times, the compressor will overheat and will fail very soon.

Refrigeration cycle of VRF systems with freon subcooler

In fig. 5 schematically depicts the principle of operation of the refrigerant subcooler. In fig. 6 shows the same refrigeration cycle on the pressure-enthalpy diagram. Let's take a closer look at what happens to the refrigerant when the Variable Refrigerant Flow system is operating.

1-2: The liquid refrigerant after the condenser at point 1 is divided into two streams. Most of it passes through a counter-flow heat exchanger. It cools the main part of the refrigerant to + 15 ... + 25 ° C (depending on its efficiency), which then enters the liquid pipeline (point 2).

1-5: The second part of the liquid refrigerant stream from point 1 passes through the expansion valve, its temperature drops to +5 ° C (point 5), and enters the same counter-flow heat exchanger. In the latter, it boils and cools the main part of the refrigerant. After boiling, gaseous freon immediately enters the compressor suction (point 7).

2-3: At the outlet of the outdoor unit (point 2), the liquid refrigerant flows through the piping to indoor units... In this case, heat exchange with environment practically does not occur, but part of the pressure is lost (point 3). For some manufacturers, throttling is done partially in the outdoor unit of the VRF system, so the pressure at point 2 is less than in our graph.

3-4: Refrigerant pressure loss in an electronic expansion valve (EEV) located in front of each indoor unit.

4-6: Evaporation of refrigerant in the indoor unit.

6-7: The pressure loss of the refrigerant when it is returned to the outdoor unit through the gas pipeline.

7-8: Compression of gaseous refrigerant in a compressor.

8-1: Cooling of the refrigerant in the outdoor unit heat exchanger and its condensation.

Let's take a closer look at the section from point 1 to point 5. In VRF systems without refrigerant subcooler, the process from point 1 immediately goes to point 5 (along the blue line in Fig. 6). The specific capacity of the refrigerant (supplied to the indoor units) is proportional to the length of the line 5-6. In systems where a subcooler is present, the useful refrigerant capacity is proportional to line 4-6. Comparing the lengths of the lines 5-6 and 4-6, the operation of the freon subcooler becomes clear. The cooling efficiency of the circulating refrigerant is increased by at least 25%. But this does not mean that the performance of the entire system has increased by 25%. The fact is that part of the refrigerant did not flow to the indoor units, but immediately went to the compressor suction (line 1-5-6).

This is precisely the balance: by what amount the productivity of freon supplied to the internal blocks has increased, and the performance of the system as a whole has decreased by the same amount.

So what is the point of using a refrigerant subcooler if it does not increase the overall performance of the VRF system? To answer this question, let's go back to Fig. 1. The point of using a subcooler is to reduce losses on long routes of Variable Refrigerant Flow systems.

The fact is that all the characteristics of VRF systems are given for a standard pipe length of 7.5 m.That is, compare VRF systems different manufacturers according to the catalog it is not entirely correct, since the actual lengths of the pipelines will be much longer - as a rule, from 40 to 150 m. The more the length of the pipeline differs from the standard one, the greater the pressure loss in the system, the more the refrigerant boils up in liquid pipelines. The capacity losses of the outdoor unit along the length are shown on special charts in the service manuals (Fig. 7). It is on these graphs that it is necessary to compare the efficiency of the systems in the presence of a refrigerant subcooler and in its absence. The performance loss of VRF systems without a subcooler on long runs is up to 30%.

conclusions

1. The refrigerant subcooler is essential element for VRF systems operation. Its functions are, firstly, to increase the energy capacity of the refrigerant supplied to the indoor units, and secondly, to reduce the pressure loss in the system on long routes.

2. Not all VRF system manufacturers supply their systems with a refrigerant subcooler. Particularly often, OEM brands are excluded from the subcooler to reduce the cost of construction.

19.10.2015

The degree of subcooling of the liquid obtained at the outlet of the condenser is important indicator, which characterizes the stable operation of the refrigeration circuit. Subcooling is the temperature difference between liquid and condensation at a given pressure.

At normal atmospheric pressure, water condensation has a temperature of 100 degrees Celsius. According to the laws of physics, water that is 20 degrees is considered 80 degrees Celsius supercooled.

Subcooling at the outlet of the heat exchanger varies as the difference between the temperature of the liquid and the condensation. Based on figure 2.5, hypothermia will be 6 K or 38-32.

In air-cooled condensers, the subcooling indicator should be from 4 to 7 K. If it has a different value, then this indicates unstable operation.

Interaction of condenser and fan: air temperature difference.

The air blown by the fan has an indicator of 25 degrees Celsius (Figure 2.3). It takes heat from freon, due to which its temperature changes up to 31 degrees.

Figure 2.4 shows a more detailed change:

Tae is the temperature mark of the air supplied to the condenser;

Tas - air with new condenser temperature after cooling;

Tk - condensation temperature reading from the pressure gauge;

Δθ is the difference in temperature indicators.

The calculation of the temperature difference in an air-cooled condenser is carried out according to the formula:

Δθ = (tas - tae), where K has a range of 5–10 K. On the graph, this value is 6 K.

The difference in temperature difference at point D, that is, at the exit from the capacitor, in this case is 7 K, since it is in the same limit. The temperature head is 10-20 K, in the figure it is (tk-tae). Most often the value this indicator stops at 15 K, but in this example, 13 K.

Undercharging and recharging the system with refrigerant

As statistics show, the main reason for the abnormal operation of air conditioners and the failure of compressors is improper charging of the refrigerant circuit with refrigerant. Lack of refrigerant in the circuit may be due to accidental leaks. At the same time, excess refueling, as a rule, is the result of erroneous actions of personnel caused by their insufficient qualifications. For systems that use a thermostatic expansion valve (TRV) as a throttling device, subcooling is the best indicator to indicate a normal refrigerant charge. A slight hypothermia indicates that the charge is insufficient, a strong one indicates an excess of refrigerant. Charging can be considered normal when the liquid subcooling temperature at the outlet of the condenser is maintained within 10-12 degrees Celsius with an air temperature at the inlet to the evaporator close to the nominal operating conditions.

Subcooling temperature Tp is determined as the difference:
Tp = Tk - Tf
Тк is the condensation temperature read from the HP pressure gauge.
Tf is the temperature of the freon (pipe) at the outlet of the condenser.

1. Lack of refrigerant. Symptoms

The lack of freon will be felt in every element of the circuit, but this disadvantage is especially felt in the evaporator, condenser and liquid line. As a result of an insufficient amount of liquid, the evaporator is poorly filled with freon and the cooling capacity is low. Since there is not enough liquid in the evaporator, the amount of steam produced there drops dramatically. Since the volumetric capacity of the compressor exceeds the amount of steam coming from the evaporator, the pressure in it drops abnormally. A drop in evaporation pressure leads to a decrease in the evaporation temperature. The evaporation temperature can drop to a minus mark, as a result of which the inlet tube and the evaporator will freeze, and the overheating of the steam will be very significant.

Overheating temperature T overheating is determined as the difference:
T overheating = T f.i. - T suction.
T f.i. - temperature of freon (pipe) at the outlet of the evaporator.
T suction. - suction temperature read from the LP pressure gauge.
Normal overheating is 4-7 degrees Celsius.

With a significant lack of freon, overheating can reach 12-14 о С and, accordingly, the temperature at the compressor inlet will also increase. And since the cooling of the electric motors of hermetic compressors is carried out with the help of suction vapors, in this case the compressor will overheat abnormally and may fail. Due to an increase in the temperature of the vapor in the suction line, the temperature of the vapor in the discharge line will also be increased. Since there will be a shortage of refrigerant in the circuit, it will also not be enough in the subcooling zone.

Thus, the main signs of a lack of freon:
• Low cooling capacity
• Low evaporation pressure
• High superheat
• Insufficient hypothermia (less than 10 degrees Celsius)

It should be noted that in installations with capillary tubes as a throttling device, subcooling cannot be considered as a determining indicator for assessing the correct amount of refrigerant charge.

2. Excessive refueling. Symptoms

In systems with expansion valves as a throttling device, liquid cannot enter the evaporator, so the excess refrigerant is in the condenser. Abnormally high level liquid in the condenser reduces the heat exchange surface, cooling of the gas entering the condenser deteriorates, which leads to an increase in temperature saturated vapors and an increase in condensation pressure. On the other hand, the liquid at the bottom of the condenser remains in contact with the outside air for much longer, and this leads to an increase in the hypothermia zone. Since the condensing pressure is increased and the liquid leaving the condenser is perfectly cooled, the subcooling measured at the condenser outlet will be high. Because of high blood pressure condensation, there is a decrease in mass flow through the compressor and a drop in refrigeration capacity. As a result, the evaporation pressure will also rise. Due to the fact that overcharging leads to a decrease in the mass flow of vapors, cooling electric motor compressor will deteriorate. Moreover, due to the increased condensing pressure, the electric motor current of the compressor rises. Deterioration of cooling and an increase in current consumption leads to overheating of the electric motor and, ultimately, to the failure of the compressor.

Bottom line. The main signs of refrigerant recharging are:
• Cooling capacity dropped
• Evaporation pressure increased
• Condensing pressure increased
• Increased hypothermia (more than 7 o C)

In systems with capillary tubes as a throttling device, excess refrigerant can enter the compressor, causing water hammer and eventually compressor failure.

Work options refrigeration unit: work with normal overheating; with insufficient overheating; strong overheating.

Work with normal overheating.

Refrigeration plant diagram

For example, the refrigerant is supplied at a pressure of 18 bar, the suction pressure is 3 bar. The temperature at which the refrigerant boils in the evaporator is t 0 = −10 ° С, at the outlet from the evaporator the temperature of the pipe with the refrigerant is t t = −3 ° С.

Useful superheat ∆t = t t - t 0 = −3− (−10) = 7. This is normal operation of the refrigeration unit with air heat exchanger... IN evaporator freon boils away completely in about 1/10 of the evaporator (closer to the end of the evaporator), turning into gas. Then the gas will be heated by the room temperature.

Insufficient overheating.

The outlet temperature will be, for example, not −3, but −6 ° С. Then the overheating is only 4 ° C. The point where the liquid refrigerant stops boiling moves closer to the outlet of the evaporator. Thus, most of the evaporator is filled with liquid refrigerant. This can happen if the thermostatic expansion valve (TRV) supplies more freon to the evaporator.

The more freon is in the evaporator, the more vapors will be formed, the higher the suction pressure will be and the boiling point of freon will increase (let's say not −10, but −5 ° С). The compressor will start filling with liquid freon, because the pressure has increased, the refrigerant flow has increased and the compressor does not have time to pump out all the vapors (if the compressor does not have additional capacities). This operation will increase the cooling capacity, but the compressor may be damaged.

Severe overheating.

If the performance of the expansion valve is less, then less freon will enter the evaporator and it will boil off earlier (the boiling point will move closer to the evaporator inlet). The entire expansion valve and tubes after it will be frozen over and covered with ice, and 70 percent of the evaporator will not freeze up at all. Freon vapors in the evaporator will heat up, and their temperature can reach the room temperature, hence ∆t ˃ 7. At the same time, the cooling capacity of the system will decrease, the suction pressure will decrease, heated freon vapors can damage the compressor stator.

Consider the circuit in Fig. 2.1, sectional view of an air-cooled condenser in normal operation. Let us assume that the R22 refrigerant is supplied to the condenser.

Point A. R22 vapors, overheated to a temperature of about 70 ° C, leave the compressor discharge pipe and enter the condenser at a pressure of about 14 bar.

Line A-B. The superheat of the vapors is reduced at constant pressure.

Point B. The first drops of R22 liquid appear. The temperature is 38 ° C, the pressure is still about 14 bar.

Line В-С. Gas molecules continue to condense. More and more liquid appears, less and less vapor remains.
The pressure and temperature remain constant (14 bar and 38 ° C) according to the pressure-temperature relationship for R22.

Point C. The last gas molecules condense at a temperature of 38 ° C, except for the liquid in the circuit there is nothing. Temperature and pressure remain constant at about 38 ° C and 14 bar, respectively.

Line C-D... All the refrigerant has condensed, the liquid continues to cool under the action of the air cooling the condenser with a fan.

Point D. R22 at the outlet of the condenser only in the liquid phase. The pressure is still about 14 bar, but the temperature of the liquid has dropped to about 32 ° C.

For the behavior of blend refrigerants such as hydrochlorofluorocarbons (HCFCs) with a large temperature glide, see section B in section 58.
For the behavior of refrigerants such as hydrofluorocarbons (HFCs) such as R407C and R410A, see section 102.

The change in the phase state of R22 in the capacitor can be represented as follows (see Fig. 2.2).

From A to B. Reduction of superheat of vapors R22 from 70 to 38 ° C (zone A-B is the zone of removing overheating in the condenser).

At point B, the first drops of liquid R22 appear.
B to C. R22 condensation at 38 ° C and 14 bar (zone B-C is the condensation zone in the condenser).

At point C, the last vapor molecule condensed.
From C to D. Subcooling of liquid R22 from 38 to 32 ° C (zone C-D is the subcooling zone of liquid R22 in the condenser).

Throughout this entire process, the pressure remains constant, equal to the reading of the HP pressure gauge (in our case, 14 bar).
Let us now consider how the cooling air behaves in this case (see Fig. 2.3).

Outside air, which cools the condenser and enters the inlet with a temperature of 25 ° C, heats up to 31 ° C, taking away the heat generated by the refrigerant.

We can represent the changes in the temperature of the cooling air as it passes through the condenser and the temperature of the condenser in the form of a graph (see Fig. 2.4) where:

tae- air temperature at the inlet to the condenser.

tas- the temperature of the air at the outlet of the condenser.

tK- condensation temperature read from the HP pressure gauge.

A6(read: delta theta) temperature difference.

In general, in air-cooled condensers, the temperature difference over the air is A0 = (tas - tae) has values ​​from 5 to 10 K (in our example 6 K).
The difference between the condensing temperature and the air temperature at the outlet of the condenser is also on the order of 5 to 10 K (in our example, 7 K).
Thus, the total temperature difference ( tK - tae) can be from 10 to 20 K (as a rule, its value is near 15 K, and in our example it is 13 K).

The concept of complete temperature head very important, since for a given capacitor, this value remains almost constant.

Using the values ​​given in the above example, we can say that for an outside air temperature at the inlet to the condenser equal to 30 ° C (i.e. tae = 30 ° C), the condensation temperature tk should be equal to:
tae + DBfull = 30 + 13 = 43 ° С,
which will correspond to an HP pressure gauge reading of about 15.5 bar for R22; 10.1 bar for R134a and 18.5 bar for R404A.

One of the most important characteristics during the operation of the refrigeration circuit, without a doubt, the degree of subcooling of the liquid at the outlet of the condenser is.

Supercooling of a liquid is the difference between the temperature of condensation of a liquid at a given pressure and the temperature of the liquid itself at the same pressure.

We know that the condensation temperature of water at atmospheric pressure is 100 ° C. Therefore, when you drink a glass of water with a temperature of 20 ° C, from the standpoint of thermophysics, you drink water supercooled by 80 K!

In a condenser, subcooling is defined as the difference between the condensation temperature (read from the HP pressure gauge) and the temperature of the liquid measured at the outlet of the condenser (or in the receiver).

In the example shown in Fig. 2.5, hypothermia P / O = 38 - 32 = 6 K.
Normal refrigerant subcooling in air-cooled condensers is generally in the range of 4 to 7 K.

When the amount of subcooling is outside the normal temperature range, it often indicates an abnormal working process.
Therefore, below we will analyze various cases of abnormal hypothermia.

One of the biggest difficulties in the work of a repairman is that he cannot see the processes taking place inside the pipelines and in the refrigeration circuit. However, measuring the amount of subcooling can provide a relatively accurate picture of the behavior of the refrigerant inside the circuit.

Note that most designers size air-cooled condensers so as to provide subcooling at the outlet of the condenser in the range from 4 to 7 K. Consider what happens in the condenser if the amount of subcooling is outside this range.

A) Reduced hypothermia (usually less than 4 K).

In fig. 2.6 shows the difference in the state of the refrigerant inside the condenser under normal and abnormal hypothermia.
Temperature at points tB = tc = tE = 38 ° C = condensing temperature tK. Measurement of temperature at point D gives a value of tD = 35 ° С, hypothermia is 3 K.

Explanation. When the refrigeration circuit is operating normally, the last vapor molecules condense at point C. Further, the liquid continues to cool and the pipeline along its entire length (zone C-D) is filled with a liquid phase, which allows achieving a normal value of subcooling (for example, 6 K).

In the event of a shortage of refrigerant in the condenser, zone C-D is not completely filled with liquid, there is only small area this zone is completely filled with liquid (zone E-D), and its length is not enough to ensure normal hypothermia.
As a result, when measuring hypothermia at point D, you will definitely get its value below normal (in the example in Fig. 2.6 - 3 K).
And the less refrigerant there is in the installation, the less will be its liquid phase at the outlet of the condenser and the less will be its degree of subcooling.
In the limit, with a significant shortage of refrigerant in the circuit of the refrigeration unit, at the outlet of the condenser there will be a vapor-liquid mixture, the temperature of which will be equal to the condensation temperature, that is, supercooling will be equal to O K (see Fig. 2.7).

Thus, an insufficient refrigerant charge always leads to a decrease in subcooling.

It follows that a competent repairer will not recklessly add refrigerant to the installation without making sure that there are no leaks and not making sure that the hypothermia is abnormally low!

Note that as refrigerant is added to the circuit, the liquid level at the bottom of the condenser will rise, causing an increase in subcooling.
Let us now proceed to consider the opposite phenomenon, that is, too much hypothermia.

B) Increased hypothermia (usually more than 7 k).

Explanation. Above, we made sure that the lack of refrigerant in the circuit leads to a decrease in subcooling. On the other hand, an excessive amount of refrigerant will accumulate at the bottom of the condenser.

In this case, the length of the condenser zone, completely filled with liquid, increases and can occupy the entire section E-D... The amount of liquid in contact with the cooling air increases and the amount of supercooling, therefore, also becomes larger (in the example in Fig. 2.8 P / O = 9 K).

In conclusion, we point out that measurements of the subcooling value are ideal for diagnosing the process of functioning of a classic refrigeration unit.
During a detailed analysis typical malfunctions we will see how, in each specific case, to correctly interpret the data of these measurements.

Too little subcooling (less than 4 K) indicates a lack of refrigerant in the condenser. Increased subcooling (over 7 K) indicates an excess of refrigerant in the condenser.

Due to gravity, the liquid accumulates at the bottom of the condenser, so the vapor inlet to the condenser must always be at the top. Therefore, options 2 and 4 are at least a weird solution that won't work.

The difference between options 1 and 3 is mainly in the temperature of the air that blows over the hypothermia zone. In the 1st variant, the air that provides hypothermia enters the subcooling zone already warmed up, since it has passed through the condenser. The design of the 3rd variant should be considered the most successful, since it implements heat exchange between the refrigerant and air according to the counterflow principle.

This option has best performance heat transfer and the design of the installation as a whole.
Consider this if you have not yet decided which direction of the cooling air (or water) to flow through the condenser.