Analysis of cases of abnormal hypothermia. Subcooling in air-cooled condensers: what is its norm? Refrigerant subcooling

19.10.2015

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

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

The 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, the hypothermia will be 6 K or 38-32.

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

Interaction between condenser and fan: air temperature difference.

The air pumped by the fan has a temperature of 25 degrees Celsius (Figure 2.3). It takes heat from freon, causing its temperature to change to 31 degrees.

Figure 2.4 shows a more detailed change:

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

Tas – air with a new condenser temperature after cooling;

Tk – readings from the pressure gauge about the condensation temperature;

Δθ – temperature difference.

The temperature difference in an air-cooled condenser is calculated using the formula:

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

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

2.1. NORMAL OPERATION

Let's look at the diagram in Fig. 2.1, representing a capacitor air cooling during normal operation in the section. Let's assume that R22 refrigerant enters the condenser.

Point A. R22 vapors, superheated 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 vapor 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 B-C. The 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; there is nothing in the circuit except liquid. Temperature and pressure remain constant at approximately 38°C and 14 bar respectively.

Line C-D. All the refrigerant has condensed; the liquid continues to cool under the influence of air cooling the condenser using a fan.

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

For the behavior of mixed refrigerants such as hydrochlorofluorocarbons (HCFCs) with a large temperature glide, see paragraph B of section 58.
For the behavior of hydrofluorocarbon (HFC) refrigerants 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. Reducing the superheat of R22 vapor from 70 to 38 ° C (zone A-B is the zone for removing overheating in the condenser).

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

At point C the last molecule of steam has 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).

During this entire process, the pressure remains constant, equal to the reading on 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).

The outside air, which cools the condenser and enters at the inlet temperature of 25 ° C, is heated 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 condenser inlet.

tas- air temperature at the condenser outlet.

tK- condensation temperature, read from the HP pressure gauge.

A6(read: delta theta) temperature difference.

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

The concept of total temperature difference is very important, since for of this 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 condenser inlet equal to 30°C (i.e. tae = 30°C), the condensing temperature tk should be equal to:
tae + dbtot = 30 + 13 = 43°C,
which would correspond to a high pressure gauge reading of about 15.5 bar for R22; 10.1 bar for R134a and 18.5 bar for R404A.

2.2. SUBCOOLING IN AIR COOLED CONDENSERS

One of the most important characteristics During operation of the refrigeration circuit, there is no doubt that the degree of subcooling of the liquid at the outlet of the condenser is important.

We will call the supercooling of a liquid the difference between the condensation temperature of the 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 at a temperature of 20 ° C, from the point of view of thermophysics, you are drinking water that is supercooled by 80 K!

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

In the example shown in Fig. 2.5, subcooling P/O = 38 - 32 = 6 K.
The normal value of refrigerant subcooling in air-cooled condensers is usually in the range from 4 to 7 K.

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

2.3. ANALYSIS OF CASES OF ANOMALITY HYPOCOOLING.

One of the biggest difficulties in the work of a repairman is that he cannot see the processes occurring 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 within the circuit.

Note that most designers size air-cooled capacitors to provide subcooling at the condenser outlet in the range of 4 to 7 K. Let's look at what happens in the condenser if the subcooling value 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 during normal and abnormal supercooling.
Temperature at points tB = tc = tE = 38°C = condensation temperature tK. Measuring the temperature at point D gives the value tD = 35 °C, subcooling 3 K.

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

If there is 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 occupied by liquid (zone E-D), and its length is not enough to ensure normal supercooling.
As a result, when measuring hypothermia at point D, you will definitely get a value lower than normal (in the example in Fig. 2.6 - 3 K).
And the less refrigerant there is in the installation, the less its liquid phase will be at the outlet of the condenser and the less its degree of subcooling will be.
In the limit, with a significant lack of refrigerant in the circuit refrigeration unit, at the exit from the condenser there will be a vapor-liquid mixture, the temperature of which will be equal to the condensation temperature, that is, the subcooling will be equal to O K (see Fig. 2.7).

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

It follows that a competent repairman will not recklessly add refrigerant to the unit without ensuring that there are no leaks and without making sure that the subcooling is abnormally low!

Note that as refrigerant is added to the circuit, the liquid level in the lower part of the condenser will increase, causing an increase in subcooling.
Let us now move on to consider the opposite phenomenon, that is, too much hypothermia.

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

Explanation. We have seen above that a lack of refrigerant in the circuit leads to a decrease in subcooling. On the other hand, excessive 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 subcooling, therefore, also becomes greater (in the example in Fig. 2.8 P/O = 9 K).

In conclusion, we point out that measuring the amount of subcooling is ideal for diagnosing the process of functioning of a classical refrigeration unit.
In the course of a detailed analysis of typical faults, we will see how to accurately interpret the data of these measurements in each specific case.

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

Due to gravity, liquid accumulates at the bottom of the condenser, so the vapor inlet into the condenser should always be located at the top. Therefore, options 2 and 4 are at least a strange solution that will not work.

The difference between options 1 and 3 lies mainly in the temperature of the air that blows over the hypothermic zone. In the 1st option, the air that provides subcooling enters the subcooling zone already heated, since it has passed through the condenser. The design of the 3rd option 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 characteristics heat transfer and plant design as a whole.
Think about this if you haven't yet decided which direction to take the cooling air (or water) through the condenser.

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

Rice. 2.6

In Fig. 2.6 shows the difference in the state of the refrigerant inside the condenser during normal and abnormal supercooling. Temperature at points tв=tc=te=38°С = condensation temperature tк. Measuring the temperature at point D gives the value td=35 °C, subcooling 3 K.

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

If there is a shortage of refrigerant in the condenser, zone C-D is not completely filled with liquid; there is only a small section of this zone completely occupied by liquid (zone E-D), and its length is not enough to ensure normal subcooling.

As a result, when measuring hypothermia at point D, you will definitely get a value lower than normal (in the example in Figure 2.6 - 3 K).

And the less refrigerant there is in the installation, the less its liquid phase will be at the outlet of the condenser and the less its degree of subcooling will be.

In the limit, if there is a significant shortage of refrigerant in the refrigeration circuit, 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, the subcooling will be equal to 0 K (see Figure 2.7).

Rice. 2.7

tв=td=tk=38°С. Subcooling value P/O = 38—38=0 K.

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

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

Note that as refrigerant is added to the circuit, the liquid level in the lower part of the condenser will increase, causing an increase in subcooling.

Let us now move on to consider the opposite phenomenon, that is, too much hypothermia.

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

Rice. 2.8

tв=te=tk= 38°С. td = 29°C, therefore hypothermia P/O = 38-29 = 9 K.

Explanation. We have seen above that a lack of refrigerant in the circuit leads to a decrease in subcooling. On the other hand, excessive 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 subcooling also increases (in the example in Fig. 2.8 P/O = 9 K).

In conclusion, we point out that measuring the amount of subcooling is ideal for diagnosing the process of functioning of a classical refrigeration unit.

In the course of a detailed analysis of typical faults, we will see how to accurately interpret the data of these measurements in each specific case.

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

2.4. EXERCISE

Choose from 4 air cooled condenser designs shown in fig. 2.9, the one you think is best. Explain why?

Rice. 2.9

Think about this if you haven't yet decided which direction to take the cooling air (or water) through the condenser.

The influence of temperature and pressure on the state of refrigerants
Subcooling in air cooled condensers
Analysis of cases of abnormal hypothermia

Undercharging and overcharging the system with refrigerant

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

The supercooling temperature Tp is defined as the difference:
Tp = Tk – Tf
Тк – condensation temperature, read from the HP pressure gauge.
Tf – temperature of freon (pipe) at the outlet of the condenser.

1. Lack of refrigerant. Symptoms

A lack of freon will be felt in every element of the circuit, but this deficiency is especially felt in the evaporator, condenser and liquid line. As a result of insufficient 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 significantly. Since the volumetric output 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 evaporation temperature. The evaporation temperature can drop to below zero, resulting in freezing of the inlet tube and evaporator, and the overheating of the steam will be very significant.

Superheat temperature T superheat is defined as the difference:
T overheat = T f.i. - T suck.
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 o C and, accordingly, the temperature at the compressor inlet will also increase. And since the electric motors of hermetic compressors are cooled using suction vapor, in this case the compressor will abnormally overheat and may fail. Due to the increase in the temperature of the steam in the suction line, the temperature of the steam in the discharge line will also be increased. Since there will be a shortage of refrigerant in the circuit, there will also be insufficient refrigerant in the subcooling zone.

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. Overfilling. Symptoms

In systems with a expansion valve as a throttling device, liquid cannot enter the evaporator, so excess refrigerant is stored in the condenser. Abnormally high level liquid in the condenser reduces the heat exchange surface, the cooling of the gas entering the condenser deteriorates, which leads to an increase in the temperature of 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 much longer, and this leads to an increase in the subcooling 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 causes a decrease in mass flow through the compressor and a drop in cooling capacity. As a result, the evaporation pressure will also increase. Due to the fact that overcharging leads to a decrease in vapor mass flow, the cooling of the electric compressor motor will deteriorate. Moreover, due to the increased condensation pressure, the current of the electric motor of the compressor increases. Deterioration of cooling and increase in current consumption leads to overheating of the electric motor and, ultimately, failure of the compressor.

Cooling capacity has dropped
Evaporation pressure increased
Condensation pressure increased
Increased hypothermia (more than 7 o C)

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

In this article we will talk about the most accurate way to refill air conditioners.

You can refill any freons. Refill - only one-component freons (eg: R-22) or isotropic (conditionally isotropic, eg: R-410) mixtures

When diagnosing cooling and air conditioning systems, the processes occurring inside the condenser are hidden from the service engineer, and often it is from them that one can understand why the efficiency of the system as a whole has decreased.

Let's look at them briefly:

Superheated refrigerant vapor passes from the compressor to the condenser
Under the influence air flow freon temperature decreases to condensation temperature
Until the last freon molecule passes into the liquid phase, the temperature remains the same throughout the entire section of the line where the condensation process occurs.
Under the influence of the cooling air flow, the temperature of the refrigerant decreases from the condensation temperature to the temperature of the cooled liquid freon

Inside the condenser, the freon pressure is the same.
Knowing the pressure, using special tables from the freon manufacturer, you can determine the condensation temperature under current conditions. The difference between the condensation temperature and the temperature of the cooled freon at the outlet of the condenser - the subcooling temperature - is usually a known value (check with the system manufacturer) and the range of these values for a given system is fixed (for example: 10-12 °C).

If the subcooling value is below the range specified by the manufacturer, then the freon does not have time to cool in the condenser - it is not enough and refilling is required. A lack of freon reduces the efficiency of the system and increases the load on it.

If the subcooling value is above the range, there is too much freon, you need to drain some of it until it reaches optimal value. An excess of freon increases the load on the system and reduces its service life.

Refueling by subcooling without using:

We connect the pressure manifold and the freon cylinder to the system.
Install a thermometer/temperature sensor on the line high pressure.
Let's start the system.
Using a pressure gauge on the high pressure line (liquid line), we measure the pressure and calculate the condensation temperature for a given freon.
Using a thermometer, we monitor the temperature of the supercooled freon at the outlet of the condenser (it should be in the range of the sum of the condensation temperature and the subcooling temperature).
If the freon temperature exceeds the permissible level (subcooling temperature is below the required range) - there is not enough freon, slowly add it to the system until it reaches desired temperature
If the freon temperature is below the permissible level (the subcooling temperature is above the range), there is an excess of freon, some of it must be slowly released until the desired temperature is reached.

Using this process simplified significantly (the connection diagram in the drawings is in the operating instructions):

We reset the device to zero, switch it to supercooling mode, and set the freon type.
We connect the pressure gauge manifold and the freon cylinder to the system, and connect the high pressure (liquid) hose through the T-piece supplied with the device.
We install the SH-36N temperature sensor on the high pressure line.
We turn on the system, the subcooling value will be displayed on the screen, we compare it with the required range and, depending on whether the displayed value is higher or lower, we slowly bleed off or add freon.

This refueling method is more accurate than refueling by volume or weight, since there are no intermediate calculations, which are sometimes approximate.

Alexey Matveev,
technical specialist at Raskhodka company

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

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

2.4. EXERCISE

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