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

Let's look at them briefly:

1. Superheated refrigerant vapor passes from the compressor to the condenser
2. Under the influence air flow freon temperature decreases to condensation temperature
3. 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.
4. 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:

1. We connect the pressure manifold and the freon cylinder to the system.
2. Install a thermometer/temperature sensor on the line high pressure.
3. Let's start the system.
4. Using a pressure gauge on the high pressure line (liquid line), we measure the pressure and calculate the condensation temperature for a given freon.
5. 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).
6. 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
7. 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 simplifies many times (the connection diagram in the figures is in the operating instructions):

1. We reset the device to zero, switch it to supercooling mode, and set the freon type.
2. We connect the pressure gauge manifold and the freon cylinder to the system, and connect the high pressure (liquid) hose through the T-shaped tee supplied with the device.
3. We install the SH-36N temperature sensor on the high pressure line.
4. 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

Improving the efficiency of refrigeration

installations due to refrigerant subcooling

Federal State Educational Institution of Higher Professional Education "Baltic state academy fishing fleet"

Russia, *****@***ru

Reducing consumption electrical energy is very important aspect life in connection with the current energy situation in the country and in the world. Reducing energy consumption by refrigeration units can be achieved by increasing the cooling capacity of refrigeration units. The latter can be achieved using various types of subcoolers. Thus, considered various types subcoolers and developed the most efficient one.

refrigeration capacity, subcooling, regenerative heat exchanger, subcooler, inter-pipe boiling, boiling inside pipes

By subcooling the liquid refrigerant before throttling, significant improvements in operating efficiency can be achieved refrigeration unit. Subcooling of the refrigerant can be achieved by installing a subcooler. The subcooler of the liquid refrigerant coming from the condenser at the condensing pressure to the control valve is designed to cool it below the condensing temperature. There are various ways supercooling: due to the boiling of liquid refrigerant at intermediate pressure, due to the vaporous agent leaving the evaporator, and with the help of water. Subcooling the liquid refrigerant allows you to increase the cooling capacity of the refrigeration unit.

One of the types of heat exchangers designed for supercooling liquid refrigerant are regenerative heat exchangers. In devices of this type, supercooling of the refrigerant is achieved due to the vaporous agent leaving the evaporator.

In regenerative heat exchangers, heat is exchanged between the liquid refrigerant coming from the receiver to the control valve and the vapor refrigerant leaving the evaporator. Regenerative heat exchangers are used to perform one or more of the following functions:

1) increasing the thermodynamic efficiency of the refrigeration cycle;

2) subcooling of the liquid refrigerant to prevent vaporization in front of the control valve;

3) evaporation of a small amount of liquid carried away from the evaporator. Sometimes, when using flooded evaporators, an oil-rich layer of liquid is deliberately diverted into the suction line to allow oil return. In these cases, regenerative heat exchangers serve to evaporate the liquid refrigerant from solution.

In Fig. Figure 1 shows a diagram of the RT installation.

Fig.1. Regenerative heat exchanger installation diagram

Fig. 1. The scheme of installation of the regenerative heat exchanger

The simplest form of heat exchanger is obtained by metallic contact (welding, soldering) between the liquid and steam pipelines to ensure counterflow. Both pipelines are covered with insulation as a single unit. To ensure maximum performance, the liquid line should be located below the suction line, since the liquid in the suction line may flow along the lower generatrix.

Shell-and-coil and shell-and-tube regenerative heat exchangers are most widespread in domestic industry and abroad. In small refrigeration machines produced by foreign companies, coil heat exchangers of a simplified design are sometimes used, in which a liquid tube is wound onto a suction tube. The Dunham-Busk company (Dunham-Busk, USA) fills the liquid coil wound onto the suction line with an aluminum alloy to improve heat transfer. The suction line is equipped with internal smooth longitudinal ribs, providing good heat transfer to the steam with minimal hydraulic resistance. These heat exchangers are designed for installations with a cooling capacity of less than 14 kW.

For medium- and large-capacity installations, shell-and-coil regenerative heat exchangers are widely used. In devices of this type, a liquid coil (or several parallel coils), wound around a displacer, is placed in a cylindrical vessel. Steam passes in the annular space between the displacer and the casing, thereby ensuring more complete washing of the surface of the liquid coil with steam. The coil is made from smooth, and more often from externally finned pipes.

When using pipe-in-pipe heat exchangers (usually for small refrigeration machines) special attention pay attention to intensifying heat exchange in the apparatus. For this purpose, either finned pipes are used, or all kinds of inserts (wire, tape, etc.) are used in the steam region or in the steam and liquid regions (Fig. 2).

Fig.2. Regenerative heat exchanger of the “pipe-in-pipe” type

Fig. 2. Regenerative heat exchanger type “pipe in pipe”

Subcooling due to boiling of liquid refrigerant at intermediate pressure can be carried out in intermediate vessels and economizers.

In low-temperature refrigeration units with two-stage compression, the work of the intermediate vessel installed between the compressors of the first and second stages largely determines the thermodynamic perfection and efficiency of the operation of the entire refrigeration unit. The intermediate vessel performs the following functions:

1) “knocking down” the superheat of the steam after the first stage compressor, which leads to a decrease in the work spent by the high pressure stage;

2) cooling the liquid refrigerant before it enters the control valve to a temperature close to or equal to the saturation temperature at intermediate pressure, which reduces losses in the control valve;

3) partial separation of oil.

Depending on the type of intermediate vessel (coil or coilless), a scheme with one or two-stage throttling of the liquid refrigerant is implemented. In pumpless systems, it is preferable to use coiled intermediate vessels in which the liquid is under condensation pressure, ensuring the supply of liquid refrigerant to the evaporative system of multi-deck refrigerators.

The presence of a coil also eliminates additional oiling of the liquid in the intermediate vessel.

In pump-circulation systems, where the supply of liquid to the evaporation system is ensured by pump pressure, coilless intermediate vessels can be used. The current use of effective oil separators in refrigeration unit circuits (flushing or cyclone on the discharge side, hydrocyclones in the evaporation system) also makes it possible to use coilless intermediate vessels - devices that are more efficient and simpler in design.

Water supercooling can be achieved in counterflow subcoolers.

In Fig. Figure 3 shows a two-pipe counterflow subcooler. It consists of one or two sections assembled from double pipes connected in series (pipe in pipe). The internal pipes are connected by cast iron rolls, the external ones are welded. The liquid working substance flows in the inter-tube space in countercurrent to the cooling water moving through the internal pipes. Pipes - steel seamless. The outlet temperature of the working substance from the apparatus is usually 2-3 °C higher than the temperature of the incoming cooling water.

pipe in pipe"), into each of which liquid refrigerant is supplied through a distributor, and refrigerant from a linear receiver enters the intertubular space; the main disadvantage is the limited service life due to the rapid failure of the distributor. The intermediate vessel, in turn, can be Use only for cooling systems running on ammonia.

Rice. 4. Sketch of a liquid freon subcooler with boiling in the annulus

Fig. 4. The sketch of supercooler with boiling of liquid Freon in intertubes space

The most suitable device is a liquid freon subcooler with boiling in the annulus. The diagram of such a subcooler is shown in Fig. 4.

Structurally, it is a shell-and-tube heat exchanger, in the inter-tube space of which the refrigerant boils, the refrigerant enters the pipes from the linear receiver, is supercooled and then supplied to the evaporator. The main disadvantage of such a subcooler is the foaming of liquid freon due to the formation of an oil film on its surface, which leads to the need for a special device for removing oil.

Thus, a design was developed in which it is proposed to supply a supercooled liquid refrigerant from a linear receiver into the annulus, and ensure (by pre-throttled) boiling of the refrigerant in the pipes. This technical solution is illustrated in Fig. 5.

Rice. 5. Sketch of a liquid freon subcooler with boiling inside the pipes

Fig. 5. The sketch of supercooler with boiling of liquid Freon inside pipes

This device diagram makes it possible to simplify the design of the subcooler, excluding from it a device for removing oil from the surface of liquid freon.

The proposed liquid freon subcooler (economizer) is a housing containing a package of heat exchange pipes with internal fins, also a pipe for the inlet of the cooled refrigerant, a pipe for the outlet of the cooled refrigerant, pipes for the inlet of the throttled refrigerant, and a pipe for the outlet of the vaporous refrigerant.

The recommended design avoids foaming of liquid freon, increases reliability and provides more intense subcooling of the liquid refrigerant, which, in turn, leads to an increase in the refrigeration capacity of the refrigeration unit.

LIST OF LITERARY SOURCES USED

1. Zelikovsky on heat exchangers of small refrigeration machines. - M.: Food industry, 19s.

2. Cold production ions. - Kaliningrad: Book. publishing house, 19 p.

3. Danilov refrigeration units. - M.: Agropromizdat, 19с.

IMPROVING THE EFFICIENCY OF REFRIGERATING PLANTS DUE SUPERCOOLING OF REFRIGERANT

N. V. Lubimov, Y. N. Slastichin, N. M. Ivanova

Supercooling of liquid Freon in front of the evaporator allows to increase refrigerating capacity of a refrigerating machinery. For this purpose we can use regenerative heat exchangers and supercoolers. But more effective is the supercooler with boiling of liquid Freon inside pipes.

refrigerating capacity, supercooling, supercooler

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 that use a thermostatic expansion valve (TEV) as a throttling device, the best indicator of normal refrigerant charge 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 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 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 lack of refrigerant in the circuit, there will also be insufficient refrigerant in the subcooling zone.

Thus, the main signs of freon deficiency are:
• 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 compressor electric motor increases. Deterioration of cooling and increase in current consumption leads to overheating of the electric motor and, ultimately, failure of the compressor.

Bottom line. The main signs of recharging with refrigerant:
• 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.

19.10.2015

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

Under normal conditions atmospheric pressure, water condensation has a temperature of 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 temperature difference 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 cross-section of an air-cooled condenser during normal operation. 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 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 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 under normal and abnormal hypothermia.
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 refrigeration circuit works 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 allows us 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, 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 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.
During a detailed analysis 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.