Condensate overcooling is understood as a decrease in the temperature of the condensate against the temperature of the saturated steam entering the condenser. It was noted above that the amount of supercooling of the condensate is determined by the temperature difference t n -t To .

Subcooling of the condensate leads to a noticeable decrease in the efficiency of the installation, since with subcooling of the condensate, the amount of heat transferred in the condenser to the cooling water increases. An increase in condensate supercooling by 1 ° C causes excessive fuel consumption in installations without regenerative heating of feed water by 0.5%. With regenerative heating of feed water, the overconsumption of fuel in the installation is somewhat lower. V modern installations in the presence of regenerative capacitors condensate subcooling under normal operating conditions condensing unit does not exceed 0.5-1 ° C. Condensate overcooling is caused by the following reasons:

a) violation of the air density of the vacuum system and increased air suction;

b) high level condensate in the condenser;

c) excessive consumption of cooling water through the condenser;

d) the design flaws of the capacitor.

Increasing the air content in the steam-air

mixture leads to an increase in the partial pressure of air and, accordingly, to a decrease in the partial pressure of water vapor in relation to full pressure mixtures. As a result, the temperature of saturated water vapor, and, consequently, the temperature of the condensate will be lower than it was before the increase in air content. Thus, one of the important measures aimed at reducing the overcooling of condensate is to ensure a good air density of the vacuum system of the turbine unit.

With a significant increase in the level of condensate in the condenser, a phenomenon may occur that the lower rows of the cooling tubes will be washed by condensate, as a result of which the condensate will be supercooled. Therefore, it must be ensured that the condensate level is always below the lower row of cooling pipes. The best remedy warning of an impermissible rise in the condensate level is the device automatic regulation it in the condenser.

Excessive water flow through the condenser, especially at its low temperature, will lead to an increase in the vacuum in the condenser due to a decrease in the partial pressure of water vapor. Therefore, the flow rate of cooling water through the condenser must be controlled depending on the steam load on the condenser and on the temperature of the cooling water. At correct adjustment the flow rate of cooling water in the condenser will maintain an economic vacuum and the subcooling of the condensate will not go beyond the minimum value for this condenser.

Subcooling of condensate can occur due to design flaws capacitor. In some condenser designs, as a result of the close arrangement of the cooling tubes and their unsuccessful breakdown along the tube sheets, a large vapor resistance is created, reaching in some cases 15-18 mm Hg. Art. The high vapor resistance of the condenser leads to a significant pressure drop above the condensate level. A decrease in the mixture pressure above the condensate level occurs due to a decrease in the partial pressure of water vapor. Thus, the temperature of the condensate is obtained well below the temperature of the saturated steam entering the condenser. In such cases, to reduce the overcooling of the condensate, it is necessary to make structural alterations, namely, to remove some of the cooling tubes in order to arrange corridors in the tube bundle and reduce the vapor resistance of the condenser.

It should be borne in mind that the removal of a part of the cooling tubes and the consequent reduction of the condenser cooling surface leads to an increase in the specific load of the condenser. However, increasing the specific vapor load is usually quite acceptable, since the condensers of older designs have a relatively low specific vapor load.

We considered the main issues of operation of the equipment of the condensing unit steam turbine... From the foregoing it follows that the main attention during the operation of the condensing unit should be paid to maintaining the economic vacuum in the condenser and to ensuring the minimum overcooling of the condensate. These two parameters greatly affect the efficiency of the turbine. For this purpose, it is necessary to maintain a good air density. vacuum system turbine installations, ensure the normal operation of air extractors, circulation and condensate pumps, keep the condenser tubes clean, monitor the water density of the condenser, prevent the increase of suction cups raw water, ensure the normal operation of the cooling devices. The instrumentation, automatic regulators, signaling and regulating devices available at the installation allow service personnel monitor the condition of the equipment and the operating mode of the installation and maintain such operating modes that ensure highly economical and reliable operation of the installation.

Carrier

Installation, adjustment and maintenance instructions

CALCULATION OF OVERCOOLING AND OVERHEATING

Hypothermia

1. Definition

condensation of saturated refrigerant vapor (Tc)
and temperature in the liquid line (Tzh):

PO = Tk Tzh.

Collector

temperature)

3. Steps of measurement

electronic to the liquid line next to the filter
desiccant. Make sure the pipe surface is clean,
and the thermometer touches her tightly. Cover the flask or
foam sensor to insulate the thermometer
from the ambient air.

low pressure).

pressure in the discharge line.

Measurements must be taken when the unit is
operates under optimal design conditions and develops
maximum performance.

4. According to the table of conversion of pressure to temperature for R 22

find the dew point of saturated steam
refrigerant (TC).

5. Record the temperature measured by the thermometer

on the liquid line (Tl) and subtract it from the temperature
condensation. The resulting difference will be the value
hypothermia.

6. If the system is properly charged with refrigerant

hypothermia ranges from 8 to 11 ° C.
If hypothermia turned out to be less than 8 ° C, you need
add refrigerant, and if more than 11 ° C remove
excess freon.

Discharge pressure (sensor):

Condensing temperature (from the table):

Liquid line temperature (by thermometer): 45 ° C

Hypothermia (calculated)

Add refrigerant according to calculation results.

Overheat

1. Definition

Hypothermia is the difference between the temperature
suction (TV) and saturated evaporation temperature
(Ti):

PG = TV Ti.

2. Measuring equipment

Collector
Conventional or electronic thermometer (with sensor

temperature)

Filter or thermal insulation foam
Pressure to temperature conversion table for R 22.

3. Steps of measurement

1. Place the liquid thermometer flask or sensor

electronic to the suction line next to
compressor (10-20 cm). Make sure the surface
the pipe is clean and the thermometer is firmly touching its top
parts, otherwise the thermometer reading will be incorrect.
Cover the flask or sensor with foam to insulate
remove the thermometer from the ambient air.

2. Insert a manifold into the discharge line (sensor

high pressure) and suction line (sensor
low pressure).

3. After conditions stabilize, write down

pressure in the discharge line. By conversion table
pressure to temperature for R 22 find the temperature
saturated evaporation of refrigerant (Ti).

4. Record the temperature measured by the thermometer

on the suction line (TV) 10 20 cm from the compressor.
Take a few measurements and calculate
average suction line temperature.

5. Subtract the evaporating temperature from the temperature

suction. The resulting difference will be the value
overheating of the refrigerant.

6. With the correct setting of the expansion valve

overheating is from 4 to 6 ° C. For less
too much gets into the evaporator
refrigerant, and you need to close the valve (turn the screw
clockwise). With greater overheating in
too little refrigerant flows into the evaporator, and
you need to open the valve (turn the screw against
clockwise).

4. An example of calculating hypothermia

Suction pressure (sensor):

Evaporation temperature (from the table):

Suction line temperature (by thermometer): 15 ° С

Overheating (calculated)

Open the expansion valve slightly according to

calculation results (too much overheating).

ATTENTION

COMMENT

After adjusting the expansion valve, remember to
replace its cover. Change overheating only
after adjusting hypothermia.

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... V vaporizer freon boils away completely in about 1/10 of the evaporator (towards 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 capacity 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. In this case, the cooling capacity of the system will decrease, the suction pressure will decrease, heated freon vapors can damage the compressor stator.

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 growing 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

Which maximum length freon pipelines maybe a split air conditioning system? 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 given value maximum, but for systems of class VRF the maximum length of pipelines (equivalent) can be much longer - up to 190 m (total - up to 1000 m).

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

Before considering the peculiarities of VRF systems operation, let's pay attention to the scheme of the freon circuit of split systems and understand what happens to the refrigerant with long lengths of 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 at point D, with initial parameters in which (temperature 75 ° C, pressure 27.2 bar) freon enters the condenser of the outdoor unit. Freon in this moment Is a superheated gas that first cools down to the 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). The optimum subcooling value is considered to be 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 in the zone of a mixture of liquid and gas (Fig. 3). Consequently, 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 discharging 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 pressure drop 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 a lower pressure and, accordingly, the condensing temperature will drop. A lower condensing temperature will result in low temperature evaporation and freezing of the gas pipeline.

If increased pressure losses occur in the liquid pipeline, then the process is even more interesting: since we found out that freon is in a saturated state in the liquid pipeline, 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 the volume of the vapor-gas mixture and an increase in the speed of movement along the liquid pipeline. Increased speed movement will again cause an additional loss of pressure, the process will become "avalanche".

In fig. 4 is a conditional graph specific losses pressure 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 when the length of the pipelines doubles (up to 30 m), the losses increase not twice (up to 800 Pa), but seven times - up to 2800 Pa.

Therefore, it is fatal to simply double the length of pipelines in relation to standard lengths for a split system with an On-Off compressor. 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 in a 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 flow 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 a 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 line lengths 5-6 and 4-6, it becomes understandable work freon subcooler. 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 at standard length pipelines 7.5 m. That is, to 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.

The thermal balance of a surface capacitor has the following expression:

G To ( h to -h to 1)=W(t 2v -t 1v)from to, (17.1)

where h to- enthalpy of steam entering the condenser, kJ / kg; h to 1 = s to t to is the enthalpy of the condensate; from to= 4.19 kJ / (kg × 0 С) - heat capacity of water; W- consumption of cooling water, kg / s; t 1v, t 2v- the temperature of the cooling water at the inlet and outlet of the condenser. Condensed steam consumption G k, kg / s and enthalpy h to are known from the calculation of a steam turbine. The condensate temperature at the outlet of the condenser is taken equal to the saturation temperature of the steam t p corresponding to its pressure p to taking into account the subcooling of the condensate D t to: t k = t p - D t to.

Subcooling of condensate(the difference between the steam saturation temperature at the pressure in the condenser neck and the condensate temperature in the suction pipe of the condensate pump) is a consequence of a decrease in the partial pressure and saturated steam temperature due to the presence of air and the steam resistance of the condenser (Fig. 17.3).

Figure 17.3. Change in the parameters of the vapor-air mixture in the condenser: a - change in the partial pressure of steam p p and pressure in the condenser p to; b - change in steam temperature t p and relative air content ε

Applying Dalton's law to a vapor-air medium moving in a condenser, we have: p k = p n + p in, where p p and p in- partial pressures of steam and air in the mixture. Dependence of the partial vapor pressure on the pressure in the condenser and the relative air content e=G v / G k has the form:

(17.2)

When entering the condenser, the relative air content is low and p p »p k... As steam condenses, the value e increases and the partial vapor pressure drops. In the lower part, the partial air pressure is most significant, because it rises due to an increase in air density and the value e... This leads to a decrease in the temperature of the steam and condensate. In addition, the vapor resistance of the condenser takes place, which is determined by the difference

D p k = p k - p k´.(17.3)

Usually D p to= 270-410 Pa (determined empirically).

The condenser, as a rule, receives wet steam, the condensation temperature of which is uniquely determined by the partial pressure of the steam: a lower partial pressure of steam corresponds to a lower saturation temperature. Figure 17.3, b shows the graphs of the change in the steam temperature t p and the relative air content ε in the condenser. Thus, as the steam-air mixture moves to the place of suction and condensation of steam, the temperature of the steam in the condenser decreases, since the partial pressure of saturated steam decreases. This is due to the presence of air and an increase in its relative content in the vapor-air mixture, as well as the presence of vapor resistance of the condenser and a decrease in the total pressure of the vapor-air mixture.

Under such conditions, overcooling of the condensate Dt k = t p -t k is formed, which leads to a loss of heat with the cooling water and the need for additional heating of the condensate in the regenerative system of the turbine plant. In addition, it is accompanied by an increase in the amount of oxygen dissolved in the condensate, which causes corrosion of the pipe system of the regenerative heating of the boiler feed water.

Subcooling can reach 2-3 0 C. The means of combating it is the installation of air coolers in the condenser tube bundle, from which the vapor-air mixture is sucked into the ejector installations. In modern vocational schools, hypothermia is allowed no more than 1 0 C. Rules technical exploitation strictly prescribe the permissible air suction into the turbine unit, which should be less than 1%. For example, for turbines with a capacity N E= 300 MW air suction should be no more than 30 kg / h, and N E= 800 MW - no more than 60 kg / hour. Modern condensers with a minimum vapor resistance and a rational layout of the tube bundle practically do not have overcooling in the nominal operating mode of the turbine unit.