Subcooling after the condenser. Analysis of VRF systems. Refrigerant subcooling system. Measuring equipment
Carrier
Installation, adjustment and maintenance instructions
CALCULATION OF SUPERCOOLING AND OVERHEATING
Hypothermia
1. Definition
condensation of saturated refrigerant vapor (Tk)
and temperature in the liquid line (Tl):
PO = Tk Tj.
Collector
temperature)
3. Measurement steps
electronic to the liquid line next to the filter
desiccant. Make sure the pipe surface is clean,
and the thermometer touches it tightly. Cover the flask or
foam sensor to insulate the thermometer
from the surrounding air.
low pressure).
pressure in the discharge line.
Measurements must be taken when the unit
operates under optimal design conditions and develops
maximum performance.
4. According to the pressure-to-temperature conversion table for R 22
find the condensation temperature of saturated steam
refrigerant (Tk).
5. Record the temperature measured by the thermometer
on the liquid line (Tj) and subtract it from the temperature
condensation The resulting difference will be the value
hypothermia.
6. If the system is correctly charged with refrigerant
hypothermia ranges from 8 to 11°C.
If hypothermia is less than 8°C, you need
add refrigerant, and if it is more than 11°C, remove
excess freon.
Pressure in the discharge line (according to the sensor):
Condensation temperature (from table):
Liquid line temperature (thermometer): 45°C
Hypothermia (calculated)
Add refrigerant according to calculation results.
Overheat
1. Definition
Hypothermia is the difference between temperature
suction (Tv) and saturated evaporation temperature
(Ti):
PG = TV Ti.
2.Measuring equipment
Collector
Regular or electronic thermometer (with sensor
temperature)
Filter or insulating foam
Pressure to temperature conversion table for R 22.
3. Measurement steps
1. Place the flask liquid thermometer or sensor
electronic to the suction line next to
compressor (10-20 cm). Make sure the surface
the pipe is clean, and the thermometer touches its top tightly
parts, otherwise the thermometer readings will be incorrect.
Cover the bulb or sensor with foam to insulate it.
Remove the thermometer from the surrounding air.
2. Insert the manifold into the discharge line (sensor
high pressure) and suction line (sensor
low pressure).
3. Once conditions have stabilized, record
pressure in the discharge line. According to the conversion table
pressure to temperature for R 22 find the temperature
saturated refrigerant evaporation (Ti).
4. Record the temperature measured by the thermometer
on the suction line (TV) 10-20 cm from the compressor.
Take some measurements and calculate
average suction line temperature.
5. Subtract the evaporation temperature from the temperature
suction. The resulting difference will be the value
refrigerant overheating.
6. When correct setting expansion valve
overheating ranges from 4 to 6°C. With less
overheating, too much enters the evaporator
refrigerant, and you need to close the valve (turn the screw
clockwise). With greater overheating in
too little refrigerant enters the evaporator, and
you need to open the valve slightly (turn the screw against
clockwise).
4. Example of subcooling calculation
Suction line pressure (by sensor):
Evaporation temperature (from table):
Suction line temperature (thermometer): 15°C
Overheating (calculated)
Open the expansion valve slightly according to
calculation results (too much overheating).
ATTENTION
COMMENT
After adjusting the expansion valve, do not forget
put the cover back in place. Change superheat only
after adjusting the subcooling.
-> 03.13.2012 - Hypothermia in refrigeration units
Subcooling of liquid refrigerant after the condenser – essential way increasing the cooling capacity of the refrigeration unit. A decrease in the temperature of the subcooled refrigerant by one degree corresponds to an increase in the performance of a normally functioning refrigeration unit by approximately 1% at the same level of energy consumption. The effect is achieved by reducing, during supercooling, the proportion of steam in the vapor-liquid mixture, which is the condensed refrigerant supplied to the evaporator expansion valve even from the receiver.
In low-temperature refrigeration units, the use of subcooling is especially effective. In them, supercooling of the condensed refrigerant to significant negative temperatures makes it possible to increase the cooling capacity of the installation by more than 1.5 times.
Depending on size and design refrigeration units This factor can be realized in an additional heat exchanger installed on the liquid line between the receiver and the evaporator expansion valve in various ways.
Refrigerant subcooling due to external cold sources
- in a water heat exchanger due to the use of available sources, very cold water
- in air heat exchangers in the cold season
- in an additional heat exchanger with cold vapor from an external/auxiliary refrigeration unit
Subcooling due to internal resources of the refrigeration unit
- in the heat exchanger - subcooler due to the expansion of part of the freon circulating mainly refrigeration circuit- implemented in installations with two-stage compression and in satellite systems, as well as in installations with screw, piston and scroll compressors having intermediate suction ports
- in regenerative heat exchangers with cold vapor sucked into the compressor from the main evaporator - implemented in installations operating on refrigerants with a low adiabatic index, mainly HFC (HFC) and HFO (HFO)
subcooling systems using external sources cold is still quite rarely used in practice. Subcooling from cold water sources is used, as a rule, in heat pumps - water heating installations, as well as in medium- and high-temperature installations, where there is a source of cool water in close proximity to them - used artesian wells, natural reservoirs for ship installations, etc. Hypothermia from external additional refrigeration machines is implemented extremely rarely and only in very large installations industrial cold.
Subcooling in air heat exchangers is also used very infrequently, since this option of refrigeration units is still poorly understood and unusual for Russian refrigeration manufacturers. In addition, designers are confused by seasonal fluctuations in the increase in cooling capacity of installations from the use of air subcoolers.
Subcooling systems that use internal resources are widely used in modern refrigeration units, with almost all types of compressors. In installations with screw and two-stage piston compressors the use of subcooling confidently dominates, since the ability to provide suction of vapors with intermediate pressure is implemented directly in the design of these types of compressors.
The main challenge currently facing manufacturers of refrigeration and climate control systems for various purposes, is to increase the productivity and efficiency of the compressors and heat exchange equipment included in them. This idea has not lost its relevance throughout its development. refrigeration equipment from the inception of this industry to the present day. Today, when the cost of energy resources, as well as the size of the fleet of operated and commissioned refrigeration equipment has reached such impressive heights, increasing the efficiency of systems that produce and consume cold has become an urgent global problem. Taking into account the fact that this problem is complex, the current legislation of most European countries encourages developers refrigeration systems to improve their efficiency and productivity.
Refrigeration unit operation options: operation with normal overheating; with insufficient overheating; severe overheating.
Operation with normal overheating.
Refrigeration unit diagram
For example, the refrigerant is supplied at a pressure of 18 bar, and the suction pressure is 3 bar. The temperature at which the refrigerant boils in the evaporator is t 0 = −10 °C, at the outlet of the evaporator the temperature of the pipe with the refrigerant is t t = −3 °C.
Useful superheat ∆t = t t − t 0 = −3− (−10) = 7. This is the normal operation of a 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. The gas will then be heated by the room temperature.
Overheating is insufficient.
The outlet temperature will be, for example, not −3, but −6 °C. Then the overheating is only 4 °C. The point where the liquid refrigerant stops boiling moves closer to the evaporator outlet. Thus, most of the evaporator is filled with liquid refrigerant. This can happen if the thermostatic expansion valve (TEV) supplies more freon to the evaporator.
The more freon there 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 °C). The compressor will begin to fill with liquid freon because the pressure has increased, the refrigerant flow rate has increased and the compressor does not have time to pump out all the vapors (if the compressor does not have additional capacity). With this type of operation, the cooling capacity will increase, but the compressor may fail.
Severe overheating.
If the performance of the expansion valve is lower, then less freon will enter the evaporator and it will boil off earlier (the boiling point will shift closer to the evaporator inlet). The entire expansion valve and the tubes after it will freeze and become covered with ice, but 70 percent of the evaporator will not freeze at all. The 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, and the heated freon vapors can damage the compressor stator.
Improving the efficiency of refrigeration
installations due to refrigerant subcooling
FGOU VPO "Baltic State Academy of Fishing Fleet",
Russia, *****@***ru
Reducing electrical energy consumption is a very important aspect of 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, a significant increase in the efficiency of the refrigeration unit can be achieved. 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 to 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.
The most widespread in domestic industry and abroad are shell-and-coil and shell-and-tube regenerative heat exchangers. 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, ensuring good heat transfer to 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 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 heat exchangers of the “pipe-in-pipe” type (usually for small refrigeration machines), special attention is paid 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 economical 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 to supply 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 interpipe space, the main disadvantage is limited period service due to rapid failure of the distributor. The intermediate vessel, in turn, can only be used for cooling systems running on ammonia.
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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. Given technical solution 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 devices. - 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
Rice. 1.21. Sema dendrite
Thus, the mechanism of crystallization of metal melts at high cooling rates is fundamentally different in that a high degree of supercooling is achieved in small volumes of the melt. The consequence of this is the development of volumetric crystallization, which in pure metals can be homogeneous. Crystallization centers with a size larger than the critical one are capable of further growth.
For metals and alloys, the most typical form of growth is dendritic, first described back in 1868 by D.K. Chernov. In Fig. 1.21 shows a sketch by D.K. Chernov, explaining the structure of a dendrite. Typically, a dendrite consists of a trunk (first-order axis), from which there are branches - axes of the second and subsequent orders. Dendritic growth occurs in specific crystallographic directions with branches at regular intervals. In structures with lattices of face-centered and body-centered cubes, dendritic growth occurs in three mutually perpendicular directions. It has been experimentally established that dendritic growth is observed only in a supercooled melt. The growth rate is determined by the degree of supercooling. The problem of theoretically determining the growth rate as a function of the degree of supercooling has not yet received a substantiated solution. Based on experimental data, it is believed that this dependence can be approximately considered in the form V ~ (D T) 2.
Many researchers believe that at a certain critical degree of supercooling, an avalanche-like increase in the number of crystallization centers capable of further growth is observed. The nucleation of more and more new crystals can interrupt dendritic growth.
Rice. 1.22. Transformation of structures
According to the latest foreign data, with an increase in the degree of supercooling and the temperature gradient before the crystallization front, a transformation of the structure of a rapidly solidifying alloy from dendritic to equiaxed, microcrystalline, nanocrystalline and then to an amorphous state is observed (Fig. 1.22).
1.11.5. Melt amorphization
In Fig. Figure 1.23 illustrates an idealized TTT diagram (Time-Temperature-Transaction), explaining the features of solidification of alloyed metal melts depending on the cooling rate.
Rice. 1.23. TTT diagram: 1 – moderate cooling rate:
2 – very high cooling rate;
3 – intermediate cooling rate
The vertical axis represents temperature, and the horizontal axis represents time. Above a certain melting temperature - T P the liquid phase (melt) is stable. Below this temperature, the liquid becomes supercooled and becomes unstable, since the possibility of the nucleation and growth of crystallization centers appears. However, with sudden cooling, the movement of atoms in a strongly supercooled liquid may cease, and at a temperature below T3, an amorphous solid phase will form. For many alloys, the temperature at which amorphization begins - ТЗ lies in the range from 400 to 500 ºC. Most traditional ingots and castings cool slowly according to curve 1 in Fig. 1.23. During cooling, crystallization centers appear and grow, forming the crystalline structure of the alloy in the solid state. At a very high cooling rate (curve 2), an amorphous solid phase is formed. The intermediate cooling rate (curve 3) is also of interest. For this case, a mixed version of solidification is possible with the presence of both crystalline and amorphous structures. This option occurs in the case when the crystallization process that has begun does not have time to complete during cooling to temperature TZ. The mixed version of solidification with the formation of small amorphous particles is illustrated by a simplified diagram presented in Fig. 1.24.
Rice. 1.24. Scheme of formation of small amorphous particles
On the left in this figure there is a large drop of melt containing 7 crystallization centers capable of subsequent growth. In the middle, the same drop is divided into 4 parts, one of which does not contain crystallization centers. This particle will harden into amorphous form. On the right in the figure, the original particle is divided into 16 parts, 9 of which will become amorphous. In Fig. 1.25. the real dependence of the number of amorphous particles of a high-alloy nickel alloy on the particle size and cooling intensity in a gaseous environment (argon, helium) is presented.
Rice. 1.25. Dependence of the number of amorphous particles of a nickel alloy on
particle size and cooling intensity in a gaseous environment
The transition of a metal melt into an amorphous, or as it is also called, glassy state is a complex process and depends on many factors. In principle, all substances can be obtained in an amorphous state, but pure metals require such high cooling rates that cannot yet be provided by modern technical means. At the same time, highly alloyed alloys, including eutectic alloys of metals with metalloids (B, C, Si, P) solidify in an amorphous state at lower cooling rates. In table Table 1.9 shows the critical cooling rates during amorphization of nickel and some alloy melts.
Table 1.9
