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Senior refrigeration technician title paper aAnalysis of refrigeration technology
Abstract Refrigeration technology is in order to adapt to the needs of people for low temperature conditions and the emergence and development of. Refrigeration technology is to make a space or object temperature down to below the surrounding ambient temperature, and remain in the provisions of the low-temperature state of a science and technology, it is with the people of low-temperature conditions of the requirements of the improvement of social productivity and the continuous development. Refrigeration methods are many, common are the following four: liquid vaporization refrigeration, gas expansion refrigeration, vortex control cold and thermoelectric refrigeration. One of the most widely used liquid vaporization refrigeration, it is the use of liquid vaporization of the heat absorption effect and the realization of refrigeration. Vapor compression, absorption, vapor injection and adsorption refrigeration are liquid vaporization refrigeration. This article focuses on the working principle of vapor compression refrigeration and several forms.
Keywords vapor compression refrigeration pressure-enthalpy diagram ideal refrigeration cycle refrigeration coefficient? Adiabatic expansion
Two-stage vapor compression refrigeration cycle
Classification number: TB6 Literature identification code: A
First, the working principle of vapor compression refrigeration Vapor compression refrigeration system consists of a compressor, condenser, expansion valve, evaporator, with the pipeline will be connected into a closed system.
The mass in the evaporator and the object to be cooled by the heat exchange, absorb the heat of the object to be cooled and vaporized, resulting in low-pressure vapors are inhaled by the compressor, compressed and discharged at high pressure. Compression process requires energy consumption. Compressor discharge of high-temperature and high-pressure gaseous mass in the condenser by the room temperature cooling medium (water or air) cooling, condensing into high-pressure liquid. High-pressure liquid by the expansion valve throttling, into low-pressure, low-temperature wet steam, into the evaporator, where the low-pressure liquid in the evaporator again vaporization refrigeration, and so on and so forth.
Liquid into gas, solid into liquid, solid into gas to absorb latent heat. Any liquid in the boiling process will absorb heat, the boiling temperature of the liquid (i.e., saturation temperature) and the amount of heat absorbed with the pressure at which the liquid changes, the lower the pressure, the lower the boiling temperature. And the saturation pressure, boiling temperature and heat absorption of different liquids are also different. Table 1
Example: at 1 atmospheric pressure
Refrigerant Boiling point (℃) Latent heat of vaporization r (kJ / kg)
Water 100 2256
R717 (ammonia) -33.4 1368
R22 -40.8 375
According to the thermodynamic properties of the refrigeration liquid used (known as refrigerant). By creating certain pressure conditions, it is possible to obtain the required low temperature within a certain range. To realize the refrigeration cycle must have a certain equipment, and to consume energy as compensation. Vapor compression refrigeration cycle is to use the compressor and other equipment, to consume mechanical work as compensation, the state of the refrigerant cycle changes, so as to use the cold occasion to obtain continuous and stable cold and low temperature. In the refrigeration cycle, the refrigerant undergoes state change processes such as vaporization, compression, condensation, and throttling expansion. In order to analyze, compare and calculate the performance of the refrigeration cycle, must know the state parameter change law of refrigerant. Currently used refrigerants, the relationship between these state parameters have been made into a variety of charts and tables to indicate.
Thermodynamic properties of refrigerant map, commonly used thermal properties of the temperature entropy (T-S) map and pressure enthalpy (㏒p-h) map, the form of the following chart, the figure x = 0 for the line of saturated liquid, x = 1 for the line of saturated vapors, between the two lines for the wet steam area, which is dryness of the line (x = 0.1, x = 0.2?).
As the heat absorption of the constant pressure process, heat release and adiabatic compression process of the compressor's power consumption can be expressed on the ㏒p-h diagram, using the process of the initial and final state of the specific enthalpy difference calculations, so the ㏒p-h diagram in the refrigeration cycle of the thermal calculations have been widely used. As the refrigerant thermal parameters h, s, etc. are relative values, therefore, in the use of the above thermal properties of the table and chart, must pay attention to their h, s between the datum is the same, for the datum value of different or inconsistent units of the chart or table, it is best not to mix, or it must be converted and corrected.
Second, the ideal refrigeration cycle? Inverse Carnot cycle
Carnot cycle is divided into positive Carnot cycle and inverse Carnot cycle, are composed of two constant temperature and two adiabatic processes, they are an ideal cycle. The main purpose of the study of vapor compression refrigeration cycle is to analyze the various factors affecting the refrigeration cycle, and seek ways to save refrigeration energy consumption. The reverse Carnot cycle is such that the work mass (refrigerant) passes through the refrigeration unit after absorbing heat from a low-temperature heat source and compensating for it with external work, and then flows to a high-temperature heat source. Reverse cycle is a cycle of consuming work, the refrigeration cycle is carried out according to the reverse cycle, in the temperature? On a temperature-entropy or pressure-enthalpy diagram, the individual parts of the cycle are shown. entropy or pressure-enthalpy diagram, the cycle of the processes are sequentially in the counterclockwise direction.
Schematic diagram of inverse Carnot cycle equipment
2. Conditions that must be met to realize the inverse Carnot cycle:
(1) constant temperature of high and low temperature heat source;
(2) no difference in heat transfer temperature between the work material in the condenser and the evaporator and the external heat source;
(3) no internal irreversible loss of the work material when it flows through the equipment;
(4) as a result, there is no internal loss in the condenser and evaporator;
(5) no loss in the temperature of the work material when it flows through the equipment. > (4) As necessary equipment to realize the inverse Carnot cycle are the compressor, condenser, expander and evaporator.
The inverse Carnot cycle is a reversible ideal refrigeration cycle, which does not take into account the internal and external irreversible losses of the work mass in the process of flow and state change. Although the inverse Carnot cycle cannot be realized, the conclusions drawn from the analysis of this cycle are of great significance in guiding the actual refrigeration cycle.
3. Refrigeration coefficient?
A refrigeration cycle is often characterized by a refrigeration coefficient ? The coefficient of refrigeration is equal to the amount of cold produced per unit of power consumption.
? =q/?W
q: the amount of heat absorbed by 1kg of refrigerant from the cooled object at T0 temperature q (kJ/kg)
W: the amount of work consumed to circulate 1 kg of the mass
For the inverse Carnot cycle:
?c=T0/(Tk- T0)
T0: the evaporation temperature; Tk: the condensation temperature
From the formula, it can be seen that the refrigeration coefficient of the inverse Carnot cycle is only related to the temperature of the high and low temperature heat source, and has nothing to do with the thermophysical properties of the refrigerant. Because the inverse Carnot cycle does not take into account all kinds of losses, and the compressor utilizes the work output of the expander, therefore, in the constant high and low temperature heat source interval, the refrigeration coefficient of the inverse Carnot cycle is the largest, and the refrigeration coefficients of other kinds of refrigeration cycles carried out in the temperature interval are all smaller than the refrigeration coefficients of the inverse Carnot cycle.
Therefore, the refrigeration coefficient of inverse Carnot cycle can be used to evaluate the thermal perfection of other refrigeration cycles.
Third, the theoretical cycle of vapor compression refrigeration and thermal calculation
1. Theoretical refrigeration cycle is different from the inverse Carnot cycle is:
(1) refrigerant in the condenser and evaporator according to the process of isobaric cycle, and has a heat transfer temperature difference;
(2) refrigerant adiabatic throttling with the expansion valve, rather than adiabatic expansion of expansion;
(3) The compressor draws in saturated steam instead of wet steam.
< p> Throttling loss after using an expansion valve instead of an expander: not only increases the power consumption of the refrigeration cycle, but also the loss of refrigeration capacity. These two parts of the loss is bound to make the refrigeration coefficient and thermal perfection has decreased.2. Dry compression instead of wet compression after the loss of superheat include:
(1) with the expansion valve instead of the expansion machine after the loss of throttling leads to the consequences: the expansion valve throttling is an irreversible process, throttling before and after the enthalpy is unchanged; refrigerant dryness increases, the liquid content decreases, the cooling capacity decreases, the power consumption rises, the refrigeration coefficient decreases, and the degree of its reduction is known as the loss of throttling. The size of the throttling loss is related to the following factors: with the condensing temperature and evaporating temperature difference, the throttling loss increases with the increase; with the physical nature of the refrigerant, the general throttling loss of refrigerant, superheating loss is small; with the condensing pressure, condensing pressure Pk closer to the critical pressure Pkr throttling loss is greater.
(2) dry compression instead of wet compression after the saturation loss
Reason: in the actual operation of the refrigeration compressor, if the inhalation of wet vapor, it will cause liquid shock, and occupies the cylinder volume, so that the suction volume is reduced, the cooling capacity decreased. Excess liquid into the compressor cylinder, it is difficult to vaporize all, at this time, both destroy the lubrication of the compressor, but also cause liquid shock, so that the compressor is destroyed. Therefore, vapor compression refrigeration device in actual operation is strictly prohibited wet compression, the refrigerant required to enter the compressor for dry saturated vapor or superheated steam, dry compression refrigerator normal operation of an important label. How to realize dry compression, as shown below, can be added to the evaporator outlet a liquid separator. The dry saturated vapor in the upper part of the separator is sucked away by the compressor to ensure dry compression, and the state point of the refrigerant entering the compressor is located on the saturated vapor line. The adiabatic compression process of the refrigerant takes place in the superheated vapor zone. Therefore, the refrigerant is not a constant temperature process in the condenser, but a constant pressure process.
Thermodynamic calculation of refrigerant cooling capacity per unit mass in the evaporator:
q0 = h1-h4 [kJ/kg]
Adiabatic compression power dissipation per unit mass of the compressor:
W= h2- h1 [kJ/kg]
Refrigerant cooling capacity per unit volume:
Qv= q0/V[kJ/ m3]
Theoretical refrigeration coefficient: ? = q0/ W
3. Vapor compression refrigeration cycle improvement
In order to recool the liquid refrigerant in front of the expansion valve, a recooler or reheat cycle can be used.
(1) set the re-cooler for the same refrigerant, throttling loss is mainly associated with throttling before and after the temperature difference (Tk - T0), the smaller the temperature difference, the smaller the throttling loss. Generally, a re-cooler can be added after the condenser so that the cooling water passes through the re-cooler and then enters the condenser. After re-cooling can make the liquid refrigerant in the condensing pressure is re-cooled to the state point 3?, Figure 3-3? is a high-pressure liquid refrigerant in the re-cooler in the re-cooling process, re-cooling can be reached at a temperature of Tr, known as the re-cooling temperature, condensing temperature and re-cooling the temperature difference △ Tr is called re-cooling degree, this cycle with re-cooling cycle is called the re-cooling cycle.
Increase the subcooling can make the refrigeration coefficient increase: refrigerant R717 per subcooling 1 ℃, the refrigeration coefficient can be increased by 0.46%; cold refrigerant R22 per subcooling 1 ℃, the refrigeration coefficient can be increased by 0.85%.
(2) back to the heat cycle in order to make the expansion valve before the liquid re-cooling to increase, and further reduce the throttling loss, but also to ensure that the compressor suction has a certain degree of superheat, and then in the refrigeration system to set up a back to the heater. The role of the return heaters is to make the refrigerant liquid before the expansion valve and compressor suction before the refrigerant vapor heat exchange, so that the compressor suction steam has a certain degree of superheat, due to superheat (superheat △ q) increases the compressor's power consumption (△ w). Therefore, whether the refrigeration coefficient of the return heat cycle is increased, depending on the ratio of △q/△w.
The following shows the changes in refrigeration coefficient and exhaust temperature of several commonly used refrigerants after adopting heat recovery cycle.
Refrigerant R717 R22 R502
Refrigeration coefficient of increase or decrease rate of % -4.18 -1.88 +3.02
Exhaust temperature change ℃ 140.3?102 84.7?53.5 66.5?37.3
As can be seen in the table above, the refrigeration coefficient does not necessarily increase after the use of the heat recovery cycle, and the refrigerant R22 is not necessarily increased. R22 after using the return heat cycle refrigeration coefficient decreased not much but to ensure the amount of dry compression thermal expansion valve stability, so the actual use of the return heat cycle. R502 and R12 is suitable for the use of the return heat cycle. R11 and R717 because of the coefficient of refrigeration decreased a lot is not suitable for the use of the return heat cycle.
Fourth, two-stage vapor compression refrigeration cycle
For the piston refrigeration compressor single-stage refrigeration cycle, in the usual environment, generally can only produce
-25 ℃ ~ -35 ℃ above the evaporating temperature. If the single-stage refrigeration cycle to produce lower evaporating temperatures, there will be many harmful factors, such as:
(1) the compressor exhaust temperature is very high, not only increase the superheat loss, so that the refrigeration coefficient declines, but also deteriorate the effect of lubricating oil, affecting the service life of the compressor and normal operation.
(2) the compression ratio (Pk/P0) increased, in the normal ambient temperature, when the evaporating temperature T0 decreased, Pk/P0 increased, the compressor volumetric efficiency is reduced, the actual suction volume is reduced, the refrigeration capacity decreased, when the compression ratio reaches a certain value, the piston refrigeration machine at this time can not be refrigeration.
(3) increased throttling losses, refrigerant unit cooling capacity reduction, increased consumption of power, refrigeration coefficient decreased.
(4) too low evaporating temperature may make the operating conditions of the refrigeration system more than the compressor standard design and use conditions, resulting in an impermissible and dangerous situation. Such as piston compressor (refrigerant R22) compression ratio, greater than 6 (high-temperature machine) and 16 (low-temperature machine) pressure difference (Pk - P0) can not be greater than 1.6MPa; screw compressor (refrigerant R22) exhaust temperature can not be higher than 105 ° C. Refrigerant R22 when the compression ratio of ?10, using a single-stage compression, the compression ratio of > 10 when the use of two-stage compression; refrigerant R717 when the compression ratio of ?10, using single-stage compression, compression ratio > 10 when the use of two-stage compression; refrigerant R717 when the compression ratio of ?10, using single-stage compression; refrigerant R718 when the compression ratio of the compression ratio of ? R717 when the compression ratio of ?8, single-stage compression, compression ratio >8 when the use of two-stage compression. Therefore, for the piston compressor, when the T0 is lower than -25 ~ -35 ℃, the use of bipolar refrigeration cycle can make the above unfavorable effects to improve. For screw compressor, because it has a good oil cooling device, the exhaust temperature is lower than the piston compressor, the allowable compression ratio and pressure difference are larger. Therefore, the general screw compressor single-stage refrigeration cycle can produce a low temperature of about -40 ℃ (Tk at 40 ℃ ~ 45 ℃). Air source heat pump unit, the compressor should be able to evaporate at least -15 ℃ ~ +15 ℃ (two-stage compression up to -35 ℃) condensing temperature?65 ℃ under normal working conditions.
The following figure is a schematic diagram of two-stage compression refrigeration cycle:
Two-stage compression refrigeration cycle is usually used flash vapor separator (energy saver) and intermediate cooler in two forms. The following two-stage compression refrigeration cycle with intercooler. The cycle puts the refrigerant vapor from the evaporator to two compressors in series (with intercooler) or two sets of cylinders of the same compressor? Relay? Type compression. Each stage of the compression ratio, exhaust temperature and so on are in line with the conditions of use of the compressor, but also to obtain a lower evaporation temperature T0, the refrigeration coefficient than the same refrigeration capacity of the single-stage refrigeration cycle, and therefore more economical. The following is a commonly used two-stage compression refrigeration cycle.
A throttling, complete intermediate cooling of the two-stage compression refrigeration cycle, the so-called complete intermediate cooling refers to the low-pressure compression stage from the compression stage of the superheated vapors in the intermediate cooler is completely cooled to saturation as shown in the following chart:
Due to the ammonia refrigeration system, high exhaust temperature, suction superheat can not be large, so this form of circulation is widely used in ammonia two-stage refrigeration system. This system is characterized by the use of complete intermediate cooling, you can reduce the superheat loss, so the power consumption is less than a single-stage, the refrigeration coefficient is larger than a single-stage. Intermediate pressure Pm=( Pk.P0)0.5
Ammonia two-stage compression of the best intermediate temperature t Jia = 0.4 Tk + 0.6T0 + 3 ℃
T0: evaporating temperature; Tk: condensing temperature
Compression ratio = Pk/P0 Pk: condensing pressure P0: evaporating pressure
When the known refrigeration capacity of Q0, through the evaporator of the refrigerant Mass flow rate Mr, then Mr= Q0/(h1-h8)
Theoretical total power consumption of the refrigeration cycle compressor is Pth, Pth= Pth1+ Pth2
Pth1 is the theoretical power consumption of the low-pressure compressor (KW)
Pth2 is the theoretical power consumption of the high-pressure compressor (KW)
Then the theoretical coefficient of refrigerating ?th= Q0/ Pth
V. CONCLUSION
With the development of technological modernization and the continuous improvement of people's living standards, refrigeration in industry, agriculture, national defense, construction, science and other sectors of the national economy, the role and status of the increasingly important. In particular, people's living standards to improve the requirements of different food storage temperature is different, two-stage compression can meet the lower temperature requirements, people in any season can taste the fresh food. In agriculture and animal husbandry, refrigeration is used for low-temperature treatment of crop seeds; construction of artificial climate seedling room. Refrigeration plays an increasingly important role in health care and industrial production. In short, through the study of this paper, the principle of the refrigeration system has a comprehensive understanding of how to improve the refrigeration coefficient of measures have an understanding.
References
Wu Yezheng Refrigeration Principles and Equipment Xi'an Jiaotong University Press
Yuchi Bin Practical Refrigeration and Air Conditioning Engineering Handbook Mechanical Industry Press
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