Refrigerants and thermal characteristics

To implement the processes of transformation of heat in plants using various workingbodies. The working bodies may be individual substances or mixtures thereof. 
Workers of the body that is used in refrigeration, called refrigerants or refrigerants (HA).These are substances or mixtures thereof having at atmospheric pressure (= 0.1 MPa)boiling point Ts = 350-120 K (77 + -153 ° C). 
HA with Ts = 350-250 K (77 + -23 ° C) generally used in heat pump installations or combined. 
HA with Ts = 273-120 K (0 ^ -153 ° C) are used in refrigeration and air conditioning. 
Substances with Ts <120 K is called cryogenic agent. 
 
For refrigerants meet the following requirements: 
a) harmless to human health; 
b) low enough boiling temperature Ts at normal atmospheric pressure; 
c) low condensing pressure Pk at normal ambient (cooling) of the medium (water, air); 
g) Low pressure difference condensing and evaporating (Pk-P0); 
d) low freezing temperature Tz; 
e) a high critical temperature T c. It should be above the temperature of the cooling medium in the condenser; 
g) the minimum adiabatic index and the specific volume of the refrigerant vapor; 
h) as large as possible heat of vaporization and specific heat. 
 
In addition, the refrigerant must be fire-explosion-proof, neutral to construction materials, well to dissolve the water, have a low cost. 
All these requirements can not be satisfied, i.e. there is no perfect refrigerant. Therefore, the choice of refrigerant necessary to consider all of their quality and factors characterizing the installation and the operating conditions. 
The main thermal parameters characterizing refrigerants 
The main thermal parameters are: 
Ts, K - the boiling point at atmospheric pressure. Characterizes the temperature of the cold, which can be obtained with minimal evacuating the vapor space; 
Tcr, K - the critical temperature of the refrigerant. Characterizes the maximum temperature at which condensation of the refrigerant can be achieved; 
k - the adiabatic index of the refrigerant vapor. Characterizes the work of compressionand the temperature of the vapor 1k late compression process. The larger k, the higher the temperature of the vapor and 1K after the compression process; 
q0, J / kg - specific mass cooling capacity. This amount of heat which is removed from the cooled medium 1 kg of refrigerant circulating in the process of evaporation. Otherwise - islatent heat of vaporization of the working substance; 
qv, J / m - specific cold. This amount of heat which is removed from the chilledenvironment 1m refrigerant vapor generated in the evaporator and sucked by the compressor. Convenient option for a design based compressors XM 
Vo, m3 / kg - specific volume of the refrigerant vapor at the boiling temperature T0. 
Relationships between them: 
qv _-; qo _ qv ■ v (31) 
Vo 
In practice, used as refrigerants about 30-40 working substances. Most common are now(except for water and air) refrigerants such as ammonia, CFCs (freons), and varioushydrocarbons. They are widely used in the vapor-liquid chillers. 
Thermal characteristics of some of the most well-known refrigerants are given in Table.3.1. 
marking refrigerants 
International designation of working substances chillers - 
the letter R (from Refrigerant - Refrigerant) and a set of numbers that have a specifictranscript. 
Non-hydrocarbon refrigerants represent the number 7, and it is added to the molecular weight of the substance. For example: water - R718; ammonia - R717; carbon dioxide - R744, etc.  Thermal characteristics of some refrigerants
Name designation Chemical formula boiling point, °С the critical temperature, °С critical pressure, МПа K
1 Water R718 H2O 100 374,15 22,6 1,33
2 Air R729 - -194,4 140,7 3,76 1,4
3 ammonia R717 NH3 -33,35 132,4 11,5 1,31
4 dioxide ugleroda R744 CO2 -78,3(субл.) 31,2 7,5 1,32
5 freon-11 R11 CFCI2 23,7 198,0 4,5 1,13
6 freon-12 R12 CF2CI2 -29,8 112,04 4,2 1,14
7 freon-13 R13 CF3CI -81,5 28,78 3,9 -
8 freon-22 R22 CHF2CI -40,8 96,0 5,0 1,16
9 freon-134а R134а C2H2F4 -26,5 100,6 4,056 -
10 propane R290 C3H8 -42,1 96,8 4,3 1,14
11 ethylene R1150 C2H4 -103,9 9,5 5,2 1,24

 
Freon (CFCs) - a fluorine, chlorine derivatives of saturated hydrocarbons. On the international system to denote the starting hydrocarbons figures: 
- Methane number of halocarbons (methane - CH4); 
- A series of ethane (ethane - C2H6); 
21 - number of propane (propane - C3H8); 
31 - number of butane (butane - C4Hi0). 
To figure indicating the starting hydrocarbon, is attributed by the fluorine atoms in thehladagentt. In the absence of fluoride - 0 If there is a hydrogen atom, then the derivatives ofmethane to 1st digit, while the derivatives of ethane, propane and butane to 2nd digit is added by unsubstituted hydrogen atoms. The remaining unoccupied valence - chlorine.The number of chlorine atoms is not specified. 
For example, methane CH4 - R50. Refrigerants his series: 
R12 (Freon 12) - CF2Cl2 (dichlorodifluoromethane); 
R13 (Freon 13) - CF3Cl (triftormonohlormetan); 
R22 (Freon 22) - CHF2Cl (diftormonohlormetan), etc. 
Ethane C2H6 - R170. Refrigerants his series: 
R143 (Freon 143) - C2H3F3 (trifluoroethane); 
R134 (Freon 134) - CHF2-CHF2 (tetrafluoroethane), etc.

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