What are the thermal state parameters of the refrigerant

What are the thermal state parameters of the refrigerant

In the refrigeration cycle, the working fluid continuously changes its thermal state. The physical quantity describing the thermal state of the working fluid is called the thermal state parameter of the working fluid, referred to as the state parameter. For a certain state, its state parameter has a certain value. When the working fluid state changes, the difference between the initial and final state parameters is only related to the initial and final state, and has nothing to do with the process of state change.

　　 Common state parameters in refrigeration technology are: temperature, pressure, specific volume, internal energy, enthalpy and entropy, etc. These parameters are very important for the analysis and thermal calculation of the refrigeration cycle.

　　 1. Temperature: Temperature is a physical quantity that describes the hot and cold state of the thermal system, and is a parameter that marks the degree of cold and heat of an object.

　　The temperature of the object can be measured with a temperature measuring instrument. In order to make the temperature measurement accurate and consistent, there must be a scale for measuring temperature, referred to as the temperature scale. The temperature scale commonly used in engineering is: Celsius temperature scale: also known as the international Baidu temperature scale. The commonly used symbol t indicates that the unit is ℃.

　　Absolute temperature scale: The commonly used symbol T indicates, the unit is Kelvin (code name is K).

　　The absolute temperature scale and the Celsius temperature scale are only different from the starting point (t=0℃, T=273.16K), and their temperature intervals per degree are indeed the same. In engineering, its relationship can be expressed as:

　　T=273+t(K)

　　 2. Pressure: Pressure is the vertical force per unit area, usually represented by the symbol P.

　　The pressure can be measured with a pressure gauge. In the International System of Units, the pressure unit is Pascal (Pa), and in actual application

　　 can also be expressed in megapascals (MPa) or bar (bar), 1MPa=106Pa and 1bar=105 Pa.

　　 There are three types of pressure marks: absolute pressure, gauge pressure and vacuum degree. Absolute pressure refers to the actual pressure of the gas in the container, represented by the symbol P; gauge pressure (PB) refers to the pressure indicated by the pressure gauge (or vacuum gauge); and when the absolute pressure of the gas is lower than the atmospheric pressure (B) , The absolute pressure in the container is lower than the atmospheric pressure, called the degree of vacuum (PK). The relationship between the three is: P=PB+B or P=B-PK

　　The state parameter of the working fluid should be absolute pressure, not gauge pressure or vacuum degree.

　　 3. Specific volume: Specific volume refers to the volume occupied by the working fluid per unit mass, which is represented by the symbol υ.

　　 Specific volume is a physical quantity that describes the density of working fluid molecules. The reciprocal of the specific volume is the density of the working fluid, that is, the mass of the working fluid per unit volume, represented by the symbol ρ. There is a reciprocal relationship between specific volume and density.

　　 Fourth, internal energy: internal energy is the sum of molecular kinetic energy and molecular potential energy inside the working fluid, which is represented by the symbol u.

　　The kinetic energy of molecules includes the kinetic energy of linear motion, the kinetic energy of rotational motion and the vibrational energy inside the molecule.

　　 is related to the temperature of the gas. The size of molecular potential energy is related to the distance between molecules, that is, the specific volume of the working fluid.

　　 Since the internal kinetic energy of a gas is determined by the temperature of the gas and the internal potential energy is determined by the specific volume of the gas, the internal energy of the gas is a function of its temperature and specific volume. In other words, internal energy is a state parameter.

　　 V. Enthalpy: Enthalpy is a composite thermal state parameter that represents all the total energy in the system. It is the sum of internal energy and pressure. For 1kg working fluid, it can be expressed as:

　　H = u＋Pυ (kJ/kg) or (kcal/kg)

　　 where: h— enthalpy or specific enthalpy (kJ/kg or kcal/kg): υ— specific volume (m3/kg)

　　U— internal energy (kJ/kg or kcal/kg): p— absolute pressure (N/m2 or [wqp1] [wqp2] Pa)

　　In the engineering unit system, the pressure unit is usually engineering air pressure, physical atmospheric pressure and millimeter water column.

　　 Since internal energy and pressure potential energy are both temperature parameters, enthalpy is also a state parameter. To be precise, enthalpy is the total heat added by a certain mass of fluid from a certain initial state to any thermal state.

6. Entropy: Entropy is a derived thermal state parameter. The Chinese meaning of entropy is the quotient obtained by dividing heat by temperature. The original foreign name of entropy means "transformation", which refers to the degree to which heat can be converted into work. It represents the working fluid state When changing, the degree of heat exchange with the outside world. Entropy is calculated indirectly through other quantities that can be directly measured.