What is refrigeration? What is enthalpy difference diagram? What is supercooling and superheating? In this issue, we will quickly understand the relevant knowledge of refrigeration through 28 diagrams drawn by refrigeration engineers!
Saturated vapors
Supercooled liquid, superheated steam
Terminology & Units
The unit of temperature is usually expressed in °C (the freezing point of water is 0°C, the boiling point is 100°C), and the thermodynamic absolute temperature K is generally used when calculating heat.
pressure
Gauge pressure: (kg/cm2)⇒ MPa. Refers to the pressure indicated by the pressure gauge, and the atmospheric pressure is 0.
Absolute pressure :(kg/cm2abs)⇒ Mpa(abs). It is the pressure when the pressure in the complete vacuum state is set to 0, and the absolute pressure = gauge pressure + atmospheric pressure (0.1).
Supercooling, superheat
I display it by state, pressure, temperature of the refrigerant
Pressure-enthalpy diagram
It is a map of the refrigerant and shows the thermodynamic properties of the refrigerant (pressure, temperature, heat, etc.)
Enthalpy diagram function: The pressure-enthalpy diagram plots the state change of the refrigerant in the refrigeration cycle, according to which the refrigeration capacity and condensation heat can be calculated, and the operating state of the refrigeration device can be judged.
Refrigeration cycle and pressure-enthalpy diagram
The P-H diagram was used for the refrigeration cycle
The enthalpy diagram is drawn according to the following conditions:
• Refrigerant R22
• Evaporation temperature 5°C
• Condensing temperature 50°C
• Compressor suction refrigerant gas temperature 15°C (superheat 10°C)
• Expansion valve inlet refrigerant liquid temperature 45°C (subcooling degree 5°C)
Changes in the refrigeration cycle
Change in condensing temperature (high pressure).
Why does the high pressure rise, and what happens when the high pressure rises?
a. Indicates the energy efficiency ratio of the refrigeration cycle at the standard condensing temperature, and the energy efficiency ratio is Q1/W1
b. Indicates that the condensing temperature is higher and the refrigeration cycle is higher, and the energy efficiency ratio is q2/W (reduced)
c. Refrigeration cycle with low condensing temperature (good heat dissipation), energy efficiency ratio Q3/W3 (increase)
The main reason for the increase in high pressure
Change in evaporation temperature (low pressure).
Change in evaporation temperature
(a) Refrigeration cycles representing standard evaporation temperatures; the energy efficiency ratio is the energy efficiency ratio q1/W1.
(b) Refrigeration cycles with low evaporation temperatures (poor heat absorption) and energy efficiency compared to q2/W2 (less).
(c) Indicates a refrigeration cycle with a high evaporation temperature (good endothermy) and an energy efficiency ratio of q3/W3 (addition).
The main reason for the drop in low pressure
Change in evaporation temperature
Change in supercooling
a) The refrigeration cycle indicates the standard evaporation temperature, and the subcooling degree is 5~7°C;
・The degree of supercooling is basically determined by the amount of refrigerant poured into the system;
・Low amount of refrigerant (air leakage, insufficient perfusion), decreased refrigeration effect, insufficient refrigeration capacity;
・The high pressure of the refrigerant increases the degree of supercooling, but the refrigeration effect remains almost unchanged.
b) The cooling and condensation outlet liquid → the supercooling degree increases, the refrigeration capacity increases, and the energy efficiency ratio is also improved.
Change in inspiratory state
(a) The appropriate inspiratory state is slightly less overheated than the saturated vapor
(b) The suction air is exhausted in a state of overheating, and all the compressors such as motors are overheated, and the service life is reduced.
(c) The suction is in a wet state, the compressor compresses the liquid, and the valve is easily damaged.
Effect of piping pressure loss