1. Temperature: Temperature is a measure of how hot or cold a substance is.
There are three commonly used temperature units (temperature scales): Celsius, Fahrenheit, and absolute temperature.
Celsius temperature (t, ℃): the temperature we often use. Temperature measured with a Celsius thermometer.
Fahrenheit (F, ℉): The temperature commonly used in European and American countries.
temperature conversion:
F (°F) = 9/5 * t(°C) +32 (Find the temperature in Fahrenheit from the known temperature in Celsius)
t (°C) = [F (°F)-32] * 5/9 (Find the temperature in Celsius from the known temperature in Fahrenheit)
Absolute temperature scale (T, ºK): generally used in theoretical calculations.
Absolute temperature scale and Celsius temperature conversion:
T (ºK) = t (°C) +273 (Find the absolute temperature from the known temperature in Celsius)
2. Pressure (P): In refrigeration, the pressure is the vertical force on the unit area, that is, the pressure, which is usually measured with a pressure gauge and a pressure gauge.
Common units of pressure are:
Mpa (megapascal);
Kpa (kPa);
bar(bar);
kgf/cm2 (square centimeter kilogram force);
atm (standard atmospheric pressure);
mmHg (millimeters of mercury).
Conversion relationship:
1Mpa=10bar=1000Kpa =7500.6 mmHg = 10.197 kgf/cm2
1atm=760mmHg=1.01326bar =0.101326Mpa
Generally used in engineering:
1bar = 0.1Mpa ≈1 kgf/cm2 ≈ 1atm = 760 mmHg
Several pressure representations:
Absolute pressure (Pj): In a container, the pressure exerted on the inner wall of the container by the thermal motion of the molecules. The pressure in the refrigerant thermodynamic properties table is generally absolute pressure.
Gauge pressure (Pb): The pressure measured with a pressure gauge in a refrigeration system. Gauge pressure is the difference between the gas pressure in the container and the atmospheric pressure. It is generally believed that the gauge pressure plus 1bar, or 0.1Mpa, is the absolute pressure.
Vacuum degree (H): When the gauge pressure is negative, take its absolute value and express it in vacuum degree.
3. Refrigerant thermodynamic properties table: The refrigerant thermodynamic properties table lists the temperature (saturation temperature) and pressure (saturation pressure) and other parameters of the refrigerant in the saturated state. There is a one-to-one correspondence between the temperature and pressure of the refrigerant in the saturated state.
It is generally believed that the refrigerant in the evaporator, condenser, gas-liquid separator, and low-pressure circulating barrel is in a saturated state. The vapor (liquid) in a saturated state is called saturated vapor (liquid), and the corresponding temperature and pressure are called saturation temperature and saturation pressure.
In a refrigeration system, for a refrigerant, its saturation temperature and saturation pressure are in one-to-one correspondence. The higher the saturation temperature, the higher the saturation pressure.
The evaporation of the refrigerant in the evaporator and the condensation in the condenser are carried out in a saturated state, so the evaporation temperature and the evaporation pressure, and the condensation temperature and the condensation pressure are also in a one-to-one correspondence. The corresponding relationship can be found in the table of refrigerant thermodynamic properties.
4. Refrigerant temperature and pressure comparison table:
5. Superheated steam and supercooled liquid: Under a certain pressure, the temperature of the steam is higher than the saturation temperature under the corresponding pressure, which is called superheated steam. Under a certain pressure, the temperature of the liquid is lower than the saturation temperature under the corresponding pressure, which is called supercooled liquid.
The value at which the suction temperature exceeds the saturation temperature is called suction superheat. The suction superheat degree is generally required to be controlled at 5 to 10 °C.
The value of the liquid temperature lower than the saturation temperature is called the liquid subcooling degree. Liquid subcooling generally occurs at the bottom of the condenser, in the economizer, and in the intercooler. The liquid subcooling before the throttle valve is beneficial to improve the cooling efficiency.
6. Evaporation, suction, exhaust, condensation pressure and temperature
Evaporating pressure (temperature): The pressure (temperature) of the refrigerant inside the evaporator. Condensing pressure (temperature): The pressure (temperature) of the refrigerant in the condenser.
Suction pressure (temperature): The pressure (temperature) at the suction port of the compressor. Discharge pressure (temperature): The pressure (temperature) at the compressor discharge port.
7. Temperature difference: heat transfer temperature difference: refers to the temperature difference between the two fluids on both sides of the heat transfer wall. The temperature difference is the driving force for heat transfer.
For example, there is a temperature difference between refrigerant and cooling water; refrigerant and brine; refrigerant and warehouse air. Due to the existence of heat transfer temperature difference, the temperature of the object to be cooled is higher than the evaporation temperature; the condensation temperature is higher than the temperature of the cooling medium of the condenser.
8. Humidity: Humidity refers to the humidity of the air. Humidity is a factor that affects heat transfer.
There are three ways to express humidity:
Absolute humidity (Z): The mass of water vapor per cubic meter of air.
Moisture content (d): The amount of water vapor contained in one kilogram of dry air (g).
Relative humidity (φ): Indicates the degree to which the actual absolute humidity of the air is close to the saturated absolute humidity.
At a certain temperature, a certain amount of air can only hold a certain amount of water vapor. If this limit is exceeded, the excess water vapor will condense into fog. This certain limited amount of water vapor is called saturated humidity. Under saturated humidity, there is a corresponding saturated absolute humidity ZB, which changes with the air temperature.
At a certain temperature, when the air humidity reaches the saturated humidity, it is called saturated air, and it can no longer accept more water vapor; the air that can continue to accept a certain amount of water vapor is called unsaturated air.
Relative humidity is the ratio of absolute humidity Z of unsaturated air to absolute humidity ZB of saturated air. φ=Z/ZB×100%. Use it to reflect how close the actual absolute humidity is to the saturated absolute humidity.
Post time: Mar-08-2022