Some knowledge of refrigeration requires a comprehensive understanding of temperature, heat, specific heat, sensible heat, latent heat, pressure, evaporation and boiling, etc., so let's take a closer look.

1 temperature

The temperature is used to indicate the degree of cold and heat of the substance. The degree of temperature can be measured by a thermometer. For example, a glass thermometer, the liquid in the tube expands after being heated, the liquid level rises, and after the cooling shrinks, the liquid level decreases, and the level of the liquid level rises. Indicates the level of temperature.

Celsius temperature: Under standard atmospheric pressure, the freezing point of water is taken as 0 degree, the boiling point is taken as 100 degrees, and 100 squares are equalized between 0 and 100 degrees, and each square is 1 degree, which is represented by the symbol °C.

Fahrenheit temperature: Under standard atmospheric pressure, the freezing point of water is set to 32 degrees, and the boiling point is set to 212 degrees. The equilibrium between the two is engraved into 180 grids, each grid being 1 degree, expressed by the symbol oF.

Kelvin temperature (also known as absolute temperature): It is based on the Celsius temperature scale, the freezing point of water is set to 273.16 degrees, and the boiling point of water is set to 373.16 degrees. Theoretically, the point at which all molecules in the material stop moving is taken as 0 degrees. The symbol K is indicated.

2 heat

The temperature of the object indicates the degree of thermal motion of the material molecules of the object. The temperature also indicates the energy of the object. This energy is called thermal energy. When two objects with different temperatures are in contact, the temperature of the two components tends to be consistent. The thermal energy is transferred from the object with higher temperature to the object with lower temperature. At this time, the energy released or absorbed by the object is called heat.

Card: 1 gram of water is heated or cooled under standard atmospheric pressure. When the temperature is raised or lowered by 1 ° C, the amount of heat added or removed is called 1 card, expressed by the symbol cal. Because the card unit is too small, the project often uses its 1000 times kcal or big card to indicate. The symbol is kcal.

British thermal unit: Heating or cooling 11b (lbs) (11b = 0.445kg) of water at a standard atmospheric pressure, the temperature of which rises or decreases by 1oF, and the amount of heat added or removed is called a British thermal unit. The symbol is Btu.

Joule: In the International System of Units, the unit of heat taken is consistent with the unit of work, expressed in joules. Joule is equivalent to the work done by moving the force of 1 m (m) in the direction of the force with a force of 1 N (Newton). Therefore, in the International System of Units, Joule is the unit of work and energy. The use of this unit simplifies the calculation. The sign of Joule is J. The legal heat unit in China is Joule.

3 specific heat

Any substance that adds heat will increase its temperature. But different substances of the same quality, when the same temperature is raised, the heat added is not the same. For comparison, the heat required to raise the temperature of 1 kg of water by 1 °C was 4.19 kJ. Taking this as a standard, the ratio of the heat required by other substances to it is called specific heat.

Knowing the specific heat value of the material, you can calculate the amount of heat that needs to be removed to cool it. For example, to cool 5kg of 70°C water to 15°C, the heat to be removed is: Q=mcD t = 5×4.19×(70-15)=1152.25 kJ

4 sensible heat

Heating a solid, liquid or gaseous substance, as long as its shape is constant, the temperature of the substance is increased after the heat is added, and the amount of added heat can be displayed at the temperature, that is, without changing the form of the substance. The heat whose temperature changes is called sensible heat.

If the liquid water is heated, as long as it remains in a liquid state, its temperature rises; therefore, sensible heat only affects the temperature change surface without causing a change in the form of the substance.

5 latent heat

When the liquid water is heated, the temperature of the water rises. When the boiling point is reached, although the heat is continuously added, the temperature of the water does not rise and stays at the boiling point, and the added heat only turns the water into water vapor, that is, The liquid state changes to a gaseous state. This type of heat that causes a change in state of matter (also known as a phase change) without changing the temperature of the substance is called latent heat.

6 pressure

Gas is composed of molecules. In the irregular movement, hundreds of millions of molecules frequently hit the inner wall of the container, and the force generated perpendicularly on the surface area of ​​the inner wall is called pressure. The common unit for measuring gas pressure in engineering is: kg/cm 2 or mmHg (mm Hg), and the legal unit in China is Pa (Pascal).

Atmospheric pressure: The pressure that the layer of air surrounding the Earth forms on the surface area of ​​the Earth is called atmospheric pressure. Usually indicated by B. The unit is expressed in PaPa or kPa.

Absolute pressure: local atmospheric pressure ten working pressure.

Working pressure (gauge pressure): The pressure measured by the meter.

7 evaporation and boiling

Evaporation: refers to the process of gasification on the free surface of a liquid.

Boiling: refers to the intense gasification process in the form of bubbles inside the liquid.

8 heat release coefficient

When freezing a substance, such as blowing on the surface, it freezes faster than when it is not blown. The physical quantity indicating the heat exchange capacity of different substances in different states is called the heat release coefficient, and its value is equal to the hourly and square meter area, which is transmitted when the temperature difference between the fluid and the solid wall is 1 °C. The heat. Expressed by the symbol a, the unit is kcal/(m2h°C), and the international system of units is W/(m2k) or J/(m2h°C). The conversion relationship between the two is: 1W/(m2K)=0.860kcal/( M2h°C)

9 heat transfer coefficient

Heat is transferred from the high temperature side fluid through the flat wall to the low temperature side fluid. In addition to the temperature difference between the two sides and the heat transfer area, the heat transfer capability is related to the thermal conductivity of the flat wall, the thickness of the flat wall and the heat release coefficient on both sides of the wall. Put all the factors into an equation, namely:

Q=KFD t (kJ/h)

Where: Q: heat transferred (kJ/h); F: surface area of ​​the flat wall (m2); D t : temperature difference D t=t1-t2 (°C); K: heat transfer coefficient kJ/(m2h°C)

K is the heat transfer coefficient, which is numerically equal to the heat transferred from one side of the hot fluid to the other side of the cold fluid when the temperature difference between the two sides is 1 ° C and 1 h passes through the heat transfer area of ​​1 m 2 . The unit is kJ/(m2h°C) or W/(m2k).

10 specific volume and density

The mass of wet air per unit volume is called density. Expressed by the symbol ρ. The volume occupied by the unit mass of humid air is called a specific volume and is represented by the symbol V.

11 humidity

Humidity is a physical quantity indicating how much water vapor is contained in the humid air. There are three ways to express it.

Absolute humidity: The mass of water vapor in each cubic meter of wet air. The symbol is Z and the unit is g/m3. Absolute humidity is not convenient to use. It does not directly reflect the dryness and humidity of the wet air.

Moisture content: The amount of water vapor contained per kg of dry air is called the moisture content, and the symbol is d, and the unit is g/kg (dry).

Relative Humidity: The ratio of the partial pressure of water vapor in wet air to the partial pressure of saturated steam at the same temperature, symbolized.

12 dew point temperature

At a certain atmospheric pressure, the water vapor in the air condenses to the temperature of water (condensation) when the moisture content is constant. When d is constant, the air temperature drops from the unsaturated state to the saturated state, at which time the relative humidity of the air = 100%. In the air conditioning technology, the air is cooled to the dew point temperature to achieve the purpose of dehumidifying the dry air.

13ç„“

ç„“ is an important parameter of wet air. It is a composite state parameter of the sum of internal energy and pressure potential energy.

During the air conditioning process, the state of the humid air often changes, and it is convenient to determine the amount of heat exchange during the change of the state. The process of changing the wet air is a constant pressure process, and the enthalpy difference is equal to the heat exchange amount.

14 static pressure, dynamic pressure, full pressure

Static pressure (Pi): The pressure generated by impinging on the wall of a pipe due to irregular movement of air molecules is called static pressure. In the calculation, the static pressure calculated by the absolute vacuum is called the absolute static pressure. Static pressure at zero atmospheric pressure is called relative static pressure. The static air pressure in the air conditioner refers to the relative static pressure. Static pressure is positive when it is higher than atmospheric pressure, and negative when it is lower than atmospheric pressure.

Dynamic pressure (Pb): refers to the pressure generated when the air flows. As long as the air in the air duct flows, it has a certain dynamic pressure, and its value is always positive.

Full pressure (Pq): Full pressure is the algebraic sum of static and dynamic pressure: Pq = Pi + Pb Total pressure represents the total energy of 1 m3 of gas. If atmospheric pressure is used as the starting point for calculation, it can be positive or negative.

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