"How much do you know about the enthalpy diagram, the secret weapon of the HVAC industry? Imagine a bunch of horizontal, vertical, and oblique lines shaking in front of your eyes, and the straight, curved, and twisted graphics dazzle you. Don't worry, this article will take you into the wonderful world of enthalpy and wet diagrams. Let's unveil the mystery of enthalpy hygrogram together!"
1 What is an enthalpy psychrome diagram
Enthalpy Wettygram: A line diagram that represents the relationship between the various state parameters of the air. In this diagram, other parameters of the air can be found based on certain parameters.
2 Air parameters
Dry bulb temperature (°C): referred to as temperature, it is the temperature measured by temperature.
Moisture content (g/kg): The mass of water vapor that coexists with one kilogram of dry air in moist air.
Relative humidity: The percentage of water vapor pressure in the air compared to the saturated water vapor pressure at the same temperature.
Enthalpy (kj/kg): How much kilojoule of energy is contained in one kilogram of substance.
Heat-humidity ratio: the ratio of the change in enthalpy (△h) to the change in moisture content (△d).
3 Contours
Isotherms: The temperatures on this line are all the same. Its parallel lines are also isotherms.
Enthalpy line: The enthalpy values on this line are all the same. Its parallel lines are also isoenthalpy lines.
Isohumidity line: The humidity on this line is the same. Its parallel lines are also isohumid lines.
Isorelative humidity line: The relative humidity on this line is the same. Its parallel lines are also isorelative humidity lines.
4 One of the applications of enthalpy psychrogram
Dew point temperature: The temperature at which water vapor in the air becomes dewdrops. The temperature at point A in Figure 2 is 35°C, the relative humidity is 100%, the enthalpy value is 130kj/kg, and the moisture content is 36.6g/kg.
At this time, if the temperature drops to 30°C, the moisture content and air pressure remain unchanged. Point A is the state of point B (virtual point). At this time, when the relative humidity is greater than 100%, the excess water will condense from the gaseous state into water droplets until the relative humidity is less than or equal to 100%.
5 Enthalpy Psychrogram Application II
To illustrate, the use of air conditioners and the enthalpy hygrogram correspond to changes in winter and summer.
Point A: Room without air conditioning in summer, temperature: 30°C, relative humidity: 60%, moisture content: 13.6g/kg.
A → C （夏天家用空调降温线）
Moisture content decreases: The water vapor "spitting" by people and objects in the room < drained from the external unit of the air conditioner
Enthalpy reduction: The heat emitted by people and objects in the room < the cooling capacity of the air conditioner
Temperature decrease: A decrease in enthalpy means that the air energy is less, and the temperature is lower.
Relative humidity increases: the lower the temperature, the lower the desire of the air to "drink" water.
At point A, the air can "drink" 3g of water, 1.5g of water in the air belly, and the relative humidity is 50%.
Point B, after cooling, you can only "drink" 1g, and the drain pipe discharges 0.8g, leaving 0.7g, and the relative humidity is 70%.
Because the air has a low desire to "drink" water, it is generally not too dry to turn on the air conditioner in summer.
B → D (Heating line of household air conditioner in winter)
Enthalpy increase: The heat lost in the room < the heat added by the air conditioner
Temperature increases: The air is heated, the energy increases, and the enthalpy becomes larger.
Increased moisture content: As the temperature increases, the air has a higher desire to "drink" water and constantly absorbs moisture from the air.
Decrease in relative humidity: Constantly "drinking" water from the air still cannot satisfy the desire to "drink" water that increases greatly with the increase in temperature, and the air is still "thirsty".
❻ Expand your knowledge
▶ Wet-bulb temperature
relative to dry-bulb temperature. In layman's terms, the wet-bulb temperature is the lowest temperature that can be reached by the current environment simply by evaporating water.
▶ Absolute humidity
In the standard state, the mass of water vapour contained in each cubic meter of moist air, i.e. water vapour density, in g/m³.
▶ Sensible heat Latent heat
Sensible heat: The heat required to increase or decrease the temperature of an object without chemical or phase change is called sensible heat.
Latent heat: refers to the heat absorbed or emitted by a substance from one phase to another under isothermal and isobaric conditions.
▶ There are a few points to note
1. The saturated empty line is the isorelative humidity line with a relative humidity of 100%. Features: Dry bulb temperature, wet bulb temperature, dew point temperature are equal.
2. The iso-bulb temperature line is basically parallel to the iso-enthalpy line, so the iso-bulb temperature line is approximately replaced by the iso-enthalpy line in engineering.
3. The isomoisture content line is the isodew point temperature line. This is because the definition of dew point temperature already indicates that the dew point temperature is the same at the state point with the same moisture content.
▶ How to find air dry-bulb temperature, wet-bulb temperature, and dew point temperature on an enthalpy psychrome graph
▶ The embodiment of each function in the air conditioning unit in the enthalpy and humidity diagram