Awareness of basic terms in HVAC systems
The basic terms of HVAC mainly include temperature, moisture content, humidity, density, specific volume, dry bulb temperature, wet bulb temperature** temperature, heat, specific heat capacity, sensible heat, latent heat, total heat of matter, sensible heat ratio, pressure, pressure, melting, cooling capacity, energy efficiency ratio, etc.
1.Temperature. Temperature is a physical quantity that indicates the degree of heat and cold of a substance, which is used to represent the state of moist air, which is the air temperature, and the commonly used unit is Celsius, which is represented by symbols. Relationship between the temperature of a substance and the thermodynamic energy of a substance: Temperature is not the only sign of a change in thermodynamic energy. Temperature scales are used to represent the scale of temperature, and the commonly used ones are Celsius, Fahrenheit, and Kelvin. Commonly used temperature detection tools include ordinary thermometers, temperature sensors, thermometers, thermometers (spot thermometers, infrared thermometers), etc.
Celsius temperature ( ) The freezing point temperature of water at 1 atmosphere is set at 0, the boiling point is set at 100, and the middle is divided into 100 equal parts, each equal part is 1.
Fahrenheit ( ) The freezing point temperature of water at 1 atmosphere is set at 32 and the boiling point is set at 212, and it is divided into 180 equal parts with 1 equal part each.
Thermodynamic temperature scale (k): The minimum temperature of water is set at -27315k, each aliquot is the same size as Celsius, the thermodynamic temperature is the temperature scale that objectively reflects the properties of the substance, the speed of the molecules of the substance. One can only reduce the temperature as close to thermodynamic zero as possible, but it will never reach zero, let alone below thermodynamic zero.
The conversion relationship between Fahrenheit and Celsius and thermodynamic temperature is as follows.
Celsius = 5-9 (Fahrenheit - 32).
Fahrenheit = 9 5 degrees Celsius + 32
Thermodynamic temperature = Celsius +273
2。Moisture content.
Moisture content is used to indicate the amount of water vapor in the air, and it is measured in g kg, which means the mass of water vapor in moist air along with every 1kg of dry air.
3.Humidity. Humidity is divided into relative humidity and absolute humidity. Absolute humidity refers to the amount of water vapor contained in 1m3 of air in kg m3 or g m3. Relative humidity is the ratio of the amount of water vapor actually contained in the air to the maximum amount of water vapor that can be contained at the same temperature and pressure, expressed as a percentage.
4.Density and specific volume.
The mass possessed by a unit volume of moist air is known as density.
The volume occupied by a unit mass of moist air is called specific volume.
Specific volume and density are reciprocal to each other.
5.Dry-bulb temperature vs. wet-bulb temperature.
The dry bulb temperature is the real temperature of the air, which can be measured directly with an ordinary thermometer, and the thermometer is exposed to the air during the measurement, but not directly exposed to the sun. The temperature unit (or ) is often followed by the letter "db" to distinguish it from the wet-bulb temperature.
Wrap the thermosensitive part of the ordinary mercury thermometer with wet gauze, immerse the lower end of the gauze in the water tank, maintain a certain amount of air circulation around the gauze, keep water in the water tank, and after the indication reaches stability, the reading displayed by the thermometer at this time is called the wet bulb temperature. The temperature unit (or ) is often followed by the letter "wb" to distinguish it from the dry-bulb temperature.
According to the difference between the dry and wet bulb temperatures, we can get the relative humidity value of the air by looking up the meter.
6.Temperature.
The temperature at which the air begins to condense, which is related to the amount of water vapor in the air.
With these state parameters, we have a quantitative grasp of the state of the moist air.
7.Heat vs. specific heat capacity.
Heat is a form of energy and is a physical quantity that indicates how much heat an object absorbs or emits heat. The unit of heat is usually expressed in calories (cal) or kilocalories (kcal also known as kcal). ikcal is 1 liter of pure water that raises or decreases 1 amount of heat absorbed or released. In the International System of Units (SI), heat is often expressed in joules (J). 1j=0.2389calWhen the temperature of an object changes, the heat q absorbed or emitted by the object is related to its temperature change δt, the mass of the object, and the material properties of the object.
q=cmδt
where c is the specific heat capacity of the object. For gases, the specific constant pressure heat capacity cp when absorbing and discharging heat under constant pressure is different from the specific constant volume heat capacity cv when absorbing and discharging heat under constant volume.
8.Sensible heat. The heat that needs to be absorbed or emitted by an object when the temperature rises or decreases during heating or cooling is called sensible heat. Sensible heat is characterized by a change in temperature and a change in phase. Changes in temperature can be measured with a temperature measuring device.
9.Latent heat. During the heating or cooling of an object, its phase changes but the temperature does not change, which is called latent heat. Latent heat is characterized by a change in phase and a constant temperature. Latent heat includes heat of melting, heat of solidification, heat of vaporization, and heat of liquefaction.
When the humidity in the computer room is very high, it takes a lot of cooling capacity to condense the water vapor into water. A room with high humidity and a room with moderate humidity will consume a lot of energy if you use air conditioning to cool and dehumidify to achieve the same temperature and humidity conditions. This is also an important reason why the machine room needs to be well sealed.
10.Total heat of matter.
The addition of sensible heat and latent heat of an object is called the total heat of matter.
Total heat = sensible heat + latent heat.
11. Sensible heat ratio.
Sensible heat factor (SHF) is the ratio of sensible heat to total heat.
Sensible heat ratio = sensible heat Total heat = sensible heat Sensible heat + latent heat.
Household or building air conditioning and other equipment, mainly to provide a comfortable environment for personnel, its sensible heat ratio is usually relatively low, about 70%. In data centers and other computer rooms, more than 90% of the heat is sensible heat, and high sensible heat ratio units are required.
12. Pressure and pressure.
Compression is the force that acts heavily on the surface of an object by the weight of a substance. Pressure is the pressure of the pressurizer per unit area.
1) Static pressure (PJ), the pressure generated by the impact on the pipe wall due to the irregular movement of air molecules is called static pressure. When calculating, the static pressure with the absolute vacuum as the zero point is called the absolute static pressure. The static pressure with the atmospheric pressure at the zero point is called the relative static pressure. The static pressure of the air in the air conditioner refers to the relative static pressure. The static pressure is positive when it is above atmospheric pressure and negative when it is below atmospheric pressure.
2) Dynamic pressure (PD).The pressure created when the air flows is called dynamic pressure. As long as the air flow in the air duct has a constant dynamic pressure, its value is always positive.
3) Full pressure (PQ). The total pressure is the algebraic sum of the static and dynamic pressures: pq=pj+pdThe total pressure represents the total energy possessed by 1m3 of gas. If the calculation is based on atmospheric pressure, it can be either positive or negative.
For air pressure, it refers to the pressure formed by the layer of air surrounding the earth on the surface area of the earth, also known as the total pressure of moist air.
In the field of air conditioning and refrigeration engineering, the so-called "pressure" is actually the concept of "pressure". This can be seen from the unit used to express its value, which is the unit of physical quantity pressure.
13. Melt and Melting Difference.
Melting is a universal parameter that characterizes the energy of a system of matter, and it is also applicable to air. The melting of moist air is equal to the sum of the melting of dry air and the melting of water vapor.
The physical meaning of melting is: the total energy of the gas in a certain state, which is equal to the sum of the internal energy and the pressure potential energy of the gas. Melt is denoted by the symbol h.
h-u+pv
where h is melting, u is internal energy, p is pressure, and v is volume.
The so-called internal energy (u) includes the kinetic energy of linear motion, rotational motion, kinetic energy of vibration around equilibrium position, and potential energy due to gravitational and repulsive forces between molecules.
14. Refrigeration capacity.
Air conditioners often use cooling capacity to describe their cooling capacity. People often use the number of horsepower (p) to describe the cooling capacity of an air conditioner. In fact, in addition to P, there are other units of cooling capacity, and in the precision air conditioner of the computer room, we often use KW to express the cooling capacity. kw is also a unit of electrical power and is dimensionally equivalent. The unit of pi that we commonly use is not a very precise value, and the cooling capacity of an IP air conditioner is about 2The cooling capacity of 5kw is comparable. For example, 75KW precision air conditioner is equivalent to the cooling capacity of 3P household air conditioner.
15. Energy efficiency ratio.
The energy efficiency ratio is often represented by the letter EER, and the full name in English is Energy Efciency Ratio. There is a difference between the amount of electricity used by an air conditioner and the amount of cooling produced. The energy efficiency ratio is the ratio of the amount of cold produced to the amount of electrical power consumed. For example, if an air conditioner with a cooling capacity of 7000W actually consumes 2400W of electricity during refrigeration, then its EER is 7000W 2400W = 292。
The energy efficiency ratio is a relative value, which varies with the specific conditions under which the air conditioner operates, generally speaking, the higher the ambient temperature, the lower the energy efficiency ratio of the air conditioner.
However, in terms of product standards, the energy efficiency ratio is an absolute value.
At present, the average energy efficiency of air conditioners in China's market is relatively low, and in household appliances, some products will be marked with energy efficiency marks.
The energy efficiency mark provides a visual indication of the energy efficiency level of the product being consumed. The higher the energy efficiency of the product, the better the energy saving effect and the more electricity saving.