What are the commonly used physical unit parameters of air compressors?
pressure
The force acting on a base area of 1 square centimeter under standard atmospheric pressure is 10.13N. Therefore, the absolute atmospheric pressure at sea level is approximately 10.13x104N/m2, which is equal to 10.13x104Pa (Pascal, the SI unit of pressure). Or use another commonly used unit: 1bar=1x105Pa. The higher (or lower) you are from sea level, the lower (or higher) the atmospheric pressure is.
Most pressure gauges are calibrated as the difference between the pressure in the container and the atmospheric pressure, so to get the absolute pressure, the local atmospheric pressure must be added.
temperature
Gas temperature is very difficult to define clearly. Temperature is a symbol of the average kinetic energy of the molecular motion of an object and is the collective manifestation of the thermal motion of a large number of molecules. The faster the molecules move, the higher the temperature. At absolute zero, motion stops completely. Kelvin temperature (K) is based on this phenomenon, but uses the same scale units as Celsius:
T=t+273.2
T = absolute temperature (K)
t=Celsius temperature (°C)
The picture shows the relationship between temperature in Celsius and Kelvin. For Celsius, 0° refers to the freezing point of water; while for Kelvin, 0° is absolute zero.
Heat capacity
Heat is a form of energy, manifested as the kinetic energy of disordered molecules of matter. The heat capacity of an object is the amount of heat required to increase the temperature by one unit (1K), also expressed as J/K. The specific heat of a substance is widely used, that is, the heat required for unit mass of substance (1kg) to change unit temperature (1K). The unit of specific heat is J/(kgxK). Similarly, the unit of molar heat capacity is J/(molxK)
cp = specific heat at constant pressure
cV = specific heat at constant volume
Cp = molar specific heat at constant pressure
CV = molar specific heat at constant volume
The specific heat at constant pressure is always greater than the specific heat at constant volume. The specific heat of a substance is not a constant. Generally speaking, it increases as the temperature rises. For practical purposes, the average value of the specific heat may be used. cp≈cV≈c for liquid and solid substances. The heat required from temperature t1 to t2 is: P=m*c*(T2 –T1)
P = thermal power (W)
m=mass flow (kg/s)
c=specific heat (J/kgxK)
T=temperature(K)
The reason why cp is larger than cV is the expansion of gas under constant pressure. The ratio of cp to cV is called the isentropic or adiabatic index, К, and is a function of the number of atoms in the molecules of a substance.
achievement
Mechanical work can be defined as the product of the force acting on an object and the distance traveled in the direction of the force. Like heat, work is a type of energy that can be transferred from one object to another. The difference is that force replaces temperature. This is illustrated by the gas in the cylinder being compressed by a moving piston, i.e. the force pushing the piston creates compression. Energy is therefore transferred from the piston to the gas. This energy transfer is thermodynamic work. The results of work can be expressed in many forms, such as changes in potential energy, changes in kinetic energy, or changes in thermal energy.
Mechanical work related to volume changes of mixed gases is one of the most important processes in engineering thermodynamics.
The international unit of work is Joule: 1J=1Nm=1Ws.
power
Power is the work done per unit time. It is a physical quantity used to calculate the speed of work. Its SI unit is watt: 1W=1J/s.
For example, the power or energy flow to the compressor drive shaft is numerically equal to the sum of the heat released in the system and the heat acting on the compressed gas.
Volume flow
The system volumetric flow rate is a measure of the volume of liquid per unit time. It can be calculated as: the cross-sectional area through which the material flows multiplied by the average flow velocity. The international unit of volumetric flow is m3/s. However, the unit liter/second (l/s) is also often used in compressor volumetric flow (also called flow rate), expressed as standard liter/second (Nl/s) or free air flow (l/ s). Nl/s is the flow rate recalculated under “standard conditions”, that is, the pressure is 1.013bar (a) and the temperature is 0°C. The standard unit Nl/s is mainly used to determine mass flow rate. Free air flow (FAD), the output flow of the compressor is converted into the air flow under inlet conditions (inlet pressure is 1bar (a), inlet temperature is 20°C).
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