|■ About adiabatic expansion and adiabatic compression|
Heat pump is a system which utilize heat energy of gas by repeating expansion and compression of gases forcefully and the expansion and compression of gases is based on mechanism of adiabatic expansion and adiabatic compression. Expansion and compression of gas without heat supplied from outside is called adiabatic expansion and adiabatic compression.
Temperature of gas drops in adiabatic expansion where gases expand under adiabatic condition. It is because liquid in gas is evaporated by expanding the gas forcefully and temperature goes down by absorbing the surrounding heat energy during its state transition. This heat is called heat of evaporation or heat of vaporization.
On the other hand, temperature of gas rises in adiabatic compression where gases are compressed under adiabatic condition. It is because liquid in gas is condensed by compressing the gas forcefully and temperature rises by generating heat energy during its state transition. This heat is called heat of condensation or liquification heat. In heat pump system, temperature of gases is changed according to its pressure which is increased by being compressed or decreased by being expanded, and this is defined in Boyle Charle’s law as “temperature of gas rise when pressure is increased and the temperature drops when pressure is decreased” and in the second law of thermodynamics as “heat moves from hot stuffs to cold stuff and the other way is not possible”.
Also, it is not true that all expansion is done by expansion valve and all compression is done by compressor in the heat pump system. In expansion valve, temperature does down by evaporation and expansion of certain amount of liquid after gas moves from higher pressure to lower pressure, however, liquid additionally absorbs surrounding heat at heat exchanger by being evaporated and temperature drops here as well. In the same way, heat of condensation is generated at compressor by state change from gas to liquid during the compression, however, temperature is raised by liquification at heat exchanger as well.
|Adiabatic expansion||Decrease temperature by expanding gas without heat supply from outside.|
|Adiabatic compression||Increase temperature by compressing gas without heat supply from outside.|
|■ About adiabatic expansion and adiabatic compression of steam (saturated steam)|
While heat source of KENKI DRYER is steam (saturated steam), adiabatic expansion and adiabatic compression affects to steam dryness and the dryness depends on steam pressure.
Pressure is decreased by adiabatic expansion of saturated steam and temperature is also decreased though, latent heat increases since liquid in the steam evaporates and dryness rises. Temperature of saturated steam becomes low as pressure of saturated seam is lowered, however, amount of heat energy increases since sensible heat decreases and latent heat increases.
In adiabatic compression, on the other hand, pressure increases and temperature rises. However, steam dryness becomes low because the steam is compressed and liquefied. Even though temperature raised according to rise in pressure, amount of heat energy decreases since latent heat decreases and sensible heat increases.
Amount of heat that saturated steam has is sum of sensible heat and latent heat, and steam dryness affects amount of heat energy of the whole steam.
For example, steam with 50% dryness has 50% of latent heat that steam with 100% dryness has. It means amount of steam with 50% dryness has to be twice as much as the amount of steam with 100% dryness when the same amount of heat energy is necessary.
The steam dryness is calculated by ratio of wight of the gas in the whole steam.
Steam dryness [%] = 100[%] – Steam wetness [%]
Please refer to the following table for comparison of latent heats that steam has in different steam dryness.
|Condition of saturated steam||Steam dryness
A heat pump type dryer of KENKI DRYER heats and dries materials by injecting saturated steam which is compressed by adiabatic compression in compressors. Since steam dryness of the injected steam affects to drying efficiency, this process is very important.