Gas expansion refrigeration, vortex refrigeration, adsorption refrigeration
Gas expansion refrigeration.
When a gas flows through a pipeline, the pressure drops significantly due to local resistance, such as when it encounters a necking and regulating valve, a phenomenon known as throttling. In engineering, because when the gas passes through the valve and other flow resistance devices, the flow rate is large and the time is short, and it is too late to exchange heat with the outside world, so it can be treated approximately according to the adiabatic process, which is called adiabatic throttling.
When the gas is adiabatic throttling, the specific enthalpy values before and after throttling remain unchanged. This is the main feature of the throttling process. It is a typical irreversible process due to the frictional resistance loss inside the air flow during throttling.
Experiments have found that the temperature before and after the actual gas throttling generally changes. The change in temperature of a gas during throttling is called the Joule-Thomson effect (charred-soup effect for short), and the reason for this phenomenon is that the specific enthalpy of the actual gas is not only a function of temperature, but also of pressure. Most real gases have a cooling effect during throttling at room temperature, i.e., the temperature decreases after passing through the throttling device, a temperature change known as the positive Joule-Thomson effect. The temperature of a few gases that throttle at room temperature and then rises, a temperature change known as the negative Joule-Thomson effect.
Vortex refrigeration. The essence of the vortex cooling effect is to use artificial methods to create a vortex, so that the gas is divided into two parts: cold and hot. Refrigeration can be achieved by using the separated cold air flow.
The separation of the gas into two parts by the vortex current is carried out in the vortex chamber of the vortex tube. The internal shape of the vortex chamber is an Archimedean spiral, and the gas that is compressed and cooled to room temperature (usually with air, but also with other gases, such as carbon dioxide, ammonia, etc.) enters the nozzle and expands, and then enters the vortex chamber at a very high speed along the tangential direction to form a free vortex, which is separated into two parts with different temperatures after the exchange of kinetic energy. The air flow in the central part flows out through the orifice plate, that is, the cold air flow; The airflow from the edge part is from the other end through the control valve, i.e., the hot airflow. Vortex tubes can produce both hot and cold effects at the same time. According to the test, when the temperature of the high-pressure gas is room temperature, the temperature of the cold air stream can reach -50 -10 and the temperature of the hot gas stream can reach 100 130. The control valve is used to change the pressure of the gas in the hot end tube, thus adjusting the flow ratio of the two parts of the gas flow and thus changing their temperature.
The advantages of eddy current tube cooling are simple structure, easy maintenance, fast start-up, and can reach relatively low temperature; Its main disadvantage is its low efficiency. Eddy current tubes should only be used in small cryogenic test equipment that is not used frequently. The application of the heat recovery principle and the injector to reduce the pressure of the cold air flow of the vortex tube can not only further reduce the low temperature that can be obtained by the vortex tube, but also improve the economy of the vortex tube. In order to achieve lower temperatures, multi-stage vortex tubes can also be used.
Adsorption refrigeration. The adsorption refrigeration system is an energy conversion system powered by thermal energy, and its principle is: a certain solid adsorbent has an adsorption effect on a certain refrigerant gas, and the adsorption capacity varies with the change of the temperature of the adsorbent. Taking advantage of this property, the adsorbent is periodically cooled and heated to make it alternately adsorption and desorption: during adsorption, the refrigerant liquid evaporates to produce refrigeration; During desorption, the refrigerant gas is released and condensed into a liquid.