The medium passes through the orifice

This post was last edited by 3983596_FPPZ on 2019-7-27 09:13. The temperature of the gas should remain basically constant, as does that of the liquid. Although some loss occurs during throttling, which results in an increase in temperature, this effect is minimal and can be ignored. The position of the throttle orifice can be viewed as simply a conversion between kinetic energy (flow velocity) and potential energy (pressure); the total energy of the fluid remains essentially constant (assuming no losses). The decrease in pressure after throttling occurs because throttling increases the resistance in the pipeline, which alters the distribution of various losses within it, and as a result, the pressure and flow velocity in different sections also change.

The second floor is incorrect; throttling to reduce pressure can also be referred to as expansion refrigeration. This is the principle behind air conditioners and cryogenic chambers.

I thought the same thing at the time, but aside from air conditioners, I haven’t seen any other device where frost or dew forms behind the throttle valve – is it because the pressure is too low and thus not enough heat is absorbed?

That refers to specially designed refrigerants, such as Freon, which use vaporization and liquefaction to absorb or release heat for cooling; I’m talking about normal refrigerants here

When a gas or liquid flows through a constriction or a valve in a pipe, its flow is hindered, and the fluid forms vortices, experiences collisions, and friction at the valve. For the fluid to flow through the valve, it must overcome these resistances, which is reflected in the pressure P2 behind the valve being much lower than the pressure P1 in front of the valve. This process in which pressure drops significantly due to local resistance encountered by the flow is commonly referred to as a “throttling process”. In fact, when fluid flows through pipes and equipment, there is also flow resistance that causes the pressure to decrease. However, its pressure drop is relatively small and occurs gradually. Moreover, the pressure drop during throttling by the throttle valve is large and occurs suddenly. During throttling, the fluid neither does work on the outside nor exchanges heat with its surroundings; it is therefore an adiabatic process. According to the law of conservation of energy, the total internal energy (enthalpy) of the fluid remains constant before and after throttling. However, each of the three components of enthalpy – the kinetic energy of molecular motion, the potential energy of molecular interactions, and the flow energy – can change. After throttling, the pressure decreases, the specific volume increases, the distance between molecules increases, and the potential energy of molecular interactions rises. Since the kinetic energy of flow generally remains unchanged, it is only possible to convert it into potential energy by reducing the kinetic energy of molecular motion. A decrease in the speed of molecular motion is reflected in a drop in temperature. When a gas is throttled, the pressure decreases, causing the gas volume to expand. As a result, the distance between molecules increases, the potential energy between them rises, while their kinetic energy decreases. Since the magnitude of a molecule’s kinetic energy reflects the temperature, in general, the temperature of a gas always drops after it is throttled. Not all fluids cool down after throttling expansion. For example, hydrogen will cause the temperature to rise. It is inappropriate to use gas equations to explain the throttling process, because such equations do not take energy changes into account, whereas increases and decreases in temperature are related to the energy of the substance. For most gases, since the throttling process is a process of decompression and expansion, the gas does work on its surroundings through this expansion, resulting in a decrease in the internal energy of the system and thus a drop in temperature. This explanation does not apply to gases with very low molecular weights. For gases: whether the temperature rises or falls during throttling depends on the Joule-Thomson coefficient as well as on the current conditions (P, V) ； That is, whether the gas throttling temperature decreases or increases depends on the state of the gas before throttling. Such as hydrogen and helium, whose temperature increases after throttling. That is also why the risk of hydrogen leakage is relatively high. This is because an increase in the throttling temperature of hydrogen can cause a flame or explosion. How do the pressure, temperature, flow rate, and density of the gas change before and after it passes through the throttle valve? As is well known, the pressure of a fluid necessarily decreases after throttling, but little attention is paid to changes in temperature, flow rate, and density. Starting with temperature, according to thermodynamic principles, compression releases heat while expansion absorbs heat; that is, as the pressure of a fluid increases, its temperature also rises as it releases heat, and as the pressure decreases, its temperature drops as it absorbs heat from the surroundings. This is particularly evident in gases, so after throttling, the temperature of gases decreases. For gases at room temperature, a greater degree of throttling results in a larger pressure drop and thus a greater decrease in temperature. It is for this reason that frost often forms in gas pipelines after throttling. Speaking of changes in flow velocity, for liquids, the effect of pressure changes on volume can be ignored; therefore, with a constant flow rate, the flow velocity is determined by the pipe diameter, that is, the cross-sectional area of the flow channel. If the pipe diameters before and after the throttle valve are the same, then the fluid’s flow velocity should remain unchanged. This is not the case for gases: as the pressure of a gas decreases, its volume necessarily increases, which means the relative flow rate at that pressure rises (the actual flow rate remains constant). As a result, the flow velocity increases after throttling, and the volume increases under the reduced pressure, leading to a decrease in density. These are the changes in the parameters of a gas as it passes through a throttle valve: reduced pressure, reduced temperature, increased flow velocity, and decreased density.

Will the temperature of that liquid also drop after passing through the orifice?

The pressure drop is relatively small, and the liquid has a high heat enthalpy, so the temperature change is not significant; however, the conversion of potential energy into thermal energy is certain.