Throttling is one of the four main processes that are indispensable for a compression refrigeration cycle. The throttling mechanism has two functions: one is to throttle and depressurize the high-pressure liquid refrigerant coming out of the condenser to evaporation pressure, and the other is to adjust the amount of refrigerant liquid entering the evaporator according to the change of system load.
Commonly used throttling mechanisms include manual throttle valves, float throttle valves, thermodynamic expansion valves, choke expansion valves (capillaries), etc. Their basic principle is to make the high-pressure liquid refrigerant be forced to flow through a small overflow cross-section, resulting in a suitable local resistance loss (or loss along the way), so that the refrigerant pressure drops sharply, and at the same time, a part of the liquid refrigerant vaporizes, absorbs latent heat, and makes the throttled refrigerant become a low-pressure and low-temperature state.
1. Manual throttle valve
The difference between the structure of the manual throttle valve and the ordinary globe valve is mainly the structure of the valve core and the threaded form of the valve stem.
Usually, the spool of the globe valve is a flat head, and the stem is a common thread, so it can only control the on-off of the pipeline and roughly adjust the flow rate, and it is difficult to adjust it to an appropriate overflow cross-sectional area to produce a proper throttling effect. The spool of the throttle valve is a needle-shaped cone or a notched cone, and the stem is a fine-tooth thread.
Therefore, when the handwheel is rotated, the distance of the valve core is not large, and the cross-sectional area of the overflow can be adjusted more accurately and conveniently.
The opening degree of the throttle valve is adjusted according to the change of evaporator load, and the opening degree is usually 1/8 to 1/4 of the handwheel, and cannot exceed one week. Otherwise, if the opening is too large, it will lose its swelling effect. As a result, it cannot be automatically adapted to changes in the heat load of the evaporator, and is almost entirely empirically combined with the reactions in the system and operated manually.
At present, it is only installed in ammonia refrigeration units, where thermostatic expansion valves are widely used for automatic adjustment.
2. Float type throttle valve
1. The working principle of float type throttle valve: Float type throttle valve is a kind of self-adjusting throttle valve. Its working principle is to use a steel float as the power of opening and closing the valve, rely on the float to rise and fall in the float chamber with the liquid level, control the size change of the opening degree of a small valve and automatically adjust the liquid supply, and at the same time play the role of throttling, when the liquid level in the container is lowered, the float descends, the throttle hole opens by itself, and the liquid supply increases; When the liquid level rises to the specified height, the throttle hole is closed to ensure that the container will not be overliquid or lacking.
2. Structure type and installation requirements of float type throttle valve: Float throttle valve is used for automatic adjustment of liquid supply volume of evaporator, liquid separator and intercooler with free liquid level. A low-pressure float valve is widely used in ammonia refrigeration systems. The low-pressure float valve has two types: straight-through type and non-straight-through type according to the way the liquid flows in it, and the characteristic of the straight-through float throttle valve is that all the liquid refrigerant entering the container first enters the float chamber through the valve hole, and then enters the container. Therefore, the structure and installation are relatively simple, but the liquid level of the float chamber fluctuates greatly. Non-straight-through float throttles are characterized by the fact that the seat is installed outside the float chamber and the throttled refrigerant does not need to pass through the float chamber and goes directly into the vessel along the pipe. Therefore, the liquid level of the float chamber is relatively stable, but its structure and installation are more complex.
At present, the float type throttle valve produced by the continental refrigeration factory is this non-straight-through type. The structure of this float throttle valve is composed of a housing, float, lever, seat, balance pipe, spool and cover.
The requirement for the installation of the float throttle valve is that the gas balance pipe of the float chamber should be connected to the cylinder body, not to the suction pipe of the liquid separator. The liquid balance pipe should not be connected to the liquid supply pipe between the liquid separator and the evaporator, nor should it be connected to the ammonia pump suction pipe of the low-pressure circulating liquid storage cylinder, so as to avoid excessive fluctuation of the liquid level in the float chamber. The liquid in the evaporator is often in a bubble boiling state, resulting in a significant decrease in the density of the gas-liquid mixture, resulting in the actual liquid level in the evaporator is higher than the liquid level of the float chamber, so when the float throttle valve is installed on the evaporator, it is best to lower the float throttle valve appropriately. Generally, a liquid filter (250 holes/cm2 steel wire mesh) should be installed in the pipeline system of the float throttle valve to ensure that the liquid entering the float valve is free of impurities and avoid valve blockage. In addition, a bypass manual throttle valve is installed so that the liquid supply can continue in the event of a failure of the float throttle or the cleaning of the filter.
3. Thermodynamic expansion valve
The thermostatic expansion valve is a freon refrigeration device that regulates the amount of liquid refrigeration entering the evaporator according to the degree of overheating of the inhaled vapor, and at the same time throttles the liquid from the condensation pressure to the evaporation pressure.
There are many types of thermostatic expansion valves, but they are roughly the same in structure. According to the structure of the force transmission parts in the power chamber of the induction mechanism in the expansion valve, it can be divided into two types: diaphragm type and bellows type, and according to different use conditions, it can be divided into two types: internal balance type and external balance type. At present, most of the commonly used small freon thermostatic expansion valves are diaphragm type internal balance thermostatic expansion valves.
1. Internal balanced thermal expansion valve: The internal balanced thermal expansion valve is generally composed of a valve body, a valve seat, a valve pin, an adjusting rod seat, an adjusting rod, a spring, a filter, a transmission rod, a temperature sensing bag, a capillary, an air box cover and an induction film.
The capillary tube is made of a copper tube with a very thin diameter, and its function is to transmit the pressure change caused by the temperature change caused by the temperature change in the temperature sensor bag to the corrugated film in the power room. The corrugated film is made of very thin (0.1~0.2mm) alloy sheets, and the cross-section is wavy, and it can have a displacement deformation of 2~3mm. The displacement of the corrugated film due to the change of pressure in the power chamber is transmitted to the valve pin through the transmission rod below it, so that the valve pin moves together with the up and down movement of the transmission rod to control the opening of the valve hole. The function of the adjusting rod is to adjust the compression degree of the spring to adjust the opening superheat of the expansion valve during the commissioning and operation of the system, and the cap on the adjusting rod seat should be screwed on after the system works normally, so as to prevent the refrigerant from leaking from the packing.
The filter is installed at the inlet end of the expansion valve to filter foreign matter in the refrigerant and prevent the valve hole from clogging.
As for its working principle, let's first analyze the force of the corrugated film when the thermostatic expansion valve is working. The metal corrugated film is subjected to three forces: above the diaphragm, the saturation pressure P of the liquid in the thermoreceptor (corresponding to the temperature it feels) produces a downward thrust P on the diaphragm, and below the diaphragm, the low-pressure liquid behind the valve seat and the evaporator produces an upward thrust P0 (evaporation pressure of the refrigerant) and the tension W of the spring. In addition, there are factors such as friction between moving parts, which are negligible in the analysis because their values are so small. From the above analysis, it can be seen that when the three forces are in equilibrium, that is, when P=P0+W is satisfied, the diaphragm does not move, and the valve port is in a certain degree of opening. When any of these forces change, the original equilibrium is disrupted, and the opening of the valve port changes until a new equilibrium is established.
When the external situation changes, such as due to insufficient liquid supply or heat load increase, the superheat of the return air of the evaporator increases, the temperature felt by the temperature of the temperature sensing bag also increases, and the saturation pressure P also increases, so that the formation of: P>P0+W, which will lead to the diaphragm to move downward, so that the opening degree of the valve port increases, and the flow rate of the refrigerant also increases, until the liquid supply and evaporation amount are equal to reach another balance. On the contrary, if the superheat of the return air of the evaporator decreases due to excessive liquid supply or the reduction of heat load, and the temperature felt by the temperature of the temperature sensing bag is also reduced, the saturation pressure P will also be reduced, so that P<P0+W will be formed, which will lead to the upward movement of the diaphragm, so that the opening of the valve port will be reduced, and the amount of liquid supplied to the refrigerant will be reduced, until it matches the heat load of the evaporator. The working principle of the thermostatic expansion valve is to use the change of P force related to the superheat of the return air to adjust the opening degree of the valve port, so as to control the flow of refrigerant and achieve automatic adjustment.
In addition, it can be seen from the above relationship that by adjusting different spring tensions W, different superheats can be obtained to open the valve port. The superheat of the refrigerant corresponding to the set spring tension W is called the static assembly superheat (also known as the shut-off superheat). Generally, it is hoped that the superheat of the evaporator will be maintained in the range of 3~5 °C.
2. External balance thermostatic expansion valve: The external balance thermodynamic expansion valve and the internal equilibrium thermodynamic expansion valve are slightly different in structure, the difference is that the lower space of the induction film is not connected with the outlet of the expansion valve, and it is connected to the evaporator outlet through a small-caliber balance pipe. In other words, the refrigerant pressure at the lower part of the diaphragm of the external equilibrium thermostatic expansion valve is not the evaporation pressure after the valve is throttled, but the refrigerant pressure at the outlet of the evaporator. In this way, the influence of the evaporator resistance loss can be avoided, and the superheat can be controlled within a certain range, so that the heat transfer area of the evaporator can be fully utilized.
If the pressure loss of the refrigerant in the evaporator is large, when the internal balanced thermostatic expansion valve is used, the liquid supply of the evaporator will be insufficient, and the superheat of the gaseous refrigerant at the outlet will increase. Therefore, in practical application, when the pressure loss of the evaporator is small, the internal balance thermal expansion valve is generally used, and when the pressure loss is large (when the pressure drop from the expansion valve outlet to the refrigerant at the outlet of the evaporator is reduced by more than 2~3 °C), the external balanced thermal expansion valve should be used.
3. Problems that should be paid attention to when installing thermal expansion valves
(1) First of all, check whether the expansion valve is in good condition, and pay special attention to check whether the temperature sensing power mechanism is leaking.
(2) The expansion valve should be installed vertically and is not allowed to be inverted.
(3) The temperature sensing package is installed on the outlet pipe of the evaporator, tightly wrapped on the horizontal pipe section without liquid accumulation, wrapped with heat insulation material, or inserted into the temperature sensing sleeve on the suction pipe.
(4) When the diameter of the horizontal return pipe is less than 25mm, the temperature bag can be tied to the neck of the air return pipe, and when the diameter of the horizontal air return pipe is greater than 25mm, the temperature bag can be tied at 45° on the lower side of the air return pipe, so as to prevent oil accumulation at the bottom of the pipe and other factors from affecting the correct temperature sensing of the temperature bag.
(5) The balance pipe of the external balance expansion valve is generally installed on the return air pipe at 100mm behind the temperature sensing bag, and should be led out from the top of the pipe to prevent the lubricating oil from entering the valve.
(6) When there are multiple expansion valves in one system, the outer balance pipe should be connected to the outlet of their respective evaporators.
4. Capillary: In small refrigeration equipment such as refrigerators and air conditioners, capillary tubes are commonly used as throttling devices, which mainly rely on the size of its pipe diameter and length to control the flow of liquid refrigerant so that the evaporator can work under appropriate conditions. In refrigeration engineering, it is generally said that the purple steel pipe with an inner diameter of about 0.5~2mm and a length of about 1~4m is a capillary. Compared with the throttle valve, the advantages of the capillary as a throttling device are that there are no moving parts, no wear, not easy to leak, easy to manufacture, cheap and trouble-free, and the disadvantage is that the flow rate is small and can not be adjusted at will.
After the inner diameter and length have been determined, the flow rate of the capillary is mainly affected by the pressure difference between the inlet and outlet sides, that is, the high and low pressure, and is also related to the degree of supercooling of the incoming liquid, the amount of flash gas contained, the degree of tube bending, and the number of coiling turns. Therefore, when the unit system is certain, it is not possible to arbitrarily change the working conditions or replace the capillary of any specification. According to the relevant experiments, under the same working conditions and the same flow conditions, the length of the capillary is approximately proportional to the 4.6 power of its inner diameter, that is, L1/L2=(d1/d2)4.6
When the ambient temperature rises or the refrigerant charge is excessive, the condenser pressure becomes high, and the capillary flow rate increases the evaporator pressure and evaporation temperature. On the contrary, when the ambient temperature decreases or the refrigerant charge is insufficient, the condenser pressure becomes low, and the capillary flow rate decreases, which will reduce the evaporator pressure and evaporation temperature, resulting in a decrease in refrigeration capacity, or even less than the required temperature.
Therefore, the refrigeration equipment using capillary tube must strictly control the amount of refrigerant according to the design requirements. For example, the amount of R12 added to a refrigerator of about 200L is about 150 grams, and the deviation between up and down is not more than 5 grams. The first charge M of a typical system can be determined approximately as follows:
M=20+0.6V （克）
式中：V—蒸发盘管内容积(cm3)。