First, the manual expansion valve Manual expansion valve is similar to the structure of the ordinary valve, except that its valve core is a needle cone or cone with a V-shaped notch. The valve stem adopts fine thread. When the handwheel is rotated, the opening degree of the valve can be slowly increased or the alkali is small, thereby ensuring good adjustment performance.
Its distinctive feature is not easy to be bad. The management personnel should manually adjust the opening of the expansion valve according to the changes in the thermal load of the evaporator and other factors, so the operation and management are inconvenient and require high experience. Nowadays, automatic expansion valves are often used, and manual expansion valves are used only. On the bypass pipe, as an auxiliary function.
Second, the float expansion valve Float expansion valve is mostly used for flooded evaporator, this evaporator requires the liquid surface to maintain a certain height, is in line with the characteristics of the float expansion valve.
Depending on the flow conditions of the liquid refrigerant, the float expansion valve can be divided into two types: straight-through and non-straight-flow. Each has its own advantages and disadvantages. The straight-through expansion valve supplies the liquid to the evaporator. First, it all passes through the float chamber and then enters the evaporator through the liquid balancing tube. Therefore, it has the characteristics of simple structure, but the liquid level of the float chamber fluctuates greatly and impacts the valve core. The force is also large, the valve core is easily damaged; in addition, a large diameter balance tube is also required. Non-straight-through float ball expansion valve, the valve mechanism is located outside the float ball, the throttling refrigerant does not go through the float ball room, but enters the evaporator directly along the pipe, so the float ball room liquid level is stable, but in the construction and installation complex.
Third, thermal expansion valve and floating ball expansion valve is different, it does not control the liquid level, but the vaporizer to control the evaporator superheat at the outlet to control the flow of refrigerant into the evaporator. Because a portion of the evaporator area must be used to superheat the gaseous refrigerant, it is widely used in all non-liquid evaporators in air conditioning or cryogenic systems (especially Freon refrigeration systems).
Thermostatic expansion valves have different balance methods, or different ways of evaporating pressure leading into the inner cavity of the mold. There are two kinds of internal expansion type and external balance type.
The installation position of the thermal expansion valve must be close to the evaporator. The valve body should be placed vertically. It cannot be tilted, and it cannot be installed upside down. It is mainly the installation of the temperature-sensing package. Usually it is wrapped around the suction pipe, close to the pipe wall and tightly packed. The contact surface should be cleaned of the oxide scale to reveal the true nature of the metal pipe. If necessary, a layer of aluminum paint can be applied as protection. Layer to prevent rust.
Cold storage expansion valve commissioning technology and troubleshooting 1 Overview As we all know, thermal expansion valve is one of the four components in the refrigeration system, in the system responsible for the refrigerant from the condensing pressure to the evaporation pressure, and proportional control of the refrigerant flow. The quality of a thermal expansion valve in a system will directly affect the performance of the entire system, so the timely removal of the malfunction of the thermal expansion valve and proper and correct selection will have important implications for the operating life, cooling effect, and operating cost of the air conditioning system. .
2 thermal expansion valve works.
The thermal expansion valve controls the flow of refrigerant into the evaporator by sensing the degree of superheat of the vapor refrigerant at the outlet of the evaporator. According to different balance methods, the thermal expansion valve is divided into external balance and internal balance, and is often used in central air-conditioning systems. Outer balance type. It consists of induction mechanism, actuator, adjustment mechanism and valve body. During operation, a temperature-sensing package fixed on the outlet pipe of the evaporator induces an overheating temperature at the outlet of the evaporator, so that a pressure is generated in the temperature-sensing package and is transmitted from the capillary to the upper space of the diaphragm. The diaphragm is elastic under the effect of pressure. The deformed mode transmits the signal to the ejector pin (actuator) to adjust the opening of the valve and control the flow of the refrigerant.
3 Several failure analysis of thermal expansion valve work 3.1 Blockage failure of thermal expansion valve 3.1.1 Causes of blockage Blockage failure of thermal expansion valve in refrigeration system occurs frequently, including "dirty block" and "ice block". The main reason is the existence of impurities in the system, such as welding slag, copper shavings, iron filings, fibers and so on. The reason for ice blockage is that the system contains too much moisture (humidity). The ways to generate moisture are:
1) During the installation, the vacuum time of the system is not enough, and the moisture in the pipeline cannot be exhausted; the welding process of the pipeline connection is not good and there is a leakage point.
2) When filling the system with refrigerant, the air in the connecting hose is not blown out of the hose.
3) When the system is replenished with oil, it will enter the air.
3.1.2 Location where clogging occurs Normally, dirty clogging occurs on a dry filter, and impurities in the system are trapped by the filter, resulting in dirty plugging. When it happens, the system firstly shows that the return air temperature rises and the superheat rises. After the fault is serious, the system stops running. If the system does not remove the impurities, the system cannot be turned on again. Ice blockage generally occurs at the orifice of the expansion valve, for example, because it is the lowest temperature in the entire system and the smallest hole size. As the system is not cooling, the overall temperature of the system will rise. As the temperature increases, the ice block will gradually melt, and then the system will recover the cooling capacity. As the overall temperature of the system decreases again, ice blocking will occur. Therefore, ice clogging is an iterative process.
3.1.3 How to remove the blockage How do you remove the blockage? For dirty plugging, if it is not severe, replace it with a dry filter. If it is very serious, it is necessary to re-clean the impurities in the system piping, vacuum, and refill the refrigerant. For a slight ice block, a hot towel can be used to cover the ice block. If the degree of ice blockage is relatively serious and the normal operation of the system has been affected, the filter dryer must be replaced, and the water in the system piping must be replaced. Refill the refrigerant.
3.2 Temperature Sensing Package Failure 3.2.1 Temperature Sensing Package Failure Common Causes When there are many times when the expansion valve is supplied with liquid, the expansion valve is not closed, the superheat degree and the degree of undercooling are not correct. The reason may be the temperature sensing package. There has been a breakdown. include:
1) The temperature-sensitive capillary tube breaks, and the charge in the temperature-sensing package is missed, so that the correct signal cannot be transmitted to the thermal expansion valve actuator.
2) The temperature sensor is not correctly wrapped.
3.2.2 Temperature Sensing Package Troubleshooting Methods In general, the temperature sensing package should be installed on the return air pipe of the horizontal section of the evaporator. It should be far away from the suction port of the compressor and close to the evaporator, and should not be installed vertically. When the horizontal return air pipe diameter is less than 7/8" (22mm), the temperature sensing package should be installed on the top end of the air return pipe, that is, "one o'clock" of the suction pipe. When the horizontal return pipe diameter is greater than 7/8", the sense The temperature package should be installed about 45 degrees below the axis of the return air pipe and the horizontal axis, ie the "3 o'clock" position of the suction pipe. Because installing the bulb in the upper part of the suction pipe will reduce the sensitivity of the reaction and may cause too much refrigerant in the evaporator. Mounting the bulb in the bottom of the suction pipe will cause fluid supply disorder because there is always a small amount The liquid refrigerant flows to the location where the bulb is installed, resulting in a rapid temperature change of the bulb. When installing, temperature-sensing bag needs to be wrapped with copper sheet, and the surface of the gas return pipe should be rust-removed. If it is a steel pipe, the surface is de-rusted and painted with silver paint to ensure good contact between the temperature-sensing package and the return gas pipe. The bulb should be lower than the top of the valve top diaphragm, and the head of the bulb should be placed horizontally or downward. When the relative position is higher than the upper chamber of the diaphragm, the capillary should be bent upward into a U shape to prevent the liquid from entering the membrane. On-chip cavity.
3.3 Improper adjustment 3.3.1 About expansion valve adjustment Related concepts First, we must understand several concepts (1) Superheat degree of expansion valve: The thermal expansion valve is at a certain opening degree, and the corresponding degree of superheat is called working superheat degree. That is, the degree of superheat of the thermal expansion valve. Includes static superheat (SS) and open superheat (OS).
(2) Static superheat: When the thermal expansion valve is in the open position, the spring force is minimum. At this time, the superheat degree controlled by the thermal expansion valve is the smallest, which is called the static superheat SS.
(3) Dynamic superheat: When the valve hole of the expansion valve is opened, the opening of the valve hole increases with the increase of the superheat of the outlet steam. From the opening of the valve hole to the full opening, the value of the increase in superheat is called dynamic superheat OS. .
3.3.2 Correct adjustment method of expansion valve (1) Before adjusting the thermal expansion valve, it must be confirmed that the cold storage refrigeration abnormality is caused by the thermal expansion valve deviating from the optimal working point, not because of low freon, dry filter clogging, and strainer , fan, belt and other reasons. At the same time, it is necessary to ensure the correctness of the sampling signal of the temperature sensing package. The temperature-sensing installation position must be correct. It must not be installed directly under the pipe to prevent the bottom of the pipe from collecting oil and other factors from affecting the temperature sensing package.
(2) Cautions when adjusting the thermal expansion valve: The adjustment of the thermal expansion valve must be performed under the normal operation of the cooling device. Since the surface of the evaporator cannot be placed on the thermometer, the suction pressure of the compressor can be used as the saturation pressure in the evaporator, and the approximate evaporation temperature can be found by looking up the table. The temperature of the return air pipe was measured with a thermometer and compared with the evaporation temperature to check the degree of superheat. During adjustment, if the degree of superheat is too small, the adjusting screw can be turned clockwise (ie, increasing the spring force, reducing the opening degree of the thermal expansion valve), so that the flow rate can be reduced; otherwise, if the degree of superheat is felt to be too large, If the supply of liquid is insufficient, the adjustment screw can be turned in the opposite direction (counterclockwise) to increase the flow rate. Due to the fact that the thermal expansion valve temperature sensing system in practice has a certain degree of thermal inertia, the signal transmission is delayed and the operation is basically stable before the next adjustment can be made. Therefore, the entire adjustment process must be patient and careful, adjusting the number of turns of the screw should not be too much too fast.
(3) Specific adjustment steps of thermal expansion valve:
1) Stop. Insert the probe of the digital thermometer into the insulation layer at the return air port of the evaporator (corresponding to the position of the temperature sensing package). Connect the pressure gauge to the tee of the compressor low pressure valve.
2) Start the machine, let the compressor run for more than 15 minutes, enter the stable operation state, make the pressure indication and temperature display reach the stable value.
3) Read the temperature T2 of the digital thermometer temperature T1 and the pressure measured by the pressure gauge. The degree of superheat is the difference between the two readings T1-T2.
Note that both readings must be read at the same time. Thermal expansion valve superheat should be between 5-8 Â°C, if not, make appropriate adjustments. The adjustment steps are: first remove the protective cover of the thermal expansion valve, then turn the adjustment screw 2-4 turns, wait for the system to run stably, read again, calculate the degree of superheat, whether in the normal range, if not, repeat the previous operation until the Requirements, the adjustment process must be careful.
4 The correct choice of thermal expansion valve 4.1 The purpose of the correct choice of thermal expansion valve Selection of thermal expansion valve pairs plays an important role in the performance of the entire system, the correct choice of thermal expansion valve will maximize the use of the evaporator, and Allow the evaporator to always match the thermal load.
4.2 Phenomenon when the thermal expansion valve does not match the system: When it does not match, the refrigerant flow rate of the system will be too much, resulting in large and small cooling capacity of the thermal expansion valve. When the cooling capacity is too small, the evaporator will supply Insufficient fluid will generate excessive heat and will adversely affect the performance of the system. When the cooling capacity is too large, it will cause oscillations, intermittently excessive supply of liquid to the evaporator, resulting in severe fluctuations in the suction pressure of the compressor, even A liquid refrigerant enters the compressor, causing a liquid strike (wet stroke) phenomenon.
4.3 Selection basis: According to the type of refrigerant in the refrigeration system, the evaporation temperature range and the size of the evaporator overheating load.
4.3.1 Selection Methods and General Procedures The following describes the selection method for the lower expansion valve. Since ceiling air coolers are used nowadays, the evaporation pressure drop is generally relatively large. Therefore, external balance type thermal expansion valves are selected. According to the evaporation temperature and the pressure difference between the inlet and outlet of the thermal expansion valve, check the product selection list given by the supplier.
The general steps are as follows:
1) Determine the refrigerant type of the system.
2) Determine evaporator evaporation temperature, condensing temperature and cooling capacity.
3) Differential pressure at the inlet and outlet of the thermal expansion valve.
4.3.2 For example, there is an evaporation coil (4DW4/10F-50x50.3A), the refrigerant is R407C, the cooling capacity is 96KW, the evaporation temperature is 8Â°C, and the condensing temperature is 50Â°C. What type of thermal expansion valve is selected? Danfoss products as an example).
First determine the pressure difference PÎ” between the inlet and outlet of the expansion valve.
P k is the condensing pressure.
P 0 is the evaporation pressure.
1 PÎ” is the pressure drop of the copper supply pipe.
2 PÎ” is the pressure drop of the dispenser and the dispensing capillary.
P k (condensation pressure), P 0 (evaporation pressure) can be found in the wetting chart from the given known parameters.
P k=17.5 5 10Ã—P a, P 0=6.5 5 10Ã—P a A)
The pressure drop of the liquid supply copper tube, due to the coil selection software used, has a pressure drop in the supply tube in the calculated data. It is known that 1 PÎ” = 0.0031 5 10 Ã— Pa. The pressure drop of the re-distributor split copper tube has an empirical value of 2 PÎ”=1 5 10Ã—Pa. "g
When the refrigerant is R407C, the cooling capacity is 96KW, the evaporation temperature is 8Â°C, and the condensation temperature is 50Â°C, 1 PÎ”.
For 10bar, select model TDEZ26 thermal expansion valve (take Danfoss products as an example).
5 Conclusions The stability of the air conditioning unit is related to the superheat of the refrigerant at the outlet of the evaporator. Therefore, it is important to accurately and timely exclude the failure of the thermal expansion valve and properly select the thermal expansion valve that matches the system.
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