Filled Thermostatic Valves
Pressure limits TXVs are designed to reduce the flow of the liquid in the evaporator whenever the evaporator pressure rises above a preset maximum specified value. This is achieved by temporarily taking control of the needle from the sensing bulb. Such a strategy-the strategy of overload of the compressor motor during periods of high diermal load and excludes liquid flood back into the compressor due to overeating evaporator at startup. Maximum operatingpressure (MOP), the evaporator is generally limited by the use of TXV having filled remote sensing bulb.
In gas-filled TXVs pressure limiting characteristic is the result of the prosecution in its sensing bulb. Refrigerant in the sensing bulb is completely evaporated when overheating exceeds the temperature-related system SS. As soon as the refrigerant in gas sensing bulb is completely converted to vapour, any further increase in the light bulb temperature is increase of superheat little impact on the lamp's pressure. Therefore, limiting the amount of charge in the sensing bulb, the maximum pressure that can be exerted by sensing bulb for TXVs aperture is also limited.
Restrictions pressure TXVs sensing bulb also limits the SS.
This is because the valve equilibrium is set only when the light pressure (Pi) is the sum of the evaporator pressure () and superheat set point springs (P3). So any time the evaporator pressure exceeds the SS, the amount of the evaporator and set a spring force exceeds bulbs pressure.
Therefore, the valve will modulate in direction closed. For example, suppose the system is equipped with a gas-filled TXV with MOP 36 psia (248 kPa) and overheating 10 F (5.6C). In this application, the sensing bulb collected in a form that causes the refrigerant to be 100% saturated vapor bulbs when the temperature reaches the saturation temperature corresponding to 43.7 psia (301 kPa). This value is the sum of the maximum working pressure (36 psia, 248 kPa) plus spring pressure equivalent to 10 F (5.6C) overheating (7.7 psi, 53 kPa). When the ball reaches the temperature, any additional suction superheat vapor has little effect on the pressure light. Therefore the speed of the refrigerant flowing through the valve can not be increased. If the evaporator pressure exceeds 36 psia (248 kPa), the amount of the evaporator and spring pressure causes needles for modulation in the direction of " closed". However, each time the pressure in the evaporator below 36 psia (248 kPa), the amount of pressure in the evaporator and superheat spring force is smaller, than a lamp maximum pressure. In these conditions, the sensing bulb pressure contributes to the modulation of the needle and TXV is responsible, as usual, changes in the evaporator and superheat.
Because its pressure-limiting characteristics, filled TXV provides protection from overload of the compressor and flood-back. Since the pressure in the evaporator is limited to bulb maximum pressure, any change in the overheating causes MOP change. Because pressure light is always equal to the pressure in the evaporator plus overheating springs (P3), increase of superheat setting decreases the MOP evaporator, because P2 plus P3 is always equal to P. On the contrary, decrease of superheat increases MOP evaporator.
In view of the critical charge is used in gas-filled light bulbs, some precautions should be observed when installing the gas TXV in the system. Expansion valve body must be installed in a warmer place than remote sensing bulb. Similarly, the tube connecting the sensing bulb valves authorities of the head should not be allowed to touch any surface is colder than the sensing bulb. If one of these conditions is not observed, the charge
in the flask will condense, causing TXV to fail due to lack of liquid in the sensing bulb. Care must be taken to find the sensing bulb so that drain the liquid refrigerant from the lamp under the influence of gravity.
The importance of pressure valves restrictions understood if we recognize diat many refrigeration systems are subject to periodic pull-down loads. These loads are significantly higher than the load of the system during normal operation. Starting with evaporating pressure and abnormally high temperatures in the period of the unfolding periods, power and power consumption of the compressor increase, often resulting in a temporary overload of the compressor motor. There are two solutions to this problem: increase the size of the compressor and engine so that it has sufficient strength to withstand a load in die overload period or limit the MOP, to avoid overload of the compressor. The best solution for the application depends on the specific requirements and operating conditions. In systems where the rapid contraction of space or the temperature of the product requires the use of more powerful compressor motor is normally selected.
This design strategy increases the initial purchase and maintenance costs of the system, but these effects are acceptable in connection with the requirements of the process. And Vice versa, in applications where rapid decrease of the load is not required, it is usually more practical to limit the maximum pressure in the evaporator using pressure limitation of the expansion valve. This strategy usually makes use of a smaller compressor motor, thereby reducing the initial purchase and maintenance costs of the system. As a rule, a pressure-limiting the expansion valve is chosen to have MOP approximately 5 to 10 psi (34.5 to 70 kPa) above the average pressure in the evaporator, arising in the ordinary load operation. The desired SS must be specified when ordering pressure limitations of TXV. Pressure limit expansion valves are widely used in air-conditioning applications.
In addition to protection against over pressure limit TXVs, these valves are also reduce the chance of liquid flood back to the compressor during startup. This reaction occurs because the pressure in the evaporator must be reduced below the SS to TXV can open. Therefore, TXV minimizes the flow of refrigerant to the level of diat allows suction pair for cooling sensing bulb before the valve is allowed to fully open...