The Constant Pressure Of The Expansion Valve
Constant pressure, characteristic for these metering valves results from the interaction of the forces generated by the pressure in the evaporator coil and set point spring. Pressure in the evaporator and the closing of the spring force on the one side of the diaphragm, which acts to move the needle in the side of the seat, reducing the die refrigerant flow. On the contrary, force created on the basis of specified spring is applied to the other side of the diaphragm and acts to move the needle from the seat, increase the flow of refrigerant. The net result of these opposing forces the position of the needle in place, which maintains constant pressure in the evaporator for all normal load conditions.
In CPXV reacts to changing load conditions flooding the evaporator during light loads and starving the evaporator during heavy loads. Floods coil decreases the effective coil volume, thereby reducing the effective area of heat transfer surface. This causes pressure evaporation of the coolant grow because the refrigerant vapor is compressed in die smaller amount of used coil.
On the contrary, the increase of the useful coil volume reduces the evaporation pressure, allowing the pair to expand to occupy the increase of the useful coil. In General, CPXV alternately floods and dies evaporator refrigerant to keep the pressure set point is determined by the position of the adjusting screw. To illustrate this answer, consider CPXV calibrated to the point of 10 psi (69 kPa). As the cooling load in the conditioned space increases, the pressure in the evaporator also rises.
When the evaporator pressure rises above the desired set point 10 psig (69 kPa), in effect it produces on the diaphragm is combined with the return spring, to break the power of die set point spring. This action causes the needle to move in the direction of the seat, reducing the flow of liouid of the refrigerant in the evaporator. Reducing the flow of refrigerant causes the liquid in the coil vaporize, to increase the useful volume and surface area of the heat transfer in the evaporator. The increase in the volume produces a corresponding decrease of pressure in the evaporator. As the evaporator pressure decreases, the force it creates on the diaphragm is also reduced, which allows you to set the vernal move the needle from the seat until the point is reached when the forces of the return valve in the equilibrium state.
Die when the load in the conditioned space decreases, the evaporator pressure decreases and the power of the set point spring exceeds the combined force created by the pressure in the evaporator and spring return. This action the position of the diaphragm in a manner that causes the needle to move away from place to die, thereby increasing the flow of liquid refrigerant in the evaporator. This is more flow of the refrigerant can not evaporate because the thermal energy is not present in the conditioned space. Therefore, additional refrigerant remains in a liquid state and floods die part of the evaporator. This action reduces the amount of useful, and, dierefore, die of heat-exchange surface of the coil. Because of incompressible liquid vapour pressure of the refrigerant in the remaining volume of the springs are compressed. The liquid refrigerant continues to flood the evaporator until the vapor pressure rises to a value that restores the balance between the forces through the diaphragm.
If the heat load of the system is reduced too much, constant pressure of the expansion valve can start to overfeed evaporator in an attempt to raise his head, so that it is equal to the given point. This response may liquid refrigerant type suction line and be transported into the compressor, where it can cause damage to the valve or grease washout. Therefore, thermostat control wired system for cycle die compressor off before the space or product temperature decreases to a level that would flood back to happen.
Forces in CPXV will automatically stop die flow of the refrigerant, die when the compressor cycles off. This is because the evaporation of liquid refrigerant in the evaporator lasts for a short time after the compressor cycles off. During this period, the pressure in the coil rises, because the couple is not removed from the evaporator to the compressor. Thus, the pressure in the evaporator and the closing of the spring force increase until it exceeds the force generated by setpoint spring. This response causes the needle to tightly sealed against the valve seat. Valve remains closed to prohibit the flow of refrigerant until the compressor cycles.
When the compressor cycles, the evaporator pressure decreases rapidly as the steam flows into the suction line. As soon as the pressure in the evaporator is reduced to a point at which a point of the vernal more power than the closing and coil pressure, the arrow moves away from the die seat, allowing the liquid in the evaporator. Valve needle continues to regulate the flow of refrigerant in the evaporator until equilibrium condition occurs between the valve forces...