Answer: Thermal expansion valves and capillaries are throttling mechanisms of refrigeration systems. In cold dryers, the supply and regulation of evaporator refrigerant are achieved through throttling mechanisms. The throttling mechanism changes the refrigerant from a high-temperature and high-pressure liquid to a low-temperature and low-pressure liquid and enters the evaporator.
A capillary tube is a small-diameter tube with a specified length. It has no moving parts and can produce a predetermined pressure drop in the refrigeration system. It is generally used as a throttling element for refrigerators, air conditioners and small cold storages.
The capillary tube produces a pressure drop along the length direction according to its flow resistance to control the flow of the refrigerant and maintain the pressure difference between the condenser and the evaporator. When a refrigerant with a certain degree of supercooling enters the capillary tube, pressure and state changes will occur along the flow direction. First, the supercooled liquid will gradually decrease with the pressure, and will first become a saturated liquid under the corresponding pressure. This section is called the liquid phase section, and its pressure drop is not large and changes linearly; from the appearance of the first bubble to the end of the capillary tube, it is a gas-liquid coexistence section, also known as a two-phase flow section. The saturated vapor content in this section gradually increases along the flow direction, so the pressure drop changes nonlinearly. The closer to the end of the capillary tube, the greater the pressure drop per unit length. When the pressure drops to the saturated pressure at the corresponding temperature, flash phenomenon will occur, causing the liquid to evaporate and cool itself, that is, as the pressure decreases, the temperature of the refrigerant also decreases accordingly, that is, it decreases to the saturated temperature under the corresponding pressure.
When the load changes, the thermal expansion valve adjusts the valve core opening by detecting the compressor suction superheat temperature, thereby controlling the refrigerant supply entering the evaporator.
