热力膨胀阀

《制冷原理与技术》讲义 陈江平 上海交通大学制冷研究所
一、节流机构四大件之一
二、节流机构的主要类型
1. 固定流通面积节流机构：毛细管（冰箱、空 调、制冷等小型制冷装置）、节流小孔 和节流短管 2. 自动膨胀阀：定压阀、浮球膨胀阀
替以方便调试） （汽车空调） 液式蒸发器和水箱式蒸发器，控制制冷剂液位） （很多场合使用，多以手动膨胀阀代
（用于满
节流降压 控制冷媒流量,使蒸发器保持一定液体量 等焓过程
3. 热力膨胀阀 4. 电子膨胀阀
2.1 Capillary Tubes
Simple and inexpensive Applications in small refrigeration systems
特点
一般是1-6m长，0.5-2mm内径 由于摩擦和流体加速造成压降 液量主要取决于入口压力 调节性能差 结构简单，价格低 无运动部件，寿命长
适用范围
家用空调器 冰箱 除湿机组
Pressure and Temperature of Refrigerant in Capillary Tube
Capillary Tubes - Limitations
Only for one fixed set of operating conditions. Deviation from design conditions will result in drop in operating efficiency. Sudden large reduction in thermal load may result in unevaporated liquid reaching the compressor and cause damages.
Liquid
Vapour
Critical flow

供液量 2.2 节流短管Orifice Tube
• Starved evaporator– not enough refrigerant to provide enough cooling capacity • Overfed evaporator – too much refrigerant for the amount of cooling needed, resulting in slugging of the compressor (liquid drops enter the compressor)
Calibrated Restrictor Different color = different Orifice size. Mesh Filter Screen. Meters refrigerant into evaporator as low pressure liquid.
Constant Pressure Expansion Valve
2.3Automatic expansion valve
An automatic expansion valve maintains a constant pressure in the evaporator. Normally this valve is used only with direct expansion, dry type of evaporators. In operation, the valve feeds enough liquid refrigerant to the evaporator to maintain a constant pressure in the coils. This type of valve is generally used in a system where constant loads are expected. When a large variable load occurs, the valve will not feed enough refrigerant to the evaporator under high load and will overfeed the evaporator at low load. Compressor damage can result when slugs of liquid enter the compressor.
The valve is operated by the
evaporator or valve outlet pressure to regulate the mass flow
rate of liquid refrigerant entering the evaporator. To maintain the evaporating pressure of a refrigeration system as steady as possible. When the instantaneous evaporating pressure is less than the control point, the valve will open widely. When the evaporating pressure is higher than the control point, the valve will then close gradually. Such a characteristic is good for suction pressure.
protecting the compressor from being overloaded owing to high
2.4 Low-Side Float Expansion Valve
The low-side float expansion valve (fig. 6-23) controls the liquid refrigerant flow where a flooded evaporator is used. It consists of a ball float in either a chamber or the evaporator on the lowpressure side of the . The float actuates a needle valve through a lever mechanism. As the float lowers, refrigerant enters through the open valve; when it rises, the valve closes.
2.5 High-Side Float Expansion Valve
high-side float expansion valve (fig. 6-24), the valve float is in a liquid receiver or in an auxiliary container on the high-pressure side of the system. Refrigerant from the condenser flows into the valve and immediately opens it, allowing refrigerant to expand and pass into the evaporator. Refrigerant charge is critical. An overcharge of the system floods back and damages the compressor. An undercharge results in a capacity drop.