1 一、节流机构?四大件之一 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 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（很多场合使用，多以手动膨胀阀代 替以方便调试）和节流短管（汽车空调） 2.自动膨胀阀：定压阀、浮球膨胀阀（用于满 液式蒸发器和水箱式蒸发器，控制制冷剂液位） 3.热力膨胀阀 4.电子膨胀阀 《制冷原理与技术》讲义 陈江平 上海交通大学制冷研究所 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 VapourLiquid Pressure and Temperature of Refrigerant in Capillary Tube Critical flow 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. 2 ? 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) 供液量 2.2 节流短管 Orifice Tube Calibrated Restrictor Different color = different Orifice size. Mesh Filter Screen. Meters refrigerant into evaporator as low pressure liquid. 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. Constant Pressure Expansion Valve 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 protecting the compressor from being overloaded owing to high suction pressure. 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 low- pressure 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. 3 三、热力膨胀阀 —内平衡阀 —外平衡阀 3.1 热力膨胀阀 为什么要控制蒸发器出口 过热度？ 避免湿压缩 避免过热度过大，导致 ?蒸发器相变换热面积减 小 ?压缩机吸气比容过大， COP下降 调节冷媒流量以适应负荷 需要 热力膨胀阀外观 热力膨胀阀 的安装形式 Sensing Bulb Evaporator 热 力 膨 胀 阀 工 作 原 理 热 力 膨 胀 阀 的 结 构 4 内平衡式热力膨胀阀 感应薄膜 阀体 螺母 阀座 阀针 调节杆座 填料 调节杆 填料压盖 螺母 过滤器 毛细管 感温包 传动杆 弹簧 传 动 杆 内平衡式热 力膨胀阀 P 弹簧 ＝P B －P 0 P B 是过热温度 下的冷媒饱和 压力 P B 阀体 螺母 阀座 阀针 调节杆座 填料 调节杆 填料压盖 螺母 过滤器 毛细管 感温包 传动杆 P 0 P 弹簧 车用内平衡膨胀阀（球阀） 外平衡 接头 感应薄膜 外平衡式热 力膨胀阀 出口 入口 外平衡 接头 调节机构 汽车空调H型膨胀阀结构 汽车空调用F型外平衡膨胀阀 外平衡式热力膨胀阀的接法 5 外平衡式热力膨胀阀的应用 适用于蒸发器出入口压力降大的情况： 蒸发盘管细长 通过分液器并联多根盘管 一般制冷量在5KW以上的系统需要用分液器， 此时必须用外平衡阀。 温包充注方式 1. Liquid Charge 2. Gas Charge 3. Liquid-Cross Charge 4. Gas-Cross Charge 5. Adsorption Charge 温包充注方式 2 Static Superheat . static superheat is the amount of superheat necessary to overcome the spring and equalizer pressures so that any additional superheat will cause the valve to open. Opening Superheat . opening superheat is the amount of superheat required to move the valve pin away from the seat after the spring and equalizer pressures have been overcome to permit refrigerant flow. Operating Superheat . operating superheat is the superheat at which the TEV operates on a refrigeration sys- tem. Operating superheat is the sum of static and opening superheats. The valve capacity versus operating superheat curve is referred to as the valve gradient. MOP 热力膨胀阀的控制特点 蒸发温度变化后，过热度不可能不变，只能不致偏离过多 感温包有热惯性，存在控制动作的滞后 供液能力(流量)和过热度(开度)之间为非线性关系 问题： 用于R22制冷系统的热力膨胀阀，蒸发温 度5℃时过热度为5℃。如果用于制冰工 况，蒸发温度为－20℃，此时过热度还 可以保持在5℃左右吗？ 如果蒸发温度上升到20℃呢？ 问题： 膨胀阀前出现冷媒气化对膨胀阀的性能 有何影响？ 蒸发温度的变化对膨胀阀的容量有何影 响？ 有什么办法可以避免蒸发温度偏高时压 缩机过载？ 怎样保证感温包能反映真正的过热度？ 6 四、 电子膨胀阀 热惯性小 调节动作不依赖于蒸发 压力，可自由确定不同 蒸发温度下的过热度 适用于热泵系统 价格贵