Do you know the six throttling devices for refrigeration equipment?
The function of the throttling mechanism: throttling and reducing the pressure of the high-pressure liquid refrigerant to ensure the pressure difference between the condenser and the evaporator, so that the refrigerant in the evaporator evaporates at the required low pressure to achieve the purpose of refrigeration; The flow rate of refrigerant supplied to the evaporator is adapted to the change in the heat load of the evaporator.
The types of throttling devices can be divided into， not the same as throttling device in refrigerator:
(1) Manual expansion valve;
(2) Floating ball expansion valve;
(3) Thermal expansion valve;
(4) Electronic expansion valve;
(6) Throttle short pipe;
1. Manual expansion valve:
Only used for ammonia refrigeration system, experimental device, bypass backup, etc.
2. Floating ball expansion valve:
In addition to throttling, reducing pressure and adjusting flow, it can also maintain a certain level of liquid in the evaporator. Applicable to: evaporator with free liquid surface, such as:
Low pressure circulating liquid storage tank;
Features: simple structure; large fluctuations in the liquid level of the float chamber, and large impact force transmitted by the float to the valve core, which is easy to damage.
3. Thermal expansion valve:
Working principle: The opening of the valve is controlled by the superheat of the gaseous refrigerant at the outlet of the evaporator. Used for: non-filled evaporator. Type: internal balance, external balance.
The pressure collected by the externally balanced thermal expansion valve is the outlet pressure of the evaporator; the pressure collected by the internally balanced thermal expansion valve is the outlet pressure of the expansion valve.
The external balance thermal expansion valve is mainly used in systems where the evaporation pressure loss or pressure drop is large, the flow resistance is large, the evaporation coil is long, the temperature fluctuation is large, and the evaporation pressure after throttling is much higher than the pressure at the outlet of the evaporator.
Let’s take a case to illustrate: two identical refrigeration systems, expansion valve 1 is internally balanced, expansion valve 2 is externally balanced, and the superheat is set to 8k. After the system is running, the expansion valve starts to adjust automatically. If the evaporator outlet is really superheated at 8k, the external balance thermal expansion valve will keep running in this state. Due to the pressure drop of the evaporator, the internal balance valve feels the pressure. If it is too high, the valve needs to be closed. When the internal balance valve feels that it is 8k, the real superheat at the outlet of the evaporator is already greater than 8k, which will cause the evaporator to not be fully utilized.
When selecting and matching thermal expansion valves, consider:
Type of refrigerant;
Evaporating temperature range;
The maximum cooling capacity of the evaporator after the valve;
Pressure difference before and after the valve;
4. Electronic expansion valve
The electronic expansion valve uses the electrical signal generated by the adjusted parameter to control the voltage or current applied to the expansion valve to achieve the purpose of adjusting the liquid supply. The adjustment range of the liquid supply is wide, and the adjustment response is fast.
Control signal of electronic expansion valve
(1) Superheat adjustment: used for dry evaporators to set a temperature sensor or pressure sensor at the evaporator outlet to collect the superheat of the refrigerant at the evaporator outlet, and feedback to adjust the opening of the control valve;
The feedforward and feedback compound adjustment can eliminate the lag of superheat control caused by the heat capacity of the evaporator tube wall and the sensor, improve the quality of system regulation, and control the superheat within the target range in a wide evaporating temperature range;
(2) Designated adjustment procedures: heat pump unit defrosting, compressor discharge temperature control.
(3) Liquid level adjustment: used for flooded evaporator.
The slender red copper pipe with a diameter of 0.7~2.5mm and a length of 0.6~6m is widely used in small fully enclosed direct cooling devices. The liquid supply capacity depends on: the state of the refrigerant at the inlet of the capillary tube (pressure, temperature), and the geometry of the capillary tube (length, inner diameter).
Simple structure and low price;
No moving parts;
The system does not need to be equipped with a liquid reservoir, and the refrigerant charge is low;
After the compressor stops, the pressure can reach equilibrium quickly, reducing the starting load of the motor;
Poor adjustment performance, liquid supply cannot be adjusted with changes in working conditions;
It is suitable for occasions where the evaporation temperature does not change much and the working conditions are relatively stable.
6. Throttle short tube
The throttling short tube is a throttling device with a fixed cross-section throttling orifice, which has been used in some automobile air conditioners, chillers, and heat pump units.
The main advantages: low price, simple manufacturing, good reliability, easy installation, the elimination of the temperature sensing bulb used to judge the increase of the cooling load in the thermal expansion valve system, etc., with good interchangeability and self-balancing ability.