How to select the right Thermal expansion valve and Electronic expansion valve? In this article, we will explain in a simplified way what is the role of the expansion valve in the water chiller system. We will also let you know the differences between a thermostatic expansion valve and an electronic expansion valve.
Part I – How to Select the Right Thermal expansion valve?
When it comes to thermal expansion valve selection, We need to make sure clear some basic parameters.
- Evaporating temperature
- Condensing temperature
- Subcooling degree
- The pressure drop of the liquid distribution head
- Refrigerant, cooling capacity
- Connection size,
- Whether the MOP function is required
- Evaporating temperature range of use
Here, We should need to clarify several concepts:
1.The nominal cooling capacity of the thermal expansion valve.
TG valve, for example, in the top part of the valve body will have the valve’s nominal cooling capacity of 32Kw, where the 32Kw is the thermal expansion valve in accordance with the ASERCOM standard operating conditions, static superheat 4 degrees, open superheat 4 degrees of the actual cooling capacity of the valve corresponding to R410A refrigerant measured, the capacity of the valve and the size of the superheat is approximately proportional to the linear adjustable range 30% ~ 120% and the valve’s static superheat 4 degrees can be adjusted in the range of 0 ~ 8 degrees, which is a large space in the selection of models.
At the same time, the test criteria and the actual application of water-cooled chiller units closer to the working conditions, the capacity of other working conditions will certainly be very different, the greater the difference between high and low pressure, the greater the capacity of the valve will be.
For the above reasons, we need to know that our usual 10Hp water chiller system, selected with a 10Hp thermal expansion valve is not accurate.
2: The evaporating temperature and condensing temperature.
Valve selection used in the low and high pressure is the corresponding valve outlet and inlet pressure, and water chiller units in the evaporating temperature and condensing temperature we often refer to the two heat exchanger outlet refrigerant pressure, selection in which we need to consider the condenser outlet to the thermal expansion valve inlet liquid tube and all accessories installed in the pipeline pressure drop, such as a reservoir, sight glass, solenoid valve, filter drier, etc. On the low-pressure side, we need to consider the pressure drop generated by the evaporator itself, If the cooling system has a separator head and capillary tube, this should also be taken into account.
3: Supercooling degree.
The greater the undercooling degree, the greater the capacity of the thermal expansion valve will be. If the size of the designed undercooling degree of the system cannot be determined, it can be estimated according to experience. The undercooling degree of the air-cooled system is generally about 2 degrees, and the water-cooled system is generally about 4 degrees. Systems with economizers need to consider the size of the economizer match.
After understanding the above basic concepts, Please make the sample selection based on the following steps:
- Determine the type of valve filling. For example in the evaporation temperature -30 degrees cold storage applications, we can only choose N series -40 ~ +10 ℃.
- Determine whether the need for MOP function, unless very special applications, the normal refrigeration, and air conditioning system does not require MOP function.
- Determine the application of refrigerant, capacity, and evaporation temperature.
- Calculate the pressure difference between the two sides of the valve.
5.Correct the required capacity according to the subcooling degree.
- Select the appropriate thermal expansion valve according to the data table in the sample.
7.Determine the code of the valve against the sample.
Two experiments are required to determine if the valve is properly sized.
- The working condition with the highest refrigerant flow rate.
- The working condition with the lowest refrigerant flow rate.
If possible, you also need to do defrost experiments, in the two extreme operating conditions, the thermal expansion valve still has a margin to be able to adjust the degree of superheat, in order to prove that the selected valve is the right size valve.
Deviations in valve selection can be adjusted by adjusting the static superheat, depending on the application, according to the current ambient temperature and water temperature at the time of adjustment.
If the thermal expansion valve is incorrectly selected will lead to the following phenomena and failures.
Selection is too large:
Compressor back to the liquid, low pressure is high, high pressure is slightly low, the compressor shell dew or frost, back to the compressor shell of the liquid diluted lubricating oil, resulting in reduced lubrication, serious cases can lead to the compressor bearing surface of the greatest force wear hold shaft, and even burn the motor.
Selection is too small:
The system’s low pressure is too low, high pressure is high, superheat is large, the exhaust temperature is too high, serious cases can lead to oil carbonization, compressor burned.
Part II- How to select the right electronic expansion valve?
- Design principles
1.Reliability principle: electronic expansion valve is a perishable part, the product itself and the use of high- reliability requirements, in the development of new products as far as possible to use the matured products.
2.General principle: electronic expansion valve is mainly designed for the selection, in the first selection need to consider the principle of industry commonality, as far as possible in the standard models provided by the manufacturer to choose.
- Determine the drive mode of the electronic expansion valve
- From the point of view of the accuracy of the control, reducer type electronic expansion valve fully open pulse number of 2000 pls and 1400 pls two, the adjustment is extremely accurate, while the direct-acting electronic expansion valve generally maximum open degree of 500 pls, the adjustment is not as accurate as of the reducer type, but generally can meet the needs of our chiller system.
- Considering the cost, the current price of a direct-acting electronic expansion valve with the same cooling capacity is relatively lower than that of the reducer type.
3.Considering the cold capacity, the nominal capacity of the reducer type electronic expansion valve is larger compared to the reducer type, but multiple reducer type electronic expansion valves can be considered in parallel.
Based on the above recommendations, direct-acting electronic expansion valves are preferred.
- Electronic expansion valve specifications
Determine the diameter of the electronic expansion valve
- Preliminary selection of electronic expansion valve specifications according to the cooling capacity characteristics of different caliber electronic expansion valves: In general, the cooling capacity of the electronic expansion valve at 40% of the number of fully open pulses to determine the caliber of the valve. If the capacity of a single electronic expansion valve is not enough, 2 or more electronic expansion valves can be used in parallel.
- Direct-acting electronic expansion valve (CAM, HAM type) cooling capacity settings (condensing temperature: 38 ℃, subcooling degree: 0 ℃, evaporating temperature: 5 ℃, superheat degree: 0 ℃, pressure loss: 0
- Deceleration type electronic expansion valve (EDM type) (condensing temperature: 38 ℃, subcooling degree: 0 ℃, evaporating temperature: 5 ℃, superheat degree: 0 ℃, pressure loss: 0)
- Select the appropriate flow characteristics according to the matching needs of the cooling system
At present, most manufacturers can provide a variety of flow characteristics of the electronic expansion valves, you can choose the appropriate flow characteristics of the electronic expansion valve according to our requirements for refrigerant flow variation and noise requirements.
The direct-acting electronic expansion valve (CAM, HAM type) airflow characteristics
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