Zhuojin Pipe Fitting Co., LtdExperimental Study on the Action Characteristics of Three-Way Proportional Control Valves in Parallel-Type Temperature and Humidity Control Systems
Literature Overview
The paper by Cui Jian-Ning and colleagues, published in Journal of Refrigeration (2011, Vol. 32, No. 5), investigates the operational characteristics of three-way proportional control valves used in parallel-type temperature and humidity control units. These units have gained widespread application in underground engineering projects due to their energy efficiency and high performance. The three-way proportional control valve serves as a critical component that regulates refrigerant flow distribution between parallel condensers, and its action characteristics directly influence the overall system performance.
The authors constructed a dual-condenser parallel-type temperature and humidity control system and tested the effects of three-way proportional control valve opening variations on system performance. The study provides quantitative relationships between valve opening and key performance parameters, offering practical guidance for system optimization.
Key Experimental Results
The experimental results establish clear performance characteristics across different valve opening ranges:
| Performance Parameter | Optimal Valve Opening Range | Key Finding |
|---|---|---|
| Supply Air Temperature Regulation | 30%–80% | Continuous temperature adjustment achievable |
| Dehumidification Capacity | 50%–65% | Maximum dehumidification, minimum input power |
| Specific Power Dehumidification | 50%–65% | Highest efficiency |
| Refrigerant Flow Rate | 50%–70% | Maximum flow rate achieved |
These findings reveal that the valve opening range of 50%–65% represents the optimal operating window for the parallel-type temperature and humidity control system, where maximum dehumidification capacity is achieved with minimum input power consumption. The broader range of 30%–80% provides the full operational envelope for supply air temperature control.
Piping and Fitting Analysis
From a piping and fitting engineering standpoint, the three-way proportional control valve is essentially a specialized three-way pipe fitting with integrated flow control capability. The design and fabrication of this component require careful attention to several engineering principles that are fundamental to pipe fitting technology.
The three-way valve in this application functions as a refrigerant flow splitter between two parallel condenser circuits. The internal geometry of the valve—specifically the branch angle, the bore dimensions, and the flow control element—determines the flow distribution characteristics and pressure drop across the valve body. The optimal valve opening range of 50%–65% for maximum dehumidification suggests that the valve's internal flow geometry is designed with a specific flow coefficient (Kv) that matches the system's refrigerant circuit requirements.
The pressure drop characteristics across the three-way valve are directly related to the piping layout of the parallel condenser system. According to the Bernoulli equation and the Darcy-Weisbach equation, the pressure drop across the valve body is proportional to the square of the flow velocity, which in turn depends on the bore diameter and the valve opening. The experimental observation that maximum refrigerant flow occurs in the 50%–70% opening range indicates that the valve's flow coefficient is calibrated such that this opening range provides the optimal balance between flow capacity and pressure drop.
The welding connections between the three-way valve body and the system piping are critical for maintaining system integrity. Refrigerant piping systems typically operate at pressures ranging from 1.0 to 3.5 MPa for the high-pressure side, and the joints must withstand these pressures over extended service life. The welding process used for these connections—typically GTAW (Gas Tungsten Arc Welding) for copper or aluminum piping, or SAW (Submerged Arc Welding) for steel piping—must produce full-penetration welds with minimal heat-affected zone distortion to avoid creating stress concentrations that could lead to fatigue failure under cyclic pressure loading.
FMEA Analysis of Three-Way Valve Failures
Applying Failure Mode and Effects Analysis (FMEA) methodology to the three-way proportional control valve in this application reveals several potential failure modes:
| Failure Mode | Potential Cause | Effect | Severity | Occurrence | Detection | RPN |
|---|---|---|---|---|---|---|
| Valve sticking | Refrigerant oil degradation, debris accumulation | Loss of flow control, system imbalance | 8 | 4 | 6 | 192 |
| Sealing leakage | Wear of valve stem packing, thermal cycling | Refrigerant loss, efficiency degradation | 7 | 5 | 5 | 175 |
| Positioner malfunction | Electrical signal failure, actuator wear | Incorrect flow distribution | 8 | 3 | 4 | 96 |
| Internal erosion | High-velocity refrigerant flow, particulate contamination | Reduced Kv, increased pressure drop | 6 | 4 | 5 | 120 |
The highest Risk Priority Number (RPN) of 192 corresponds to valve sticking, which is the most critical failure mode. This failure is primarily caused by refrigerant oil degradation and debris accumulation in the valve bore, which can be mitigated by regular maintenance schedules, the use of high-quality refrigerant oils with good thermal stability, and the installation of upstream filters in the piping system.
The internal erosion failure mode (RPN = 120) is particularly relevant from a materials and piping design perspective. The high-velocity refrigerant flow through the valve's internal passages can cause erosion of the valve body material, particularly at the branch junctions where flow impingement occurs. The selection of valve body material—typically brass, bronze, or stainless steel for refrigerant applications—must account for the erosive effects of high-velocity two-phase flow. The internal geometry should be designed with smooth transitions at branch junctions to minimize flow separation and impingement.
Engineering Practice and Design Recommendations
The experimental findings from this paper have direct implications for the design and specification of three-way proportional control valves in refrigeration and HVAC piping systems. The optimal operating window of 50%–65% valve opening for maximum system efficiency suggests that the system's piping layout should be designed to ensure that normal operating conditions correspond to this valve opening range. This requires accurate hydraulic calculation of the refrigerant circuit piping to determine the required flow coefficient and to select a valve with appropriate Kv rating.
The piping connections to the three-way valve should be designed with minimal bends and fittings in the immediate vicinity of the valve to ensure uniform flow distribution and to minimize pressure fluctuations that could affect valve stability. The use of concentric reducers rather than eccentric reducers at connections to the valve body helps maintain flow symmetry. The welding joints connecting the valve to the system piping should be inspected using non-destructive testing methods—preferably dye penetrant testing (PT) for copper piping or radiographic testing (RT) for steel piping—to ensure full penetration and absence of porosity or lack of fusion defects.
The study also highlights the importance of valve maintenance intervals in piping system design. The piping layout should provide adequate access for valve removal and replacement, with flanged or threaded connections at the valve body ends to facilitate disassembly without cutting the main piping runs. The inclusion of isolation valves on both the inlet and outlet sides of the three-way valve, along with drain connections, is essential for maintenance accessibility.
This literature demonstrates that the performance of a three-way control valve is not an isolated component characteristic but is deeply integrated with the overall piping system design. The hydraulic interactions between the valve and the connected piping determine the system's operational envelope and efficiency. Engineers designing refrigeration and HVAC piping systems should approach three-way valve selection and installation as a system-level problem rather than a component-level selection, ensuring that the valve's flow characteristics are harmonized with the piping layout to achieve optimal system performance.