Experimental Study of Three-Way Proportional Control Valve Characteristics in Parallel Temperature-Humidity Dehumidification Units

Literature Overview

Published in Journal of Refrigeration (2011, Vol. 32, No. 5), this paper by Cui Jian-ning, Han Xu, Jia Yong-jie, and Zhou Sen-lin investigates the operational characteristics of a three-way proportional control valve used as the key component in a parallel dual-condenser temperature-humidity control dehumidification system. The study was motivated by the growing demand for energy-efficient dehumidification solutions in underground engineering applications. Experimental tests were conducted on a dual-condenser parallel system to quantify how three-way valve opening affects supply air temperature, dehumidification capacity, and refrigerant flow rate.

System Architecture and Valve Function

The parallel temperature-humidity control dehumidification unit employs two condensers operating in parallel, with the three-way proportional control valve serving as the primary means of refrigerant flow distribution between the two condenser circuits. The valve modulates the split ratio of refrigerant flow, thereby controlling the effective condensing capacity and, consequently, the supply air temperature and dehumidification performance of the unit.

From a piping and fitting standpoint, the three-way proportional control valve functions as a critical tee-type component in the refrigerant piping system. Unlike a simple structural tee fitting, this valve actively modulates flow between the inlet and two outlet branches. The valve body geometry, port sizing, and internal flow path design directly influence the flow characteristics, pressure drop, and control authority of the valve. The refrigerant piping connections at the valve inlet and outlets must be properly sized to minimize additional pressure losses that could degrade the valve's proportional control performance.

Experimental Findings Summary

The experimental results reveal that the three-way valve's effective control range spans from 30% to 80% opening. Below 30%, the valve exhibits limited modulation authority due to laminar flow effects and seat leakage; above 80%, the flow becomes nearly fully open, reducing the valve's ability to make fine adjustments. The optimal operating window for dehumidification performance is 50% to 65% opening, where the unit achieves the highest dehumidification capacity per unit of input power.

Flow Distribution Analysis at the Three-Way Valve

The three-way proportional control valve presents a unique challenge in refrigerant piping systems: it must simultaneously distribute flow between two branches while maintaining stable pressure characteristics across a wide range of operating conditions. The valve's performance is sensitive to several piping-related factors:

1. Inlet piping straightness: Turbulence or vortex formation upstream of the valve inlet, caused by nearby elbows or tees, can create asymmetric flow entry conditions that degrade the valve's proportional response. A minimum of 5 to 10 pipe diameters of straight pipe should be maintained upstream of the valve.
2. Outlet branch sizing: The two outlet branches of the valve connect to the respective condenser circuits. Any significant difference in the hydraulic resistance of these branches—due to different pipe lengths, diameters, or fitting counts—will shift the valve's effective flow split ratio away from the design value.
3. Valve orientation and mounting: The physical orientation of the three-way valve affects the gravity component of the refrigerant flow, particularly in systems handling liquid refrigerant or two-phase mixtures. Improper mounting can cause flow stratification at the valve body, leading to inconsistent control characteristics.

Engineering Practice Implications

This study has direct relevance to engineers designing refrigerant piping systems that incorporate three-way control valves. The key insight is that the valve's performance envelope—particularly the 30% to 80% effective control range—must be respected in the system design. Operating the valve outside this range leads to degraded control performance and potential instability.

For the piping design, the following practices are recommended:

• Maintain adequate straight pipe length (at least 5D upstream, 3D downstream) at all valve connections to ensure uniform flow profiles.
• Use long-radius 90-degree elbows instead of standard-radius elbows in the condenser circuits to reduce pressure drop and maintain the valve's designed flow split characteristics.
• Ensure symmetric piping layout between the two condenser branches to preserve the valve's proportional control authority.
• Install vibration-isolated supports near the valve to prevent mechanical fatigue of the valve actuator and piping connections.

Material and Welding Considerations for Refrigerant Piping

The refrigerant piping in such systems typically uses seamless copper tubes or stainless steel tubes, depending on the refrigerant type and operating conditions. The three-way valve connections are usually brazed or welded joints, and the integrity of these joints is critical for preventing refrigerant leakage. For copper tube systems, silver brazing with a filler alloy containing at least 45% silver provides adequate joint strength and leak tightness. For stainless steel systems, GTAW (Tungsten Inert Gas Welding) with argon backing is the preferred process, with careful control of heat input to avoid sensitization of austenitic grades.

The valve body itself is typically manufactured from cast brass or cast stainless steel, with the internal flow passages machined to precise tolerances. Any corrosion or erosion at the valve seat or flow passages will degrade the proportional control characteristics over time, necessitating periodic inspection and maintenance.

Summary

The experimental study of the three-way proportional control valve in a parallel dehumidification system demonstrates that valve opening directly governs supply air temperature, dehumidification capacity, and refrigerant flow distribution. The optimal operating window of 50% to 65% opening for maximum dehumidification efficiency provides a clear design target for system controllers. For piping engineers, the key takeaway is that the performance of three-way control valves is inseparable from the quality of the connected piping—proper straight pipe lengths, symmetric branch layouts, and appropriate fitting selection are essential to preserving the valve's proportional control authority across its 30% to 80% effective range.