Stainless Steel Coil Tube (Coiltube) — HVAC Dry-Condition Fan Coil System
Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered coiled stainless steel piping product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for integration into Fan Coil Units (FCUs) operating under dry-condition configurations within Heating, Ventilation, and Air Conditioning (HVAC) systems. The coil tube serves as the primary heat exchange medium, facilitating efficient thermal transfer between the circulating fluid (typically water or refrigerant) and the surrounding air stream.
In a dry-condition fan coil water system, the coil tube operates at a surface temperature above the dew point of the ambient air, thereby preventing condensation formation on the coil surface. This configuration is particularly advantageous in environments where humidity control is critical, such as hospitals, laboratories, clean rooms, and commercial office buildings. The stainless steel coil tube ensures long-term reliability, resistance to corrosion, and maintenance of thermal performance over extended service life.
Material & Properties
Base Material
- Material Grade: Austenitic Stainless Steel — AISI 304 (UNS S30400) or AISI 316L (UNS S31603), selected according to application environment requirements.
- Alternative Grades: AISI 321 (UNS S32100) for elevated temperature applications; Duplex Stainless Steel (e.g., UNS S31803) for high-corrosion environments.
Key Mechanical Properties
- Tensile Strength: ≥ 520 MPa (AISI 304); ≥ 550 MPa (AISI 316L)
- Yield Strength: ≥ 205 MPa (AISI 304); ≥ 275 MPa (AISI 316L)
- Elongation at Break: ≥ 40% (minimum)
- Impact Toughness: ≥ 34 J at −40°C (Charpy V-Notch)
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (AISI 304); 14.6 W/(m·K) at 20°C (AISI 316L)
- Coefficient of Thermal Expansion: 17.3 × 10⁻⁶ /°C (AISI 304); 16.0 × 10⁻⁶ /°C (AISI 316L)
Corrosion Resistance
- Pitting Resistance Equivalent Number (PREN): ≥ 19 (AISI 316L)
- Resistance to: Atmospheric corrosion, waterborne chlorides, mild acids, alkaline solutions, and condensate-induced oxidation.
- Surface Passivation: Acid pickling and passivation treatment per ASTM A967 to ensure a chromium oxide passive film of minimum 50 Å thickness.
Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) |
| Product Code | 1343 |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd |
| Outer Diameter (OD) | 6.35 mm (¼"), 9.53 mm (⅜"), 12.70 mm (½") |
| Wall Thickness | 0.50 mm – 1.50 mm (customizable) |
| Coil Inner Diameter | 150 mm – 400 mm (configurable per FCU design) |
| Coil Length | Up to 30 m continuous coil (spooled) |
| Number of Rows | 1 to 6 rows (standard); up to 8 rows (custom) |
| Tube Pitch (Cross-flow) | 12.7 mm (½") standard; 15.9 mm (⅝") for enhanced airflow |
| Tube Pitch (Longitudinal) | 15.9 mm – 25.4 mm (adjustable) |
| Operating Temperature Range | −40°C to +150°C (continuous) |
| Maximum Operating Pressure | 1.6 MPa (standard); 2.5 MPa (reinforced) |
| Surface Finish | Mirror polished (Ra ≤ 0.4 μm) or satin finish (Ra ≤ 0.8 μm) |
| End Connection | Flared, threaded (BSP/NPT), or welded (butt-weld / orbital weld) |
| Standards Compliance | ASTM A213/A269, EN 10217-7, GB/T 14976 |
Manufacturing Process
Step 1 — Raw Material Selection & Incoming Inspection
High-purity austenitic stainless steel strip or seamless tubing is procured from certified mills. Incoming material undergoes spectrographic analysis (per ASTM E1951) to verify chemical composition, and ultrasonic testing to confirm internal soundness free of laminations and inclusions.
Step 2 — Coil Forming
The straight stainless steel tubing is fed through a precision multi-roller coiling machine that progressively bends the tube into the specified helical geometry. The forming process employs controlled reduction angles and support rollers to prevent ovality distortion, ensuring the coil maintains dimensional accuracy within ±0.5 mm tolerance. The coiling speed and roller configuration are optimized to avoid plastic deformation or work-hardening that could compromise material ductility.
Step 3 — Heat Treatment & Stress Relief
Following coiling, the tube assembly undergoes solution annealing at 1050°C – 1150°C with rapid water quenching to dissolve carbides, restore full austenitic microstructure, and relieve residual stresses introduced during the forming process. This treatment ensures maximum corrosion resistance and mechanical ductility.
Step 4 — Pickling & Passivation
The annealed coil tube is treated with a citric acid or nitric acid-based pickling solution to remove oxide scale, followed by passivation to form a uniform chromium oxide passive layer. This process conforms to ASTM A967 and ensures the surface is free of free iron contamination.
Step 5 — Precision Trimming & End Forming
The coil ends are trimmed to exact length using high-precision cutting tools. End connections (flaring, threading, or welding prep) are formed using automated equipment to ensure uniformity and leak-tight integrity.
Step 6 — Quality Inspection & Testing
Each coil tube undergoes comprehensive quality verification including:
- Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of OD, wall thickness, coil diameter, and pitch.
- Pressure Testing: Hydrostatic test at 1.5× the maximum operating pressure, held for 30 minutes with zero leakage tolerance.
- Surface Inspection: Visual and magnetic particle inspection (MPI) per ASTM E1444 for surface defects.
- Corrosion Testing: Salt spray test per ASTM B117 for minimum 500 hours without pitting or red rust.
Applications
Primary Applications — Dry-Condition Fan Coil Systems
The stainless steel coil tube is specifically engineered for dry-condition fan coil water systems, where the following design principles are applied:
- Enthalpy-Based Control: The system operates using enthalpy sensors to maintain the coil surface temperature above the dew point, eliminating condensation.
- Water Temperature Modulation: Variable-speed pumps and mixing valves regulate water supply temperature to the coil tube, ensuring precise thermal output without moisture generation.
- Improved Indoor Air Quality (IAQ): By preventing condensate formation, the system eliminates the primary vector for microbial growth (bacteria, mold, and Legionella) on the coil surface.
- Reduced Energy Consumption: Elimination of latent heat removal reduces fan energy consumption by 15%–30% compared to conventional condensing coil systems.
Industry Applications
| Sector | Application | Key Benefit |
|---|---|---|
| Hospital & Healthcare | Patient rooms, operating theaters, ICU units | Eliminates condensation-related microbial contamination; meets ISO 14644 clean room standards |
| Pharmaceutical & Biotech | Clean rooms, GMP environments, cold chain storage | Corrosion-resistant; hygienic surface finish; compliant with FDA and EU GMP regulations |
| Commercial Office Buildings | VAV (Variable Air Volume) terminal units | Enhanced IAQ; reduced maintenance cycles; energy-efficient dry operation |
| Food & Beverage Processing | Cooling rooms, packaging areas, storage facilities | Sanitary-grade stainless steel; resistant to food-grade cleaning chemicals |
| Marine & Offshore | Ship HVAC systems, offshore platform accommodations | Resistance to high-chloride marine atmosphere; extended service life |
| Data Centers | Precision cooling for server rooms | Condensation-free operation prevents moisture damage to sensitive electronics |
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system and holds the following certifications relevant to this product:
- ISO 9001:2015 — Quality Management System certification for manufacturing excellence.
- ISO 14001:2015 — Environmental Management System certification for sustainable manufacturing practices.
- ISO 45001:2018 — Occupational Health and Safety Management System certification.
- EN 10204 3.1 — Material Test Reports (MTR) issued by authorized inspection personnel, providing full traceability from raw material to finished product.
- CE Marking — Compliance with European Union directives for HVAC components.
- ASME B31.9 — Compliance with pressure piping standards for refrigeration and air conditioning systems.
- UL Listed — Safety certification for use in North American HVAC systems.
- RoHS & REACH Compliance — Restriction of Hazardous Substances and Registration, Evaluation, Authorization, and Restriction of Chemicals compliance for global market access.
Unique Selling Points
1. Dry-Condition Optimization: The coil tube geometry and surface finish are specifically engineered for dry-condition operation, enabling enthalpy-based HVAC control that eliminates condensation while maximizing sensible heat transfer efficiency.
2. Superior Corrosion Resistance: The use of high-purity austenitic stainless steel (AISI 304/316L) with a precision passivated surface provides exceptional resistance to waterborne chlorides, acidic condensates, and atmospheric pollutants — significantly outperforming copper and aluminum coil alternatives in humid environments.
3. Enhanced Indoor Air Quality: By preventing condensate formation on the coil surface, the system eliminates the biofilm growth medium that causes microbial contamination, ensuring compliance with stringent IAQ standards (ASHRAE 62.1, EN 16798).
4. Extended Service Life: The stainless steel construction provides a design service life exceeding 25 years without degradation of thermal performance or structural integrity, compared to 10–15 years for copper coil alternatives in corrosive environments.
5. Energy Efficiency: Dry-condition operation reduces the fan power requirement by 15%–30% and eliminates the latent cooling load penalty, resulting in overall system energy savings of 10%–20%.
6. Full Traceability: Each coil tube batch is accompanied by a complete material traceability dossier including mill certificates, heat treatment records, and non-destructive testing reports per EN 10204 3.1.
Technical Reference
This product specification is informed by the technical study "Discussion on Dry-Condition Fan Coil Water Systems" (干工况风机盘管水系统探讨), which provides the theoretical foundation for the design parameters, operating principles, and performance optimization of coil tubes in condensation-free HVAC configurations. The study emphasizes the importance of precise water temperature control, enthalpy-based sensor placement, and coil surface temperature management to maintain dry operation across varying ambient humidity conditions.