Stainless Steel Coil Tube for Fan Coil Units (FCU) — Dehumidification-Optimized Series

Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.

Product Catalog No.: 725

Classification: Technical Literature & Product Engineering Reference

Technical Focus: The Relationship Between Fan Coil Unit (FCU) Dehumidification Capacity and Average Supply/Return Water Temperature

1. Product Overview

The Stainless Steel Coil Tube is a precision-engineered heat exchange component designed for integration into Fan Coil Units (FCUs) — the core terminal equipment in commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) systems. This coil tube serves as the primary evaporator or condenser element within the FCU, facilitating efficient thermal transfer between the circulating chilled or heated water and the process air stream.

The fundamental engineering principle underlying this product centers on the dehumidification capacity of the FCU, which is directly and significantly governed by the average supply and return water temperature (T_avg = (T_supply + T_return) / 2). A lower average water temperature increases the temperature differential (ΔT) across the coil surface, thereby enhancing both sensible and latent heat transfer rates. This, in turn, maximizes the moisture removal capability (latent cooling load) of the unit, which is critical for maintaining indoor relative humidity (RH) within acceptable comfort ranges of 40–60% RH.

Key Engineering Insight: The dehumidification performance of a fan coil unit is nonlinearly dependent on the average supply/return water temperature. A reduction in T_avg by even 1–2 °C can result in a 15–30% increase in latent heat transfer capacity, provided the coil surface temperature remains below the dew point temperature of the incoming air. The stainless steel coil tube is engineered to maintain structural integrity and thermal efficiency across this critical operating range.

2. Material & Properties

The coil tube is manufactured from high-grade austenitic stainless steel, selected for its superior corrosion resistance, thermal conductivity, and long-term mechanical stability in humid, chemically aggressive environments typical of HVAC applications.

Base Material: Austenitic Stainless Steel — Grades 304 (S30408) and 316L (S31603) conforming to ASTM A213 / ASME SA-213 specifications
Corrosion Resistance: Excellent resistance to chlorides, condensation water, and atmospheric pollutants; pitting resistance equivalent number (PREN) ≥ 24 for Grade 316L
Thermal Conductivity: 14.9–16.3 W/(m·K) at 20 °C — optimized for rapid heat exchange between the refrigerant water circuit and the air stream
Mechanical Strength: Minimum tensile strength of 515 MPa (Grade 304) and 485 MPa (Grade 316L); yield strength ≥ 205 MPa
Operating Temperature Range: −40 °C to +250 °C, accommodating both extreme chilled water conditions (down to 2 °C) and heating applications (up to 80 °C)
Surface Finish: Electropolished or passivated inner surface (Ra ≤ 0.4 μm) to minimize fouling and maintain long-term hydraulic efficiency
Hygienic Compatibility: FDA-compliant; suitable for environments requiring high sanitary standards

3. Technical Specifications

Parameter	Specification	Standard / Reference
Tube Outer Diameter (OD)	6.35 mm (1/4"), 9.52 mm (3/8"), 12.7 mm (1/2")	ASTM A213
Tube Wall Thickness	0.50 mm – 1.00 mm	ASME B36.19M
Tube Inner Diameter (ID)	5.35 mm – 11.70 mm (dependent on OD and wall thickness)	—
Coil Pitch (Center-to-Center Spacing)	6.35 mm – 19.05 mm	ASHRAE Handbook — HVAC Systems & Equipment
Number of Rows	2 rows – 8 rows (configurable)	—
Fin Type	Aluminum corrugated fins, louvered or micro-channel	—
Fin Density	10 FPI – 18 FPI (Fins Per Inch)	—
Maximum Operating Pressure	1.6 MPa (232 psi) — water circuit side	EN 12828
Hydrostatic Test Pressure	2.4 MPa (348 psi) — 1.5× maximum operating pressure	ISO 5149-2
Recommended Water Flow Velocity	0.5 m/s – 1.5 m/s	ASHRAE Guideline 34
Heat Transfer Coefficient (Air Side)	25 – 55 W/(m²·K)	ASHRAE Fundamentals
Heat Transfer Coefficient (Water Side)	2,500 – 5,000 W/(m²·K)	ASHRAE Fundamentals
Overall Heat Transfer Coefficient (U-value)	60 – 120 W/(m²·K) (typical operating conditions)	—
Dehumidification Capacity Range	10 – 85 g/h per kW of total cooling capacity	ASHRAE Standard 90.1
Coil Length (Standard)	300 mm – 2,000 mm (custom lengths available)	—
Coil Height (Standard)	250 mm – 750 mm	—
Connection Type	Flanged, threaded (G, BSP), or welded headers	ISO 7-1 / BSPP

4. Manufacturing Process

The production of the stainless steel coil tube follows a rigorously controlled multi-stage manufacturing workflow to ensure dimensional accuracy, thermal performance, and long-term reliability:

Raw Material Procurement & Inspection: Stainless steel tubes are sourced from certified mills and subjected to spectroscopic analysis (PMI — Positive Material Identification) to verify chemical composition compliance with ASTM/ASME grade requirements.
Tube Cutting & End Preparation: Precision plasma or laser cutting to specified lengths; tube ends are deburred and chamfered to facilitate seamless insertion into headers.
Tube Bending & Coil Forming: Cold-forming processes using CNC-controlled bending machines to achieve precise coil pitch and curvature radii without inducing work-hardening or micro-cracking in the tube walls.
Fin Application: Aluminum fins are mechanically bonded to the stainless steel tubes using high-performance adhesive or crimping techniques, ensuring low thermal contact resistance at the fin-to-tube interface.
Header Assembly & Brazing: Tubes are inserted into stainless steel or copper headers and sealed using phosphorus-free silver brazing alloy (e.g., BAg-25Cu) to maintain the corrosion resistance of the stainless steel system.
Fin Stacking & Coil Assembly: Individual finned tube sections are assembled into the final coil stack with precise row and circuit configuration per design specifications.
Hydrostatic Pressure Testing: Each completed coil is pressure-tested at 2.4 MPa for a minimum of 15 minutes to verify leak-tight integrity.
Surface Finishing & Passivation: Final passivation treatment using citric acid or nitric acid solution to remove any free iron contamination and establish a protective chromium oxide layer on the exposed stainless steel surfaces.
Performance Testing: Select coils undergo thermal performance verification in a climate chamber to validate heat transfer and dehumidification capacity against design targets.

5. Applications

The Stainless Steel Coil Tube for Fan Coil Units is deployed across a broad spectrum of HVAC applications where reliable, long-lasting, and high-performance heat exchange is essential:

Commercial HVAC Systems: Office buildings, hotels, retail spaces, and hospitals utilizing VAV (Variable Air Volume) or constant air volume FCU configurations.
Residential HVAC: High-end residential installations requiring low-maintenance, corrosion-resistant heating and cooling terminal units.
Cleanroom Environments: Pharmaceutical manufacturing facilities, semiconductor fabrication plants, and laboratory spaces where particulate control and chemical resistance are paramount.
Food & Beverage Processing: Processing plants and cold storage facilities where hygienic standards mandate stainless steel construction.
Marine & Coastal Applications: Shipboard HVAC systems and coastal installations where chloride-induced corrosion is a persistent concern.
Data Centers: Precision temperature and humidity control in server rooms, where consistent dehumidification performance is critical to equipment reliability.

6. Dehumidification Performance Analysis

The core technical differentiator of this coil tube product lies in its optimized design for latent heat transfer — the process by which moisture is condensed from the air stream. The following relationship governs the dehumidification capacity (Q_latent) of the FCU:

Q_latent ∝ (T_dew − T_surface) × A × U_overall

Where T_dew is the dew point temperature of the incoming air, T_surface is the coil surface temperature (directly influenced by T_avg), A is the effective heat transfer area, and U_overall is the overall heat transfer coefficient.

Key performance characteristics:

When T_avg ≤ 7 °C, the coil surface temperature drops below the typical indoor dew point (10–15 °C), enabling maximum condensation and moisture removal.
When T_avg ≥ 12 °C, the surface temperature approaches or exceeds the dew point, significantly reducing dehumidification capacity and potentially leading to indoor humidity complaints.
The stainless steel coil tube's low fouling factor and high surface finish quality ensure that the U-value remains stable over the product lifecycle, unlike carbon steel alternatives that suffer from internal corrosion and scaling.
The optimized fin geometry (louvered or micro-channel configurations) increases the air-side turbulence and effective heat transfer area, further enhancing both sensible and latent capacity.

7. Quality Certifications & Standards Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems and holds the following certifications and standards compliance for this product line:

ISO 9001:2015 — Quality Management Systems certification ensuring consistent production quality and process control.
ISO 14001:2015 — Environmental Management Systems certification demonstrating commitment to sustainable manufacturing practices.
ASME SA-213 / ASTM A213 — Compliance for seamless stainless steel tubes used in heat exchangers and heat transfer equipment.
EN 10216-5 — European standard for welded austenitic stainless steel tubes for heat exchangers.
GB/T 14976 — Chinese national standard for seamless stainless steel tubes for heat exchangers (applicable for domestic market compliance).
UL Listing — Underwriters Laboratories certification for safety compliance in North American HVAC applications.
CE Marking — Conformity with European directives including the Pressure Equipment Directive (PED 2014/68/EU).
Material Traceability: Full mill test certificates (MTC) provided with every batch, including chemical composition analysis, mechanical property verification, and heat number traceability.

8. Unique Selling Points

Superior Corrosion Resistance: Grade 304/316L stainless steel construction eliminates the risk of internal tube corrosion and scaling, ensuring consistent thermal performance over a service life exceeding 20 years.
Optimized Dehumidification Performance: Precision-engineered fin geometry and coil configuration maximize latent heat transfer, directly addressing the critical relationship between average water temperature and moisture removal capacity.
Hygienic Design: Electropolished inner surfaces (Ra ≤ 0.4 μm) prevent bacterial adhesion and biofilm formation, making the coil suitable for pharmaceutical and food-grade applications.
Customizable Configurations: Available in multiple tube diameters, fin densities, row configurations, and coil dimensions to match specific FCU design requirements and space constraints.
Full Quality Traceability: Every coil is manufactured with complete material traceability, hydrostatic testing documentation, and performance certification.
Energy Efficiency: The high thermal conductivity and low fouling factor of stainless steel contribute to lower pumping energy requirements and reduced overall system energy consumption, supporting compliance with ASHRAE 90.1 energy efficiency standards.

9. Technical Recommendations

Design Note: To maximize the dehumidification capacity of fan coil units equipped with this stainless steel coil tube, engineers should design the chilled water system to maintain a supply water temperature of 6–7 °C (with a return temperature of 12–13 °C), yielding an average water temperature (T_avg) of approximately 9–10 °C. This operating range ensures the coil surface temperature remains sufficiently below the indoor air dew point to achieve optimal latent heat transfer while avoiding excessive supercooling that would result in unnecessary energy consumption.

For further technical inquiries, product customization, or detailed performance data, please contact the engineering team at Wenzhou Jixing Metal Materials Co., Ltd.