1. What is a Stainless Steel Evaporator?

A stainless steel evaporator is an evaporating heat exchanger coil—typically using 304 or 316L stainless steel tubes and, depending on environment, stainless or coated aluminum fins—where liquid refrigerant (NH₃, CO₂, HFO, or hydrocarbon) enters, boils inside the tubing, and absorbs heat from air, glycol, or brine passing over the coil bank.

Why stainless exists: traditional copper-tube + aluminum-fin (Cu/Al) coils prematurely fail in aggressive atmospheres. Stainless steel eliminates the galvanic couple (Al/Cu), resists chloride and chemical attack, and tolerates frequent high-pressure sanitation.

2. How does a stainless steel evaporator work?

Refrigerant side

• Low-pressure liquid enters via distributor → feeders → coil circuits.

• Heat flows through tube wall; refrigerant evaporates and becomes saturated vapor.

• Vapor exits to the suction header and compressor.

Air / secondary fluid side

• Air/glycol/brine crosses fins; heat is extracted; outlet temperature drops.

• In low-temp rooms, frost forms → defrost strategy must protect U-value.

Key design reality: performance depends on the overall heat-transfer coefficient (U)—dominated by air-side convection, fin geometry, and distribution—not only on base metal conductivity.

3. Main types of stainless steel evaporator assemblies

4. Why stainless vs. copper/aluminum?

4.1 Conductivity vs. system U-value

Despite lower λ, designers achieve the same or better coil capacity by tuning fin density, louver geometry, face velocity, circuiting, and tube wall—because U is largely constrained by the air side and frost, not by tube conductivity alone.

4.2 Failure modes avoided

4.3 Total cost of ownership (TCO)

Even with +50–80% CAPEX uplift, stainless coils typically deliver 25–40% lower 10-year TCO by avoiding early coil replacements, product loss, and downtime.

5. Applications where stainless is the “correct answer”

• Coastal cold rooms / blast freezers: chloride-laden air destroys Al fins; SS fin survives.

• Ammonia refrigeration (NH₃): copper is incompatible; SS is preferred.

• Meat/seafood processing: daily alkaline foams + high-pressure washdowns.

• Battery & electronics cooling rooms: sulfur compounds accelerate Cu corrosion.

• Pharma & clean facilities: hygiene, CIP cycles, and audit requirements.

• Petrochemical HVAC: mixed corrosive gases and solvent cleaning routines.

Observed service life: SS fin/tube coils 8–12+ years in harsh sanitation; Cu/Al often 2.5–4 years under the same regime.

6. Design & selection checklist (engineering level)

6.1 Refrigerant distribution

• Equal-length feeders, precision orifices, or EEVs to prevent starvation.

• NH₃/CO₂ require strict circuit balance; consider distributors with verified flow maps.

6.2 Air-side geometry

• Fin spacing: 8–12 FPI for coolers; 6–8 FPI for low-temp; 3.5–5 FPI in heavy frost rooms.

• Face velocity: 2.0–3.0 m/s typical; derate for frost loads to maintain defrost interval.

• Coil rows: 3–6 rows depending on capacity and ΔT; check pressure drop vs. fan curve.

6.3 Tube geometry

• Common OD: 9.52 mm or 12.7 mm; wall thickness sized for pressure code and corrosion allowance.

• Enhanced inner surfaces are less common in SS; rely on circuiting & distribution for boiling HTC.

6.4 Materials & interfaces

• Prefer 316L in chloride or sanitation-heavy sites; 304 acceptable inland.

• Avoid Al support plates with SS tubes (galvanic couple). Use SS plates & fasteners.

• Passivation/polish inside/outside where hygiene is audited; specify surface roughness if needed.

6.5 Defrost & drainage

• Hot-gas/electric/glycol defrost sized for coil mass and frost rate; verify pan slope & drain diameter.

• Design air baffles for even melt and minimal re-entrainment.

6.6 System controls

• Superheat setpoints tuned to refrigerant and load profile; CO₂ booster systems are sensitive.

• Fan staging/VFD for humidity and frost management; interlock with defrost logic.

7. Installation & commissioning

• Provide coil slope to ensure oil return; verify suction gas velocity at risers.

• Leave physical clearance for sanitation; consider removable guards and hinged fan panels.

• Verify distributor orientation (gravity & vapor carry-over direction).

• Leak test with dry nitrogen; dehydrate; charge per manufacturer’s circuit map.

• Tune EEV/orifices for uniform frosting during trial run; re-balance if a bank starves.

8. Maintenance & serviceability

• High-pressure cleaning (150–200 bar) approved for SS fins/tubes; use compatible detergents.

• Track coil ΔP and approach temperature to predict fouling; schedule defrost and deep-clean.

• In ammonia plants, verify oil return protocols and separator performance quarterly.

• SS coils are weld-repairable; fin straightening is feasible; spares: distributors, EEVs, fan motors.

9. Quick selection table (copy into your RFQ)

10. Frequently asked questions (FAQ)

Q1. Stainless conducts heat worse—does that hurt performance?

Coil performance hinges on U, not base metal λ alone. Designers compensate by increasing effective area and air-side HTC; practical results match or exceed Cu/Al in the same duty—while surviving aggressive environments.

Q2. Where is stainless mandatory?

Coastal, ammonia, sanitation-heavy food plants, sulfur-bearing atmospheres, petrochemical sites, and clean facilities subject to harsh cleaning chemicals.

Q3. Will fan power go up?

It can if geometry is copied 1:1. Properly engineered SS coils adjust fin pitch, row count, and face velocity to keep pressure drop within the target, often with no net increase in fan power.

Q4. SS tube + coated Al fin vs. full SS fin—how to choose?

For mild coastal/industrial air, coated Al fin can be a cost-effective compromise. For daily HP washdowns, heavy chlorides, or long life guarantees, choose full SS fin.

Q5. What about defrost?

Match method (hot gas, electric, glycol) to frost rate and duty cycle. Ensure pan geometry, drain sizing, and airflow routing avoid re-entrainment and ice bridges.