Definition

A safety sensor located in the HVAC supply duct that prevents supply-air relative humidity from exceeding a danger threshold (typically 90–95% RH); guards against condensation, condensate accumulation and duct-hygiene issues.

Detailed Explanation

A high-limit sensor (HHL or HL) is the secondary safety layer in humidification control. While the primary control sensor (room or return duct) chases the target RH, transient overdose events (e.g. a door opening drops RH sharply → device overshoot) can push supply RH above 95%. At that point supply-air condensation risk begins (duct walls, sensors, downstream coils). The high-limit sensor typically slows the humidifier above 85% RH and shuts it down above 90% RH. It works via PLC override: the main PID is cut, the system enters safe mode.

Why It Matters

A high-limit sensor prevents four main problems: (1) IN-DUCT CONDENSATION — at 95%+ RH supply air hitting cold duct walls condenses; duct-hygiene breach, corrosion, downstream coil wetting. (2) DOWNSTREAM EQUIPMENT DAMAGE — a wet cooling coil loses pressure and efficiency; a wet HEPA can burst, breaching hygiene class. (3) SPACE HYGIENE BREACH — cleanrooms, hospitals and pharma face microbial growth + contamination risk at excessive RH. (4) CONTROL-LOOP STABILITY — overshoot lengthens PID correction; early high-limit action keeps the RH band tight.

Practical Example

A printing plant in Konya had no high-limit sensor in the original design. On cold winter mornings, when heating + humidifying ran together, supply air reached 94–96% RH for 2–3 minutes; the duct saw 50–80 ml/h of condensate drip and the downstream filter wetted. NKT retrofit: a humidity high-limit sensor was placed in the supply duct 3 m after the manifold (slow at 88%, stop at 92%). Over the next 12 months: zero drip, filter life doubled.

Engineering Note

High-limit sensor placement rules: (1) In the supply duct, at least one absorption distance + 1 m after the manifold; (2) In a developed-flow region (at least 5× duct diameter before any elbow or damper); (3) Sensor tip at duct centre, not on the top or side wall. Control logic: typical 3-band — 85% (slow, drop capacity to 50%) → 90% (stop) → 95% (alarm + BMS notify + event log). The high-limit sensor must be independent and a different brand from the primary (single-point-of-failure protection).

NKT Application Link

For this application NKT – Nem Kontrol Teknolojileri supplies the Neptronic SKE4 resistive steam humidifier, the Neptronic SKH high-pressure atomisation system and the Neptronic SKD direct steam-injection humidifier. Unit-selection support is based on space conditions, humidification load and the feed-water analysis; technical service and spare parts are provided. Design and installation of water treatment, as well as engineering design, installation and commissioning, remain within the scope of the facility's designer, contractor and water-treatment specialist.