What should the target dew point be at the cooling tunnel inlet to definitively prevent condensation?

The general rule is to keep the inlet dew point at least 3°C below the product surface temperature. In a typical chocolate cooling tunnel, internal air is 0–10°C and product surface temperature at entry is 12–18°C, giving a safe dew point window of 4–8°C. For enrobed (fully coated) products, the target must be tightened to ≤6°C due to the criticality of the thin cocoa butter film. NKT engineering positions dew point sensors at three points (inlet, middle, outlet) during commissioning, and when deviation exceeds ±1°C, automation increases the silica gel rotor regeneration capacity. Adjustment based only on relative humidity is misleading when temperature fluctuates; absolute moisture control is mandatory.

How can sugar bloom and fat bloom be distinguished visually and by touch?

Fat bloom forms when cocoa butter transitions from stable Form V crystals to unstable Form VI; it leaves a grey-matte, oily film that smudges softly under finger pressure. Re-tempering can largely recover the chocolate. Sugar bloom has a completely different mechanism: condensation droplets dissolve surface sugar, and as water evaporates, sugar recrystallises as a coarse, glossy layer. It feels gritty like sandpaper, leaves no mark on the finger, and is irreversible. Practical test: press lightly with a fingertip, an oily mark means fat bloom, a hard crunchy feel means sugar bloom. Sugar bloom is always evidence of a humidity control failure, typically occurring in short windows where the dew point limit was exceeded.

How do you correctly size a silica-gel rotor dehumidifier for an enrobed chocolate line?

For enrobed chocolate (e.g., Magnum-type fully coated products) the target dew point is ≤6°C, so the only viable technology is a silica-gel rotor desiccant dehumidifier, condensation-type units cannot reach below this floor. Correct sizing requires the joint calculation of tunnel airflow (m³/h), absolute humidity differential between outdoor and tunnel air (g/kg), infiltration load through openings, and moisture evaporated from the product surface. NKT recommends TFT Italian silica-gel rotor ADP-series units (ADP 2000-9500) as standard for these applications; in coastal facilities, a 2× ADP 5500 redundant N+1 configuration delivers both backup and part-load efficiency.

Why should tunnel humidity be managed by dew point rather than relative humidity alone?

Relative humidity (%RH) is a temperature-dependent ratio; the same absolute moisture content reads 60% RH at 20°C but climbs to 95% RH at 12°C, brushing the condensation limit. Because temperature can vary 4–6°C along the inlet-to-outlet axis of a cooling tunnel, a fixed %RH target produces a different condensation risk at every point. Dew point is an absolute quantity: it expresses actual water vapour content as a single temperature value, directly comparable to the product surface temperature. Condensation occurs only when surface temperature drops below the air dew point. Therefore in industrial humidity control practice, the dew point setpoint is the only reliable reference; %RH should remain a secondary indicator for operators. On NKT automation panels both values are displayed in parallel, but the control loop operates on dew point.

How much does a buffer zone (entry vestibule) contribute to system performance?

A buffer zone placed at the tunnel inlet, forming a humidity-controlled volume between two curtains or doors, breaks direct air infiltration from the external corridor into the tunnel. Our field measurements show that a properly sized vestibule reduces infiltration load by 40–60%, directly impacting dehumidifier duty and energy use. A typical vestibule is 1.5–2.5 m long, held at slightly negative pressure (-5 to -10 Pa), and provides pre-drying. Its real value is absorbing the sudden humidity spike at door openings: corridor air enters the vestibule first and then feeds the tunnel in a controlled manner. The investment pays back in 3–5 months through energy savings. NKT sizes the vestibule as an integral part of project deliveries; retrofitting one later usually requires production downtime.

In a positive pressure strategy, what is the typical target value and how is it measured?

In cooling tunnels, the positive pressure target is +10 to +25 Pa relative to the adjacent corridor or production area. This differential blocks external moist air infiltration through openings while creating a very slow outward flow of dry air from the tunnel. Measurement is done with a calibrated micromanometer or a permanently installed differential pressure transmitter; the sensor links a reference port in the tunnel wall to a second port in the corridor via pneumatic tubing. The setpoint is kept dynamic based on seasonal door-opening frequency: +20 Pa during peak product flow, +10 Pa during stable production. Above 25 Pa, door seal load rises and closing time lengthens, reversing leakage into the production area. NKT automation modulates the supply fan inverter to hold the target when deviation exceeds ±3 Pa and logs the event.

How is the infiltration load through tunnel openings calculated and reduced?

Infiltration load is calculated in practice as Q = A × v × ΔW × ρ, where A is opening area (m²), v is average airflow velocity (m/s), ΔW is the absolute humidity difference between outside and inside air (g/kg), and ρ is air density. In a typical conveyorised tunnel, air exchange through openings ranges between 0.5–1.5 ACH, and ΔW can reach 8–12 g/kg in summer. The reduction strategy is layered: (1) plastic strip curtain plus air curtain combination cuts inlet velocity by more than half, (2) the buffer vestibule absorbs 40–60% of the remaining load, (3) positive pressure seals the last gap. Minimising conveyor belt opening height to 20–30 mm clearance above product height delivers the fastest gain in most plants. NKT field surveys measure each opening with an anemometer and include the total g/h moisture load in equipment selection.

Is the regeneration energy cost of a silica gel rotor dehumidifier realistic?

In a silica gel rotor, regeneration consists of dragging the moisture-laden sector through hot air at 100–140°C to expel the absorbed moisture; this energy is typically supplied by electric heaters, natural gas, or process waste heat. Regeneration accounts for 55–70% of total energy consumption, with the remainder being process fans and rotor drive. In a fair comparison, the excess compressor load and defrost cycles that a condensation system would draw under the same conditions to reach a 6°C dew point are close to the regeneration cost of a silica gel rotor; at low dew points (≤8°C) the rotor is usually more economical. NKT projects preferentially integrate boiler room or compressor waste heat for regeneration, preserving TFT rotor efficiency and reducing operating cost by 20–30%. Investment payback is 2–4 years.

When periodic maintenance is neglected, how and how fast does performance degrade?

Performance loss is silent and gradual, which is its most dangerous feature. In the first 3–6 months pre-filters become loaded, sub-micron dust accumulates on the rotor face, and nominal airflow drops 8–12%, dew point rises 1–2°C without the operator noticing. Between months 6 and 12, the regeneration heater's heat transfer efficiency declines, outlet temperature falls 10–15°C, and the rotor's moisture discharge capacity weakens; dew point shifts by 3–4°C and the first sugar bloom complaints appear. After month 12, gland seals harden, carryover losses increase, motor current rises, and the unit drops below 70% of nominal capacity. The NKT maintenance protocol includes 3-month filter, 6-month seal and sensor, annual rotor cleaning and regeneration heater checks; under regular follow-up the unit holds 95% of nominal performance for 8–10 years.

For a chocolate cooling tunnel, is a single dehumidifier enough, or is a redundant (N+1) configuration required?

For continuous-production plants where downtime cost is high, a redundant N+1 configuration is recommended: instead of a single large ADP 9500, two ADP 5500 units (each able to feed the line standalone) or one main unit plus a small backup. The redundant configuration is economically justified when at least one of three conditions holds: (1) continuous production (one unit stays online while another is in maintenance/cleaning, (2) high seasonal load swing) both units run in parallel at summer peak, while one is dropped offline in winter for part-load efficiency, (3) a single large unit exceeds physical delivery/installation limits, above 9,500 m³/h capacity, 2× ADP 5500 is more practical for transport and commissioning. NKT prioritises redundant configurations in plants with >1,500 kg/h output and 16+ hours/day operation; for small-mid lines, a single properly sized unit is sufficient.

Technical Article46 min read

Dehumidification in Cooling Tunnels for Chocolate and Confectionery

Why is dew point control mandatory in chocolate and confectionery cooling tunnels to prevent sugar bloom & fat bloom? Semi-open / closed / inline system architecture, condensing vs silica-gel rotor dehumidifier selection, buffer zone design, and an enrobed chocolate case study.

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Related Glossary Terms

For deeper definitions of the technical concepts in this article, browse the related entries in the NKT Glossary:

Relative Humidity (RH)Absolute Humidity Moisture Load Dew Point Psychrometric Chart Silica Gel Rotor (Desiccant Rotor)Reactivation (Regeneration)Evaporator Coil Adsorption Defrost Infiltration Sugar Bloom Calibration TCO (Total Cost of Ownership)ACH (Air Changes per Hour)PLC (Programmable Logic Controller)Setpoint (Target Value / Set Point)Sorption (Adsorption + Absorption)Fat Bloom Tempering (Chocolate Crystallization)Enrobed Chocolate Panning Buffer Zone (Vestibule)VFD (Variable Frequency Drive)Humidification Capacity (kg/h)Silica (SiO₂)Filtration (Water Filtration)Biofilm

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