Economizer Cycle

Detailed Explanation

The economizer cycle (free cooling) is a control mode that exploits outdoor cooling capacity at no cost. During shoulder seasons (spring, autumn) and cool nights, when outdoor temperature or enthalpy is below indoor design, the OA damper opens to 100% and the mechanical cooling compressor is disengaged.

Control strategies fall into two basic categories:

1. Dry-bulb temperature economizer — only dry-bulb temperature is compared. Simple, inexpensive, suitable for dry climates (Ankara, central Anatolia). 2. Enthalpy economizer — when outdoor enthalpy (h, kJ/kg) is below indoor enthalpy, OA increases. The only correct approach in humid climates (Istanbul, Izmir, coastal areas); a temperature-only check would import moisture load instead of cooling in humid summer air.

Economizer sequencing: • Mode 1: Outdoor temperature ≤ indoor − 1°C → OA 100% • Mode 2: Outdoor between indoor and indoor + setpoint → OA modulating • Mode 3: Outdoor > indoor + setpoint → OA minimum (IAQ floor)

Humidification load impact: in economizer mode the makeup ratio rises from 15-30% to 100%; humidification load grows 3-5× for the same building. For this reason "equipment capacity selection must be based on the economizer-on scenario, not the design peak day" is a core engineering rule.

Economizers are mandatory under LEED, BREEAM, ASHRAE 90.1 and the 2022 Turkish Building Energy Performance Regulation for high SEER/EER scoring.

Why It Matters

The impact of economizer mode on humidification load is often overlooked, but consequences are serious:

Example: 30,000 m³/h supply air, target 22°C, 50% RH = 8.3 g/kg.

Winter peak scenario (January, outdoor -3°C/75% RH = 2.4 g/kg, economizer off): • 20% makeup, 80% return • Δw = 0.2 × (8.3 − 2.4) = 1.18 g/kg • Load: 30,000 × 1.2 × 1.18 / 1000 = 42.5 kg/h

Shoulder-season scenario (April, outdoor 12°C/50% RH = 4.4 g/kg, economizer 100%): • 100% makeup • Δw = 8.3 − 4.4 = 3.9 g/kg • Load: 30,000 × 1.2 × 3.9 / 1000 = 140.4 kg/h

Difference: 42.5 → 140.4 kg/h = 3.3×. If equipment is sized for winter peak (45 kg/h capacity), 50% RH cannot be held in April; the room drops to the 32-35% RH band.

Real-world impact: • Complaint surge in offices below 35% RH • Paper curl in print shops • Static-electricity risk in hospital operating theatres • Wood cracking in museums, stress on artefacts

Engineering principle: sizing for winter peak to save capex causes far higher OPEX (efficiency loss, product defects, complaints). In economiser systems the humidification load is calculated separately for the outdoor conditions in which the economiser actually operates. Typical band: economizer-mode capacity is 3-5× winter peak.

Formula

Economizer activation criteria:

Dry-bulb mode: Economizer active ⟺ Toutdoor ≤ Tindoor − ΔTset ΔTset: typically 1-3°C

Enthalpy mode: Economizer active ⟺ houtdoor < hindoor h = 1.006 × T + (2,501 + 1.86 × T) × w / 1000 (kJ/kg) T: dry-bulb temperature (°C) w: absolute humidity (g/kg)

OA damper modulation: xOA = (Tindoor − Tsupply_setpoint) / (Tindoor − Toutdoor)

xOA: outdoor air ratio (0.15-1.00) Tindoor: indoor design temperature (°C) Tsupply_setpoint: supply air setpoint (°C) Toutdoor: outdoor air temperature (°C)

Humidification load in economizer mode:

Qeconomizer = mair × (windoor − woutdoor) / 1000

Ratio vs winter peak:

Factorec = Qeconomizer / Qwinter_peak = (1.0 × (windoor − woutdoor_shoulder)) / (xwinter × (windoor − woutdoor_winter))

Example (Istanbul, office): • Winter peak (-3°C, 2.4 g/kg, 20% OA): load 42.5 kg/h • Shoulder peak (12°C, 4.4 g/kg, 100% OA): load 140.4 kg/h • Economizer multiplier: 140.4 / 42.5 = 3.3×

NKT selection formula (safety margin included): Qselection = max(Qwinter, Qeconomizer) × 1.15

For the example: Qselection = 140.4 × 1.15 = 161.5 kg/h → 2 × Neptronic SKE4-90 (2 × 90 = 180 kg/h)

Engineering Note

Critical engineering points in humidification design under an economizer cycle:

• Enthalpy vs temperature selection — climates with annual average RH > 50% (Istanbul, Izmir, Bursa, Trabzon) require enthalpy economizer. A dry-bulb-only comparison imports moisture load instead of cooling on humid summer evenings. • Climate data quality — 8,760-hour typical meteorological year (TMY) data must be used; single-point decisions mislead. • Shoulder-season peak — certain hours in April and October constitute the true peak of the humidification load; not only "winter peak" but "economizer peak" must be evaluated. • Humidifier technology — economizer mode requires high capacity; a hybrid of steam (Neptronic SKE4) as base + adiabatic (Neptronic SKH/SKV) as supplementary is optimal for energy and capex. • OA damper quality — AMCA Class 1A; economizer cycling means extra damper motion, mechanical durability matters. • Freeze protection — when economizer is open in winter, preheat coil or damper restriction is needed; otherwise the coil freezes and humidifier output destabilises. • Frost-stat and humidifier interlock — when supply temperature drops below 4°C, the humidifier must shut off automatically, synchronised with the heating-coil alarm. • Sequenced control strategy — 1) economizer opens → 2) supply temperature stabilises → 3) PID humidification engages → then capacity ramps up. Otherwise "moisture rain in empty space" results.

NKT – Nem Kontrol Teknolojileri provides technical support for humidifier capacity selection taking economizer on/off scenarios into account; simulation reports and annual energy projections remain with the facility's designer.

NKT Application Link

For this application NKT – Nem Kontrol Teknolojileri supplies TFT, Neptronic and Microwell equipment. It supports unit selection with capacity calculation and technology matching based on space conditions, target temperature and RH (or dew point) and moisture load, and provides technical service and spare parts. Design, installation and commissioning remain within the scope of the facility's designer and contractor.