Design Conditions (Set Point)

Detailed Explanation

Design conditions (set points) are the indoor temperature, relative humidity and sometimes dew-point targets an HVAC system must maintain year-round. They are set by space function and are the fundamental input to engineering design; equipment capacity, control strategy and energy budget all depend on them.

Key references:

1. ASHRAE Handbook — HVAC Applications, Table 4 (Design Indoor Conditions for Various Places, Products, and Processes): T and RH references for 180+ applications: silk, carpet, wood, museum, hospital, lab, food storage, production halls. 2. TS EN 16798-1 (replacing TS EN 15251): indoor air quality categories (I-IV), temperature and RH bands for comfort. 3. TS EN ISO 7730 — analytical method for thermal comfort, PMV-PPD indices. 4. Sectoral guides — JEDEC (electronics), ICH Q1A/Q1B (pharma), AAMC (museums, libraries), ASTM D7102 (paper), ASTM D2654 (textile).

Typical design-condition examples: • Modern office: 22-24°C winter, 24-26°C summer, 40-60% RH • Hospital operating theatre: 19-23°C, 30-60% RH (surgeon comfort + static control) • Pharma tableting: 22°C ± 2°C, 35% RH ± 5 • Museum (wooden artefacts): 21°C ± 1°C, 50% RH ± 3 (ICOM standard) • Textile weaving (cotton): 22°C, 62-65% RH (for regain) • Cold storage (fresh fruit): 0-4°C, 85-95% RH • Lithium battery dry room: 23°C, dew point -40°Cdp (≈ 0.5% RH, over ice)

Design conditions are not single points but defined with deadbands; for example 22°C ± 1.5°C, 50% RH ± 5. Tighter deadbands carry higher capex/opex.

Why It Matters

Correct design-condition selection is the direct determinant of humidifier/dehumidifier capacity and annual energy budget. Even a 5% RH change for the same building shifts outcomes:

Example: 5,000 m³ office, 22°C indoor, 6,000 m³/h of 100% outdoor air, winter peak (outdoor -3°C, wOA ≈ 2.4 g/kg).

• 40% RH target: Δw = 6.5 − 2.4 = 4.1 g/kg → 30 kg/h load • 45% RH target: Δw = 7.4 − 2.4 = 5.0 g/kg → 36 kg/h load • 50% RH target: Δw = 8.2 − 2.4 = 5.8 g/kg → 42 kg/h load • 55% RH target: Δw = 9.0 − 2.4 = 6.6 g/kg → 48 kg/h load • 60% RH target: Δw = 9.9 − 2.4 = 7.5 g/kg → 54 kg/h load

60% RH requires 80% more humidification load than 40% RH; unit capacity and capex grow in proportion.

Also: • ±5% deadband → standard control, normal capex • ±2% deadband → tight PID, +15-20% control cost • ±1% deadband → dual-zone control, redundant sensors, extra units, +35-50% capex

Engineering opportunity: a customer says "I need 50% RH", but if the process actually tolerates ±5%, choosing a 45-55% band (±5%) reduces capex by 30%. Tight tolerance should be applied only when the process truly requires it.

Before the project, the real tolerance, product sensitivity and cost of deviation should be clarified with the process owner. This report is the decisive argument in capex budget discussions.

Practical Example

Consider an Istanbul pharmaceutical tableting facility. Production area 1,200 m² × 4 m height = 4,800 m³, sensitive tablet production (between-dosage form).

Interview findings: • Customer initial request: 22°C ± 1°C, 35% RH ± 2 — ICH Q1A(R2) reference • Real process requirement: tablet hardness stable below aw 0.4, suitable in 30-40% RH band • Product stability test: 24-month shelf life guaranteed at 35% ± 5 RH • Seasonal effect: customer said "35% constant", but batch tests showed product unaffected by ±5% room HVAC drift

NKT design-condition recommendation: 22°C ± 1.5°C, 35% RH ± 5 (band 30-40%). Customer accepted as ICH documentation supports the band.

Capacity comparison (winter peak, -3°C/2.4 g/kg outdoor):

Tight band (35% ± 2): • Effective setpoint: 33-37%, Δw = 5.7 − 2.4 = 3.3 g/kg (22°C/35% RH ≈ 5.7 g/kg) • ±2% PID precision required: 4 × Neptronic SKE4-60 (240 kg/h, redundant + 8-point sensors) • Capex: ~340,000 USD

Loose band (35% ± 5): • Effective setpoint: 30-40%, Δw still 3.3 g/kg; the wide band needs no redundant unit or dense sensor grid • ±5% PID precision required: 2 × SKE4-90 (180 kg/h, 4-point sensors) • Capex: ~190,000 USD • Savings: 150,000 USD capex + 35,000 USD/year opex

Result: correct design-condition selection delivered ~150,000 USD capex savings and ~570,000 USD total savings over the 12-year unit life. The NKT design-condition report is the central argument for the customer's investment committee.

Engineering Note

Engineering points in design-condition selection:

• The first input must always be the customer's request; then "real process need" is evaluated. Most customers state targets "the way they're used to" rather than "the way it should be". • Multi-layer setpoints — wide band (operating range) + tight band (action range) + alarm bands. Example: 35% RH ± 5 (operating), 32-38% (action), <28% or >42% alarm. • Temperature and RH are not independent — RH reading depends on temperature; the real control parameter is dew point. Even with dew point held within ±0.8°C, a temperature drift of ±1.5°C makes RH drift by about ±4%, and since the equilibrium moisture of a hygroscopic product follows RH, temperature stability is part of humidity control too. • Prefer dew-point setpoint — using dew point instead of RH in modern process control is more accurate for product stability. The psychrometric conversion is calculated directly from RH and temperature. • Seasonal adjustment — summer target can be 1-2°C above winter (ASHRAE thermal comfort standard, adaptive comfort model). Fixed setpoint causes seasonal energy waste. • Occupied vs unoccupied strategy — during nights, weekends and holidays the setpoint can be widened (setback control); energy savings 25-40%. • Occupancy-based control — person count (CO2 sensor) + hourly usage profile must reflect into dynamic setpoint adjustment. • Validation tests — in GMP/HACCP applications such as pharma and ICU, setpoint validation requires 3 consecutive batches + IQ/OQ/PQ documentation.

NKT project approach: design-condition specification engineering report (TR + EN), finalised with customer signature; post-project this document becomes the reference for annual performance evaluation.

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.