hvac-systems

name	hvac-systems
description	HVAC systems — psychrometrics, cooling/heating load (ASHRAE), duct sizing (velocity/equal friction), fan selection, coil performance, refrigerant selection, VAV vs. CAV, DOAS, energy efficiency.
metadata	{"priority":7,"promptSignals":{"phrases":["HVAC","heating","cooling load","duct","psychrometric","fan selection","VAV","air handling","refrigerant","chiller"],"minScore":3}}

HVAC Systems — Complete Skill

Psychrometrics

Key Properties (Moist Air)

Dry bulb temperature (T_db): thermometer reading [°C] Wet bulb temperature (T_wb): evaporative cooling limit [°C] Dew point (T_dp): temperature at which moisture condenses [°C] Relative humidity (RH): φ = p_v / p_vs [%] Humidity ratio: W = 0.622 × p_v / (p_atm - p_v) [kg_w/kg_da] Specific enthalpy: h = 1.006 T_db + W(2501 + 1.86 T_db) [kJ/kg_da]

Saturation pressure: p_vs = 0.6108 × exp(17.27 × T / (T + 237.3)) [kPa] (Magnus approximation, T in °C)

Psychrometric relations: RH = W × p_atm / (0.622 + W) / p_vs T_dp: solve p_v = p_vs(T_dp) for T_dp T_wb ≈ T_db - (T_db - T_dp) / 3 (approximate)

Air density: ρ = p_atm / (R_da × T_K × (1 + 1.608 W)) ≈ 1.2 kg/m³ (standard conditions)

Psychrometric Processes

Sensible heating/cooling: horizontal line (W constant), T changes Humidification: vertical line (T constant), W increases Cooling and dehumidification: W decreases + T decreases (toward saturation curve) Evaporative cooling: constant wet-bulb temperature line

Sensible heat ratio (SHR): SHR = Q_sensible / Q_total = (Q_sensible) / (Q_sensible + Q_latent) Typical office: SHR = 0.8-0.9; typical retail: 0.7-0.8

Cooling and Heating Loads

ASHRAE Load Methods

Peak cooling load components: Q_total = Q_external + Q_internal + Q_ventilation

External: Q_conduction = U × A × CLTD (Cooling Load Temperature Difference method) Q_solar = SC × A × SHGF × CLF (Solar Heat Gain Factor × Cooling Load Factor)

Internal: Q_people = N × (sensible gain/person + latent gain/person) Q_lights = W_lights × 3.412 [BTU/hr/watt] × CLF_lights [ASHRAE Tables] Q_equipment = Equipment wattage × CLF × diversity factor

Ventilation (OA): Q_vent = ṁ_OA × Δh = 1.2 × V̇_OA × (h_outside - h_supply) [kW]

Simplified total: Q_cooling [kW] = Σ all gains [kW]; then add safety factor (1.1-1.25) Rule of thumb: ~100 W/m² (office), 150-200 W/m² (data center), 30-50 W/m² (residential)

Heating Load

Q_heating = Q_envelope + Q_infiltration - Q_internal Q_envelope = U × A × ΔT (no solar credit for design day) Q_infiltration = ṁ_inf × c_p × ΔT ASHRAE 99% design dry bulb for winter (coldest 1% of hours)

Ventilation (ASHRAE 62.1)

V_oa = R_p × P_z + R_a × A_z R_p = per-person ventilation rate [L/s/person]: 10 L/s/p (office) R_a = per-area ventilation rate: 0.3 L/s/m² (office) Zone efficiency E_z: varies by diffuser type (0.8-1.2)

Duct Design

Velocity Method

Select air velocity by duct location:

• Main trunk: 6-10 m/s (low-pressure); < 5 m/s (noise critical)
• Branches: 4-6 m/s
• Supply outlets: 2-3 m/s

From Q = V_duct × A: A = Q / V_duct → select rectangular or round duct

Equal Friction Method (Most Common)

Select friction loss per unit length: typically 0.8-1.0 Pa/m Use Moody chart or duct friction chart (ASHRAE Fundamentals): Δp/L = f × (ρ V²/2) / D_h

Colebrook-White (duct): 1/√f = -2 log(ε/(3.7D) + 2.51/(Re√f)) Duct roughness ε: galvanized steel = 0.15 mm, flex duct = 1.2 mm

Equivalent diameter (rectangular to circular): D_eq = 1.30 × (a×b)^0.625 / (a+b)^0.25 (same hydraulic resistance)

Fitting losses: Σ K × ½ρV² (K from ASHRAE Handbook Fundamentals, Chapter 21) Elbow K ≈ 0.2-0.5; tee main→branch K ≈ 0.5-1.5; entry K = 0.5

Pressure Drop Summary

Total system static pressure = friction losses + fitting losses Fan must overcome: supply duct + return duct + coil + filters + diffusers

Fan Selection

Fan Laws

Q ∝ N, ΔP ∝ N², P_fan ∝ N³ Q ∝ D³, ΔP ∝ D², P ∝ D⁵ (same speed, different size)

Fan total efficiency: η_t = Q × ΔP_t / P_shaft Centrifugal forward-curved: η = 55-70%; backward-inclined: η = 70-82%; axial: η = 60-75%

Fan selection process:

1. Calculate required Q [m³/s] and total static pressure [Pa]
2. Select fan from manufacturer curve at operating point (intersection of system curve and fan curve)
3. Check BHP; verify Q at SP matches design; check noise [dB]

Affinity laws for VFD speed control: At 50% speed: Q = 50%, SP = 25%, P = 12.5% (cube law savings) VFD payback: excellent for VAV systems with varying load

Coil Performance

Cooling Coil

Apparatus dew point (ADP) — temperature to which air must be cooled to reach desired conditions Bypass factor (BF) = (T_leaving - T_ADP) / (T_entering - T_ADP) BF = 0.05-0.20 (lower for more coil rows)

Coil capacity: Q = ṁ_air × (h_in - h_out) [kW] Coil rows: 6-8 rows for typical DX; 4-6 rows for chilled water

LMTD for coil: LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂) ΔT₁ = entering air - leaving fluid ΔT₂ = leaving air - entering fluid

Chiller Performance

COP = Q_evaporator / W_compressor Chiller EER [BTU/hr/W] = COP × 3.412 IPLV (Integrated Part Load Value): weighted average at 100%, 75%, 50%, 25% load

Typical performance: Air-cooled chiller: COP = 2.5-3.5 (full load) Water-cooled centrifugal: COP = 5-7 (excellent) VRF heat pump: COP = 3-5 (heating and cooling simultaneously)

System Types

VAV (Variable Air Volume) — Most Common Commercial

AHU maintains constant static pressure; VAV boxes modulate airflow Supply temperature reset: raise T_supply when loads low (improves humidity control) Minimum position: maintain minimum ventilation at low load Energy savings: fan power reduces with cube of speed

CAV (Constant Air Volume)

Constant airflow; control by varying supply temperature Simpler controls; higher fan energy at part load Used: labs, clean rooms, areas requiring constant pressure control

DOAS (Dedicated Outdoor Air System)

Separate system handles 100% outdoor air ventilation; dehumidifies to neutral condition Zone units (fan coils, VRF) handle sensible load only Benefit: better humidity control; decoupled latent and sensible

VRF (Variable Refrigerant Flow)

Direct expansion system; one outdoor unit → multiple indoor units Heat recovery VRF: simultaneous heating and cooling in different zones Limitation: refrigerant pipe length limits (~100m equivalent); not suitable for all climates

Energy Efficiency

ASHRAE 90.1 Compliance

Envelope: U-values for wall, roof, glazing by climate zone HVAC: minimum efficiencies for chillers (kW/ton), RTUs (EER), boilers (AFUE/Ec) Lighting: LPD (W/m²) limits by space type

Energy Recovery

Heat wheel (rotary): 70-80% sensible + latent recovery Plate HX: 60-75% sensible only Runaround coil: 40-55%; useful for separated supply/exhaust streams

ASHRAE 90.1: ERV required when Q_oa > 70% of supply and building in most climate zones

Output

Provide: psychrometric state points (T_db, W, h) at all process endpoints, peak cooling load Q [kW] and heating load Q [kW], airflow Q [m³/s], duct size [mm] for given velocity/friction, fan total SP [Pa] and power [kW], chiller COP, refrigerant selection recommendation.