HVAC Mediflow Quick Reference Calculator

Mediflow — HVAC Quick Reference Calculator

HVAC Quick Reference · Field-Verified

Humidity Ratio

W = 0.62198 × Pw / (P − Pw)

Partial pressure of water vapour Pw kPa — from psychrometric tables or wet-bulb reading

Total (barometric) pressure P kPa — standard sea level = 101.325 kPa

Humidity Ratio W

—

kg/kg dry air

W (g/kg)

—

g/kg dry air

✓ Constant 0.62198 = ratio of molar masses: water (18.015) ÷ dry air (28.966). Field-verified standard.

Relative Humidity

φ = (Pw / Pws) × 100%

Actual vapour pressure Pw kPa

Saturation pressure at same temp Pws kPa — 20°C ≈ 2.339 · 25°C ≈ 3.169 · 30°C ≈ 4.246

Relative Humidity φ

—

Moist Air Enthalpy

h = 1.005T + W(2500 + 1.88T)

Dry-bulb temperature T °C

Humidity Ratio W kg/kg dry air

Enthalpy h

—

kJ/kg dry air

2500 kJ/kg = latent heat of vaporisation at 0°C · 1.005 = Cp of dry air · 1.88 = Cp of water vapour.

Specific Volume & Density

v = 0.287(T + 273.15)(1 + 1.607W) · ρ = 1/v

Dry-bulb temperature T °C

Humidity Ratio W kg/kg dry air

Specific Volume v

—

m³/kg dry air

Air Density ρ

—

kg/m³

Sensible Heat — Metric

Qs (kW) = 1.23 × Q(L/s) × ΔT(°C) ÷ 1000

Airflow Q L/s

Temperature differential ΔT °C

Sensible Heat Qs

—

Sensible Heat

—

Btu/hr

Airflow

—

CFM

Metric constant 1.23 W/(L/s·K) = ρ × Cp = 1.2 kg/m³ × 1.025 kJ/(kg·K) at standard conditions. Imperial equivalent: 1.08 Btu/hr per CFM·°F.

Latent Heat — Metric

QL (kW) = 3010 × Q(m³/s) × ΔW(kg/kg) · or 3.01 × Q(L/s) × ΔW

Airflow Q L/s

Humidity ratio difference ΔW kg/kg — supply minus return

Latent Heat QL

—

Latent Heat

—

Btu/hr

Airflow

—

CFM

Total Heat, Cooling & Sensible Heat Factor

QT = Qs + QL · kW_refrig = QT · SHF = Qs / QT

Sensible Heat Qs kW

Latent Heat QL kW

Total Heat QT

—

Cooling Load

—

tons of refrigeration

SHF

—

sensible heat factor

Total Heat

—

Btu/hr

Enter sensible and latent heat values above.

Velocity, Airflow & Duct Area

V = Q/A · Q = V·A · A = Q/V

Airflow Q L/s

Duct cross-section area A m² — round duct: π×(D/2)²

Velocity V

—

m/s

Velocity

—

FPM

Airflow

—

CFM

Typical duct velocities: main ducts 3–6 m/s · branches 2–4 m/s · grilles 1.5–2.5 m/s.

Air Changes per Hour

ACH = Q(m³/s) × 3600 / Volume(m³)

Supply airflow Q L/s

Room volume m³ — length × width × ceiling height

Air Changes per Hour

—

ACH

Airflow

—

CFM

Volume

—

ft³

Enter airflow and room volume.

Duct Friction Loss (Darcy-Weisbach)

hf = f × (L/D) × (V² / 2g)

Friction factor f Moody — smooth duct ≈ 0.01–0.02

Duct length L m

Hydraulic diameter D m — round: actual dia; rect: 4A/P

Air velocity V m/s

Head Loss hf

—

Head Loss

—

in w.g.

Head Loss

—

mm w.g.

Reynolds Number

Re = V · D / ν

Velocity V m/s

Hydraulic diameter D m

Kinematic viscosity ν m²/s — air at 20°C ≈ 1.53×10⁻⁵

Reynolds Number Re

—

dimensionless

Enter velocity and duct dimensions.

Refrigeration Cycle — Vapour Compression

RE = h₁ − h₄ · Wc = h₂ − h₁ · COP = RE / Wc

Evaporator outlet enthalpy h₁ kJ/kg — superheated vapour at compressor inlet

Compressor outlet enthalpy h₂ kJ/kg — hot discharge gas

Expansion valve inlet h₄ kJ/kg — subcooled liquid after condenser

Refrigeration Effect RE

—

kJ/kg

Compressor Work Wc

—

kJ/kg

COP

—

coefficient of performance

Condenser Duty h₂−h₄

—

kJ/kg

Enter enthalpy values from P-h chart.

Refrigerant Mass Flow Rate

ṁ = Qe / RE

Evaporator (cooling) capacity Qe kW

Refrigeration Effect RE kJ/kg — from cycle calculation above

Mass Flow Rate ṁ

—

kg/s

Mass Flow Rate

—

kg/hr

Log Mean Temperature Difference (LMTD)

ΔTlm = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)

Hot-end temperature difference ΔT₁ °C

Cold-end temperature difference ΔT₂ °C

LMTD ΔTlm

—

°C

When ΔT₁ = ΔT₂ the logarithm is undefined — LMTD equals ΔT₁ in that case.

Heat Transfer & Required Surface Area

Q = U · A · ΔTm · A = Q / (U · ΔTm)

Heat duty Q kW

Overall heat transfer coefficient U W/(m²·K) — air-water 20–100 · water-water 800–1500

Mean temperature difference ΔTm °C — use LMTD from above

Required Area A

—

m²

Required Area

—

ft²

Fan Power

P (kW) = Q(m³/s) × ΔP(Pa) / (1000 × η)

Airflow Q L/s

Static pressure ΔP Pa — 1 in w.g. ≈ 249 Pa

Fan efficiency η 0.0–1.0 — centrifugal: 0.65–0.80

Fan Brake Power

—

Fan Brake Power

—

Air Power (ideal)

—

Airflow

—

CFM

Air Power (ideal) = Q × ΔP / 1000 with no efficiency loss. Brake power = Air Power ÷ η.

Fan Affinity Laws — Speed Change

Q₂ = Q₁(N₂/N₁) · ΔP₂ = ΔP₁(N₂/N₁)² · P₂ = P₁(N₂/N₁)³

Original speed N₁ RPM

New speed N₂ RPM

Original airflow Q₁ L/s

Original static pressure ΔP₁ Pa

Original power P₁ kW

New Airflow Q₂

—

L/s

New Pressure ΔP₂

—

New Power P₂

—

Power Saving

—

% reduction

Enter speed and original performance values.

Single-Phase Power

P = V · I · cosφ · Q = V · I · sinφ · I = P / (V · cosφ)

Voltage V V — AU single phase = 230 V · US = 120 V / 240 V

Current I A

Power factor cosφ 0.0–1.0 — HVAC motors typically 0.80–0.90

Real Power P

—

Apparent Power S

—

Reactive Power Q

—

VAR

Three-Phase Power

P = √3 · V · I · cosφ · I = P / (√3 · V · cosφ)

Line-to-line voltage V V — AU three phase = 415 V · US = 208 V / 480 V

Real power P W

Power factor cosφ 0.0–1.0

Line Current I

—

A per phase

Apparent Power S

—

kVA

Reactive Power Q

—

kVAR

CO₂ Steady-State Indoor Air Quality

C_indoor = C_outdoor + G / Q

Outdoor CO₂ concentration ppm — global ambient ≈ 422 ppm (2024)

CO₂ generation rate G m³/s — sedentary adult ≈ 0.004–0.006 m³/s per person

Ventilation airflow Q m³/s — outdoor air supply rate

Steady-State CO₂

—

ppm

Rise Above Outdoor

—

ppm rise

Enter CO₂ generation and ventilation rate.

ASHRAE 62.1 target: below 1000 ppm indoor. WELL standard: below 900 ppm. Values above 1100 ppm indicate inadequate outdoor air supply.

Based on field-verified HVAC formulas. Metric constants: sensible heat 1.23 W/(L/s·K); humidity ratio 0.62198; ρ·Cp ≈ 1.206 kJ/(m³·K). Imperial: 1.08 Btu/hr·CFM·°F. Always confirm unit consistency before applying results to design. Refer to AS 1668, ASHRAE Handbooks, and a licensed mechanical engineer for detailed engineering design. Prepared by Mediflow.

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