# Hvac Equations (U.S./Metric)

The heating, ventilation, and air conditioning (HVACequations.

## AIR EQUATIONS

### Velocity

#### U.S. UNITS

or for standard air (d = 0.075 lb/cu ft)

To solve for “d”:

V = Velocity (fpm)

Vp = Velocity Pressure (in. w.g.)

d = Density (lb/cu ft)

Pb = Absolute Static Pressure (in. Hg)
(Barometric pressure + static pressure)

T = Absolute Temp. (460° + °F)

#### METRIC UNITS

or for standard air (d = 1.204 kg/m3)

To solve for “d”:

V = Velocity (m/s)

Vp = Velocity Pressure (Pascals or Pa)

d = Density (kg/m3)

Pb = Absolute Static Pressure (kPa)
(Barometric pressure + static pressure)

T = Absolute Temp. (273° + °C = °K)

### Heat Flow

#### U.S. UNITS

Q (sens.) = 60 x Cp x d x cfm x Δt

or for standard air (Cp = 0.24 Btu/lb – °F):

Q (sens.) = 1.08 x cfm x Δt

Q (lat.) = 4750 x cfm x ΔW (lb.)

Q (lat.) = 0.67 x cfm x ΔW (gr.)

Q (total) = 4.5 x cfm x Δh

Q = A x U x Δt

R = 1/U

Q=Heat Flow (Btu/hr)

Cp = Specific Heat (Btu/lb · °F)

d = Density (lb/cu ft)

At = Temperature Difference (°F)

AW = Humidity Ratio (lb or gr H2O/lb dry air)

Ah = Enthalpy Diff. (Btu/lb dry air)

A = Area of Surface (sq ft)

U = Heat Transfer Coefficient (Btu/sq ft · hr * °F)

R = Sum of Thermal Resistances (sq ft· hr · °F/Btu)

P = Absolute Pressure (lb/sq ft)

V = Total Volume (cu ft)

T = Absolute Temp. (460° + °F = °R)

R = Gas Constant (ft/°R)

M = Mass (lb)

#### METRIC UNITS

Q (sens.) = 60 x Cp x d x l/s x Δt

or for standard air (Cp = 1.005 kJ/kg – °C):

Q (sens.) = 1.23 x l/s x Δt

Q (lat.) = 3 x l/s x ΔW (lb.)

Q (total) = 1.2 x l/s x Δh

Q = A x U x Δt

R = 1/U

Q=Heat Flow (watts or kW)

Cp = Specific Heat (kJ/kg – °C)

d = Density (kg/m3)

At = Temperature Difference (°C)

AW = Humidity Ratio (g H2O/kg dry air)

Ah = Enthalpy Diff. (kJ/kg dry air)

A = Area of Surface (m2)

U = Heat Transfer Coefficient (W/m2 . °C)

R = Sum of Thermal Resistances (m2 . °C/W)

P = Absolute Pressure (kPa)

V = Total Volume (m3)

T = Absolute Temp. (273° + °C = °K)

R = Gas Constant (kJ/kg °R)

M = Mass (kg)

### Total Pressure

#### METRIC UNITS

TP = Vp + SP

l/s = 1000 x A x V

TP = C x Vμ

TP = Total Pressure (Pa)
Vp = Velocity Pressure (Pa)
SP = Static Pressure (Pa)
V = Velocity (m/s)
Vm = Measured Velocity (m/s)
d = Density (kg/m3)
A = Area of duct cross section (m2)
C = Duct Fitting Loss Coefficient

## FAN EQUATIONS

#### U.S. UNITS

cfm = Cubic feet per minute

rpm = Revolutions per minute

P = Static or Total Pressure (in. w.g.)

bhp = Brake horsepower

d = Density (lb/cu ft)

#### METRIC UNITS

I/s = Litres per second

m3/s = Cubic metres per second

P = Static or Total Pressure (Pa)

kW = Kilowatts

d = Density (kg/m3)

## PUMP EQUATIONS

#### U.S. UNITS

gpm = Gallons per minute

rpm = Revolutions per minute

D = Impeller diameter

bhp = Brake horsepower

#### HYDRONIC EQUIVALENTS

• a. One gallon water = 8.33 pounds
• b. Specific heat (Cp) water = 1.00 Btu/lb °F (@ 68°F)
• c. Specific heat (Cp) water vapor = 0.45 Btu/lb °F (@ 68°F)
• d. One ft. of water = 0.433 psi
• e. One ft. of mercury (Hg) = 5.89 psi
• f. One cu.ft. of water = 62.4 lb = 7.49 gal.
• g. One in. of mercury (Hg) = 13.6 in.w.g. = 1.13 ft. w.g.
• h. Atmospheric Pressure = 29.92 in.Hg = 14.696 psi
• i. One psi = 2.31 ft. w.g. = 2.04 in.Hg

#### METRIC UNITS

I/s = Litres per second

m3/s = Cubic metres per second

D = Impeller diameter

BP = Brake horsepower

## HYDRONIC EQUATIONS

#### U.S. UNITS

gpm = Gallons per minute
Q = Heat flow (Btu/hr)
Δt = Temperature diff. (°F)
ΔP = Pressure diff. (psi)
Cv = Valve constant (dimensionless)

whp = Water horsepower
gpm = Gallons per minute
bhp = Brake horsepower
Sp. Gr. = Specific gravity (use 1.0 for water)
Ep = Efficiency of pump

NPSHA = Net positive suction head available
Pa = Atm. press. (use 34 ft w.g.)
Ps = Pressure at pump centerline (ft w.g.)

V2/2g = Velocity head at point Ps (ft w.g.)
Pvp = Absolute vapor pressure (ft w.g.)
g = Gravity acceleration (32.2 ft/sec2)
f = Friction factor (dimensionless)
L = Length of pipe (ft)
D = Internal diameter (ft)
V = Velocity (ft/sec)

#### Converting pressure in inches of mercury to feet of water at various water temperatures

 Water Temperature degrees F F F 60 ∘ 60 ∘ 60^(@) 150 ∘ 150 ∘ 150^(@) 200 ∘ 200 ∘ 200^(@) 250 ∘ 250 ∘ 250^(@) 300 ∘ 300 ∘ 300^(@) 340 ∘ 340 ∘ 340^(@) Ft. head differential per in. Hg. differential 1.046 1.046 1.046 1.07 1.07 1.07 1.09 1.09 1.09 1.11 1.11 1.11 1.15 1.15 1.15 1.165 1.165 1.165

#### METRIC UNITS

Q = Heat flow (kilowatts)

Δt = Temperature diff. (°C)

ΔP = Pressure diff. (Pa or kpa)

Cv = Valve constant (dimensionless)

m3/s = Cubic metres per second

l/s = Litres per second

WP = Water power (kW) or (W)
m3/s = Cubic metres per second
I/s = Litres per second
Sp. Gr. = Specific gravity (use 1.0 for water)
BP = Brake power (kW)
E, = Efficiency of Pump
H = Head (Pa) or (m)

NPSHA = Net positive suction head available
Pa = Atm. press. (Pa – Std. Atm. press. = 101,325 Pa)
Ps = Pressure at pump centerline (Pa)

V2/2g = Velocity head at point Ps (m)
Pvp = Absolute vapor pressure (Pa)
g = Gravity acceleration (9.807 m/sec2)
f = Friction factor (dimensionless)
L = Length of pipe (m)
D = Internal diameter (m)
V = Velocity (m/sec)

## ELECTRIC EQUATIONS

#### U.S. UNITS

I = Amps (A)

E = Volts (V)

P.F. = Power factor

R= ohms (Ω)

P = watts (W)

Bhp = Brake horsepower

#### METRIC UNITS

kW = Kilowatts

I = Amps (A)

E = Volts (V)

P.F. = Power factor

R = ohms (Ω )

P. = watts (W)