AHU air side cooling load calculation

Sensible cooling/heat load

The simple equation which we use for calculating sensible load is

$Q_{s} = 1.08 * CFM * \Delta T$

The above equation is derived from

$Q_{s} = m * C_{p} * \Delta T$

Where,
$Q_{s}$ - Sensible load
m - Mass flow rate of air
$C_{p}$ - Specific heat of air
$\Delta T$ - Temperature difference

To get the sensible load in Btu/hr by entering the air flow rate in cfm (cubic feet per minute), the other variables should have the below specified units.

mass flow rate in lb/ft3-hr,
Specific heat in Btu/lb-°F,
Temperature difference in °F

• Mass flow rate of air = mass/volume
• Density of air = 0.075 $lb/ft^{3}$ at atmospheric pressure and $70^{o} F$
• 1 cfm = 60 $ft^3/hour$
• Specific heat of air = 0.24 $Btu/lb-^{o}F$

Substitute the above values and we get

$\begin{aligned} Q_{s} &= (0.075 * cfm * 60) * 0.24 * \Delta T \\ &= 1.08 * cfm * \Delta T \end{aligned}$

Latent cooling/heat load

The simple equation which we use for calculating latent load is

$Q_{l} = 0.69 * CFM * \Delta W$

The above equation is derived from

$Q_{l} = m * L * \Delta W$

Where,
$Q_{l}$ - Latent load
m - Mass flow rate of air
L - Latent heat of water
$\Delta W$ - Difference in humidity ratio

To get the sensible load in Btu/hr by entering the air flow rate in cfm (cubic feet per minute), the other variables should have the below specified units.

mass flow rate in lb/ft3-hr,
Latent heat of water in Btu/lb

• Latent heat of water vapour = 1094 Btu/lb at atmospheric pressure and $75^{o} F$
• Latent heat of condensed water = 18 Btu/lb at atmospheric pressure and $75^{o} F$
• Latent heat of vapourizing water = 1094-18 = 1076 Btu/lb at atmospheric pressure and $75^{o} F$
• Density of air = 0.075 $lb/ft^{3}$ at atmospheric pressure and $70^{o} F$
• 1 cfm = 60 $ft^3/hour$
• Specific heat of air = 0.24 $Btu/lb-^{o}F$

Substitute the above values and we get

$\begin{aligned} Q_{l} &= (0.075 * cfm * 60) * 1076 * \Delta W \\ &= 4842 * cfm * \Delta W \end{aligned}$

Where,

$\Delta W$ is in lb/lb

$\begin{aligned} Q_{l} &= 4842 * cfm * \Delta W / 7000 \\ &= 0.69 * cfm * \Delta W \end{aligned}$

Where,

$\Delta W$ is in gr/lb

$\Delta W$ can be calculated from Humidity ratio, when you have DBT & WBT at inlet/outlet.

Total cooling/heat load

Total load can be calculated by adding above two values viz., sensible load and latent load.

$Q_{t} = Q_{l} + Q_{s}$

OR we can calculate using below equation

$Q_{t} = m * \Delta H$

Where,
$Q_{t}$ - Total load
m - Mass flow rate of air
$\Delta H$ - Difference in Specific Enthalpy

$\begin{aligned} Q_{t} &= 0.075 * cfm * 60 * \Delta H \\ &= 4.5 * cfm * \Delta H \end{aligned}$

Where,
$Q_{t}$ - Total load in Btu/hr
$\Delta H$ - Difference in Specific Enthalpy in Btu/lb

Sensible Heat Factor (SHF)

Sensible heat factor can be calculated using below equation

$SHF = \dfrac {Q_{s}} {Q_{s} + Q_{l}}$