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Master the Total Cooling Equation

The Total Cooling Equation

The total cooling equation calculates the total cooling load in an HVAC system.

The Total Cooling Equation:

Qt = 4.5 x CFM x dH

In the HVAC total cooling equation, Q represents the total cooling capacity in British Thermal Units per hour (BTU/hr).

Here's the breakdown:

  • Q: Total cooling capacity (BTU/hr)

  • CFM: Cubic feet per minute, which is the airflow rate through the system

  • dH: The enthalpy difference between two state points (measured in BTU per pound of dry air)

The 4.5 in this equation is not a constant. Instead, it is the product of the density(p) of the air at "standard air" conditions and the conversion factor of 60 minutes per hour.

4.5 = p × 60 min/hr =  0.075 lb/ft3 x 60 min/hour

"Standard air" has historically been defined by ASHRAE as having a density of 0.075 lb/ft3, which equates to air density at sea level (barometric pressure of 29.92 in. Hg) and 70F DB temperature.

Please note: Air conditions and elevations other than “standard air”  will cause this factor to change.

So, by multiplying the air flow rate (CFM) by the enthalpy difference (dH) and the factor (4.5), you get the total cooling capacity of the HVAC system.

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