Sensible Heat Load Calculator

Sensible heat is the energy exchanged when air changes temperature at constant moisture content. Applied to an airstream it is mass flow times specific heat times temperature difference. Converting mass flow to volumetric flow at standard conditions produces the constant every HVAC technician memorizes: 60 minutes per hour times 0.075 pounds per cubic foot of standard air times 0.24 BTU per pound per degree F equals 1.08. So Q in BTU/hr equals 1.08 times CFM times delta-T, and dividing by 12,000 gives tons of refrigeration, a unit defined as the heat absorption rate of one short ton of ice melting over 24 hours, or 288,000 BTU per day. That 1.08 constant is valid only for sea-level standard air. Density falls with the barometric pressure ratio, so the effective constant drops to roughly 0.90 at 5,000 feet and about 0.84 at 7,000 feet; sizing a Denver or Albuquerque coil with 1.08 overstates capacity by nearly 20 percent. Typical design conditions are a 20 degree F supply-air temperature difference for cooling, around 55 degrees F supply into a 75 degrees F space, and a 40 to 60 degree F rise for heating. Safety factors of 10 to 15 percent cover uncertainty in occupancy and equipment gains, but going much past 20 percent is counterproductive because oversized single-stage cooling short-cycles and dehumidifies poorly. Two limits apply: this is an airside energy balance for a known airflow, not a building load calculation covering envelope, solar, infiltration, lighting, and plug loads, and it excludes latent load entirely.

The sensible and latent distinction rests on Joseph Black experiments at Glasgow in the 1760s, separating heat that raises temperature from heat absorbed during a change of state, and on nineteenth-century determinations of the specific heat of air, whose constant-pressure value near 0.24 BTU per pound per degree F came from calorimetric work by Henri Victor Regnault and his successors. The ton of refrigeration entered the vocabulary from the natural ice trade, since early mechanical plants were marketed by the tonnage of harvested ice they displaced. Codifying the arithmetic for buildings was the work of the professional societies. The American Society of Heating and Ventilating Engineers, founded in 1894, merged with the American Society of Refrigerating Engineers in 1959 to form ASHRAE, which has published load-calculation procedures in its Handbook of Fundamentals ever since. Those methods evolved from simple degree-day and equivalent temperature difference approaches through the cooling load temperature difference tables of the 1970s to the radiant time series and heat balance methods used in modern simulation software. For residential work the Air Conditioning Contractors of America issued Manual J, which carried the same physics to contractors and is now referenced by many building codes as the required sizing method. The altitude correction became routine practice as air conditioning spread into the mountain West, where designers found equipment selected on sea-level constants delivering materially less capacity than expected.