Building Sensible Heat Thermal Load with Air Exchange Calculator

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Building Sensible Heat Thermal Load with Air Exchange Calculator

Heat Transfer Engineering
Thermodynamics
HVAC Design and Engineering

Outdoor air introduced into a building constitutes a large part of the total space-conditioning load, which is one reason to limit air change rates to the minimum required. Air exchange typically represents 20 to 50% of a modern, ventilation-code-compliant building’s thermal load in nontemperate climates. The effect on heating loads tends to be much larger than on cooling loads (McDowell et al. 2003).

Air exchange increases a building’s thermal load in several ways. First, incoming air must be heated or cooled from the outdoor air temperature to the indoor or supply air temperature. The rate of energy consumption by this sensible heating or cooling is

Eq. 1
q_s = 60 · Q · ρ · c_p · ΔT

where

q_s = sensible heat load, Btu/h
Q = airflow rate, cfm
ρ = air density, lbm/ft³ (about 0.075 at or near sea level)
c_p = specific heat of air, Btu/lbm·°F (about 0.24)
ΔT = temperature difference between indoors and outdoors, °F

and at or near sea-level air density, with an adjustment for typical room air humidity, this equation is commonly presented for design use as

Eq. 2
q_s = 1.1 · Q · ΔT

Equations (1) and (2) are known as the sensible heat equation.

HVAC designers typically assume sea-level air pressure for locations with altitudes of 2000 ft or lower.

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Source

2016 ASHRAE Handbook - HVAC Systems and Equipment, ASHRAE: 2016.