Air Conditioning Load

The air conditioning load is the rate at which heat and moisture must be removed from (cooling load) or added to (heating load) a space to maintain specified indoor design conditions of dry-bulb temperature, relative humidity, and air quality. A cooling load calculation separates the load into sensible and latent components. Sensible loads raise the dry-bulb temperature and include heat conduction through the envelope (walls, roof, windows, floor), solar radiation transmitted and absorbed through glazing, internal gains from people, lighting, and equipment, and ventilation or infiltration air. Latent loads add moisture without changing temperature and come from people (respiration and perspiration), cooking, showers, processes, and humid outdoor air brought in by ventilation or infiltration. The total cooling coil load is the sum of space sensible and latent loads plus the outdoor air load and any duct or fan gains. The sensible heat ratio SHR = Q_sensible / Q_total indicates how much of the coil load is sensible; it determines the required apparatus dew point and coil selection. The envelope conduction load is estimated with Q = U·A·ΔT, where U is the overall heat transfer coefficient, A the surface area, and ΔT the design temperature difference, often corrected with a cooling load temperature difference (CLTD) to account for thermal mass and time lag. Solar heat gain through glazing is Q_solar = SHGC · A · q_solar, where SHGC is the solar heat gain coefficient and q_solar the incident solar irradiance. Internal gains from people are typically 70–100 W sensible and 40–60 W latent per person for office activity. Ventilation air load is Q = m_dot · Δh for total load, or Q_s = m_dot · c_p · ΔT and Q_l = m_dot · h_fg · Δω for sensible and latent components. Peak cooling load occurs when outdoor temperature, solar gain, and occupancy coincide, and it determines the chiller and air-handling unit capacity. Oversizing increases capital cost, short-cycling, poor humidity control, and energy use, so the load must be calculated for each zone rather than estimated by floor area alone.