Two kinds of heat in every airstream Any heated or cooled air carries energy in two forms. Sensible heat changes the dry-bulb temperature of the air without changing its moisture content. Latent heat is the energy tied up in water vapour: it moves when moisture condenses or evaporates, and it can change without any temperature […]
Psychrometrics Basics: Reading Air Before You Recover Its Heat
Air is a mixture, not a single number Sizing any heat recovery system starts with the state of the air. Four properties define it: Dry-bulb temperature (DB): the ordinary thermometer reading. Wet-bulb temperature (WB): thermometer with a wet wick; tracks the cooling effect of evaporation. Humidity ratio (W): mass of water vapour per mass of […]
Psychrometric Processes in Heat Recovery: What Happens on the Chart
Four basic processes On the psychrometric chart, ventilation air moves along predictable lines: Heating: horizontal line to the right (dry-bulb up, moisture constant). Sensible cooling: horizontal line to the left (dry-bulb down, moisture constant). Humidification: line upward (more moisture). Dehumidification: line downward (condensation, latent heat released). Sensible recovery = a move along the constant-humidity line […]
The epsilon-NTU Method: Predicting Heat Exchanger Performance
Effectiveness, the practical metric Effectiveness (epsilon) is the ratio of actual heat transferred to the maximum thermodynamically possible: epsilon = actual_heat_transfer / (Cmin x (Thot_in – Tcold_in)) It is the number you see quoted as “70% efficient” for a recovery core, and it is directly related to the efficiency metrics reported for air-to-air units. NTU: […]
Dew Point and Frost Point: The Temperatures That Decide Core Survival
Dew point: where water appears The dew point is the temperature at which air, cooled at constant pressure and humidity, becomes saturated and water begins to condense. If a recovery core surface drops below the exhaust dew point, condensate forms on it – normal in cooling duty, a design input for drainage. Frost point: where […]
Air Changes per Hour (ACH): The Ventilation Rate Behind Recovery Load
Definition Air changes per hour is the volumetric airflow divided by the room volume, expressed per hour: ACH = (volumetric_flow_rate / room_volume) x 3600 An ACH of 6 means the room air is theoretically replaced six times in an hour. Typical values Classrooms: about 4-6 ACH (see school ventilation) Laboratories and fume hoods: 6-12 ACH […]
Outdoor Air Ratio: Why 100% Outside-Air Spaces Need Recovery Most
Definition The outdoor-air ratio (OAR) is the share of supply air that is genuinely fresh outdoor air: OAR = outdoor_air_flow / total_supply_air_flow A space with no recirculation runs at OAR = 1 (100% outside air). Where OAR is forced to 1 Some spaces cannot recirculate for safety or code reasons: Cleanrooms and graded pharmaceutical suites […]
Specific Heat and Density of Air: The Constants Behind Every Recovery Calculation
The two numbers you reuse constantly Specific heat of air, cp: about 1.006 kJ/kg.K (or 1006 J/kg.K). Density of air, rho: about 1.2 kg/m3 at 20 degC and 1 atm. Together they give a volumetric heat capacity of roughly 1200 J/m3.K – the energy stored in each cubic metre of air per degree of temperature. […]
Seasonal Energy Recovery Efficiency: Why Nameplate Effectiveness Is Not the Year
Rated vs real A core may be rated at 75% effectiveness, but the energy saved over a year is lower, because recovery is most valuable in extreme weather and can even hurt in mild weather (it reheats supply you wanted cool). The seasonal efficiency weights effectiveness by the hours and temperature lifts actually seen. What […]
Thermal Transmittance (U-Value): The Building-Side Number Recovery Reduces
Definition The U-value is the rate of heat transfer through a building element per square metre per degree of temperature difference, in W/m2.K. A low-U wall or window loses little heat; a high-U one loses a lot. U-value vs the exchanger k-value Do not confuse them. The exchanger overall conductance (UxA in the epsilon-NTU method) […]