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 […]
Face Velocity: The Single Number That Sets Core Size and Pressure Drop
Definition Face velocity is the approach speed of air entering the core face: face_velocity = volumetric_flow / core_face_area It is measured in metres per second (m/s). Typical range For HVAC air-to-air cores, face velocity usually sits between 1.5 and 3.5 m/s. Lower is gentler on the core and the fan; higher packs more flow into […]
Specific Heat & Density of Air in Recovery Math
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. […]
Rotary Wheel Purge Sector: How Wheels Reduce Carryover
What a purge sector is A rotary energy wheel spins exhaust and supply airstreams through the same matrix, so a small amount of exhaust gas and moisture is inevitably carried into the supply. A purge sector is a slice of the wheel (typically 5-15 degrees) where a portion of the clean supply air is directed […]
Air Leakage & Cross-Contamination in Cores
Two streams, one box Every recovery device brings exhaust and supply into one casing. The question is whether they can mix. Leakage is the fraction of one stream that escapes into the other through seals, the matrix or carryover. How the technologies compare Plate (sensible): streams separated by solid walls; leakage is only through perimeter […]
Seasonal Efficiency: Beyond Nameplate Ratings
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 […]