This simulator follows outdoor, return, exhaust and mixed-air paths through linked dampers, a mechanical cooling coil, a supply fan and a freeze-protection sensor, so you can compare differential dry-bulb and differential enthalpy economizer control logic and see why cool-but-humid outdoor air can fool a temperature-only decision.
• Equipment laboratory tab: a real-time 3D cutaway of the outdoor-air intake/damper, return-air damper, relief/exhaust path, mixed-air chamber, mixed-air sensor/freeze stat, mechanical cooling coil, supply fan, supply-air outlet and economizer controller, with Home view, Focus selected part, Show full enclosure / cutaway, Exploded view, Auto rotate and Expand camera controls plus a clickable component list with callouts. Controls include outdoor dry-bulb temperature, outdoor relative humidity, return-air temperature, return-air relative humidity, minimum outdoor-air fraction, an economizer decision select (differential enthalpy / differential dry bulb / manual outdoor fraction), manual outdoor-air fraction, supply-air target, mechanical coil capacity, and an Outdoor damper stuck fully open fault checkbox, alongside live stats, a What is happening? sequence readout, operating-history chart, switch-state tokens and a cell-readings table. • Curves & measurements tab: a mixed-air/supply-air temperature chart, an outdoor-vs-return enthalpy chart, the model equations, and snapshot measurement readouts. • Experiments tab: four preset investigations (cool dry outdoor air, cool-but-humid air under enthalpy control, the same weather under dry-bulb control, and a stuck-open damper in cold weather that trips freeze protection) plus a Run model checks verification bench and a timestamped event log with trial-report export. • Learn & assess tab: lesson cards on qualifying outdoor air, mixing moisture and energy, integrating mechanical cooling, and protecting against a stuck-open damper, a knowledge-check quiz with reset, and a scope/references note.
The economizer controller decides how much outdoor air to admit above the configured minimum ventilation fraction, based on whether outdoor conditions are more favorable than return-air conditions. In differential dry-bulb mode it compares only temperatures; in differential enthalpy mode it compares total heat content (temperature plus moisture) using humidity ratio w = 0.62198·pv/(Patm − pv) and enthalpy h = 1.006·T + w·(2501 + 1.86·T). Outdoor and return streams then mix on a dry-air mass basis — humidity ratio and enthalpy are each blended by outdoor fraction before mixed dry-bulb temperature is derived from the mixed enthalpy and humidity ratio.
The mechanical cooling coil, modeled with a 7°C apparatus dew point and a capacity limit, supplements or fully provides the remaining cooling needed to reach the supply-air target once mixing is complete. If mixed air falls below 5°C, a latched freeze-protection sensor trips the supply fan — a fault you can force by setting the outdoor damper stuck fully open in cold weather, and it must be cleared and reset before normal operation resumes.
The outdoor-vs-return enthalpy chart is the key to understanding differential enthalpy control: cool outdoor air is not always low-enthalpy air, because high humidity can add more heat content than a low dry-bulb temperature saves. That's why the same cool-but-humid weather can produce very different admitted outdoor-air fractions depending on whether the controller is in enthalpy mode (holds at minimum ventilation when outdoor enthalpy exceeds return enthalpy) or dry-bulb mode (admits more outdoor air based on temperature alone, potentially increasing the coil's latent load).
The mixed-air/supply-air chart shows how close the mechanical coil gets to the supply-air target — when required cooling exceeds coil capacity, supply temperature is not forced to target, reflecting a capacity-limited coil rather than an idealized one. The Run model checks button in the Experiments tab exercises independent fresh models — leaving your current trial untouched — to confirm invariants in the mixing and control-logic equations. This is a moist-air mixing model at 101.325 kPa with constant dry-air flow (2.4 kg/s) and a saturation-pressure approximation; minimum ventilation, freeze setpoint and enable limits are teaching settings, not code or manufacturer sequences.
Differential dry-bulb control compares only outdoor and return air temperatures to decide whether to bring in more outdoor air. Differential enthalpy control instead compares total heat content (temperature and moisture together, via h = 1.006·T + w·(2501 + 1.86·T)), so it correctly holds back on humid outdoor air even when that air feels cool.
Enthalpy accounts for both sensible heat (temperature) and latent heat (moisture content). If outdoor humidity is high enough, its latent heat content can outweigh a lower dry-bulb temperature, giving it higher total enthalpy than warmer but drier return air — which is exactly the scenario the "cool but humid" experiment preset demonstrates.
If mixed-air temperature drops below 5°C — for example from a stuck-open outdoor damper admitting too much cold outdoor air — a latched freeze stat trips the supply fan. You must clear the damper fault and then use the reset freeze protection action before the fan can resume.
The Run model checks button runs independent, freshly-initialized models to confirm invariants in the moist-air mixing and economizer decision logic, such as dry-air-mass-weighted mixing of enthalpy and humidity ratio behaving consistently across outdoor-air fraction and control-mode changes.