AHU Controls Simulator

Single-zone recirculating AHU. RAT equals room temperature exactly. The mixed-air damper represents coordinated outdoor and return dampers: its actual position is assumed equal to outside-air fraction. MAT = OAT × outside fraction + RAT × return fraction. This idealization does not model real damper flow characteristics or pressure balance.

Manual damper control uses the requested position. Economizer mode uses at least Minimum outdoor air; it adds outdoor air when OAT is more than 1°C below RAT and SAT SP is below RAT, estimating the mixing ratio needed for SAT SP. Otherwise it returns to the minimum. This is a simple dry-bulb teaching sequence, without humidity/enthalpy, freeze protection or code-required ventilation calculations.

Fan speed follows its command over a 20-second full travel. Airflow and duct static pressure come from the intersection of an illustrative fan curve and a lumped duct resistance. At resistance 1, full speed gives rated airflow at 500 Pa; at 60% speed the pressure is 180 Pa. Changing resistance changes the operating point. This is a single supply duct referenced to a zero-pressure room, without a distributed duct/return pressure network or individual VAV dampers.

Auto fan control uses an independent conventional pressure PI, with editable PB and error-proportional integral gain. Manual speed or Fan Off disables and clears this loop; re-entry starts its integral from actual fan speed and may produce a command step. An unreachable pressure target leaves the fan at 100% and reports the shortfall. Actual pressure is a model result, not forced to its setpoint.

Coil heat transfer uses configured water temperatures and conductance with an 8-second discharge-temperature response. The room uses 30 MJ/K heat capacity, supplied-air sensible heat, internal load and envelope exchange. Air heat capacity is 1206 J/(m³·K). Fan heat, humidity, condensate, water hydraulics, return/relief fan control and equipment protection are not modeled.

HCV and CCV commands cannot be positive together. Fan Off or zero speed closes both commands immediately; actual fan and damper positions run down over their travel time. Coils also inhibit below 20 CFM, an educational model operating limit. At zero flow SAT and MAT are unavailable. Capacity shortfall leaves temperature targets unmet; errors stop the run rather than hiding invalid results.

Start captures the run settings. Pause/Resume preserves state; Reset restores run-start settings and clears history. Initial room temperature and duration lock after Start. Apply queues live edits for the next simulated step; paused edits wait for Resume. Every numerical step is one second, independent of playback speed. Background tabs pause playback.

This simplified simulator is not physically validated. It does not emulate manufacturer firmware, implement equipment safeguards or establish compliance with standards. All defaults are teaching choices.