Condenser-Water Cooling Loop — Control Bench

What this is

A transient, physics-based simulation of the building’s condenser-water loop — a control and efficiency test bench for rehearsing the cooling-tower sequence, trying setpoints, and weighing pump and fan staging against energy use. It is not a data historian. The same calibrated engine runs here and in the headless model used for analysis.

The real system it represents

A ~560-ton open evaporative tower — two counterflow cells (CT-1 / CT-2), 564 nominal tons, with (2) 25 HP induced-draft fans — rejects building heat to outdoor air. Two 75 HP vertical-inline pumps (CTP-1 / CTP-2, lead/standby, one running at a time) circulate condenser water to the building’s water-source heat pumps, AHU compressors, and computer-room cooling. One cell is currently out of service, which is why single-cell operation is a first-class mode here. Note: the in-service tower is currently packed with excess fill media, so it is air-side-limited and under-performs its nameplate — the model is calibrated to the tower as it runs today, to be re-fit when the corrected-media tower comes online.

What’s under the hood

The bench is modeled on the actual installed equipment, not just automation data. The pump head curve is the manufacturer submittal; the cooling-tower capacity, fan power, and basin volume are the tower nameplate and engineering data; the condenser-water piping is the real Sch 40 runs — sizes and lengths taken off the building prints; and the setpoints and control logic are the building’s own sequence of operations. The BAS field trends then calibrate the loop resistances and validate the result against measured flow, pressure, and temperature — but the equipment skeleton is the real plant.

Hydraulic resistances, the pump differential-pressure curve, the loop transport volumes, and the tower air-side cap were fit to BAS field trends (pump DP, loop flow, supply and return temperature, wet-bulb). The tower’s peak effectiveness is anchored to the tower nameplate (ε_max = 0.588) and held there pending better data — it is not fit to field. At a single-cell point on 18 June the model reproduced loop flow and ΔT to within ~1 % at the 55 % pump speed; at 62 % the hydraulic anchor runs about 3 % low on flow.

How to drive it

Press Pause/Run and let the sequence operate the plant, or take any actuator to HAND to override it (including over safeties). Watch the faceplate and the supply-temperature trend; use the power draw figure to weigh the energy cost of staging and setpoint choices.

Known limitations — read before drawing conclusions

• Wet-bulb sensitivity is under-modeled. Across the wet-bulb range the modeled tower effectiveness moves only about a fifth of what the field data shows, so against the current (over-packed) tower the model reads roughly 5 °F optimistic at the low-wet-bulb corner. This is a model-form limitation — a tower-physics revision is planned, pending a fixed-fan wet-bulb sweep — so the bench is not yet reliable for absolute free-cooling or economizer-hours predictions.
• Loop load is treated as equal to tower heat rejection. The compressor heat-of-rejection uplift (~×1.2) is not applied yet, so rejection and tower duty read conservative relative to the true condenser load.
• The differential-pressure loop is modeled as a function of flow. That is appropriate for this constant-flow building (no per-unit two-way valves — the loads are always flowing), but it cannot reproduce a measured DP-reset schedule.
• The occupied-load floor is calibrated to a single cool, occupied day — and was set from the same day it is checked against, so that match is by construction. A second trend day is needed before trusting the building-load estimate on mild days.
• Thermal response is quasi-steady. Steady-state supply temperature tracks the field trend to about 0.7 °F RMS. The building and return nodes carry no heat capacitance, so a sudden load step gives a slightly sharp return-temperature response; transient accuracy is not yet characterized. Steady-state behavior is the trustworthy part.