Chilled Water Coil
Approach Auditor
Audit leaving air wet-bulb temperatures against entering chilled water baselines to diagnose micro-fouling boundaries.
Hydronic Fluid Heat Transfer Physics & Coil Approach Thermodynamics Overview
In large commercial chilled water networks and air handling units, cooling performance depends entirely on the efficiency of the hydronic heat exchanger interface. The approach temperature represents the mathematical delta or differential gap between the leaving air wet-bulb temperature ($T_{\text{air out}}$) and the entering chilled water supply temperature ($T_{\text{water in}}$). In a perfectly clean system, this approach window stays tight—typically hovering between 4°F to 6°F. As water travels through the small copper tube lines, it picks up dissolved carbonates, calcium scales, and microscopic organic materials. Over months of constant operation, these impurities precipitate out of the liquid and attach to the inner tube walls. This thin layer of scale acts as a highly resistant thermal barrier, restricting heat transfer, widening the approach temperature gap, forcing chillers to run longer, and driving up facility utility bills.
Frequently Asked Questions
A: A high approach temperature specifically tells you that heat cannot cross the tube walls. If the waterside is fouled with scale, the water remains cold because it can’t absorb heat from the air, causing the leaving air to stay warm. Conversely, if the problem is poor airside performance (like a slipping fan belt or clogged filter), the leaving air temperature drops rapidly because the reduced air volume stalls in the coil too long, which narrows the approach window rather than widening it.
A: Chilled water loops are mostly closed-circuit arrays, but any fresh makeup water introduced to replace valve leakage brings in raw minerals like silica and calcium. Without automated chemical corrosion inhibitors, biocide treatments, and routine eddy-current tube testing, these minerals plate onto the inner copper pipe surfaces. Because calcium scale has an incredibly low thermal conductivity index compared to pure copper, a tiny layer of scale just 1/32 of an inch thick can drop overall coil heat transfer metrics by nearly 30%.