Chilled Water Plant That Cannot Reach Design Tons Without a Third Chiller
A campus chilled water plant has two identical 400-ton centrifugal chillers in a primary-variable configuration serving 60 air handlers with two-way modulating coil valves. Design is 44°F supply, 56°F return, a 12°F delta-T. On a 90°F day at roughly 680 tons of load, both chillers run at about 60% capacity, supply temperature holds 44°F, and return temperature is 50°F. The lead chiller's evaporator approach is 2.5°F against a 1.0°F design. The most remote air handler's coil valve is 100% open while that coil's air-side delta-T is normal and its water-side delta-T is 6°F. A differential pressure transmitter at the far end of the loop reads 4 psi against a 12 psi setpoint, and operators say a third chiller is routinely started before the plant reaches design tonnage.
1. Calculate design and actual evaporator flow. Does this plant have a capacity shortfall or a delta-T problem?
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Design flow: 680 tons × 12,000 / (500 × 12°F) = 8,160,000 / 6,000 ≈ 1,360 gpm.
Actual flow implied by the measured delta-T: 8,160,000 / (500 × 6°F) ≈ 2,720 gpm - roughly double design.
That is the whole story. The plant is producing the required tons at the required supply temperature; it is simply moving twice the water needed to do it. Two consequences follow:
- Chillers. Each machine sees far above its evaporator design flow (typically limited to about 0.7–1.2× design). At very high flow the evaporators are usually flow-limited or trip on high velocity, and the load-percentage readout can mislead. Regardless, the low return temperature (50°F instead of 56°F) is direct evidence that the coils are not extracting the design delta-T.
- Pumps and sequencing. The pumps run at maximum to hold 4 psi at the far end, and a third chiller is started because the operating staff read low plant delta-T as insufficient capacity. The third chiller adds capacity the plant does not need and worsens part-load efficiency.
The 2.5°F evaporator approach against 1.0°F design is a separate finding: it points to tube fouling or reduced heat transfer, which limits each chiller's usable capacity and should be confirmed with a tube inspection and a log of approach versus load and flow.
2. The remote coil valve is 100% open yet its water-side delta-T is only 6°F with normal air-side performance. What coil-level causes fit, and how would you discriminate among them?
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A wide-open valve with half the design delta-T means the coil is passing far more water than it should relative to the heat it is absorbing. Candidates:
- Valve sizing and authority. An oversized valve with low authority (say below 0.3) is effectively an on/off device; it dumps flow even near the closed end. Check the valve Cv against the coil design flow and the pressure drop ratio across the valve versus the rest of the branch.
- Excessive differential pressure. A 12 psi loop setpoint is high for a modern two-way system. High available pressure across a wide-open valve drives excess flow. Verify by reading pressure drop across the coil and valve at the remote unit and comparing with design.
- Coil fouling or air-side bypass. Normal air-side delta-T with a low water-side delta-T argues against fouling of the water side, but internal fouling reduces heat transfer while still passing flow; check the coil's entering and leaving water temperatures at stable load and confirm the coil is not bypassed by the damper or ductwork.
- Control behavior. If the valve is commanded open by an over-aggressive PI loop, the valve may be at 100% during periods when the load does not require it. Trend the command against zone temperature.
Discriminate by measuring, at several coils: valve position, valve command, pressure drop across the valve, pressure drop across the coil, and water delta-T at stable load. If most coils sit part-open with low delta-T, the problem is system-wide (DP setpoint and valve sizing). If only this coil shows low delta-T at 100% open, the problem is local to the coil or its valve.
3. What operating changes and capital fixes would you sequence to restore design delta-T, and how would you verify success?
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Operating changes first, because they are reversible and reveal whether hardware is at fault:
- Lower the differential pressure setpoint and enable valve-position-based DP reset, so the loop holds the setpoint that keeps the worst-case valve at about 90–95% open. Expect the remote pressure to fall well below 12 psi and excess flow to drop.
- Re-sequence chillers on actual tons and evaporator flow, not on low delta-T symptoms; add flow limiting per chiller so each stays within its evaporator window.
- Trend plant delta-T, total flow, and per-chiller approach at fixed intervals. A rising approach at constant flow is fouling.
Capital and mechanical work:
- Clean or inspect lead chiller evaporator tubes to restore the 1.0°F approach.
- Replace or re-trim oversized coil valves with characterized valves of adequate authority.
- Search for and eliminate uncontrolled bypasses, including chiller minimum-flow bypasses that fail open and decoupler lines that never close.
- Clean or replace coils and verify air-side performance.
Verification: the plant should carry about 680 tons with two machines at design flow near 1,360 gpm and a return near 56°F, with the remote valve below 100% open. If delta-T recovers but the approach stays at 2.5°F, the chiller still cannot be loaded and the tube work is the remaining limit.