Constant Pressure Water Make-up System

Why Closed Loops Lose Pressure — and What That Costs Operators

Every closed chilled-water or condenser-water loop loses pressure over time. Some losses are visible — a pump seal weeping, a small drip from a flanged joint — but the more common ones are not. Evaporation from open cooling towers, the slow seepage at expansion-vessel bladders that have lost elasticity, and the modest but continuous losses through bleed-off and chemical-dosing pumps all add up. In a mid-sized district-cooling loop running 3,500 TR, the cumulative loss can be 1.5–3.0 % of the loop volume per day.

When static head drops, the first thing operators notice is the relief valve cycling, then the expansion tank level falling, then — if nothing is done — the pumps losing their NPSH margin and cavitating. By the time cavitation noise is audible, the bearings are already taking damage. A make-up system is normally the simplest fix, but the choice between on-off solenoid metering, variable-speed pump with pressure transducer, and a bladder-equipped stabiliser changes the operating profile significantly.

The constant-pressure variant keeps the loop at a defined setpoint (typically 1.0–2.5 bar, project-specific) regardless of how the load above it behaves. It does this with a pressure transducer feeding a variable-speed pump, paired with a small break-tank sized to the local water-supply requirements and backflow-prevention standard. The result is stable differential pressure across the terminals, no relief-valve cycling, and a slower bleed rate because the system is no longer being topped up reactively.

In practice the energy saving compared with a properly sized but unsupplemented system is modest; the real saving is equipment life. Pump bearings, seal faces, valve stems and control-valve actuators all last longer when they are not working against a pressure profile that drifts by a bar over the course of a season. For plant rooms that already run tight on operating hours, that is usually the more decisive argument.

Selection Parameters We Use on a HyApexFlow Quote

Send the parameters above with your chiller / condenser model numbers and water-quality report. We reply within 24 hours with a sized configuration and a delivered-cost estimate.

Make-up System Questions, Answered

Does a constant-pressure unit replace the expansion tank?

No — they serve different functions. The expansion tank absorbs thermal-volume changes; the make-up unit replenishes water losses. In most plant rooms both are required, with the make-up line discharging into the expansion tank rather than directly into the loop.

Can it be retrofit to an existing loop without draining?

Yes, in nearly all cases. The break-tank and pump skid is installed on a bypass with an isolation valve arrangement; the loop stays pressurised throughout. Typical installation requires two hot-tapping points on the common return header and a 220 V / 380 V supply.

What is the maintenance interval?

Six-monthly inspection (pressure transducer calibration check, seal inspection, strainer cleaning) and annual servicing (pump bearing condition, valve-actuator cycling test). Consumables are standard off-the-shelf items; no proprietary spares are required.

Does it handle glycol / brine loops?

Yes, with material upgrades on the seal faces and pump elastomers. Tell us the concentration (ethylene glycol, propylene glycol, or calcium chloride brine) when requesting a quote.