Abstract
The decision to retrofit or replace a mission-critical chiller involves complex technical and financial modeling. It must be governed by thermodynamic headroom, asset criticality, and lifecycle cost analysis.
Thermodynamic Headroom Analysis
Before replacement, existing isentropic efficiency must be benchmarked against a modern equivalent. If an existing constant-speed centrifugal chiller is operating at 0.75 kW/TR, but a modern magnetic-bearing chiller could achieve 0.45 kW/TR on the same load profile, thermodynamic headroom justifies capex. If the machine is already running near its theoretical limit for the facility's ambient profile, replacement is unwarranted.
Reliability Engineering and MTBF
Analyzing the MTBF and mapping failure rates against a Weibull distribution curve determines whether a machine has entered the 'wear-out' phase of its lifecycle. A statistically unstable machine serving a Tier IV data centre mandates replacement regardless of its thermal efficiency.
Financial Modeling (NPV)
The capital decision requires an NPV calculation over a 15-year horizon, incorporating cost of capital, utility inflation, maintenance overhauls, and exact energy savings validated by the site-specific part-load profile.
NPV = Sum( R_t / (1+i)^t ) - C_0