Prescription · Full case
Sequence three 75 kW VFD compressors — stop part-load pile-up
Three 75 kW VFD compressors were observed modulating simultaneously when a two-unit strategy would have met demand, adding ~130 MWh/year of avoidable energy.
Potential savings
₹1,08,000
per month
₹12,96,000 / yr modeled
Root-cause analysis
- No central sequencer—each compressor obeys its own pressure switch.
- Overlapping set-points keep all units cutting in.
- Operator habit leaves all three enabled to "avoid dips," sacrificing efficiency.
Recommended action
- Immediate set-point reset (no CAPEX): raise unload band and extend timer so only lead+lag stay online.
- Operator playbook: rotate lead weekly; never leave all three enabled 'to avoid dips'.
- Long-term: sequencer PLC + receiver buffer (~₹6–8.5L) for sub-1-year payback.
Event snapshot (evidence)
| Unit | Power (kW) | % of 75 kW | Comment |
|---|---|---|---|
| Unit-1 | 43 | 57% | Part-load |
| Unit-2 | 54 | 72% | Part-load |
| Unit-3 | 68 | 91% | Near loaded |
| Combined | 165 | — | Three online at partial load |
Three machines stayed online while demand fit a sequenced two-unit strategy. The third unit idling/modulating adds approximately 130 MWh/year.
During a similar load window, shut the lowest-loaded unit under supervision. Stable pressure proves two units suffice when sequenced correctly. Log header pressure, kW, and alarms for 15–30 minutes to confirm.
Cost–benefit & ROI
≈130,000 kWh/yr from the third unit idling/modulating.
| Tariff (₹/kWh) | Annual saving |
|---|---|
| 7.00 | ₹9.1L |
| 8.00 | ₹10.4L |
| 10.00 | ₹13.0L |
Capex for sequencer + sensors: ₹6L–₹8.5L ⇒ ~9-month payback at ₹8/kWh. Side gains include trimmed peak kW, fewer lag run-hours, quieter operation, and more headroom.
- Trimmed peak kW on shared incomer
- Fewer lag run-hours → extended bearing life
- Quieter plant floor during off-peak
- 75 kW of peak capacity freed for process loads
Risks & mitigations
| Risk | Mitigation |
|---|---|
| Pressure dip on step-loads | Add +2 m³ receiver and keep Lag-1 armed with 30 s delay. |
| Operator override | SOP at panel, weekly audit of set-points, alarm if all three online >10 min. |
| Sensor drift | Quarterly calibration with dual-sensor plausibility check (±0.1 bar). |
KPIs & alerts
| KPI | Target | Alert |
|---|---|---|
| Minutes with 3 units ON | ≤ 5% | >5% in any 24 h |
| Time outside 6.5–7.3 bar | ≤ 5% | >5% |
| Starts per shift per unit | ≤ 10 | >10 |
| Unloaded time per unit | ≤ 10% of run time | >10% |
Commissioning & verification
- Calibrate header sensor and confirm 6.5–7.3 bar band.
- Run two-unit trial (Lag-2 disabled) to prove stability under new rules.
- Acceptance (24 h): ≤5% minutes outside band, ≤10 starts/shift, ≤10% unload time, ≥95% minutes with ≤2 units.
- Lock in set-points, store PLC recipe, and pin SOP at panel.
Manager takeaway
Two 75 kW units can carry the load. Sequencing discipline now plus a PLC and small receiver recovers roughly ₹10L–₹12L per year with <1-year payback and boosts reliability.