Compressor Heat Recovery Payback
A plant is considering heat recovery from a 75 kW oil-injected screw compressor. The vendor states: “75 kW × 80% recoverable × 6,000 h = 3,60,000 kWh per year, worth about ₹24 lakh at current LPG cost, so the ₹6.5 lakh unit pays back in under four months.” Plant records show the compressor averages 68 kW input over 5,000 h/yr at 70% load. The factory’s hot-water demand is 30 kW for only 3,000 h/yr; at other times the existing LPG boiler runs at low fire. Recovered heat is available as 55 °C oil, the existing hot-water loop returns at 45 °C, and the LPG boiler is about 80% efficient. The recovery unit needs a 5 kPa back-pressure allowance and a new 2.2 kW circulation pump.
1. Audit the vendor’s calculation and identify the assumptions that are not supported by plant data.
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The vendor uses rated power, a fixed 80% recovery fraction, and 6,000 hours, none of which matches the records. The measured average input is 68 kW over 5,000 hours. Even accepting 80%, that gives 68 × 0.8 × 5,000 = 272,000 kWh, not 360,000 kWh. More importantly, the calculation ignores the demand side. Heat that cannot be used has zero value; the plant needs 30 kW for only 3,000 hours, so the useful recovery is capped at 30 × 3,000 = 90,000 kWh per year. The vendor’s figure is therefore more than three times the physically usable energy, which is the main reason the payback claim is unrealistic.
The LPG saving should be calculated on fuel displaced, not on recovered kilowatt-hours: 90,000 kWh = 324 GJ; at 80% boiler efficiency the fuel input is 405 GJ; at 46 MJ/kg this is about 8,800 kg of LPG. At ₹85/kg the gross saving is about ₹7.5 lakh per year, not ₹24 lakh. The vendor’s own ₹24 lakh also does not reconcile with its stated energy quantity at that LPG price, which is a further warning that the proposal has not been internally audited.
2. Calculate a defensible simple payback and state whether the project should proceed.
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Useful heat = 30 kW × 3,000 h = 90,000 kWh = 324 GJ. Fuel displaced = 324/0.80 = 405 GJ; LPG mass = 405,000 MJ/46 MJ/kg ≈ 8,800 kg; saving at ₹85/kg ≈ ₹7.5 lakh per year. Deduct the circulation pump: 2.2 kW × 3,000 h = 6,600 kWh, about ₹0.05 lakh at ₹7.5/kWh if electrically driven. Deduct a compressor penalty: 5 kPa back pressure may cost roughly 1–2% of 68 kW over 5,000 h, about 5,000–7,000 kWh, or ₹0.04–0.05 lakh. Net saving is about ₹7.4 lakh per year, giving a simple payback of 6.5/7.4 ≈ 0.88 year, or about 10–11 months. With maintenance and fouling allowance, a 12–15 month payback is realistic.
That is still attractive, so the project should proceed - but on the corrected basis, with a 30 kW-matched heat exchanger and buffer storage, not on the vendor’s four-month claim. If only 70% of the demand can actually be captured, the payback extends to about 15–18 months, which is still acceptable but should be stated to management.
3. What technical and operational conditions must be met before the unit is accepted?
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The heat exchanger must be sized for 30 kW with the 55 °C oil and 45 °C return water, giving a hot-end approach of about 5 K; this is feasible with a plate exchanger but needs a realistic fouling allowance and a bypass for when the compressor is off. A 30 kW load corresponds to about 5.2 m³/h of water at a 5 K rise, so the loop and buffer tank must be sized for that, with the existing boiler retained as backup and a control valve to prevent the compressor oil from being over-cooled.
Confirm that 55 °C oil can actually meet the process requirement; if 60 °C hot water is needed, this source cannot deliver it alone. Check that the 5 kPa back pressure does not upset compressor control or oil carry-over, and verify the compressor manufacturer’s approval. Install meters on recovered heat, make-up water, and LPG consumption so the saving is measured, not assumed. Acceptance should require a 30-day trial showing the design heat recovery at the design water flow, no compressor alarms, and a documented saving close to the corrected estimate.