I. Background Analysis

This client is a factory engaged in the production and processing of auto parts. The factory is equipped with 12 machining equipment units, and a GA160W air compressor has been installed for these machines. The surface of the machined parts contains a large amount of aluminum chips and coolant mixed oil, so they need to be spray-cleaned with hot water and chemical cleaning agents. For this purpose, the factory has also installed a hot water boiler with a maximum power of 500 kW for the parts cleaning line.

As we know, 100% of the electrical energy consumed by the air compressor is dissipated in the following forms:

1. 75% of the electrical energy is converted into heat stored in the hot oil and removed by the oil cooler;
2. 10% of the electrical energy is converted into heat contained in the compressed air and removed by the air cooler;
3. 10% of the electrical energy is converted into heat lost through radiation and uncontrollable internal compression losses;
4. 5% of the electrical energy is lost as motor heat.

It can be seen from the above that for the GA type oil-injected screw compressor, approximately 75% of the energy is consumed in the hot oil circuit. The heat recovery unit is designed to recover most of the above heat in the form of hot or warm water without any negative impact on the compressor performance, with a recovery rate of 65%~75% of the actual input shaft power.

II. Energy Recovery Analysis and Calculation

The actual working pressure of the GA160 air compressor currently used by your company is 6.5 bar. According to the standard data of Atlas Copco, the actual input shaft power is approximately 160 kW.

The recoverable energy calculated at 70% is:

160×70%=112 kW

Based on full-load operation of a single air compressor, the daily recoverable energy (24 hours) is:

112×24=2688 kWh

Converted to heat energy:

2688×3600/4.2=2304000 kcal

Considering a 15% heat loss in the external hot water pipelines (depending on insulation and transmission distance), the effective daily heat energy available from one GA160 (24-hour full load) is approximately:

H1​=2304000×85%=1958400 kcal

With a return water temperature of 65°C (inlet temperature to the compressor), the system can continuously produce water with a 10°C temperature rise (up to 75°C) at a rate of about:

1958400/1000/10∘C/24=8.16 tons/hour

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Main Technical Parameters of Our customer’s current Hot Water Heating System conditions:

1. Actual set outlet temperature range of the hot water boiler: 70°C ~ 78°C; the return water temperature drops by about 9°C ~ 10°C.
2. In one automatic heating cycle, the heating time is about twice the non-heating time (ratio 2:1).
3. Hot water pump flow rate: approx. 36 m³/h.
4. If the outlet temperature is set to 75°C, the daily gas consumption is about 65–75 m³/24h.At the current LPG price of 15.80 RMB/m³, the gas cost for the hot water boiler is approximately 1185 RMB/day.

The calorific value of gaseous LPG is 22,000–29,000 kcal/m³; we take 27,000 kcal/m³.

The boiler thermal efficiency is taken as 95%.

The actual daily heat energy consumption (24 hours) of your current hot water system is:

H2​=27000×75×95%=1923750 kcal

From the above:

H1​>H2​

Based on the above thermal analysis and calculation, recovering heat from one GA160 under full-load operation can basically meet the current thermal demand for hot water in your company.

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III. Return on Investment

Daily LPG savings:

1958400/27000≈72.5 m3

Assuming 8,000 operating hours per year, the annual cost saving is approximately:

72.5/24×8000×15.8=381833.00 RMB

IV. Design flow chart for heat recovery unit

The above thermal energy recovery system diagram is provided for the client’s reference only and shall not be regarded as a standard design drawing for the preheating of boiler feed water at the client’s site.

The retrofit design of the specific external pipelines shall be completed by the client based on the actual pipeline configuration.

We can provide indicative data of the air compressor heat recovery system for the client’s pump set selection and design, as follows:

1. Pump Set Selection:
   • For the hot water application of GA160 for boiler feed water preheating, the flow rate varies with the outlet water temperature.The water flow rate at a temperature rise of 10 ℃ is approximately: 8.16 t/h = 2.3 L/s.
   • Head and external pipeline resistance
2. Water pressure drop of the internal heat recovery device in the air compressor:According to the curve chart, the internal water-side pressure drop at 8.16 t/h = 2.3 L/s is approximately: 0.35 bar.

V. Project Effect

Upon completion of this thermal energy recovery renovation project, the customer’s hot water boiler will only need to be operated in winter. For the rest of the year, the heat recovered from the air compressor will be stored in a hot water storage tank and then supplied to the cleaning machine via a plate heat exchanger, making the boiler basically unnecessary. This project achieves a payback period of three years and starts generating net benefits from the fourth year onward.