Case Study: Heat recovery at air compressor

Area of Improvement:

2 Nos Lubricated screw air compressor of 1000cfm at 7 barg was being used at plant for compressed air requirement.

Observation:

Air compressor used for generating compressed air is one of the least energy efficient machines. Most of the energy used in compressed air generation is lost in the form of heat. During compression of air heat is produced, which is cooled down with the help of fluid cooler either air cooled or water cooled heat exchangers are used in case of Lubricating screw air compressor. The heat distribution of compressed air compressor is shown in below pie chart. The heat of compression can be recovered to preheat water from ambient temp to about 70 Deg C.

Total Heat recoverable in lubricated screw air compressor is approx 76% of input power. Means if 100KW of energy is supplied for air compressor 76KW of heat can be recovered from lubricating oil. Some of the compressor manufacturer claim heat recovery of 80% and high if inlet temperature of water being heated is low.

Note: – In case of non lubricating screw air compressor heat of compression can also be recovered, investment of heat recover in case of non lubricating compressor is higher than lubricating compressor as larger setup is required.

Energy Conservation Measure:

Heat generated at air compressor and transferred at lubricating fluid is recovered with the help of heat exchanger, water is supplied at 30 Deg C and outlet temperature achived was 70 Deg C, Total 104KW of heat is recovered with the heat exchanger which was approx 65% of heat input to the compressed air machine.

Nowadays almost all OEM of air compressor are offering Heat recovery unit with air compressor. In old compressor Heat recovery unit can be placed externally if space is not provided inside cabinet for Heat recovery Unit. Payback in our case was below 1 year months, Payback in case of non lubricating air compressor heat recovery is approx 2 years.

Limitation:

The only limitation for air compressor heat recovery is outlet temperature of water or heat recovering media cannot be increased above 70 Deg C and  inlet water or heat recovering media should be lower than 50 deg C for better design.

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