In theory or for most people, the recovery of waste heat from air compressors is a simple engineering task. However, in my practical work, I have also discovered many problems, especially how to calculate the water production rate of waste heat recovery using various algorithms. We will briefly explain the waste heat recovery of screw air compressors as an example.

There is a well-known enterprise in Shanghai that mainly produces computer peripheral products. Our company provides energy-saving services for air compressor systems, including waste heat recovery projects. This project needs to meet the bathing water needs of about 1000 people, with a water temperature of around 40-45 degrees, a waterway length of about 200 meters, 4 floors, and 2 customized energy-saving 90kW air compressors.

In theory, this is a very common and ordinary waste heat recovery project for air compressors. However, after we implemented it, the result was that the hot water temperature was not enough and the air compressor occasionally had high temperatures. The main reason for this problem was actually that the waste heat recovery equipment manufacturer provided us with a smaller waste heat recovery machine. As a professional manufacturer of waste heat recovery equipment, we should not make such a low-level mistake. The reality is that this problem can be seen everywhere, and sometimes it's just that customers haven't noticed it. Based on the author's understanding, the reason for this issue may be due to the use of incorrect methods and calculation formulas when estimating the amount of waste heat recovery, or purely based on experience. In order to accurately calculate the heat recovery water production of an air compressor, we must master the following parameters: compressor power, loading rate, operating time, recovery medium (oil or gas), specific heat capacity of water, heat exchange efficiency, season, geographical location (south or north), etc. Therefore, the author has classified the methods for calculating the water production of screw air compressor waste heat recovery obtained from various online or offline channels, and calculated them through actual cases, for reference only in designing air compressor waste heat recovery projects.

The first type: Type A

Taking 6 250KW air compressors with a loading rate of 80% and 1-hour heat production as an example, demonstrate the calculation process:

1. Power of heat recovery: 250kW * 6 units x 80% x 1 hour=1200kW. 2. Calculated to large truck: 1200kW x 860=1032000Kcal (860 is converted from kW to Kcal coefficient)

So: The calculation formula for Y tons of water with a temperature increase of X degrees is as follows: (1 ton of water with a temperature increase of 1 degree will consume 1000Kcal of heat)

X=1032000Kcal/1000/Y or Y=1032000Kcal/1000/X

3. Conclusion: In 1 hour, the water production of 6 250KW air compressors is:

Summer water production: temperature rise of 30 degrees (from 30 degrees to 60 degrees), total heat production: 1032000/1000/30=34.4 tons;

Winter water production: temperature rise of 40 degrees (from 20 degrees to 60 degrees), total heat production: 1032000/1000/40=25.8 tons.

Note: Fully utilize thermal energy recovery

Second type: Type B

Taking one 315kW air compressor (with a heat conversion rate of 80%), a loading rate of 80%, and a 1-hour heat production rate as an example, demonstrate the calculation process:

Hot water produced in 1 hour=(maximum heat load of the machine 252KW/H × 3600KJ) ÷ (specific heat capacity of water 4.2KJ × temperature rise of water 10-60 ℃) × heat exchange efficiency 60% × air compressor loading rate 80%=2.074T

Note: Make full use of thermal energy recovery, with a temperature rise of 50 degrees (10 degrees to 60 degrees)

Third type: C-type

Taking the 1-hour water production of a 250KW air compressor as an example, demonstrate the calculation process:

According to actual test data statistics and analysis, 75% of the energy consumption of the air compressor is carried away by the high-temperature and high-pressure lubricating oil through the cooler. Calculated based on a loading rate of 100% and a recovery rate of 95% during the operation of the air compressor, the recoverable heat load is:

The heat recovery load of a 250KW air compressor is: 250kW × 75% × 100% × 95%=178kW

Temperature rise of 55 degrees (from 5 degrees to 60 degrees), hourly heat production:

In the formula, C is the specific heat capacity, m is the water volume, and Δ t is the temperature change value.

The specific heat capacity of water is 4.2, which means that 1kg of water absorbs 4.2kJ of heat and the temperature will rise by 1 ℃.

24 hours available

The production of hot water is 2.77t/h * 24h=66.48 tons/day

Fourth type: D-type

Calculation of water production and consumption in summer and winter using 5 132kW air compressors with a loading rate of 80% (Figures 1 and 2):

5 air compressors generate heat

=5×132×860×80%×40%×24×(1-5%)=4,141,209.6kcal

Summer heat requirement=100 × (60-25) × 1000=350000000 kcal

Winter heat requirement=100 × (60-5) × 1000=5500000kcal

When carrying out waste heat recovery projects, we can refer to the calculation methods for hot water volume in waste heat recovery of screw air compressors mentioned above, and input the corresponding data into the formula to quickly calculate the amount of hot water. Of course, actual engineering will be more complex, and in order to calculate more accurately, more factors need to be considered.

In the industrial field where energy utilization and cost efficiency are increasingly required, air compressor waste heat recovery is a profitable business for industrial enterprises. The energy recovered from the air compressor forms a supplementary or new heat source, with low input and high output. Not only can it maintain the optimal operating temperature of 82-96 ℃ for the air compressor unit, improving the performance of lubricating oil and reducing losses, but it can also generate 45-60 ℃ for use in employee dormitories, 60-70 ℃ for winter heating in offices, and other excess hot water can be used as preheating for process hot water such as reverse osmosis pure water production, achieving energy recovery and energy-saving effects.

However, in order to fully utilize the waste heat recovery system of air compressors, create customer value and economic value, it is necessary to improve the technical strength and professional level of employees in the air compressor industry, waste heat recovery equipment manufacturers, and the attitude of honest operation, which is more beneficial to the healthy development of the industry.

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