Comparative analysis of heat exchange methods for air compressor waste heat recovery

       During the operation of the air compressor, oil waste heat and gas waste heat will be generated. In principle, if these two kinds of heat can be recovered, the energy saving benefit will be greatly improved.

       In order to ensure the normal functioning of the three major functions of the air compressor lubricating oil, the operating temperature of the air compressor has a reasonable and optimal temperature range: 60℃～85℃. If the working of the air compressor exceeds 85℃, the oil of the air compressor will be aging and coking, the lubricating effect will be reduced, and the sealing effect will be deteriorated, which will lead to the failure of the air compressor and affect the normal operation of the air compressor. In addition, most air compressor manufacturers set the fan operating temperature to start at 85°C and stop at 75°C. The air output of the screw air compressor will decrease with the increase of the operating temperature of the unit.

           In actual use, the mechanical efficiency of the air compressor will not work stably at the gas production rate calibrated at 80 °C. When the temperature increases by 1°C, the gas production decreases by 0.5%, and when the temperature increases by 10°C, the gas production decreases by 5%. Generally, the air compressors with air cooling and heat dissipation are operated between 88 and 96 ℃, and the decline rate is 4-8%, especially in summer.

           In the compression process of the air compressor, the main shaft of the equipment is mainly driven to drive the compression process. Due to the friction between the spindle and the bearing bush during the operation, the temperature of the spindle rises. The elevated temperature is very harmful to the equipment in operation. This part of the heat depends on the lubricating oil to take away the heat in the process of lubricating the running parts. The heat taken away is finally transferred to the lubricating oil, which increases the temperature of the lubricating oil. Therefore, if the thermal energy utilization transformation is carried out, the exhaust operating temperature and oil temperature of the air compressor unit are controlled within the optimal temperature range, and the operating time of the cooling fan of the screw air compressor is reduced, which can save a lot of electric energy, and at the same time better guarantee air volume.

         Air compressor waste heat recovery is a very environmentally friendly and efficient energy-saving device. It has no adverse effect on the air compressor itself. On the contrary, it can effectively reduce the operating temperature of the air compressor, keep the air compressor at a good working temperature, and improve the air compressor. The efficiency of the machine is improved, and the original cooling fan is stopped to save power. As a new type of high-efficiency energy-saving equipment, the air compressor waste heat recovery equipment completely produces hot water from the heat generated by the air compressor, without subsequent energy consumption.

Plate Air Compressor Waste Heat Recovery

        In the field of air compressor waste heat recovery, plate air compressor waste heat recovery. It is the most popular and widely used air compressor waste heat recovery technology in the industry. The main principle is to use the heat exchanger to recover the oil waste heat and gas waste heat in the air compressor. In the waste heat recovery of air compressors, the use of plate heat exchangers accounts for the largest proportion. The reason is that the manufacturing process is simple, the production cycle is short, the cost is low, and the price is cheap, so many projects prefer plate heat exchangers.

Advantages of plate waste heat recovery:

1) High heat transfer efficiency: stainless steel with thin wall has high heat transfer efficiency;

2) Small footprint: the plate heat exchanger has a compact structure;

3) Light weight: the plate thickness of the plate heat exchanger is only 0.4-0.8mm;

4) Low price: Plate heat exchangers are cheap.

However, due to the influence of its own structure, the plate-type waste heat recovery equipment also has the following disadvantages:

1) The sealing performance is poor, easy to leak, and the gasket needs to be replaced frequently;

2) The operating pressure is subject to certain restrictions, generally not exceeding 1MPa;

3) The use temperature is limited by the temperature resistance of the gasket material;

4) Easy to block, not suitable for fluids containing suspended solids;

5) The fluid resistance is larger than the tubular type.

        The main reason is that the hardened water is heated by the high temperature oil medium, which is easy to produce scale. The plate heat exchanger has a narrow flow channel gap, and if there is scale, it is very easy to block, resulting in a decrease in the efficiency of the heat exchanger.

   Because of this situation, tubular heat exchangers are on the market again.

Tubular heat exchanger and its characteristics

        The wound tube heat exchanger is a new type of heat exchanger, and its structure is different from the shell and tube heat exchanger widely used in the industrial industry. The heat exchange tube of the heat exchanger is spiral wound and wound in multiple layers. A certain distance is maintained between each layer and the previous layer through the spacer plate successively, and the winding direction between layers is opposite. Since the length of the heat exchanger in the shell can be lengthened, the shell size of the heat exchanger can be shortened and the heat transfer efficiency can be improved

       Since the first design and manufacture of the coiled tube heat exchanger, many new types of coiled tube heat exchangers have appeared in the form of coiled tubes, but the main structures are similar.

       Compared with plate heat exchangers, tubular heat exchangers have some incomparable advantages: wide applicable temperature range, adaptability to thermal shock, self-elimination of thermal stress, and high compactness. There is a flow dead zone, especially by setting up multiple pipe passes (single shell side), it is possible to meet the simultaneous heat exchange of multiple fluids in one device. Therefore, the tubular heat exchanger is an efficient and compact heat exchanger with a complex structure. Although the cost is higher, it has many advantages:

1. Precise heat exchange tube spacing. Due to its material, stainless steel pipes are difficult to have plasticity when bending or winding. In order to achieve uniform spacing of all heat exchange tubes, it requires complete technology and rich experience in production and processing.

2. Reasonable number of tube layers and layer spacing. During heat exchange, the relative tube diameters of each layer are changing, and the angles must be kept the same. It is indeed difficult to ensure that the lengths of each heat exchange tube are basically the same. The designer designed a different number of heat exchange tubes for each layer, as well as a reasonable layer spacing, which perfectly overcomes this problem, and also solves the flow channel requirements under complex heat exchange conditions.

3. Welding process. In the process of equipment production, fully automated robot welding is used to ensure that all welding points, especially the welding of tube sheets, have uniform standards and high safety performance.

4. Wide application range, compact structure, and large heat transfer area per unit volume. For heat transfer tubes with a diameter of 8 to 12 mm, the heat transfer area per cubic meter of volume can reach 100-170 square meters.

5. There is no dead angle of flow, and a single device can carry out heat transfer of various media at the same time, which has little resistance to heat transfer in the gas path and is not easy to scale.

6. The use of fully automated robot welding ensures high operating pressure in the pipe, and the current maximum operating pressure can reach more than 20 MPa.

7. The thermal expansion of the heat transfer tube can be compensated by itself.

8. The heat exchanger is easy to achieve large-scale development, and is suitable for oil-free air compressors and centrifuges for gas circuit heat exchange.

9. Low probability of impurity deposition, low scaling tendency and long service life

To sum up, with the continuous progress of air compressor waste heat recovery technology, the waste heat recovery of tubular air compressors is much better than plate waste heat recovery, which is easy to leak and block. It is believed that it will contribute more power to escort the operation of air compressors and create more benefits for users.