Analysis of Energy Saving Potential of Cooling Towers

With the enhancement of economic awareness, energy saving and consumption reduction have attracted more and more attention. The energy saving of the thermal system and equipment of the power plant brings obvious economic benefits to the operation and operation of the power plant. At present, energy saving and consumption reduction are mainly concentrated in three major equipment and complex systems. After theoretical research and extensive application, great economic benefits have been achieved. But for a long time we have not paid enough attention to the cooling tower in the circulating water system. On the one hand, it is considered that the inlet temperature of the circulating water of the condenser is a single-valued function of environmental factors; on the other hand, its maintenance is relatively heavy and complicated, and many power plants even ignore it due to the lack of understanding of the energy saving potential of the cooling tower.

Kunshan Cooling Tower analyzes and discusses the energy-saving potential and thermal performance influencing factors of natural ventilation cooling towers, which are helpful and reference for optimal operation and maintenance of power plants.

1. Analysis of energy saving potential of cold water tower

The energy saving potential of the circulating water temperature difference of 1oC

The working process of the cooling tower is that the circulating water absorbs the exhaust heat from the condenser, and is sent to the cold water tower at the temperature t1 and is divided into the water distribution tank through the pressure pipeline. On the top, along the height and depth of the packing layer, the heat exchange is completed with the cold air by means of evaporation, conduction and convection. The air absorbs heat and moisture, and its temperature and humidity gradually increase close to saturation and escape from the top of the tower, and the cooled circulating water returns to the condenser at temperature t2. It can be seen that the outlet water temperature of the cooling tower directly affects the exhaust pressure and cycle thermal efficiency of the steam turbine. In the power plant in operation, the cold water tower often works in an environment deviating from the design conditions. The water temperature at the outlet of the tower is higher than the design value, resulting in a drop in vacuum and a reduction in the economy of the unit. Table 2 shows the impact of 6 types of units on the economic performance of the unit due to the reduction of the cooling capacity of the tower, resulting in a 1oC increase in the outlet water temperature.

It can be seen that the energy saving potential associated with a 1oC increase in the temperature of the circulating water inlet of the condenser of the operating power plant. At present, most of the cold water towers lack performance testing. Due to the increase of heat load or improper maintenance, the output of the cooling tower is insufficient, and the high outlet temperature is a common phenomenon. For example, the circulating water of our company's 135MW unit is turbid, the water filling is seriously fouled, and the water temperature at the outlet of the tower is 4oC higher than the design temperature under the same conditions. The annual loss of standard coal for this unit is about 2706t, which is the only economic loss. About 550,000 yuan (the coal price is 200 yuan/t).

Therefore, choosing a water-spraying filler with excellent performance can reduce the water temperature of the tower and have a smaller ventilation resistance. According to the literature, the replacement of high-performance film fillers for both co-current and counter-current cooling towers can lead to a 5-8 oC reduction in cooling water, which is equivalent to a 50% increase in cooling capacity or more for existing cooling towers. It is an important means to improve the thermal performance of cooling towers by attaching importance to the operation and maintenance of water-spraying fillers and reducing the freezing of cooling towers and the damage of fillers.

1.3 Potential energy-saving benefits of water spray density

The water spray density refers to the amount of cooling water passed by the water filling per unit area, and it is also one of the main factors affecting the output of the cooling tower. Due to improper operation and untimely maintenance, the nozzles are blocked, the packing is damaged and algae grow, resulting in a decrease in the heat exchange area and an increase in the water spray density. The attached figure shows the change of the water temperature at the outlet of the tower with a relative reduction of 1% to 25% in the water spray area.

Another important parameter that affects cooling tower performance is circulating water flow. Increasing the circulating water volume is beneficial to the heat exchange on the condenser side, but there is an optimal circulating water volume for the cooling tower. When the relative humidity of the air out of the tower is not saturated, increasing the amount of circulating water can make the air out of the tower gradually become saturated. If the circulating water volume continues to increase, the temperature of the water exiting the tower will rise rapidly, because the heat absorbed by the air has reached saturation, and the heat released by the excess hot water can no longer be absorbed by the air. In addition, the increased pump power consumption has little effect on the efficiency of the steam turbine. In fact, the power consumption of the circulating water pump is used to compensate the water temperature rise at the outlet of the cooling tower.

To sum up, cooling towers have huge energy-saving potential, and the performance of cooling towers should be studied as the main energy-saving methods for operating power plants. According to the specific conditions of each plant, practical measures to increase the output of the cooling tower are formulated to ensure the safe and economical operation of the unit.

2 The main reasons for the reduction of cooling tower performance

The thermal performance of the cooling tower is related to whether the design output of the tower is reasonable, the manufacture and installation of the device, the quality of operation and maintenance, and the quality of overhaul. It must be analyzed according to the specific conditions of each tower. Common causes of cooling tower performance degradation include:

1) The drenching packing is damaged and shedding and growing algae, and garbage or scaling is accumulated on the packing, which reduces the heat exchange area and increases the density of the drenching water, resulting in an increase in the temperature of the outgoing water.

2) The water distribution tank is blocked at different elevations, the water tank overflows, the nozzle is blocked and falls off, and the splash plate is not medium, which causes uneven water distribution. The function of the water distribution system is to spread the hot water evenly on the entire water filling, so as to prolong the contact time and contact area of ​​the soda-water heat exchange. Uneven water distribution reduces the cooling effect of the tower.

3) The installation gap of the filler near the beam and column of the ventilator is too large, and the holes in the ventilator have poor sealing or unequal heights, resulting in insufficient pumping force. In addition, the accumulation of scale, grease and algae on the fill or water trap and the excessive load of water flowing into the fill will also cause the reduction of air flow, the weakening of steam-water heat exchange, and the performance of the cooling tower.

4) The abnormal treatment of circulating water sewage and dosing makes the concentration ratio of circulating water high, so that the precipitates containing calcium bicarbonate, magnesium bicarbonate and calcium sulfate condense on the inner surface of the heat exchange equipment, and sometimes deposit on the water spray plate. These foulants block the pipes of the heat exchange equipment and reduce the cooling efficiency of the tower.

5) Improper operation of cooling towers in cold and severe cold areas in winter causes the air inlets and water filling to freeze, which directly affects the thermal performance of the tower. Frequent start and stop of cooling towers in winter seriously affect the operating efficiency and service life of the equipment, and increase maintenance costs.

It can be seen that the above factors all affect the economy of the cooling tower and reduce the efficiency of the unit, especially in the summer high temperature and lack of rain season, the performance of the cooling tower reduces the load of the unit to maintain its safe operation, thereby limiting the output of the unit and reducing the equipment utilization rate .

In the operation of the cooling tower, the most important thing is to monitor whether the thermal performance of the cooling tower is normal. If it can be adjusted to the best working condition, it will undoubtedly benefit the economical operation of the unit. In order to maintain the cooling tower working in the best condition, on the one hand, we should strengthen supervision and maintenance, analyze and evaluate many factors that cause performance degradation, and choose the high performance of the cooling tower operation at the lowest maintenance cost; on the other hand, because If the performance of the equipment is degraded due to long-term operation or the output of the cooling tower is insufficient due to the increase in heat load, it should be considered that technical transformation has improved its thermal performance.

The economical operation of the cooling tower is closely related to the efficiency of the steam turbine, the power consumption of the circulating water pump, the maintenance cost of the cooling equipment, the amount of supplementary water and the water quality treatment cost. In fact, all cooling towers can be improved to improve the efficiency, and the investment depends on the efficiency of the unit. return.

3 Conclusion

The cooling tower is an important auxiliary equipment in the thermal cycle of the thermal power plant. It contains considerable energy-saving potential. The thermal performance of the cooling tower is directly related to the economic efficiency of the power plant. The chiller tower with excellent performance can make the unit output the maximum power with the minimum energy consumption, which is the prerequisite to ensure the high thermal efficiency, safe operation and full-load power generation of the steam turbine. Strengthening the performance monitoring and energy-saving diagnosis of cooling towers, modifying equipment and adjusting parameters in a timely manner will help reduce power generation costs. Cooling towers are key equipment that cannot be ignored for energy saving and consumption reduction.