3.1. Effect of Hot Air Recirculation (HAR) on Heat Transfer Performance of the Air-Cooled Condenser (ACC)
HAR means that the hot air discharged from the ACC is returned to the air-cooling fan suction port, which causes the air temperature of the inlet of the ACC to rise. The hot recirculation rate (HRR) represents the strength of the HAR, the greater the HRR, the stronger the HAR. On account of the complexity of the direct ACC itself and the overall arrangement of its surrounding buildings, the flow field embracing them is very complicated; therefore, it is very necessary to study the wind or confirm the influence law of different ambient winds on the HRR. We selected the ambient temperatures of 34 °C and 38 °C, different incident wind directions of west-north-west (WNW), south-south-east (SSE) and wind speeds of 5 m/s and 7 m/s to complete the numerical simulation of the impact of ambient wind on HRR.
For quantitative analysis of thermal reflow, the HRR is defined as follows:
where,
Ti is the average temperature at the inlet of the fan and
Ta and
To represent the ambient temperature and the average temperature at the outlet of the ACC, respectively.
For the ACC, the wall heat conduction thermal resistance and the steam condensation heat resistance are neglected. It is considered that the exhaust steam temperature is equal to the tube bundle average wall temperature, that is, the temperature of the exhaust steam and the pressure of the exhaust steam are calculated according to the tube bundle average wall temperature.
where,
Ts is the exhaust steam temperature and
q and h represent the heat flux per square meter and the heat transfer coefficient, respectively.
Ta1 and
Ta2 represent the average temperature of the inlet air and outlet air of the ACC, respectively.
The heat transfer capacity of a single unit under a design condition is about 820.86 MW, and the calculated heat flow per square meter is 47,400.47 W/m
2.
Ta1,
Ta2 and
h were obtained by three-dimensional (3D) model calculation. The HRR and the back pressure of the ACC can represent the level of the cooling performance of the ACC, a higher HRR and back pressure will have a lower cooling performance of the ACC. Combining different wind speeds, ambient temperature and wind directions, a total of eight working conditions were obtained. The calculation and analysis of these working conditions were carried out, and the corresponding back pressure and HRR were obtained to further study the effect of different environmental conditions on the heat transfer performance of the ACC. The calculated data is shown in
Table 2 below.
It can be seen from
Table 2 that the change of wind direction affects the average temperature at the fan inlet and the exhaust temperature at the fan outlet. The wind direction of WNW increases the average temperature at the fan inlet. From
Figure 4, we can see that under the effect of HAR, the back pressure of the unit was higher than 24 kPa under the ambient temperature of 38 °C. Under the conditions of the ambient temperature of 34 °C, wind speed of 7 m/s and wind direction of SSE and WNW, the back pressure of the unit was also higher than 24 kPa, which exceeded the safety back pressure (24 kPa) at the steam exhaust port of the turbine. At the same ambient temperature and the same wind direction, HRR and back pressure increased as the wind speed increased from 5 m/s to 7 m/s. Similarly, an increase in ambient temperature can cause a significant increase in back pressure. Among them, the increase of ambient temperature led to the performance degradation of the ACC, and the increase of wind speed also caused the same situation to occur, but the influence was less than the increase of ambient temperature and greater than the change of wind direction.
3.2. Influence of Summer Extreme Conditions on Heat Transfer Performance of the Direct ACC
As the ambient temperature rose, especially in the extreme high-temperature conditions in summer, the heat transfer performance of the direct ACC would be correspondingly reduced, resulting in an increase of turbine back pressure. The output of the units was not guaranteed, and even affected the safe operation of the units. Therefore, the heat transfer performance of the direct ACC under high temperature and unfavorable wind direction in summer was studied to understand the law of the influence of extreme meteorological conditions on the ACC’s cooling performance. Summer extreme conditions and unfavorable wind direction and wind speed were corresponding to the ambient temperature of 38 °C, the summer-dominant wind direction of SSE and the wind speed of 7 m/s. For the ACC, under the extreme summer conditions, the average temperature at the inlet of the fan was 39.82 °C, and the cooling triangle outlet average temperature was 65.534 °C. According to the formula, the back pressure of the ACC can be calculated to be 30.40 kPa, and the HRR of the ACC was 6.614%.
The ACC has a total of 128 cooling units with 16 columns and 8 rows. We represented
X as a column and
Y as a row, for example,
X8
Y2 represents the heat exchange unit in the eighth row and the second column, and so on.
Figure 5 shows the transverse section temperature distribution of the ACC and it can be seen that the temperature gradient decreased from the windward side to the leeward side. The temperature on the windward side is significantly higher than the temperature on the leeward side. We can use Equation (8) to calculate the HAR of the ACC; therefore, the HAR in the upstream was significantly higher than the downstream of the wind field, so the heat transfer efficiency of the ACC heat exchange units on the windward side was reduced.
Figure 6 showed the ACC and MCTs transverse section velocity distribution. Because of the influence of HAR, the heat exchange units of the ACC on the windward side upstream of the wind field produced significant eddy currents. It can be seen in
Figure 6 that there was no obvious eddy current around the wind field of the MCTs, so there was no mutation in the velocity at the outlet of the MCTs. Therefore, it can be concluded that the MCTs were not obviously affected by the ambient wind. Besides, it can be indicated from
Figure 6 that the influence of ambient wind on the ACC was significantly greater than that of MCTs.
Figure 7 shows the cooling triangle temperature distribution of the ACC by comparing the
X15
Y4 and
X16
Y4 cooling units. Due to the influence of eddy currents, the volume flow of the units decreased from 327.622 m
3/s to 242.931 m
3/s and the fan power dropped from 76.95 KW to 49.12 KW. Therefore, under summer extreme conditions, due to the effect of HAR, hot air intrusion occurred at the edge of the ACC, resulting in the reduced cooling performance of the ACC.
3.3. Effect of MCTs on the Cooling Performance of the ACC
In power plants, MCTs and the ACC are coexisting, and there is bound to be an interaction between them. The exhaust of the MCTs will partially enter the ACC under the effect of the ambient wind, which will affect the cooling performance of the ACC. Under different environmental conditions, MCTs will have different effects on the cooling performance of the ACC. Ambient air enters the MCTs through the air inlet. After a series of heat exchange with the high-temperature steam, it is discharged into the environment by the fan at the air outlet. The temperature of this part of the exhausted air is often higher than the ambient air, and the exhausted air always entrains high-temperature steam. The increase in temperature causes the air density to become smaller, so the discharged air will rise. Under the influence of ambient wind, this part of the exhausted air will pass through the air inlet of the ACC, causing the temperature of the air sucked by the ACC to increase. According to the heat exchange formula, the heat exchange temperature difference is reduced, and the ACC needs to do more work to complete the same heat transfer quantity, which greatly affects the heat transfer performance of the ACC, especially the entrained water mist, which may have serious consequences for the ACC. Similarly, the air discharged from the ACC will also affect the heat transfer performance of the MCTs. Therefore, when MCTs and the ACC work together in the same power plant, it is very indispensable to study the interaction between them. According to the numerical simulation, the effect of the MCTs on the cooling performance of the ACC and the influence of the presence or absence of the MCTs on the cooling performance of the ACC were studied, which provides a feasible scheme for the research on the ACC. In order to better analyze the effect of MCTs on the cooling performance of the ACC, we chose to be in summer extreme conditions (ambient temperature of 38 ℃, wind speed of 7 m/s, wind direction of SSE), which can weaken other influencing factors, focusing on the impact of MCTs on the cooling performance of the ACC.
Two typical cooling units,
X10
Y7 and
X11
Y7, were selected to investigate the area where the ACC was affected by the MCTs. In the area where the ACC was not affected by the MCTs, we selected two typical cooling units,
X7
Y7 and
X8
Y7, for analysis.
Figure 8a,b shows the velocity and temperature distribution of the influence of MCTs on the ACC, respectively. We can see that under the condition that the ambient temperature of 38 °C, the wind speed of 7 m/s and the wind direction of SSE, the exhaust of the MCTs partially entered the ACC under the effect of the ambient wind. In the affected cooling unit
X10
Y7, the HRR was calculated to be 0.593%, and in the cooling unit
X11
Y7, the HRR was 0.583%. In the nearby unaffected cooling unit
X7
Y7, the HRR was 0.036%, and the cooling unit
X8
Y7 had a HRR of 0.280%. Focusing on
Figure 9, the HRR of the cooling units affected by MCTs increased, which meant that MCTs had a great effect on the cooling performance of the ACC. However, in the cooling units not affected by MCTs, the increase of HRR was not large, which indicated that the presence of MCTs had a local influence on the cooling performance of only two cooling units, and then slightly impacted the overall cooling performance of the ACC, which provides a good insight into the arrangement optimization of the ACC and MCTs.
In the above discussion, we separately discussed the effects of MCTs on the affected and non-affected areas of the ACC. For a better study, the influence of the overall cooling performance of the ACC, under the same environmental conditions, the HRR of the ACC both in the absence and presence of MCTs were calculated separately. Through such a comparative study, theoretical support was provided for the study of the effect of the MCTs on the cooling performance of the ACC.
Figure 10a,b shows the cooling triangle temperature distribution of the ACC in the presence and absence of MCTs respectively, under the condition that the ambient temperature is 38 °C, the wind speed is 7 m/s and the wind direction is SSE, the exhaust of the MCTs partially entered the ACC under the effect of ambient wind. The temperature at the outlet of the ACC was 65.534 °C, the back pressure of the ACC was 30.396 kPa and the HRR was 6.614%. If the MCTs were turned off at this time, the temperature at the outlet of the ACC would change to 65.488 °C, the back pressure would be 30.335 kPa and the HRR would drop to 6.477%. In order to analyze the influence of the presence and absence of MCTs on the heat transfer performance of the ACC, we obtained the total average heat transfer coefficient and the average heat transfer coefficient of each column in the presence and absence of MCTs. From
Table 3, we can see the comparison of the heat transfer performance of the ACC in the absence and presence of MCTs. It was very clear to see that in the absence of MCTs, the total average heat transfer coefficient of the ACC and most of the average heat transfer coefficient per column are slightly higher, while the back pressure and the HRR of the ACC were lower than in the presence of MCTs; however, the magnitude of the reduction was relatively small, so the presence of MCTs had a minor impact on the overall cooling performance of the ACC. The main reason was that after the ambient air entered the MCTs, the two phases of gas and water were directly contacted, which was mainly based on latent heat transfer, and the dry bulb temperature was not greatly increased, while the ACC was dominated by sensible heat transfer, and the oncoming flow air dry bulb temperature played a leading role in its thermal performance. Secondly, the cooling air volume (1089.55 kg/s) of the MCTs was significantly smaller than that (59,017.36 kg/s) of the ACC; therefore, the presence of MCTs had a relatively small effect on the overall thermal characteristics of the ACC.