What Is a Cooling Pond?

A facility in which water is cooled in a cooling pond. The ponds, reservoirs, and lake-specific pools used to cool circulating water are collectively called cooling ponds. A deep cooling pond refers to a cooling pond with a depth of more than 4m and a significant stable wet differential flow. Shallow cooling ponds generally have a depth of less than 3m, and only produce weak wet differential reflows or no wet differential reflows in local pool areas.

There are many types of cooling structures. According to different control methods of hot water and air contact, cooling equipment can be divided into two categories, namely cooling pools and cooling towers. Cooling pools can be divided into two types: natural cooling pools and water spray cooling pools. Among these two types of cooling structures, cooling towers have the most forms and the most complicated structures. According to whether the circulating water in the circulating water supply system is in direct contact with the air, the cooling tower is divided into three types: open (wet), closed (dry) and mixed (dry and wet). Of these three types of cooling towers, the most widely used are open (wet) cooling towers.

Hot water flows into natural lakes or artificial pools and reservoirs near the factory. In the process of water flow, in addition to the mixing of hot water with the original water in the pool to cool down, it also conducts heat through the contact and contact with the air on the water surface and evaporates heat to cool the water. This kind of cooling equipment is called natural cooling pool. Water surface is generally divided into the following two types.

Water bodies with limited water surface area include shallow water cooling ponds (ponds, shallow reservoirs, shallow lakes, etc.) with a water depth of less than 3m and deep water cooling ponds (deep reservoirs, lakes, etc.) with a water depth of more than 4m. The shallow water cooling pond is mainly dominated by plane flow, which produces only weak temperature-differential heavy currents in local areas or does not generate any special currents at all. There is an obvious and stable temperature difference heavy flow in the deep water cooling pond.

Water bodies with large water surface areas or water bodies with large water surface areas relative to the amount of cooling water, including river channels, large lakes, and bays.

The water spray cooling pool is an open-type pool that uses nozzles to spray water for cooling. A water distribution pipe system is arranged on the pool, and the pipes are equipped with nozzles.

Cooling pool physical model and analysis model

The physical model of the cooling pool can be used to understand, study and analyze the thermal and hydraulic characteristics of the cooling pool, and analyze the effects of mixing of drainage outlets, diversion facilities and heat retaining walls. However, the physical model is difficult to meet the similar requirements of the heat transfer process. At the same time, it is impossible to simulate the transient changes of meteorological conditions and the huge heat storage effect of the deep cooling pool under laboratory conditions. Therefore, the physical model has certain limitations.

The analysis model has certain assumptions and simplifications, but the analysis model can calculate the heat dissipation of various flow patterns, and at the same time, it can flexibly study the transient parameters of the cooling pool under different weather conditions according to the engineering design conditions. In engineering design, a physical model, an analysis model, or a combination of the two should be used according to the engineering conditions and design stages.

Thermal calculation standard and method of cooling pool

1. The traditional thermal calculation of cooling pool adopts steady state method. First determine the calculation standard for the maximum calculated temperature of the cooling water. Generally, one of the following methods is used:

(1) Deep cooling pools can use the average annual hottest month-to-month average natural water temperature and corresponding meteorological conditions for many years; shallow cooling pools can use the annual average hottest year-to-year average natural water temperature and corresponding weather conditions for many years.

(2) Consider the residence time of cooling water in the pool, and determine the calculation standard in combination with other factors.

2. It is impossible to use the above-mentioned traditional methods to obtain the true operating characteristics of the cold-ground pool, determine the water temperature of the intake water, and the water temperature distribution in the pool. In particular, when a comprehensive utilization reservoir is used as a cooling pond, more detailed calculation results are needed to meet the requirements of the optimal design of the circulating water system and the assessment of the impact of thermal drainage on water bodies and aquatic organisms. The steady-state calculation method cannot meet the above requirements.

3.The following methods and steps can be used when designing the cooling pond

(1) According to the site conditions, the steady-state calculation method is used to initially select several sets of feasible cooling pool schemes and corresponding parameters, such as area, water depth, and condenser temperature rise.

(2) Select a typical design year, use transient calculation method, input meteorological and hydrological data daily, and calculate the above several feasible cooling pond schemes. The frequency distribution curve of the water temperature and the vertical distribution of the monthly water temperature in the pond are obtained.

(3) Select the most likely solution to implement long-term calculations using transient calculation methods. The so-called long-term refers to the choice of several continuous flat dry years or the entire economic operation period of the power plant (about 20 years). The calculation results are used to verify the original frequency curve of water temperature and the distribution of water temperature in the pond. These results can be used for optimal design of the circulating water system and ecological environment assessment.

Cooling pool area and water depth selection

The complete cooling pool design should use the transient calculation method. It is reasonable to find the frequency curve of the multi-year intake water temperature and the vertical distribution figure of the annual temperature of the whole pool. It is reasonable to combine other factors to determine the area and depth of the cooling pool.

The cooling pool should be designed as deep if possible. Thereby making full use of the heat storage effect of the water body of the cooling pond, and reducing the temperature of the water taken under adverse weather conditions. Generally speaking, a water supply system with a cooling pond should adopt a lower cooling multiple, ie, a smaller amount of water and a higher condenser temperature rise, while meeting environmental protection and turbine temperature requirements. In this way, the number of cooling pools is small, which is advantageous for the formation of stratified flow. Properly increasing the heat load per unit area of the cooling pool can reduce the area of the cooling pool, in addition to reducing the cost, it also has the following benefits: ^[2]

1. When the water temperature is high, the cooling efficiency of the cooling pool is significantly improved;

2. The amount of replenishment water required is less, because the amount of natural evaporation reduced by the reduction in area is sufficient to compensate for the smaller amount of evaporation due to the increase in water temperature.

3. Higher thermal debt load will promote the formation of stratified flow. Thereby, the surface area is fully utilized.

Cooling pool drainage water inlet design

The design and layout of drainage outlets vary from project to project, and the following points are general principles:

1. The outflow velocity of the drainage outlet should be low to obtain a smaller degree of dilution. The density Froude number of the exit should be less than 0.5.

2. The outflow from the drainage outlet must be smoothly connected with the water surface of the cooling pond. The aspect ratio of the drain should be as low as possible, preferably less than 0.1.

The water level of the deep cooling pool changes greatly, and it is advisable to construct a multi-level drop or other forms of structures so that the drainage can be leveled at any water level.

Slowly flow into the cooling pool. In shallow cooling ponds, the water level variation is generally small. A diving dyke can be built in the pond to make the water flow out smoothly.

3. Although it is possible for the surface weight flow to extend the hot water to the entire pool, the following layout principles should still be followed:

(1) The arrangement of drainage outlets should make the entire pool of water flow form a contraction flow, which can reduce vortex and backflow.

(2) Drainage to avoid sudden spread in the pool.

(3) The outlet direction of the water outlet should be away from the water inlet.

(4) When the comprehensive utilization reservoir is used as the cooling pond, the drainage outlet should be set near the reservoir outlet so as to bring the hot water downstream in time and reduce the thermal load of the reservoir.

(5) The water intake should be set at the deepest part of the cooling pool in order to make full use of the heat storage effect of the water body. The underwater terrain in front of the water intake should be relatively open, so as to avoid local topography hindering the lowest temperature water in the reservoir.

(6) The water intake is generally a chest wall type water intake building. The submerged depth of the upper edge of the water intake opening under the chest wall and the water inlet flow velocity of the water intake are designed according to the principle of heat retaining walls. The heat retaining wall can be regarded as a special case of the chest wall type water intake building.

(7) The volume of water body behind the heat shield wall should not be too large. If the water exchange time is more than a few hours, the daily changes of meteorological conditions may re-affect the water temperature. ^[2]