Everything about open circuit and closed circuit cooling towers!

Power plants, refineries, industrial processes, and various sectors (the food industry, etc.) use cooling towers, a form of heat exchanger, to cool the water required in the condenser of air conditioning systems.
These devices come in a variety of kinds based on their functions, body shape and material, water and air flow configuration, air flow suction type, etc., and each has advantages and disadvantages as well as a range of uses.

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Cooling towers are divided into two categories in terms of cooling performance:

1. Open Circuit Cooling Towers
2. Closed Circuit Cooling Towers

Building and industrial engineers, designers, and employers can profit from both open-loop and closed-loop cooling towers' distinct sets of advantages when it comes to heat rejection technology.
For the intended use, open-loop and closed-loop cooling towers can offer adequate thermal efficiency, dependability, and longevity if they are built properly. The task of deciding whether an open circuit or closed circuit cooling tower is better suited for a particular application requires knowledge of each one's functionality, features, and applications. It is the responsibility of the relevant engineers and those with a fundamental understanding of the requirements. They are used for what was meant. The introduction of open-circuit and closed-circuit cooling towers will be discussed in this respect, and the traits and constraints of each distinct point of view will be looked at and contrasted.

1. Open circuit cooling towers

How do open-circuit cooling towers work?

An open circuit cooling tower is a heat exchanger that uses direct contact between air and water to cool water. In the open circuit cooling tower, the incoming water (requiring cooling) is pumped to the upper part of the cooling tower and enters the tower from there. The nozzles distribute this water on the packings of the cooling tower and the water flows as a thin and uniform layer on the packings; This creates a very large contact surface (heat exchange) with the air. The fan also circulates atmospheric air on the packings. The incoming air cools the water in two different ways; Part of the heat is removed as a result of air convection (contact between hot water and colder air), but the majority of cooling occurs due to the evaporation of a part of the water entering the system. In this way, water can be cooled down to a lower temperature than the ambient temperature. After the water falls from the packings, the cooled water accumulates in the tower pan and finally returns to the cycle of the project system. The air, which is saturated with moisture, leaves the upper part of the cooling tower. The drip catcher installed on top of the nozzles also prevents water droplets from leaving the cooling tower.

The structure and operation of the open circuit cooling tower can be seen in the picture below.

Specifications and drawbacks of open circuit cooling towers

The open circuit cooling tower is the most typical type of cooling tower that is used for heat removal on a big scale today. This is because open circuit cooling towers have a broad range of capacities and existing structures, low investment costs, and high thermal efficiency.
Open cooling towers typically have a lower initial investment cost for the buyer if the cooling tower is connected to the water chiller and the blockages and sediments produced inside the pipes do not present a particular issue. Additionally, compared to closed circuit cooling towers, open circuit cooling tower operating expenses are lower, at least initially. Nevertheless, the fouling of heat exchangers, condenser tubes, and communication tubes has the potential to considerably raise the overall cost. Because of this, engineers must also take into account the running costs of water filtration and the maintenance costs of descaling, even though open-circuit cooling towers are less expensive to purchase than closed-circuit cooling towers. Since evaporative cooling results from direct contact between water and air, open-circuit cooling towers typically produce cooler water and have better thermal efficiency. In general, it can be said that open-circuit cooling towers work better overall because they can cool the outlet water to a temperature that is close to the environment's bubble temperature based on the design conditions in the specific geographic area. As was previously stated, the water that needs to be cooled in an open circuit cooling tower comes into direct contact with the air once it has entered the cooling tower, making it vulnerable to all of the pollutants in the atmosphere. As a result, it is essential to maintain the open circuit cooling tower's equipment fully and correctly due to the pollution in the air, the pollution that enters the tower through the compensation water, and the presence of absorbed oxygen. The significance of filtering and water purification in the system's circulation is also increased by this problem. Because the open circuit cooling tower's energy efficiency will gradually decline over time due to fouled pipes if the water in the pan is not fully purified and maintained. Therefore, a strict and regular cleaning plan must be taken into consideration to keep optimal heat transfer in open-circuit cooling towers. Additionally, open cooling towers need compensatory water treatment, which raises running costs. 

2. Closed circuit cooling towers
How do closed-circuit cooling towers work?

A heat converter that cools water by indirectly cooling air and water is a closed-circuit cooling tower. To prevent direct interaction between air and water and the resulting issues, closed-circuit cooling towers must cool the water inside coils made of copper, stainless steel, etc. Moreover, after being pushed to the tower's summit, the water in the pan is also sprayed by nozzles onto the water-filled coils. When a cooling tower is closed, some of the cooling that is wanted is accomplished through indirect contact and heat exchange between the air that enters the tower and the water in the coils.

The structure and operation of the closed circuit cooling tower can be seen in the picture below.

Characteristics and drawbacks of closed circuit cooling towers

The use of coils in the closed circuit cooling tower separates the water that needs to be cooled from the water in the cooling tower, and in this way, contact between the water coming out of the cooling tower and the air is prevented. Therefore, in cases where the water that needs to be cooled cannot be in contact with the air (for example, in the food industry, etc.), it is necessary to use a closed-circuit cooling tower.
In a closed circuit cooling tower, due to the indirect contact of the water inside the coil with the air and the lack of need to supply compensating water, no pollution, impurities, or salts are added to the cooling water, and this water (or any other process fluid) in a completely closed cycle It flows cleanly and without any pollution, as a result, if closed-circuit cooling towers are used, deposits will not form on the internal surfaces of the thermal coils inside the tower and other equipment related to the cooling tower, such as condensers and heat exchangers. Therefore, since the use of an open circuit cooling tower can have a significant risk in terms of sedimentation of the equipment in the cooling facility, in areas where the water hardness is very high, the closed circuit cooling tower is the best option. Also, due to the significant reduction of sedimentation and clogging in these towers, the use of closed circuit technology in cooling towers can help to reduce or eliminate the need for physical/chemical purification of water in the cooling tower system.
The overall thermal performance of closed-circuit cooling towers is more stable and because the heat transfer surfaces have less deposition, closed-circuit cooling towers have higher thermal efficiency in the long run. Therefore, some cooling applications need these towers due to the long-term operation of the closed-circuit cooling tower at peak efficiency. For example, buildings equipped with water source heat pumps—which are widely used in office buildings, hotels, and medical centers—are one of the largest applications of closed-circuit cooling towers. It is also common to use closed-circuit cooling towers for information centers, storage units, greenhouses, high-efficiency compression chillers, and various types of process industries to reduce the temperature of process fluids in factories.
In the closed circuit cooling tower, due to the non-evaporation of the cooled water inside the coils, there is no need to add compensating water, and the water waste related to the evaporation process in the closed circuit cooling tower is also reduced, depending on the type of cooling equipment connected to the desired cycle. found or reaches zero. Therefore, water consumption in closed-circuit cooling towers is much less compared to open-circuit cooling towers. For this reason, the use of closed-circuit cooling towers in dry and low-water areas is a more appropriate option, despite their higher investment cost compared to open-circuit cooling towers.
Also, closed-circuit cooling towers usually require less horsepower to pump water than an open-circuit cooling tower of the same capacity. In general, in closed-circuit cooling towers, significant savings are achieved in connection with the reduction of horsepower required for pumping, the elimination of expensive valves, and additional equipment and piping related to the supply of compensation water. In addition, closed circuit cooling towers bring savings in the field of operation such as reducing the need for physical and chemical water treatment, reducing water consumption, and reducing maintenance operations. Therefore, although the investment cost of closed-circuit cooling towers due to having equipment such as heat exchange coils and additional water pumps is much higher compared to open-circuit cooling towers, comparing the total cost of open-circuit cooling towers and tower Closed-loop cooling is not the whole story from an investment cost standpoint alone, and operating costs, maintenance costs, and additional equipment installation costs in an open-loop cooling tower should also be considered when compared from an economic perspective.
Because the cool and clean water provided in the closed circuit cooling tower reduces the need for maintenance and coating in the pipes and all equipment related to the cooling tower, the life of the closed circuit cooling tower is longer. Also, reducing the need for maintenance operations leads to less downtime, which is very important for information centers and some applications that require essential cooling.
Closed-circuit cooling towers also provide advantages for cooling systems by operating in near-zero ambient temperatures. While some types of closed-circuit cooling towers may still require some degree of freeze prevention, all open-circuit cooling towers must include pan heaters and a drain-return or recirculation system for system shutdowns. Be equipped in freezing conditions.
Closed circuit cooling towers can provide dry cooling in cold seasons. Therefore, if the outside weather conditions are favorable (in winter and autumn) so that the cooling operation in the closed-circuit cooling tower can be performed only by the tower fans, the energy flow of the pump that sends the water to be sprayed by the nozzles from the pan to The upper side of the transfer tower can be cut off, and this not only reduces water consumption to a great extent but also saves energy by turning off the circulation pump.
The performance of open circuit cooling towers is mainly dependent on the ambient temperature, and air humidity is not an important factor in the performance of this type of cooling tower. Therefore, unlike open-circuit cooling towers, closed-circuit cooling towers can be used in areas with high relative humidity.
Also, open-circuit cooling towers do not require a lot of area and height for installation and operation, and for this reason, they can be used in environments that have limited space.

You can also read the article CLOSED CIRCUIT COOLING TOWER for more information.

The following factors should be taken into consideration if we want to compare open-circuit and closed-circuit cooling towers briefly.

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