Differences between closed-type cooling towers and open-type cooling towers
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1. The cooling principle of an open-type cooling tower is to spray circulating water in the form of a mist onto the fiberglass filler; through the contact between water and air, heat exchange takes place. Additionally, fans are used to circulate the air inside the tower, carrying away the heated air that has exchanged heat with the water, thereby achieving cooling. With this cooling method, the initial investment is relatively low, but the operating costs are high (water and electricity consumption). 2. The cooling principle of a closed-type cooling tower is, in simple terms, based on two cycles: an internal cycle and an external cycle. Without fillers, the main core part is a copper tube surface cooler. ①Internal circulation: It connects with the target equipment to form a closed-loop system (the circulating medium is soft water). Cool the target device by removing heat from it to the cooling unit. ②External circulation: In a cooling tower, it is used to cool the cooling tower itself. It does not come into contact with the internal circulating water; heat is dissipated only through the copper tube surface coolers inside the cooling tower. Under this cooling method, the operation of the motor is controlled automatically based on the water temperature setting. Two cycles are required; when the environmental temperature is high in spring and summer, both cycles need to operate simultaneously. In autumn and winter, the ambient temperature is not high, and in most cases only an internal circulation is required. A closed-type cooling tower, also known as a sealed cooling tower or a closed cooling tower, is simply referred to as a closed tower. The closed-type cooling tower originated from the evaporative cooler; it is actually a heat exchanger that combines the properties of a water-cooled cooler and a conventional cooling tower. It represents a type of heat exchanger that lies between water coolers and air coolers, which is why some manufacturers refer to it as an “evaporative air cooler”. There are now many types of such cooling equipment, all of which share the common feature of spraying water outside the shell-and-tube heat exchanger and employing forced ventilation. Heat is transferred from the fluid being cooled inside the shell-and-tube heat exchanger to the water sprayed outside via the wall surfaces, and then to the air through forced convection between the sprayed water and the air. The heat transfer from the sprayed water to the air is primarily due to the latent heat of vaporization of the water and the sensible heat exchange between the water and the air. Since the cooled fluid flows in a closed cycle between the partitioned heat exchanger and the external process equipment, it is referred to as a \"closed tower\" to distinguish it from ordinary cooling towers where the cooled fluid comes into direct contact with air; accordingly, ordinary cooling towers are called \"open towers\".
Cooling towers typically use electric motors driven by couplings, drive shafts, and reducers to operate the fans within the tower. These fans help to dissipate heat quickly from the water that enters the cooling tower. The water is then pumped under pressure and sent to the equipment that requires cooling; after being used, it returns to the cooling tower for further cooling, thus enabling the recycling of cooling water. The circulation of industrial cooling water between heat exchange equipment and cooling towers is driven by water pumps. In terms of design, manufacturing, selection, and operation, various factors such as reliability are taken into account, which results in the water pumps in this system having a large excess of head and flow rate. This is evident in the following aspects: 1. It is difficult to calculate the exact amount of water in each circulating water system. When calculating the system’s water flow rate, process engineers consider safety and other factors, and add at least 10%-20% extra to meet the maximum required flow rate, thereby ensuring that there is an adequate amount of water in the entire system. 2. Throughout the entire circulating water system, each section of piping and each elbow presents a certain level of resistance. The height at which the cooling tower is located, as well as the resistance and pressure requirements of the heat exchange components, all contribute to the resistance present in the system. It is not possible to calculate these resistances with high precision; therefore, the resistance values calculated by process engineers are only approximate figures. When selecting a pump based on these values, it is necessary to ensure that the pump can meet the production requirements safely, which means adding a margin of at least 10%-20% on top of the calculated resistance values when determining the pump’s head pressure – the head pressure throughout the entire circulating system must be sufficient. But all of this energy was wasted needlessly in the system in the past. The turbine makes full use of this excess energy to drive the fan to rotate. As the name implies, a water-driven fan is one that uses hydraulic power to operate the fan, rather than traditional electrical power. In water-driven fan cooling towers, a turbine is used in place of an electric motor as the power source for the fan. The driving force for the turbine comes from the excess flow rate and excess head of the system. After the modification, the hot water supplied by the water pump passes through the turbine, causing it to rotate. The output shaft of the turbine is directly connected to the fan, thereby driving the fan to rotate.
A cooling tower is a type of circulating heat dissipation device that absorbs the waste heat generated during the production process through circulating water. This water is sprayed onto fillers inside the tower, where it comes into contact with the atmosphere and releases its heat into the air, thereby reducing the temperature of the circulating water. By mixing and bringing together hot and cold fluids within the tower, and utilizing the temperature difference, the cooled object in contact with the cooling water causes the water vapor to evaporate into the air, thereby achieving self-cooling. What is introduced below are the standards for controlling the temperature difference between the inlet and outlet water of cooling towers: The temperature difference between the inlet and outlet water of a cooling tower is one of the most effective ways to assess its cooling efficiency, and it is also an important parameter in selecting cooling towers. Under normal circumstances, the standard design conditions for civilian cooling towers are an inlet water temperature of 37°C and an outlet water temperature of 32°C, resulting in a temperature difference of 5°C between the inlet and outlet waters℃ ； The design operating conditions for industrial cooling towers are generally divided into several ranges such as 65°C–45°C, 43°C–33°C, and 40°C–32°C; the temperature difference between the water entering and leaving the tower can range from 8°C to 20°C. During operation, it is essential to maintain unobstructed flow in all parts of the cooling system; blockages must be avoided at all costs. Common types of blockages include those in the pipes and those caused by excessive scale buildup in the packing. If a blockage occurs, it is best to remove it as soon as possible to ensure smooth water flow through the cooling tower, thereby enabling effective cooling.
The generic term “cooling tower” is used to describe direct (open-loop) and indirect (closed-loop) heat dissipation equipment. Although most people think of a “cooling tower as an open, direct-contact heat dissipation device,” the indirect cooling tower, sometimes referred to as a “closed-circuit cooling tower,” is also a type of cooling tower. A direct, or open-loop cooling tower is a system with a sealed structure, in which circulating water is sprayed in the form of mist onto fiberglass fillers. The filler provides a larger contact surface, enabling heat exchange through the contact between water and air. Furthermore, fans drive the air flow within the tower to circulate, carrying away the hot air that has exchanged heat with water, thereby achieving cooling. The filling may consist of multiple layers, primarily vertical; the water that enables the spread of wet surfaces (filling), or horizontal splash elements that create numerous small droplets with a large surface area, forming several layers of thin films (splash). Indirect or closed-circuit cooling towers do not involve direct contact between air and a liquid, usually a mixture of water or ethylene glycol, for cooling purposes. The difference lies in the open-type cooling tower, which has two separate fluid circuits. One is that the water in the external circuit is in the second channel; it is the heat fluid flow connected to the external circulation of the tube bundle (non-public coil) that is cooled and returned in a closed loop. Air is circulated to draw heat from the water cascaded around the entire heat pipe, providing a cooling effect similar to that of an open evaporation cooling tower. The heat flow in operation moves from the internal fluid circuit, through the coil tube walls, to the external circuit, and then is heated by some evaporation of air and water before reaching the atmosphere. The actions for indirect cooling towers are therefore very similar, with one exception regarding opening the cooling tower. This process is carried out by a coolant in a \"closed\" circuit, not exposed directly to the atmosphere or external circulating water. In a counterflow cooling tower, air travels upward through the fill or tube bank, while water moves downward in the opposite direction. In a cross-flow cooling tower, air moves horizontally while water moves downward through the fill. Cooling towers have another feature: mechanical ventilation cooling towers, which use aerial means of transport, rely on electrically driven fans to draw in or force air into the tower, thereby promoting energy conservation and environmental protection. The rapid growth of exhaust chimneys used in naturally ventilated cooling towers provides an air buoyancy draft. Fan-assisted natural ventilation cooling towers use mechanical draft to enhance the effect of buoyancy. Many early cooling towers relied on air drafts generated by wind direction. If the cooling water returns from the cooler for reuse, some water must be added to replace or make up for the portion that has evaporated during flow. Since evaporation involves pure water, the concentration of dissolved minerals and other solids in the circulating water tends to increase, unless some dissolution occurs, such as through solid control measures. Some water is also lost along with the droplets carried by the exhaust gases (drift), but this can usually be reduced to a very small amount by installing baffle-like devices known as drift eliminators, which collect the droplets. The amount to be made up must equal the losses due to evaporation, drift caused by wind, and other water leakage issues, in order to maintain a stable water level.