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This article will take an in-depth look at Open Loop and Closed Loop Cooling Towers.
The article will bring more understanding on topics such as:
This chapter will discuss Open Loop and Closed Loop Cooling Towers and their operation.
An open loop cooling tower is a heat rejection device in which there is direct contact between the water to be cooled down and the air.
Open loop cooling tower makes use of direct contact with the air in order to cool down the water. It is essentially a heat exchanger. In these types of cooling towers, there is the partial heat transfer due to heat exchange between air and water. Heat transfer also occurs due to the evaporation of the small amount of water that needs to be cooled. This allows cooling down to a temperature that is below the ambient temperature.
The water to be cooled goes through the upper part of the cooling tower. The water is distributed over the tower packing by the nozzles. Due to the shape of the packing, water will spread into a thin and even film over the packing. This results in an extremely large contact surface (the heat exchange surface).
The fan blows or extracts (depending on the type of fan being used) ambient air through the packing. This air cools the water in two different ways. Due to convection (the contact of hot water and cold air), the heat will be removed, but the cooling of the main part will be due to evaporation. The saturated air with humidity will be exhausted through the upper part of the cooling tower. The cooled water is gathered in the basin so it can be reused in the production process. The water drops are made sure not to leave the cooling tower by the drop eliminators that are above the nozzles.
Closed Loop Cooling Towers are heat rejection devices in which there is no direct contact between the water to be cooled down and the air that is inside the closed loop cooling tower.
In closed loop cooling towers, an additional heat exchanger is employed. This is unlike in open loop cooling towers in which there is direct contact between the water to be cooled down and the air. There are also cooling towers with piping and plate heat exchangers.
Closed loop cooling towers are similar and yet differ from open loop cooling towers. When there can’t be direct contact between the water that needs to be cooled down and the air (e.g. in food industries), it is necessary to employ a heat exchanger. The heat exchanger separates the processed water to be cooled down from the cooling tower’s evaporation water. This prevents the processed water from getting into contact with the air. In closed loop cooling towers, it might be necessary to use antifreeze, whereas in open loop cooling towers antifreeze is unnecessary.
The water to be cooled down is guided through the heat exchanger. This heat exchanger is made out of stainless-steel plates and is situated next to the cooling tower in a separate adjacent room. The transmission of the heat of the water from the process side to the cooling water on the tower’s cooling side occurs inside the heat exchanger. The process water is cooled down again, and, in the process, it can be reused as cooling water. Consequently, circulation of the cooling water occurs in a closed circuit between the heat exchanger and the consumers (condensers, production machines etc.).
When the reheated water leaves the plate heat exchanger, it is directed through the piping to the cooling tower’s upper part. It will be distributed over the tower packing by the nozzles. Through the packing, the cooled down water falls and is gathered in the basin. This is where the fresh water will be guided back to the heat exchanger through the recirculation pump. The fresh water is guided back to the heat exchanger in order to be reused. The water is cooled by the air generated by the fan or fans in counter flow. This air warms up and gets saturated soon after contact with the water that flows over the packing. Through the top of the cooling tower, the air is exhausted. The drop eliminators sit above the nozzles, making sure that there are no water drops that leave the cooling tower.
Various factors can affect the performance of open loop and closed loop cooling towers. These include:
Range is the difference between the temperature of the hot water inlet and the temperature of the cold water outlet at the tower. For instance, a design that demands hot water coming at 100°C and required to be cooled down to 80°C is said to have a range of 20°C. To reduce the capital and energy cost of the tower, an increase in the range is made.
The heat load is determined by the process being served by the cooling tower. The desired operating temperature is the one that controls the degree of cooling that is required. In most scenarios, an operating temperature that is low is desirable in order to increase the efficiency of the process or to improve the product’s quantity or quality. However, high operating temperatures are desirable in applications like internal combustion engines. Increasing the heat load also increases the cooling tower’s size and cost. Process heat loads are difficult to accurately determine because they vary according to the process involved. Refrigeration and air conditioning heat loads are easily determined with more accuracy, on the other hand.
The wet-bulb temperature depicts the site’s temperature condition. To measure it, a thin film of water is placed on the bulb of a thermometer. A reading of a thermometer that is non-wetted provides a ‘dry bulb’ temperature (DBT) reading. A determination of the relative humidity can be made from a comparison of wet and dry bulb readings, using a psychrometric chart or a table of the air properties. The temperature of the wet bulb always stays below the dry bulb value, except in situations when there is a complete saturation of the air with water – a condition called 100% relative humidity. This is when the temperatures of the wet and dry bulb are exactly alike.
A tower cannot cool the hot process water to a temperature that is lower than the temperature of the entering air on the wet bulb. The temperature of the wet bulb is also the dew point of the ambient air. Designing a cooling tower that can provide cooling water that is the same as or lower than the prevailing temperature of the air on the wet bulb is impossible. Each system on a cooling tower must be specifically sized for each geographic area’s prevailing summer wet-bulb temperature. Mechanical draft towers with high efficiency cool down the water to within 5 or 6°F of the wet-bulb temperature. In natural draft towers, the water is cooled down to within 10 to 12°F.
Generally, an assumption is made that the ambient air wet bulb temperature represents the temperature of the entering air measured by the wet bulb. As a matter of fact, this is true only if the tower is sited away from any heat sources that may raise the local temperature. The ambient wet bulb temperature can also be defined as the temperature that is measured from an elevation of 50 feet to an elevation of 100 feet upwind of the tower, without heat sources interfering at the measurement point and the tower. This is done at a height 5 feet above the tower base. This description is fit by very few cooling tower installations.
How closely the temperature of the leaving cold water approaches the temperature of the entering on the wet bulb, is simply termed as the approach. The approach is found by subtracting the ambient temperature of the wet bulb from the cold water leaving the cooling tower. If a cooling tower can produce cold water at a temperature of 86°F the moment when the ambient temperature of the wet bulb is 79°F, then that cooling tower’s approach is 7°F.
The approach is the most important factor in the indication of the performance of a cooling tower. It provides a cut-off to the temperature of the leaving cold-water, regardless of the cooling tower’s size, heat load, or range. It is impossible to cool the water below the air temperature on the wet bulb. It must be noted that when the WBT decreases, the temperature of the leaving water from the cooling tower also lowers. This relationship is linear when flow and range are uniform. The temperatures of the approach generally range between five and 20°F. This implies that the temperature of the leaving cold water shall be 5 to 20°F more than the ambient WBT regardless of the heat load’s size or the cooling tower’s size.
As the opted approach is lowered, the size of the tower increases exponentially. It is not economical to opt for a cooling tower of approaches below 5ºF and there is no guarantee in performance for approaches less than 5ºF from manufacturers.
Approach = CWT – WBT, where CWT is the cold water temperature and WBT is the wet bulb temperature
Range = HWT – CWT, where HWT is the hot water temperature
Cooling tower efficiency = range / (range + approach) * 100
This chapter will discuss the components of open loop and closed loop cooling towers together with their functions.
Most open loop and closed loop cooling towers consist of the following instrumentation systems: blow down rate; flow meters for cooling tower makeup water; water level switches for hot and cold water basins; vibration switches; high and low level switches; thermocouples for the measurement of the temperature of hot and cold water; and high and low oil level switches.
Explosion proof fan motors are required by refinery and petrochemical cooling tower applications due to the potentially leaky heat exchangers. Fan motors must be provided with protection systems for overload relay and earth fault relay.
Most of the cooling tower nozzles are made from plastic. These types of plastics include polypropylene, ABS, PVC plastics, glass filled nylon. Nozzles allow for the uniform distribution of hot water that is inside the cell of a cooling tower.
These valves regulate the flow of hot water to evenly distribute in the cells. These valves are manufactured in such a way that they stand up against corrosive environments.
They transmit power to the gear reduction unit’s input shaft, from the motor’s output shaft.
They reduce the magnitude of the speed depending on the requirements of the cooling tower. The gear reducer, motor and driveshaft are permanently alighted by the torque tube.
Cooling tower louvers are made out of asbestos sheets. Louvers have two functions which are: (i) to retain the water circulating within a cooling tower (ii) to distribute the flow of air into the fill media equally.
This is simply a supporting platform for the fan cylinders and it also provides an accessible way to the fan and the water distribution system.
It requires underground burial or it needs to be supported on the ground to prevent thrust loading of the cooling tower that is caused by the pressure of the water in the pipe and self weight.
These are one of the main parts of open loop and closed loop cooling towers. The following are materials that most cooling tower fans are made out of: class fiber; hot-dipped galvanized steel; fiber reinforced plastic (FRP); aluminum. Fiber reinforced plastic is one of the favorable choices because of its lightweight and reduces the energy requirements (the consumption of energy) of the cooling tower fan. The cooling tower fan blades’ pinch blade angles vary according to the season. For instance, the pinch angle is increased during the summer season, when the level of the density is low that it cannot permit an increase in the capacity of the fan.
Chemically treated wood is used to make most of the open loop and closed loop cooling tower structures. Some cooling towers are now also being manufactured from FRP and reinforced cement concrete depending on the cooling tower’s application.
There are two functions that are served by cold water basins and they are normally made out of RCC. One of the two functions of the cold water basin is to act as storage and collecting water from the tower. The other function of the cold water basin is to provide the main structure and the foundation of the cooling tower itself. Cold water basins are usually mounted below the ground level or on top of the soil. The elevation of the open loop and closed loop cooling tower is found by measuring the distance between the fan assembly and the water basin’s top.
Drift eliminators serve to lower the quantity of water that escapes into the discharge air inside the cooling tower. Drift eliminators project air in various directions and avoid unnecessary loss of water. They are made out of PVC. More passes through the drift eliminator cause the amount of drift loss to decrease, while also elevating the pressure drop, increasing the fan power consumption. In large industrial applications, heavy duty drift eliminators are used.
The open loop and closed loop cooling tower fill media puts air in contact with a water surface area accordingly. The fills cause the water to form in thin flowing sheets so that a large amount of the surface area of the water interacts with the air flow. The fill media is manufactured from the following materials: polypropylene, wood, or PVC. Fill media exist in three different forms: vertical offset fill; cross corrugated fill; and vertical fill.
This chapter will discuss the various types of closed loop cooling towers and open loop cooling towers.
The types of closed loop cooling towers include:
These types of closed loop cooling towers function in a manner that is similar to that of dry cooling systems, but with pre-cooling pads in addition. When water flows over porous media, the air is drawn through the pads.
These types of closed loop cooling towers are most suitable when the key considerations are water conservation and reduced maintenance. They eliminate water treatment since they do not utilize any water.
These types of closed loop cooling towers increase efficiency by combining dry and evaporative cooling while simultaneously reducing water consumption.
This type of closed loop cooling tower eliminates the need for a heat exchanger between the heat rejection equipment and the process loop. These types of cooling towers provide energy-efficient operation in a footprint that is reduced compared to dry coolers, because of evaporation being used as the main method of cooling.
Since blow down of the basin water is minimized in closed loop systems, there is an improvement in the conservation of water compared to open loop systems.
In dry mode, these units provide heat rejection until the load exceeds the dry rejection capacity. During this switch-over, the unit takes the evaporative mode, increasing the cooling capacity.
This lowers the temperature of the incoming air on the dry bulb. This in turn causes greater heat rejection. As a consequence, adiabatic systems are most suitable for hot dry environments and they use less water.
The types of open loop cooling towers include:
This type of cooling tower is the most suitable for industrial applications. In this type of open loop cooling tower, the air flows in a horizontal plane through the fills while the water flows down in a vertical plane.
As indicated by the name, the fan-less, fill-less cooling tower does not consist of a fill or fan to be used to cool the waste heat water. Instead, it makes the outside wind pass through its cooling media.
It consists of louvers made of wood, which functions as the sidewall that restricts the spillage of water. This type of open loop cooling tower is the most favorable choice economically and it requires low maintenance cost among the different types of cooling towers.
It is mostly used in dirty water applications like an oil refinery and chemical process cooling where contamination of water with ammonia compounds, fats, oil, and other contaminants can take place.
The field erected cooling tower is available for those industries or manufacturing plants that cannot find the right standardized design of cooling towers for their specific needs. This type of open loop cooling tower is custom made. It is constructed using pultruded fiberglass and it uses steel as fasteners, fiberglass reinforced polyester sheets for cladding, pultruded FRP sections.
The round or bottle cooling tower is famously known for its magnificent technology and outstanding compact design. It can be found in a variety of flute sizes. Due to its round shape, it can allow air to flow to get an even distribution and it also offers maximum heat transfer per contact surface area and unit volume.
This type of open loop cooling tower uses the counter flow induced draft technology and consists of a cross corrugated PVC film filled transfer media in its design. It is made out of fiberglass-reinforced plastics. It is usually fabricated and designed at the water cooling towers manufacturers’ factory and is assembled at the site of the job only.
This is perhaps the most famous cooling tower. This tower also utilizes the same technology used by the round cooling tower- the counter flow-induced draft technology. The heat transfer media in this cooling tower are across PVC film fills that are corrugated.
The rectangular cooling tower is made out of fiberglass reinforced plastics while its architectural components are made out of mild steel or hot-dipped galvanized steel. This type of cooling tower is available in both single celled and multi-cell.
This type of cooling tower can be used for both cooling tower projects, new and old. FRP offers more advantages compared to the traditional materials used in the construction like wood, concrete, and steel.
This chapter will discuss the applications, benefits, and efficiency improvements of open loop and closed loop cooling towers. Considerations when choosing an open loop and closed loop cooling tower will also be discussed.
The applications of open loop and closed loop cooling towers include:
The benefits of open loop cooling towers include:
The benefits of closed loop cooling towers include:
To improve the efficiency of open loop and closed loop cooling towers the following can be considered:
Installing a new pipe where it is needed will help improve energy efficiency, even if it is one piece of new piping needed.
A cooling tower must be recycling upward 98% of water. If it does not meet this requirement, it must be serviced. Recycling water improves water and energy efficiency.
You must be aware of the number of cooling cycles your tower uses. If the cycles are adjusted from three to six, it can make a significant difference in efficiency and water usage is saved.
The various considerations to be cognizant of when selecting an open loop and closed loop cooling tower include:
In closed loop cooling towers, the coefficient of heat transfer between cooling, water, and the process can be regulated at its highest design conditions; technically the coefficient of heat transfer will be a maximum with closed loop cooling towers due to the use of clean water for cooling. In some types of closed loop cooling towers, an intermediate heat exchanger can also be separately installed. This is done to allow for the ease in cleaning and to reduce capital costs. If a foul occurs in the heat exchangers, it shall only be associated with the intermediate heat exchanger which is directly taking part in the heat rejection, and this can be easily managed.
In open loop cooling towers, a lower approach can be easily achieved. A note must be made that, in open loop cooling towers, two approaches need to be counted; one with the cooling tower and the other with the heat exchanger. In the lower approach case, the open loop cooling tower will be beneficial.
Return head can be used effectively since there is no exposure of cooling water to the atmosphere in closed loop cooling towers. The head that is needed for the circulation of the cooling water shall only be the resistance of the heat exchanger and the frictional head. The static head needed by the pump can be totally eliminated. Since the cooling water used for heat transfer is not exposed to the atmosphere, corrosion and scaling problems can be eliminated.
Because of two circuits in the case of closed loop cooling towers, water volume is low for water treatment and it can be effectively managed.
In closed loop cooling towers, corrosion is totally eliminated from process heat exchangers since the water used for cooling is not exposed to the atmosphere directly. Because of the non-exposure of the cooling water to the atmosphere in closed loop cooling towers, the water does not get contaminated with airborne particles and the heat exchangers are protected from corrosion and other water related problems.
While the maintenance that a standalone open loop cooling tower requires is low; considering the full cooling system that includes pipe and heat exchangers, the open loop cooling tower maintenance is much higher than that required by closed loop cooling towers.
Since both the open and closed loop cooling towers operate based on the principle of water evaporation. For an equal amount of heat load for a cooling tower, both types of cooling towers need the same quantity of water. With a small difference in the design, an advantage of air/dry cooling can be offered by the closed loop cooling tower to reduce the water consumption rate.
Open loop cooling towers require a low amount of capital due to the unavailability of the intermediate heat exchanger.
The cost of operation in closed loop cooling towers is low due to the improved consistency in approach, reduction in power required for pumping and improved overall approach.
Open loop cooling towers can be expanded easily, while closed loop cooling towers require high skills in the design because of the use of an intermediate heat exchanger.
Each class of cooling tower, either open loop or closed loop, has different types of designs with different capabilities and advantages. Therefore when picking an open loop or closed loop cooling tower for a specific application, one must consider the design specifications that meet the application requirements.
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