Applications of Dry and Wet/Dry Cooler Systems

Applications of Dry and Wet/Dry Cooler Systems

Dry Coolers:

Another shell and tube heat exchanger method used in water cooling is the system called Dry Cooler. The basic principle is to transfer the return water load in the system to air by the aid of a heat exchanger system including fans. Its working principle is that the air sucked by fans cools the fluid within the tube while it passes through the fins. In this method, the exterior surface of the exchanger is dry. In this case, problems such as calcification and corrosion do not exist. Thanks to closed circuit operation of the system, the problem of diminishing cooling water is not observed.

Measures should be taken against freezing in Dry Coolers for winter months. Otherwise, the damage to the pipes due to freezing of the internal fluid will be irreparable. In our country, the instances where Dry Coolers rendered unusable by freezing require complete replacement are quite common. The commonly used measure against the risk of freezing is purging the water inside the Dry Cooler in cold weather conditions when the system is not used. Still, it is not possible to completely flush the water inside the Dry Cooler, due to the piping structure; antifreeze (ethylene glycol) must be added to the cooling fluid in an adequate percentage.

The glycol to be added to the cooling water must also be taken into consideration for the selection of radiators. Dry Cooler design should be made with respect to water containing 25%-35% glycol. Otherwise, the decrease in cooling capacity caused by the glycol which is added to water will cause the performance of the Dry Cooler to fall below expectations. Therefore, the value of the cooling capacity of the Dry Cooler, is nor meaningful in the absence of the design conditions and glycol-water ratio.

  

The water temperature achieved in Dry Coolers depends on the ambient dry bulb temperature; water cooled to approximately 5 ºC above dry bulb temperature can be generated. In cased where cooling water at lower temperatures is needed, Wet-Dry Coolers are used.

These systems operate under the same principle as the previously explained Free Cooling coil systems. If a chiller has already been installed in the plant and if it is wished to take advantage of Free Cooling in low ambient temperatures, dry cooler systems are ideal for this case. While dry coolers in applications within the plastics, chemicals, energy, air-conditioning etc. sectors in tandem with a cooling group, they can also be used by themselves depending on the water cooling requirements.

Wet/Dry Coolers:

Wet-Dry Coolers operate on the same basic principle as Dry Coolers. There is a water sprayer system which provides additional cooling upon demand. The sprayed water leads to adiabatic cooling of the inlet air flow. When the fluid in the system must be cooled to a lower temperature than outdoor ambient temperature, the pressure water sprayer system steps in, saturating the inlet air with moisture and lowers air temperature below ambient temperature.

The water system, by becoming active by thermostat control to provide additional cooling in only specific hours of the hottest days of summer, removing the need for an auxiliary cooling system for special circumstances. Since dry operation will be in effect at other times, the system does not have water consumption. The spryer system can also be applied to condensers of air cooled water cooling groups as well as dry coolers. Wet-dry coolers can be applied in three different ways whose basic principles are the same:

Direct Water Spray System Wet-Dry Coolers:

The water used in the sprayer system should be decalcified and filtered; otherwise the lime and sediment accumulating on exchanger fins will gradually diminish the capacity and shorten the period of usefulness of the exchanger. In order to prevent this effect, the Mesh and Spray System Wet-Dry Coolers were developed. In Wet-Dry coolers, epoxy coated fins should be used as an additional precaution against the corrosive effect. The epoxy coating has a quite high resistance to the salt and acid in the environment. The materials preferred for the unit are epoxy dust painted galvanized sheet and for highly corrosive environments stainless steel. Although Wet-Dry Coolers are exposed to the harmful effects of water to a much lesser extent than water towrs, these precautions are crucial for longevity of these units.

The amount of water spent in order to saturate the inlet air with moisture varies with the sprayer used and the pressure of spraying. The selection depends on the relative humidity of the ambient air; care must be taken to make sure the quantity and quality of the sprayed air is adequate for bringing the relative humidity as close to 100% as possible. For this reason, more water than will completely vaporise is sprayed and the remaining water stays in the environment in the liquid form. This precaution also provides security against losses which may evolve in time in the performance of the spraying system.

Figure 10. Direct water spray system wet-dry coolers

Fogging System Wet-Dry Coolers:

In this application which is similar to direct water spraying systems, the droplets of water smaller than 35 microns that are sprayed under high pressure from nozzles saturate the inlet air with moisture and bring it closer to the wet bulb temperature. In this system, as in the spraying system, the water used should be decalcified and filtered.

Mesh and Spray System Wet-Dry Coolers:

The mesh and spray system involves the spraying of the amount of water that is required by the system from nozzles placed on specific positions on the wide and fine mesh material located on the front of dry coolers, and lowering the inlet air temperature coming in contact with the heat exchanger surface by the adiabatic vaporisation of the sprayed water, thus increasing efficiency of cooling.

As explained below, the sprayed water leads to adiabatic cooling of the inlet air flow. As the specified set values are exceeded, the control system initiates the water spraying system to lower the temperature of the air entering the heat exchanger. In very arid climates, the water spraying system can provide adiabatic cooling of the inlet (ambient) air which borders on 15˚C to 20˚C. The period of operation and frequency setting of the water spray system is continuously maintained by the controlling unit in order to achieve optimization of system performance and minimization of water consumption. Since the water is not directly sprayed on the heat exchanger surface, but rather on the mesh surface, furring does not occur on the fins. In this way, drops in heat transfer efficiency are avoided. This system also renders any water softening process superfluous.

Another point which requires consideration in Dry- Wet/Dry Cooler selection is the necessity to assure the adequacy of the design to provide the cooling capacity required in conditions of high ambient temperature. However, in periods where ambient temperatures are low, operating all fans at maximum rotation to achieve the desired capacity will be superfluous and costly. In systems monitored by cooling water outlet temperature, operating fans at low rotation or disabling them will provide an air supply of sufficient flow to the system. The use of double rotation fans, speed control devices, and electronically controlled EC fans with automatic control will enable additional power saving for the system.

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