Why is spray cooling tower Better?

Industrial plants must often cool high-temperature wastewater prior to biological treatment. Process water also may need cooling to reach permitted discharge levels or to cool intake process water.

For more information, please visit spray cooling tower.

Cooling towers are often used to dissipate the heat duty associated with these streams. Still, they have a higher initial cost, are more expensive to maintain and usually require chemicals for prevention and removal of scaling. Cooling ponds are another alternative but can require as much as 20 times the land space as ponds equipped with spray coolers or other spray cooling techniques, which may prove too expensive or impractical.

Floating spray coolers and surface aerators can provide evaporative cooling of industrial wastewater as an economic alternative or supplement to either cooling towers or cooling ponds. These mechanical cooling devices are quick and easy to install and don’t require chemical addition to prevent scaling or clogging. Both spray cooling options are even more cost-effective when existing ponds are available for equalization and cooling.

Spray Coolers vs. Surface Aerators

So when should spray coolers be used instead of surface aerators? Capital costs for a spray cooler tend to be greater than for a surface aerator, with a 75-hp spray cooler costing about five times as much as a surface aerator of the same horsepower. Applications that require a high rate of heat dissipation are good candidates for spray coolers. For applications with a relative low heat duty, spray coolers are too expensive and cannot be justified economically.

Some crossover does exist, however, depending on the application. The cooling capacity of surface aerators is about 10 times less than spray coolers for a given set of operating conditions. Therefore, surface aerators make sense for applications that are borderline in terms of operating temperature and only need a relatively small amount of cooling, particularly during summer operating conditions.

Similarly, paper mills sometimes require spray coolers ahead of their secondary wastewater treatment process to optimize biological treatment performance. In this case, biological treatment wouldn’t be possible downstream unless the water was first cooled below 95°F (35°C) ahead of secondary treatment.

How Spray Cooling Works

Spray cooling is a dynamic process that depends on site conditions that change from day-to-day and throughout any given day. Most of the heat removed in any spray cooling system is removed by evaporation. For instance, under summer operating conditions, 85-90% of cooling is achieved in this manner.

The primary purpose of a spray cooling device is heat dissipation. It’s difficult to generalize the heat dissipation capacity of any type of device since design conditions vary so widely. Wind velocities, wet and dry bulb temperatures, liquid flow, and liquid temperature all affect heat dissipation.

To calculate cooling performance, the basic design parameters of the wastewater system must be established. The maximum or design flow rate should first be determined. The critical design point is at maximum flow, maximum inlet temperature and maximum wet bulb temperature. If the cooling system performs under these conditions, then power consumption can be cut during less severe conditions. Unit size and multiple-unit design should be considered to achieve near optimum power savings as well as minimum capital costs.

In addition to the above, other factors affecting spray cooling efficiency include target effluent temperature, influent hot water temperature relative to the wet bulb, surface area of the spray, relative velocities of the air and water during contact, and contact time of the water droplets in the air.

Industrial Applications

Floating spray coolers and surface aerators can be used in pulp & paper mills, power plants, steel mills, food processing plants and other industrial applications to meet cooling requirements.

Floating spray coolers are an economic option for cooling process water or wastewater from industries, especially during peak loadings. The spray cooler concept employs a multiple nozzle assembly supported on a floating platform. A pump-manifold-nozzle design produces effective heat transfer while the floating platform offers quick, flexible installation. Heated water is sprayed many times into the air to achieve the desired heat dissipation, cooling the water to within about 9-10°F (-13 to -12°C) of the actual wet bulb temperature with continuous influent flow to the cooling pond.

Spray coolers provide the optimal design for peak heat transfer efficiently, with low power consumption (individual units can be cycled on/off to save energy) and less drift loss than with wet cooling towers.

Compared to cooling towers, capital costs for spray coolers are lower with existing ponds and operating costs are lower as chemical treatment is not needed to prevent bacteria growth. Spray cooler technology also does not provide an environment conducive to Legionella bacteria growth, which has been discovered inside cooling towers and released via the cooling tower mist. While spray coolers do require more land than wet cooling towers, their footprint is less than 5% of what cooling ponds consume.

Contact us to discuss your requirements of sprinkler cooling tower. Our experienced sales team can help you identify the options that best suit your needs.

Why Are Nozzles Important?
Everyone knows how important the fill is to a cooling tower’s performance. Since fill is the heat transfer medium around which the “box” that is the cooling tower is built, you could argue that it’s the most important factor in determining just how well your cooling tower performs. However, since the nozzles are tasked with getting the hot water that you want to cool delivered to the fill, the nozzles also play a very important part in the duty of a tower. Nozzles are involved in the following aspects of a cooling tower’s performance evaluation.

Water Distribution
Of course, the most obvious impact of a nozzle is due to its primary job of getting the hot water to the fill. Ideally, the nozzles are located and sized so that they provide a uniform pattern of water over the fill with no gaps between their spray areas. You’ve spent a “few dollars” to put fill into your cooling tower, so now you need to make sure that you utilize it to its fullest extent by making sure that it is totally wetted out and all sections of fill receive a constant amount of water. All published performance data for a fill is based upon this assumption.

Gaps in spray coverage can lead to air bypass because dry fill offers lower resistance to airflow, and air will always take the path of least resistance. This will result in airflow being stolen from wet fill sections where it is needed to cool the hot water. Also, since air and water are both fluids, the gaps are not 100% constant in their location. As the competing fluids interact with each other they tend to pulse and shift around. This causes some of the fill to experience alternating cycles of wet and dry times which will promote scale formation within the fill. As scale builds up in a fill it starts to adversely affect its thermal performance capability, and in the extreme, can totally block airflow from the fill and potentially cause structural damage to the cooling tower if the fill’s weight gain from the scale is excessive.

Spray Zone
Another way that the spray system affects a cooling tower’s thermal performance is from the cooling that happens as soon as the water exits the nozzle and falls to the fill. The cooling effect in this spray zone can vary from 5-15% of the total cooling provided by the tower. The key to cooling here is to maximize the interaction between the hottest water and the coldest air. Thus, a nozzle that generates very small droplets will have a greater cooling effect than a nozzle that makes big, fat droplets. This is similar to the cooling effect of walking through a fine mist on a hot summer day as opposed to being hosed down by a fire hose.

Drift Eliminators
With ever-increasing scrutiny being paid to drift emissions from cooling towers, you have to be aware of the role that the nozzles can play on drift. Unfortunately, the fine spray of very small droplets that is beneficial from a cooling perspective creates a much more difficult challenge in preventing those same very small droplets from exiting the cooling tower as drift. This can be made even worse if the circulating water has very low surface tension due to water treatment that includes surfactants or due to compromised make-up water sources from wastewater treatment plants, such as California Title 22 water.

As the surface tension of the water decreases, it causes smaller and smaller droplets to be formed. When you combine this with a pressurized water distribution system, especially if you are overpumping it and have a higher than expected pressure at the nozzles, you can actually generate a mist. As mentioned in the November 2015 edition of The Cooling Quarterly, mist droplets can be a couple orders of magnitude smaller than the typical drift droplets for which drift eliminators are designed to capture, and drift eliminators lose their efficiency if the droplets are too small.  (This is why there are mist eliminators, which are designed differently from drift eliminators because of the greater degree of difficulty in droplet removal).

Share Article:

Are you interested in learning more about motor cooling tower? Contact us today to secure an expert consultation!