Showing posts with label evaporative cooling. Show all posts
Showing posts with label evaporative cooling. Show all posts

Friday, October 20, 2017

Wet Bulb Temperature and Cooling Tower Performance

corrosion resistant cooling tower induced draft type
Corrosion resistant evaporative cooling tower
Image courtesy Delta Cooling Towers, Inc.
Evaporative cooling towers enable many buildings across the globe to enjoy moderate interior temperatures. They serve as the final heat transfer step that moves heat from the building interior to the surrounding environment. In addition to their extensive application throughout large residential, commercial and industrial HVAC systems, their are numerous process cooling applications that employ evaporative cooling towers as an effective means of heat rejection.

Delta Cooling Towers, Inc. is a globally recognized manufacturer of corrosion resistant cooling towers, air strippers and tanks fabricated of HDPE to provide extended life service. The company posted an article entitled "Understanding Wet Bulb Temperatures And How It Affects Cooling Tower Performance". The original post is on this page of the company website, and all credit for the article goes to them. We share it below also, slightly edited for format on this forum.  From the article...

A cooling tower primarily uses latent heat of vaporization (evaporation) to cool process water. Minor additional cooling is provided by the air because of its temperature increase. Cooling tower selection and performance is based on water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb temperature. Ambient wet bulb temperature and its affect on performance is the subject of this article. Ambient wet bulb temperature is a condition measured by a device called a psychrometer. A psychrometer places a thin film of water on the bulb of thermometer that is twirled in the air. After about a minute, the thermometer will show a reduced temperature. The low point when no additional twirling reduces the temperature is called the wet bulb temperature. The measured wet bulb temperature is a function of relative humidity and ambient air temperature. Wet bulb temperature essentially measures how much water vapor the atmosphere can hold at current weather conditions. A lower wet bulb temperature means the air is drier and can hold more water vapor than it can at a higher wet bulb temperature. For example:
Since cooling tower cells cool water by evaporation, the wet bulb temperature is the critical design variable. An evaporative cooling tower can generally provide cooling water 5° - 7° higher above the current ambient wet bulb condition. That means that if the wet bulb temperature is 78°F, then the cooling tower will most likely provide cooling water between 83° - 85°F, no lower. The same tower cell, on a day when the wet bulb temperature is 68°F, is likely to provide 74° - 76°F cooling water. When selecting a cooling tower cell, the highest or the design wet bulb temperature your geographical area will encounter must be used. Highest wet bulb temperatures occur during the summer, when air temperatures and humidity is highest. For example, in Indianapolis, Indiana, the design wet bulb temperature is 78°F. Historically Indianapolis can expect less than one hour per year that the conditions exceed a 78°F wet bulb. Typically, 6,000 hours a year will have a wet bulb of 60°F or lower meaning that a cooling tower cell designed for a 78°F wet bulb will be able to make 65-67°F water for 6,000 hours per year nearly 70% of the year. Most cooling towers are capacity rated at a "standard" wet bulb temperature of 78°F. That means on the days when the wet bulb temperature is 78°F, the tower will produce its stated capacity. In other words, a tower rated to produce 135 tons of cooling will produce 135 tons of cooling at a 78°F wet bulb temperature. At a higher wet bulb temperature, the tower cell capacity to produce colder water decreases. Every location has a unique design (worst case) wet bulb temperature that is published by organizations such as ASHRAE and can be obtained easily.

What does it mean when your cooling tower water temperature is higher than the normal 5-7°F above the current wet bulb temperature?

  1. Your cooling load may be larger than the rated capacity of your cooling tower.
  2. Your cooling tower may have lost efficiency
  • Due to scale build up on the tower heat exchange surfaces.
  • Due to loss of air flow across the heat exchange surfaces.
  • Due to improper water flow from clogged nozzles or pump performance
What can you do to improve your tower performance? 

  • Add tower cell capacity
  • Check for the efficiency losses described above
  • Replace the heat exchange surfaces with new clean fill
  • Check for proper airflow
  • Check the water flow is at design
  • Check that nozzles are not clogged or broken

Cooling tower performance is tied to ambient wet bulb conditions. Higher wet bulb temperatures occur in the summer when higher ambient and relative humidity occurs. Initial system design and proper system maintenance is critical to be certain your cooling tower is providing desired cooling.

For more information, or to discuss your own heat transfer challenges, contact a product application specialist. Combine your own knowledge and experience with their application expertise to develop an effective solution.

Wednesday, August 23, 2017

Cooling Towers: Operating Principles and Systems

evaporative cooling tower made of HDPE plastic
Example of evaporative cooling tower, fabricated
from HDPE plastic to resist corrosion.
Image courtesy Delta Cooling Towers
The huge, perfectly shaped cylindrical towers stand tall amidst a landscape, with vapor billowing from their spherical, open tops into the blue sky. Such an image usually provokes a thought related to nuclear power or a mysterious energy inaccessible to the millions of people who drive by power plants every day. In reality, cooling towers – whether the hyperboloid structures most often associated with the aforementioned nuclear power plants or their less elegantly shaped cousins – are essential, process oriented tools that serve as the final step in removing heat from a process or facility. The cooling towers at power plants serve as both an adjuster of a control variable essential to the process and also as a fascinating component of the process behind power creation. The importance and applicability of cooling towers is extensive, making them fundamentally useful for industrial operations in power generation, oil refining, petrochemical plants, commercial/industrial HVAC, and process cooling.

In principle, an evaporative cooling tower involves the movement of a fluid, usually water with some added chemicals, through a series of parts or sections to eventually result in the reduction of its heat content and temperature. Liquid heated by the process operation is pumped through pipes to reach the tower, and then gets sprayed through nozzles or other distribution means onto the ‘fill’ of the tower, reducing the velocity of the liquid to increase the fluid dwell time in the fill area. The fill area is designed to maximize the liquid surface area, increasing contact between water and air. Electric motor driven fans force air into the tower and across the fill area. As air passes across the liquid surface, a portion of the water evaporates, transferring heat from the water to the air and reducing in the water temperature. The cooled water is then collected and pumped back to the process-related equipment allowing for the cycle to repeat. The process and associated dispersion of heat allows for the cooling tower to be classified as a heat rejection device, transferring waste heat from the process or operation to the atmosphere.

Evaporative cooling towers rely on outdoor air conditions being such that evaporation will occur at a rate sufficient to transfer the excess heat contained in the water solution. Analysis of the range of outdoor air conditions at the installation site is necessary to assure proper operation of the cooling tower throughout the year. Evaporative cooling towers are of an open loop design, with the fluid exposed to air.

A closed loop cooling tower, sometimes referred to as a fluid cooler, does not directly expose the heat transfer fluid to the air. The heat exchanger can take many forms, but a finned coil is common. A closed loop system will generally be less efficient that an open loop design because only sensible heat is recovered from the fluid in the closed loop system. A closed loop fluid cooler can be advantageous for smaller heat loads, or in facilities without sufficient technical staff to monitor or maintain operation of an evaporative cooling tower.

Thanks to their range of applications, cooling towers vary in size from the monolithic structures utilized by power plants to small rooftop units. Removing the heat from the water used in cooling systems allows for the recycling of the heat transfer fluid back to the process or equipment that is generating heat. This cycle of heat transfer enables heat generating processes to remain stable and secure. The cooling provided by an evaporative tower allows for the amount of supply water to be vastly lower than the amount which would be otherwise needed. No matter whether the cooling tower is small or large, the components of the tower must function as an integrated system to ensure both adequate performance and longevity. Understanding elements which drive performance - variable flow capability, potential HVAC ‘free cooling’, the splash type fill versus film type fill, drift eliminators, nozzles, fans, and driveshaft characteristics - is essential to the success of the cooling tower and its use in both industrial and commercial settings.

Design or selection of an evaporative cooling tower is an involved process, requiring examination and analysis of many facets. Share your heat transfer requirements and challenges with cooling tower specialists, combining your own facilities and process knowledge and experience with their application expertise to develop an effective solution.`