Many factors must be considered when designing or specifying a process water cooling system. Understanding these factors will help you select the cooling strategy that best fits your process. In general, industrial process water is cooled by evaporative cooling towers, air-cooled heat exchangers or refrigerant chillers. Regardless of the system chosen, it is essential to review and understand the manufacturer’s specifications for cooling water to design or specify a successful system. The specification typically is given with a water flow rate, temperature rise and heat load rating. The water temperature required for the process also will influence the type of system you choose.
Is the process especially sensitive to temperature variation, requiring tight temperature control? Does the process have both upper and lower water-temperature limits? Does the process have different cycles in which more or less water is required? Are there any material compatibility issues? Questions like these need to be asked and answered to have a chance at success.
System ChoicesEvaporative Cooling Tower System. Systems using evaporative cooling towers are common for industrial process water cooling because of their low initial expense and economical operation.
An evaporative cooling tower cools water by evaporating it in a controlled manner. Water is sprayed inside the tower on plastic fill material, and fans blow air over the fill to evaporate and cool the water. As the water is being evaporated, a mineral buildup occurs. For this reason, the process water often is separated from the tower water by a plate-and-frame heat exchanger. This design allows for a completely closed process loop of water apart from the tower water, which can become corrosive. Using a plate-and-frame heat exchanger and creating two isolated water flows (a dual-loop system) is recommended when using an evaporative cooling tower for most industrial applications.
An evaporative cooling tower has some limitations with regard to temperature. No cooling tower can cool water below the wet bulb temperature of the installation location. For design purposes, use the 1 percent summer design point for the site, available from engineering handbooks. A tower will work better in a dry climate: It is possible to get much cooler water from a tower in arid Phoenix than in muggy Chicago. In general, water can be cooled to within 5oF (3oC) of the design wet bulb temperature; the closer you need this specification, the larger and more expensive the tower will be.
A tower can be effective in cold weather if the system is designed properly. During the coldest months, the water should drain out of the tower into an external (indoor) reservoir when cooling is not required, thereby removing the risk of freezing during a protracted shutdown or power failure.
Regardless of tower construction (galvanized steel, stainless steel or high density polyethylene), temperature restrictions exist due to the materials used inside the tower. Most tower fill is made of polyvinyl chloride (PVC) and will melt if the temperature rises much above 140oF (60oC). Fills made with more heat-resistant materials exist, but it is more common to work within the system’s design constraints to keep the water temperatures in the tower well below the upper limits.
Air-Cooled Heat Exchanger System. Air-cooled heat exchangers have a simple operation and a completely closed coolant loop without the maintenance of cooling tower water. The water is pumped through coils, and air is blown over the coils by a number of fans to cool the water to near the ambient temperature.
The coils typically are made with copper tubes and aluminum fins, so corrosion is not an issue. When paired with a closed reservoir, the system is completely isolated from oxygen, which makes it difficult for rust to form. The closed-reservoir design will enhance the performance of the equipment and extend its life.
Air-cooled heat exchangers are attractive if higher process water temperatures (110oF [43oC] or above) are acceptable. However, these systems are susceptible to outside temperatures. For example, if the outside temperature is 95oF (35oC), the temperature of the water will not get down to 90oF (32oC), even if there are 100 fans blowing on the coil. As a result, processes that require water temperatures of 90oF or lower cannot be cooled reliably by an air-cooled heat exchanger in many parts of the United States.
A glycol solution can and should be used for many process water applications. The solution will provide protection from freezing and, when mixed with a corrosion inhibitor, can extend the longevity of the entire system. The heat transfer capacity of the glycol will impact the thermal design. Glycol can be used for any cooling water application except in the tower loop of an evaporative cooling tower system.
When freezing is a possibility at the installation location, a glycol solution should be used to protect the coils and equipment from freezing. Alternately, the coil can be completely drained when it is out of use during the freezing period, but this option is not always practical.
Refrigerant Chiller System. A chiller system uses a refrigerant compressor and is capable of cooling water to much lower temperatures than either cooling towers or air-cooled heat exchangers. It is not typically the first choice for a process water cooling system because of its high cost of ownership, but it is an absolute necessity for many processes. You will need to use a chiller system if your process requires water temperatures below 85oF (29oC) in most climates. The heat from the process often is dumped to the outdoors by air-cooled condensers. Chillers are effective in all climates when properly designed.
A glycol solution should be used in a chiller system because of the possibility of water freezing inside the system and damaging components.
When selecting a refrigerated chiller system, consider specifying duplex pumps. A system can include two full-size pumps (also called duplex pumps) monitored by a pressure switch or flow switch. One pump operates normally, and the second pump is a backup that can start automatically if a failure occurs. If duplex pumps are specified, have the pumps alternate occasionally to make sure the backup pump will be in working order when it is needed.
One of the most overlooked aspects of a water cooling system is the sizing of the reservoirs. The reservoirs should have sufficient volume to ensure proper operation of the pumps and to provide some thermal stability. A rule of thumb is that a process water reservoir should hold about 3.5 or 4 minutes worth of flow. For instance, if you have a 100 gal/min pump, a 400 gal reservoir is reasonable. The reservoir can be even larger if the heat will vary during the process. The reservoir provides cheap protection against temperature spikes and gives more volume to heat up in the event of a power failure while the backup pump is running.
A plant water cooling system is a reliable and cost-effective addition to your facility when it is designed correctly. Understanding the available systems and how they can benefit your particular process will help you achieve this goal.