Process cooling is the function of removing heat in order to control or maintain appropriate temperature levels. Almost every manufacturing process produces a certain amount of excess heat due to friction or process heating. From a shop compressor to the power plant just outside town, process air is required to generate the pressure and airflow necessary to decrease or maintain the proper temperatures.
Fans, both axial and centrifugal, are the primary media to provide such pressure and airflow. While many process implementations exist, this article will explore two centrifugal applications: The fan on an air pollution control unit in a power production facility (for selective catalytic reduction), and the blower on a glass-tempering oven. In both cases, industrial process fans provided effective process cooling.
Controlling Temperatures During Selective Catalytic Reduction
With the increasing demands for environmental controls on exhaust systems, selective catalytic reduction (SCR) is becoming more common on the back end of combustion turbines. Traditionally, combustion turbines have been prepared with heat recovery steam generators (HRSG) or have not been treated for NOX removal. In these applications, the exhaust temperature remained within an adequate operating range for the SCR or CO systems to achieve desired emissions reduction.
On simple cycle systems where the exhaust from the combustion turbine is ducted straight through, the temperatures are too high for reliable SCR operating ranges (850 to 1100°F [454 to 593°C]). One solution on these systems is to add ambient air into the exhaust stream ahead of the SCR catalyst. This reduces the overall exhaust temperature to acceptable ranges for NoX reduction (500 to 800°F [260 to 426°C]).
One way to accomplish this it to inject ambient air into the system by providing 13 to 20” water column of pressure with centrifugal blowers at a specified volume rating. Injection of the ambient air typically takes place several feet upstream of the catalyst face in order to fully mix and establish a uniform temperature across the gas path. The volume then is controlled depending upon the temperature setpoint of the exhaust by use of variable frequency drives (VFDs) or inlet vane dampers. The cooling blowers are usually provided on a skid arrangement: Two identical blowers are mounted to mirror each other. This allow the unit to continue to operate if one blower is out of service. Downtime is unacceptable on these systems, so redundancy is a critical necessity for the facility.
The blowers in the SCR system also can assist in satisfying the pre- and post-purge requirements of many combustion turbines. Utilizing this system can reduce the time required for pre- and post-purges.
As the demand for fast power, emissions requirements on gas pipe lines and the need for traditional temperature SCRs increases, having a properly sized cooling air-injection system is becoming a key portion of the design.
Controlling Temperatures During Glass Tempering
Manufacturing tempered glass is a detailed and unique process that relies upon the quick cooling of the glass to obtain the purposes for which tempered glass is used. The unique designs and characteristics of centrifugal fans are a critical component of the cooling process on a tempering system.
Tempered glass typically is about four times stronger than ordinary, or annealed, glass. When annealed glass shatters, the result is sharp, jagged shards that can be hazardous to a person or pet. By contrast, tempered glass shatters into small, relatively harmless pieces. Tempered glass is used in applications where human or pet safety is important such as automobile glass, patio doors, shower doors, racquetball courts or patio furniture.
During glass tempering, the centrifugal fans play a key role in the cooling process. Prior to the tempering process, the glass typically is cut to size. The glass then is fed through a heat-treat process in which it passes through a tempering oven that heats the glass to approximately 1200°F (649°C). The glass then undergoes a rapid, high pressure cooling process called quenching. This process lasts only a few seconds. A continuous blast of high pressure air — distributed through a perforated panel on which the glass is riding — cools the outer surfaces of the glass more quickly than the center. As the center of the glass cools, it pulls back from the outer surfaces, forcing it into compression. While the center remains in tension, the glass edges remain compressed, giving the tempered glass its strength.
The perforated panels upon which the glass panels ride are constructed with hundreds of holes that measure approximately 0.046” in diameter. The tempering process requires significantly high airflow pressures to effectively cool the glass in a short period of time.
Due to the short time and high pressure required to rapidly reduce the glass from approximately 1200°F (649°C) to around 500°F (260°C), two centrifugal fans tied in series are required to supply the necessary pressure and airflow. The major variable in this process is the pressure downstream from the fans. This downstream condition can create another variable to reconcile in the tempering panel and cooling system design. Fan curve characteristics can be tricky when the pressure is on the discharge side of the fan. In this situation, the fans can be pushed into the peak of the curve, which is a no-go zone for most fans. Accuracy in the fan selection is critical to the successful function of this setup.
By operating two fans in series, significant pressures can be developed along with the higher flows required for the tempering process of very thin glass. (Very thin glass is more difficult to temper.) Through this process, properly designed and installed blowers have been able to help glass-tempering companies produce tempered glass that is 0.125” thick. The cooling requirements of the thicker pieces of glass do not require the speed of the thinner glass in order to accomplish the tension versus the compression characteristic.
Once the initial tempering process is completed, the tempered glass continues through an additional cooling process. This subsequent stage requires another centrifugal fan. Overall, the glass-tempering process is unusual in the process cooling arena. At the same time, it is just one more way in which industrial fans are being used for increased manufacturing accomplishments.
In conclusion, power and glass are just two industries where centrifugal fans provide the critical pressures and airflow required for proper process cooling. In any process cooling application where centrifugal fans provide the essential flow and pressure ingredients, proper selection is critical to the efficient and accurate operation of the entire system. Factors such as the required flow rate and pressure, ambient and system air temperatures, installation elevation and system size requirements must all be carefully evaluated to ensure the appropriate fan is selected for the project. When successfully sized, selected and manufactured, centrifugal fans supply an effective process air medium for most process cooling applications.