Cooler Stacks
With warm weather approaching, aerial coolers can be both a process problem and an acoustical problem as plant operators try to get the most out of these units. In many cases good process control and reduced environmental noise place contradictory requirements on Operators who are running all fans at full speed for maximum cooling but are thereby creating maximum noise levels. However this Newsletter discusses one cooler modification which can enhance both process and acoustical performance in certain circumstances.
Process:
In cases where warm air recirculation is a problem—i.e. the exhaust air from a cooler recirculates to the fan entry as per Figure 1—it is usually advisable to place a chimney stack on the cooler exit to direct the warm air exhaust away from the cooler suction. The general rule of thumb is for these stacks to be 15 to 20 feet high. Where several coolers are crowded together in a gas plant another problem is cross-circulation—shown in Figure 2—where one cooler exhaust feeds into the suction of an adjacent cooler. Once again an exhaust stack alleviates this problem. The “stack draft” of this chimney also increase the airflow and the performance of the cooler.
Acoustics:
The environmental noise radiating from aerial coolers can be a significant contributor to noise levels exceeding the AEUB permissible maximum. Operators trying to get the most out of the process will often run cooler fans at their maximum allowable. However noise is very sensitive to fan speed. A 10% increase in speed will provide operators with only a 10% increase in airflow but at the cost of a 50% increase in noise energy.
The noise radiating from an aerial cooler distributes approximately 75% from the fan-side and 25% from the bundle-side. This means that for induced draft coolers 75% of the noise radiates from the exit and for forced draft coolers 75% radiates from the intake. For either cooler type, if a chimney stack is a necessary process addition, it is possible to design it so that the stack acts both as a hot air deflector and a silencer. Figure 3 illustrates the essence of a simple exhaust chimney silencer. The major change to the simple single-skin chimney is that the exhaust silencer stack must be double skin with at least 4” of fibreglass sound absorptive insulation and a perforated inner skin. A chimney silencer as per Figure 3 will typically reduce cooler exit noise by some 4 to 12 decibels across the normal audible spectrum with most of the reduction in the higher frequencies. If more noise attenuation is required in the lower and mid band frequencies, then center baffles (aka splitters) will need to be added.
