Noise Control For Engine Compressor Plants
Previous newsletters have discussed the relative benefits of high grade reactive silencers and low speed, low noise multi blade fans. The third noise source of consideration, in the quiet design of engine compressor systems, is the engine casing noise. Most commercial grade compressor buildings will reduce casing noise by some 10 to 25 dB (across the normal spectrum of hearing). However the acoustic containment of the building is severely impacted by operating the building with doors, windows and louvres open. These doors, windows and louvres are usually left open for good ventilation of the unit, particularly in the summer during the day. It may not be reasonable to ask operators to return to compressor sites to close these appertures at 10:00 p.m. every night when the stricter environmental noise level comes into effect.
Solutions
One answer to the dual problem of noise control and ventilation is illustrated in Figure 4. Here a duct is connected to the cooler between the fan and the cooler bundle to tap off some 0.5 to 1.5% of the total fan air flow. This will typically provide some 5 to 15 air changes per hour in the building and thus provide adequate forced ventilation to keep temperatures inside the building at a tolerable level whilst simultaneously allowing operators to keep the windows and doors closed. This works for either forced draft or induced draft units provided the tap-off at the cooler is between the bundle and fan.
Winterizing Gas Engine/Compressors
Although plant process coolers are usually winterized in some fashion, the standard gas engine drive compressor is often neglected. Figure 1 illustrates a typical engine/compressor shaft driven cooler with minimal winterization consisting of louvres only over the compressor inter and after cooler. It is considered that the compressor cooler only can hydrate and that choke louvres are necessary only on that section. These louvres are supposed to stop -40ºC air from freezing these bundles when driven by the fan at design speed. The engine jacket section often has no louvres in the belief that the thermostat will bypass the bundle with the majority of the coolant, thus maintaining constant engine temperature.
The problem with cooler fans directly connected to the engine, is that they continue to operate at full speed during the winter. As the louvres close on the compressor coolers, this diverts even more unnecessary cold air through the engine jacket water (EJW). Although 50/50 glycol water cannot freeze at -40ºC, the effect is for the thermostat to bypass over 90% of the engine coolant at the engine head. This leaves only 10% or less flowing through the actual EJW section. These jacket water coolers are normally designed with the flow in the turbulent range. This keeps the heat transfer high and tends to prevent interior scaling of the tubes. During the winter the thermostat bypass will cause the tubes to operate in the laminar flow regime. This does not matter in the winter when the ambient is very cold but leads to interior scaling which can then create problems in the summer when the thermostat tries to return all flow to this cooler bundle.
Solutions
Two solutions are possible to solve this problem. We can either install a warm air circulation system to maintain a moderate air temperature (eg. +10ºC) at the bundle face or we can disconnect the fan from the engine jack shaft and operate it with an electric motor and variable frequency drive (VFD) which will slow the fan down in cold weather. Either solution will have the effect of maintaining relatively high flow to the EJW bundle throughout the year, thus avoiding the cold weather problems associated with laminar flow and internal scaling.
Note that most of these coolers in Albertan applications are designed for maximum summer temperature in the range of 30 to 35ºC. However, the statistical average ambient temperature in Alberta on an annual basis is only about 4ºC.
Of the two solutions offered, the electric/VFD system is preferred over the warm air recirculation system if the unit is also subject to environmental noise com-plaints. As the noise is significantly affected by fan speed, the installation of electric/VFD drives have the dual advantage of reducing the fan noise at night when ambient temperatures tend to be some 12ºC cooler than daytime peaks. Figures 2 and 3 illustrate the warm air recirculation system and the electric drive system.
