While SideCar operation simply requires a reliable supply of air flow and pressure, blower type and size are important to allow flexibility, minimize power costs, maximize dependability and preclude nuisances such as noise. All three major types of blowers have been employed: PD (lobe type rotary positive displacement), Regenerative, and Centrifugal. Each type has advantages and size ranges that must be considered for each application.
SideCars require approximately 5 CFM/FT of RBC media length @ 2.5 to 2.8 PSIG at the manifold connection (depending on tank depth, media diameter and other factors) for “PROCESS AIR” or continuous routine aeration. Blowers are typically selected to best meet these requirements within the family of blowers, i.e. in the interest of staying within a blower or motor size the actual capacity may vary from slightly less than 5 to as high as 6 CFM/FT. Wastewater treatment is anything but an exact science.
For occasional “AIR SCOUR” (see these SideCar Instructions) it is necessary that air can be controlled to direct ~10 CFM/FT under a targeted stage or RBC(s). This is accomplished by valving off (shutting down) other stages or RBC’s during this occasional procedure. SideCar blowers are usually selected to provide just PROCESS AIR requirements with no standby capacity (RBCs can operate for days without or with reduced air before severe problems gradually arise.) It is logical, however, except in the smallest plants, to use two or three partial capacity blowers to supply this air to provide operational flexibility and some backup for those rare cases when a blower must be taken out of service.
To conserve energy and encourage equal air distribution, air piping losses must be minimized and blower selection must provide turndown capabilities. Oversize blowers can present operating problems and complicate system flexibility. Properly matching blower(s) to system requirements allows use of simple constant speed motors with ON-OFF controls; there is no need for expensive and complicated control systems.
PD blowers, while efficient and inexpensive, are likely to require expensive sound dampening enclosures to avoid complaints from workers and plant neighbors. Regen blowers are quiet and reasonably efficient in the 3.0 PSIG pressure range in capacities up to nearly 1000 CFM. At higher air flows multi-stage centrifugal blowers become competitive and efficient, but require scrutiny in selection and controls for varying air temperatures, barometric pressures and, most importantly, capacity control to prevent surging and run out (and high HP) at lower pressures.
Except in the rarest cases air flow to SideCars should not be throttled during normal service. An exception is in small plants where there are multiple air uses, some requiring a pressure zone higher than that of SideCar. SideCars are designed, when installed at like submergences with adequate size air supply piping, to accurately distribute the air within +/- 10% over the entire system. No balancing or flow control is required.
To summarize, this is not meant to be a complete treatise on blower selection but rather to provide guidelines and caveats to the customer that chooses to furnish the SideCar air supply blowers.