Preface: Blower-to-SideCar air piping sizing and design is most critical to the success of an RBC aeration system. Accordingly, it must be carefully designed and installed to deliver the required air flow at the prescribed pressure to the SideCar inlet connection(s) while minimizing wasteful headloss that will increase the pressure and HP of the blower(s). This writeup shares suggestions to optimize air piping from MPI’s long history in this specialized industry.
Air Flows: Each plant system must be analyzed to calculate system pressures but for this discussion assume conservative continuous “process air” requirements of 125-150 CFM@ ~3.0 psig at the inlet of each SideCar under a 25’ L RBC. To assure acceptable air distribution across the system air piping must be sized so there is <10% difference from the first to last SideCar connection. (Given the option the lead RBCs typically subjected to higher loadings and biomass growth should be the first air connections to benefit from any air imbalance).
There is absolutely no economy in under sizing air system piping that wastes power and threatens air distribution. The end game in this exercise is to plot a system head curve with selected pipe sizes and overlay blower curves to confirm compatibility. The goal, using project specific criteria, is to have the operating condition point for the blower at ~3.0 psig.
Pipe Materials and Installation: Air supply piping must be carefully aligned, supported and installed to preclude stress on the SIDECAR MANIFOLD connection or distortion of the expansion joint or flex ell. The SUPPORT BRACKETS are designed to take the weight of only the SIDECAR ASSEMBLY. DO NOT HARD PIPE TO SIDECAR! Typically, a non-restrictive valve (BUTTERFLY or GATE) is installed near the inlet connection of the MANIFOLD to isolate that system from other air uses.
In most cases for larger multi-RBC plants exposed exterior air supply piping is (Sch. 10 or 20) steel or (Sch. 5 or 10) stainless steel which can withstand high temperatures at the blower and prolonged exposure to sunlight. Because of the relatively low air pressures (<4.0 PSIG), low cost, superior flow characteristics and convenience of installation many smaller to medium plants have used PVC or CPVC air supply piping.
The biggest concern is supporting and protecting above ground horizontal runs. As there are no known tables for PVC pipe support spacing carrying relatively weightless gases a limit of 6′ is suggested in a moderate <80° F environments. Over time PVC pipe can sag between supports when subjected to higher temperatures.
Exposure to direct sunlight should be avoided to prevent UV degradation and heat rise. As the air temp rises due to adiabatic compression (approx. 10°F per PSIG increase in blower discharge pressure) a combination of hot weather, blower heat, sunlight and inadequate supports can result in noticeably sagging, unsightly pipe.
Accordingly, metal pipe and fittings are definitely preferred at the blower(s), to resist and dissipate heat and vibration and allow flexibility in selecting valve types (choices in PVC are limited and sometimes bulky). Transition to well supported PVC (or CPVC) pipe, protected from direct sun, can then be considered. Blower(s) should be located in a convenient area but as close as possible to SideCar™ to minimize cost of and losses in supply piping. AIR PIPING MUST ALWAYS BE SUPPORTED INDEPENDENT OF BLOWERS.
Below is a table of guidelines to assist in sizing air piping and max support spacing for steel pipe.
|Blower to SideCar™ Air Supply Piping Guidelines|
|Pipe Dia. (a)||Airflow||Steel Pipe (b)||PVC Pipe (c)|
Pipe sizing for estimating relatively short runs only. All systems must be designed for site-specific conditions.
- From FW Webb Co. Engineering Data. Thermal expansion of lateral pipes must be considered.
- See discussion in text above re supports for PVC pipe.
Summary Hints and Comments: Each plant must be carefully design but the following are useful in evaluating design options:
- There is no economy in under-sizing air supply piping. Use accepted methodology for diffused air systems in calculating losses.
- Apply design protocol by increasing blower discharge pipe sizes and reducing at SideCar inlet.
- Use flow efficient fittings (sweeps rather than short R elbows, for example) to conserve pressure.
- Locate blowers as close as practical to conserve pipe costs and losses.
- If calculated pipe sizes are smaller than those in above table take another look.
- Never try to support field piping on blower discharge or SideCar inlet fittings.
- A goal of ~10% friction loss (in total of blower discharge pressure) is a good starting point.