System losses, other than those of the boiler and ancillary equipment of the boiler, are normally a result of the total pressure losses from friction in the ductwork, elbows, changes in duct cross-sectional area, and losses associated with emissions control equipment. A certain amount of loss is unavoidable, but losses can be reduced significantly with simple and relatively inexpensive modifications during outage season. These modifications include turning vanes, implementing well-developed and longer sections for diverging and converging flows, redesigning compound elbows, and aerodynamically enhancing or removing obstructions in the flow path.
Causes of Total Pressure Loss
The losses in total pressure as a result of flow through a system are caused by two factors that are easily addressed during outage season — friction losses due to the viscosity as the air flows along the surface of the ducts, and system equipment and dynamic losses due to the turbulent wake caused by changes in direction and separation of flow around obstructions.
The total pressure (Pt) in a system is comprised of two components: static pressure (Ps) and velocity pressure (Pv). Pt is the summation of Ps and Pv (Pt = Ps + Pv). Ps is the component of pressure that exists by virtue of compression only and Pv is that component that exists due to motion only.
Pressure components change as the cross-sectional area changes. By conservation of energy, the sum of Ps and Pv at any point in the flow system is equal to the sum of Ps and Pv at any other point in the system, minus any losses in pressure occurring between the two points. With each conversion of energy, there is a loss of total pressure. The more abrupt the change in a cross-sectional area is, the greater the loss.
Duct Changes to Minimize Pressure Loss
Here are a few specific things that you can do to minimize pressure loss in your duct system and increase overall plant efficiency:
- Reduce the number of times the cross-sectional area changes to minimize system losses.
- Engineer more gentle changes or lower the angle of divergence or convergence in transition sections to lower losses. With the right analysis and plan, you should be able to realign your duct systems to maintain a total angle of convergence of 30° or less and a total angle of divergence of 15° or less.
- Limit the number of turns in direction since there are losses associated with every turn. The gas velocity should be limited to 4,000 fpm.
- Make sure the ducts system has as few flow or duct elbows as possible. The aspect ratio should be no less than 1:1 and no more than 4:1. Round ductwork, having the least perimeter-to-area ratio, is the optimum for minimum frictional loss.
- Install turning vanes in the elbow to reduce the loss, as well as guide the flow and keep the velocity profile uniform.
- If possible, combine elbows. For example, if a duct makes a change in elevation, use two 45° elbows if possible instead of two 90° elbows. Also, try to maintain a reasonable turning radius on the elbow. Generally, the smaller the radius of the elbow, the greater the loss will be.
- If there are any unnecessary obstructions in the duct system or any unused equipment, evaluate them and consider removing them. Taking a simple total pressure reading upstream and downstream of the obstruction or equipment will define the losses and their operational costs.
Relatively small changes to your duct system can mean big savings over time. Reducing the system pressure requirements reduces the energy (horsepower) needed to move the required volumetric flow rate through a given system, which translates to reduced operating costs.
Making outage season an opportunity to identify — and even make — efficiency upgrades can be a good use of this scheduled downtime. If you’re interested in exploring the possibilities, our Air Handling Division specializes in the design, fabrication, and installation of ductwork, as well as surrounding air handling equipment. Give us a call at 1-888-663-2028 and we’ll be happy to help.