The mathematics for properly designing emission control systems have been around for quite awhile, but studies of hundreds of systems show that many are wasting energy and lack the capacity to meet applicable codes and process objectives.
There’s a lot of money on the table in operating costs for these systems. More than 40 percent of the energy consumed in most manufacturing plants powers fans, pumps and ventilators used for emission control and ventilation systems. In some cases, the annual operating costs of a system may actually exceed the initial capital cost within two years of installation. Opportunities for improvement reside in the air power equation.
Power required for an air-handling system is computed with the following factors:
Q is the volumetric flow rate stated in cubic feet per minute.
TP is total pressure, or the resistance due to friction in ducts, hoods and ΔP of control device, etc., stated in inches of water.
df is the density factor of the gas being collected, a dimensionless value.
η is the efficiency of the fan, a dimensionless value.
These are combined into the Air Power Equation.
Small reductions in the numerator can have a significant cost impact. For example, a typical 20,000 cfm baghouse requires 60 or more horsepower for operation. A reduction of 1,000 cfm with improved hood design, or reduction of 1" static pressure with an improved duct or baghouse system, can save as much as $4,000 per year.
There are limits on what can be done, however. The process may require a certain airflow or hooding arrangement, and that will dictate air volume. Adjustments to system pressure and fan efficiency may be better places to effect reductions.
System pressure usually is affected by two factors:
- Hood and duct resistance, as a function of velocities in
the system and the inefficiencies of flow such as short radius elbows,
branch-entry angles greater than 45o, abrupt
contractions and elbows, poorly designed hoods and other interferences at fan
inlets and outlets. These are called fan system effects.
- Resistance across the emission control device. A baghouse that operates at a pressure drop of 8" w.c. will require twice the power of a collector operating at 4" w.c. However, the lower-pressure-drop collector may not provide the capture efficiency of the baghouse with high pressure drop. Pressure drop in a baghouse can be lowered by adding filter area, which requires a larger housing. More important, baghouses often perform best at high pressure drops. The key is to minimize pressure drop while still meeting emission requirements. Excess static pressure just wastes power.
4 Steps That Can HelpMinimizing flow, minimizing pressure, density control and better fan efficiency can help find that narrow range of safe, efficient operation.
Minimize Flow. Systems directly connected to a process source are inherently volume-limited, whereas systems that capture emissions with enclosures or hoods need to be optimized during the design process. Total enclosure of an emission source minimizes airflow and worker exposure. However, such enclosures can restrict visual observation of the process and hinder maintenance access.
Hoods that cannot be designed for total enclosure should be located as close to the source as possible. A side-draft hood located twice the distance from the source can require as much as four times the exhaust volumetric flow rate as a total enclosure (figure 1). Capture hoods for high-velocity emissions must be located so the opening is in the direct path of the fume, mist or dust.
However, keep in mind that other factors such as explosive limits for the gas being collected, moisture content (dewpoint) and heat content may influence the air volumetric flow rate requirements, so there may be limits to the optimization.
Minimize Pressure. Pressure offers greater opportunities to reduce energy costs. A system with good airflow characteristics (duct velocities and sizes optimized), matched with the proper pressure monitors, control device and variable-frequency drives, can help manage system pressure. Most baghouses or other collection devices will have varying pressure drops over the life of the system. Bags generally are more efficient at higher pressure drop but will use more energy. Oxidizers, scrubbers and electrostatic precipitators tend to operate at more constant resistance. A good pressure-monitoring system that controls system volumetric flow rate can significantly reduce the operating costs of even medium-sized systems. As variable-frequency drives become less expensive, they can be found on many installations, especially systems over 10,000 cfm.
Be mindful of duct inefficiencies and fan system effects (elbows at inlets and outlets, etc.). These shortcuts increase static pressure and operating costs. Figure 2 shows an example where short-radius elbows and system effects would add $6,500 per year in wasted power.
Control Density. Temperature, moisture, molecular weight, elevation and the absolute pressure in the duct or vessel affect the density of the transporting gas. A density change may affect the hardware requirements for the system. Evaporative cooling, for example, reduces volume, but the higher density air requires more power. This may be more than offset by reduced costs for smaller ducts, control devices and fans, as well as lower the value for volumetric flow rate in the equation. Cooler temperatures also may allow use of less expensive collectors, fans and peripheral devices.
Fan Efficiency. The design of the fan and its blade type can impact efficiency and power requirements. Laboratory-measured peak fan efficiency may not be the most stable point of operation. If peak efficiency coincides with the peak of the pressure curve, then there may be operational problems as volumetric flow rates vary with small changes in system pressure. The designer must consider both curves when selecting the best fan and operating point to optimize reliability and power usage. Fan type may dictate proper selection. For example, airfoil wheels, while more efficient, may not be a good choice when handling particulate-laden air.
The key to any design is proper fan selection and, more importantly, matching the fan to the system, as calculated (figure 3). Any of the three improper matches waste power and produce unsatisfactory system performance.
In summary, the power equation identifies four main areas affected by energy consumption volumetric flow rate, pressure, density and fan efficiency. The challenge for industry is to operate in the narrow functional range that ensures system effectiveness with minimum energy consumption. Attention to the air power equation can help meet those goals.
thinking about the air power equation, here are some good design goals for any
team studying new projects or system alterations.
In Addition to Energy
- Protect worker and public health by meeting local/national
standards for in-plant air and exhaust.
- Provide an efficient connection to the process through proper hood
design or direct connection to the process, while considering safety for fire,
explosion and process reactions, as well as the ergonomics of process access.
- Look for opportunities to recycle tempered air back to the plant or
process by filtering exhaust through redundant systems.
- Minimize auxiliary costs (compressed air, natural gas, water, etc.).
- Minimize replacement costs (filter bags, neutralizing chemicals,
- Provide an easily maintained and accessible system.
- Make the system simple to operate and train personnel to ensure ongoing performance.