![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Module 5: Flowcharts and Ventilation Systems - Fans - Performance
Brake Horsepower
Effect of Gas Temperature and Density on Fan Current
Summary Animation of Ventilation System
Practice Problems
Objectives
- Explain how fan motor data is used to evaluate performance of an
industrial ventilation and air pollution control system.
- Correct fan motor currents from actual to standard conditions.
It is helpful to be able to determine when the system characteristic curve (discussed in the previous lesson) has shifted. Measuring the gas flow rate is the most direct way to check fan performance to determine when the system characteristic curve has shifted. However, this process is time consuming. The fan motor current data provides an indirect, but nevertheless very useful, indication of gas flow changes from the baseline conditions. An increase in fan motor current is generally associated with an increase in the gas flow rate. Decreases in fan motor current occur when the gas flow rate drops. Unfortunately, the relationship between gas flow rate and fan motor current is not linear. The nonlinear characteristic of the relationship is indicated by the brake horsepower curve shown in Figure 1.
The fan motor current is directly proportional to the brake horsepower as indicated by Equation 1, which applies to three-phase motors.
- Where:
While the shape of the horsepower curve varies for different types of fan wheels, the general relationship applies to all centrifugal fans in their normal operating range. As the horsepower increases, the gas flow rate increases.
The fan motor current provides a good indicator of changes in the gas flow rate because it is related directly to motor horsepower as indicated by Equation 1. When the fan motor current has increased, the brake horsepower has probably increased, and, therefore, the gas flow rate has probably increased. However, the extent of the change cannot be determined from the simple relationship due to the nonlinearity of the brake horsepower curve and the variations in the power factor.
-
#1
- What are some examples of situations that might cause the fan motor current to rise?
The brake horsepower is related to the cube of the fan speed as indicated in the third fan law shown below. (The other two fan laws were covered in the previous lesson.)
- Where:
The fan motor current is measured by the fan ammeter, which is monitored either in the main control room or in a remote fan control room.
Effect of Gas Temperature and Density on Fan Current
A fan operates like a high-speed shovel. Every rotation of the fan wheel at a given operating point moves a constant volume of air. While the volume is constant, the weight of the air being moved may not be constant. The density of the gas being handled by the fan is a strong function of the gas temperature. At high gas temperatures, the gas has a low density, and the gas is relatively light. When the gas temperature is cold, for example at ambient temperature, the gas is dense, and its weight is substantial.
In addition to gas temperature, gas density is also a function of the absolute gas pressure. The absolute pressure can be taken into account using Equation 3.
- Where:
The gas density has a direct effect on the fan motor current. The current will be high when the gas stream is cold such as the times when the process is starting up. If steps are not taken to minimize gas flow during cold operating periods, the fan motor could burn out due to excessive current flow. To prevent this, the fan inlet or outlet dampers are usually partially closed during start-up to restrict the amount of "dense" air being handled. As the process heats up and the gas stream becomes less dense, the dampers can be opened to permit normal gas flow rates.
When using the fan motor current as an indicator of gas flow rate, it is important to correct the motor currents at the actual conditions back to standard conditions. This correction removes the effect that gas density changes can have on fan motor current. Thus, an operator can distinguish between whether changes in system resistance (e.g. within the fabric filter) or a decreased gas stream temperature is causing the fan motor current to fluctuate. The correction can be made using Equation 4.
- Where:
Example Problem 1.
Converting the Fan Motor Current from Actual to Standard Conditions
A fan motor is operating at 80 amps and the gas flow rate through the system is 10,000 ACFM at 300°F and -10 in. W.C. (fan inlet). What is the motor current at standard conditions?
Solution:
Step 1. Calculate the gas density at actual conditions. As a basis, use 1 lb mole of gas. Assume 1 lb mole of gas has a mass of 29 pounds.
- Convert pressure from inches of water to psia.
(1).gif)
- Calculate the gas volume at actual conditions using the ideal gas
law equation.
(2).gif)
- Calculate the gas density,
.
(3).gif)
Step 2. Calculate the gas density at standard conditions.
- Calculate the gas volume at standard conditions using the ideal gas
law.
(1).gif)
- Calculate the gas density, .
(2).gif)
Step 3. Correct the motor current for the change in gas density.
.gif)
Note 1: The problem could have been solved quickly by using tabulated values of the gas density. However, the gas density calculation provided a convenient review of the ideal gas law discussed in Module 2. This approach also reduces the risk of a gas density error caused by not taking into account the effect of pressure changes.
Note 2: The gas composition in Example Problem 1 could be taken into account by calculating the weighted average molecular weights of the constituents rather than assuming 29 pounds per pound mole, which is close to the value for air. This correction is important when the gas stream has a high concentration of compounds such as carbon dioxide or water, which have molecular weights that are much different than air. For instruction on how to make this calculation, see the lesson on Density in Module 2.
The gas temperature and gas pressure corrections for gas density must also be used when selecting a fan. The fan ratings tables are expressed in standard temperatures and pressures. These corrections are needed to ensure that the fan will deliver the necessary gas flow rates and absolute pressure increases under the actual operating conditions anticipated in the process.
The temperature of a gas may increase, decrease, or remain constant when a pressure change occurs. When the gas flow reaches the fan, the gas flow changes from negative to positive pressure. This increase in pressure can cause the temperature to increase slightly.
Summary Animation of Ventilation System
The next animation illustrates how hood performance and system parameters change as conditions within a fabric filter change. It combines many of the important concepts discussed so far about hoods and fans. Fabric filters would normally have many bags to trap dust particles. For simplification, the fabric filter in this animation has only one bag.
|
Animation #1
This animation shows (No audio)
|
 |
Note: Animations require a Netscape 4.7 or Internet Explorer
4.01 or higher browser and a Shockwave Flash plug-in (Netscape browsers)
or Shockwave Flash ActiveX Control (Internet Explorer browsers). Shockwave
Flash plug-in/ActiveX Control (version 3.0 minimum) can be obtained at
the Macromedia
web site. Before downloading or installing any software or plug-ins, please
refer to your organization's network/computer policies or check with your
system administrator.
Centrifugal fans are the most commonly used type of fan in industrial processes due to their ability to generate high-pressure rises in the gas stream. The major components of a typical centrifugal fan include the fan wheel, fan housing, drive mechanism, and inlet dampers and/or outlet dampers.
The intersection of the fan characteristic curve and the system characteristic curve is called the operating point for the fan. The factors that affect the fan characteristic curve are the type of fan wheel and blade, the fan wheel rotational speed, and the shape of the fan housing. The system characteristic curve takes into account the energy losses throughout the ventilation system. These curves are helpful indicators in determining if a change in the system has occurred. A change in the system can also be detected through the fan motor current data that corresponds with the gas flow rate, however not linearly.
The fan laws can predict how a fan will be affected by a change in an operating condition. The fan laws apply when all of the following conditions hold:
- The fans have the same design and geometric shape.
- The fans have not been altered in shape or form.
- The system characteristic curve does not change.
A fan will move a constant volume of air; however, the amount of work required to move the gas flow is dependent on the density of the gas. Two factors that affect density are temperature and pressure. The gas flow density has a direct effect on the fan motor current.
Practice Problems
Fans - Performance
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.