Determining System Impedance
Once the airflow is estimated, the system impedance or "airflow resistance" must be calculated or measured. System impedance is expressed in static pressure as a function of airflow. A typical system impedance curve is governed by equation (4):
This equation describes the relationship between static pressure and the airflow required in a particular system.
Going back to the previous example, using equation 4 we calculate static pressure through the cabinet to be 0.11 inches of water. In this application, there is up to 1 gpm of available facility water at 52°F (11°C). We need to select a fan that can provide at least 27 CFM of airflow at 0.11 inches of water and a heat exchanger that has the following performance when using water at 1 gpm or less:
ITD is the Initial Temperature Difference between the incoming hot air and cold water.
As Figure 1 below indicates, 6105 copper tube-fin heat exchanger will provide 6.9 W/°C when paired with a fan that can provide at least 27 CFM and water flow greater than 0.25 gpm. If we select an Oriental Motor fan model MU1225S as shown in Figure 2, the pink vertical and horizontal lines show that at the required 0.11 inches of water this fan will provide 39 CFM, this is well over our requirement of 27 CFM. It's important to note, however, that fan accessories such as finger guards and filters can have an impact on fan performance, as shown by the difference between airflows at points A, B, and C in Figure 2. In a high impedance system such as our example, the effect on fan performance is minimal. With a low impedance system though, the impact to fan performance can be greater. If our cabinet had a clear airflow path, system impedance would be relatively low and accessories would have a significant impact on fan performance, as shown by the airflow differences between points D, E, and F on Figure 1. Any significant drop in airflow from the required amount will impact the performance of the heat exchanger.
In addition to airflow and system impedance, other important factors must be considered when selecting a fan such as fan type, constant or variable flows, AC or DC power, air density, noise, life expectancy, EMI/RFI interference, and more. Part 2 will cover the considerations of air density, audible noise, life expectancy, and EMI/RFI interference.