June 20, 2010
June 20, 2010
June 23, 2010
15.37.1 - 15.37.23
A Guided Inquiry Approach to Teaching Fan Selection
The selection of a fan for sufficient airflow for a particular system involves much more than determining the flow requirements and selecting a fan out of a catalog. A designer must understand that the flow rate of a fan is dependent on the amount of backpressure in the system while the backpressure depends on the flow rate. The characteristic curve for a fan and the impedance curve for a system show these dependencies. The actual amount of flow that a fan will deliver to a given system is determined by the intersection of these two curves.
Students could be taught about this as part of a traditional lecture. The exercise described in this paper is an attempt to enhance their learning experience through a guided inquiry laboratory exercise. The students learn to generate both the fan and system characteristic curves. The curves are used to predict the actual flow rate of a fan in an application. Once the fan is mounted into the device the actual operating flow rate is determined. As part of the exercise the students also plot characteristic curves for fans in series and in parallel to determine what happens in those configurations. The system used for the exercise is a familiar computer power supply.
This exercise has been implemented twice in small groups. The first time the subject was introduced in a lecture which included an in class demonstration. The second time the exercise was used to introduce the topic. A lecture during the next class was used to answer any questions the students had about the topic.
This paper discusses the learning objectives, equipment, procedure and preliminary findings from the two implementations. Also, the optional in-class demonstration is discussed.
The exercise documented in this paper is part of a National Science Foundation funded project being jointly conducted at Penn State Erie, The Behrend College and Portland State University. It is one in a suite of eight exercises being developed by the authors which are intended to help teach core principles in the thermal and fluid sciences through the use of everyday devices. These include a hair dryer, a bicycle pump, a blender, a computer power supply, a toaster, straight and stepped tanks, and a pipe section with a change of area. The project was first introduced at the 2007 ASEE national convention in a paper presented in the DELOS division1. Papers are available describing some of the other exercises2,3,4. This paper focuses on the computer power supply exercise.
The performance of a fan follows a characteristic curve, known as a fan curve, which is specific to that particular fan design. Coad5 describes fan curves as “probably the most misunderstood and yet most useful tool in HVAC systems engineering”. It is easy to misinterpret catalog information on fans since they often rate fans by their maximum flow rate and/or maximum pressure output. These specifications do not actually have much value for selecting a fan for a particular application. In fact, neither of these values will ever be realized in a real application
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