Course: HET 230 - Spring 2015

Unit 7 Outline

Unit 7 Outline
Unit 7 Outline

Introduction:

Last week, we discussed the aspects of fan curves and fan tables.  An experienced technician knows where to get their information from, and this goes along with being able to look-up and gather information one would need to determine the proper fan sizing needed.  Being able to plot and or calculate fan curves is important to know when sizing or diagnosing airflow problems.   You also learned about system effects that are inherent in fan selection, as well as in the determination of transitions and proper outlet conditions.

This week, you will develop an understanding of fan laws, the five components of cubic feet per minute, revolutions per minute, Sheave diameter, and brake horsepower, including common vocabulary terms.  In addition, you will be provided a PowerPoint a presentation, weekly study materials.  You will also participate in weekly discussions and complete all assignments and a content assessment quiz.  A skilled technician understands the relationship between CFM, RPM and sheave diameter in order to control static pressure.  Knowing the capable brake horsepower of the motor is important to a technician.  It is particularly important to know the amount of torque that is being exerted on motor by the fan blades as they push or pull air through a duct system, as opposed to static pressure.  This can create increased amp draw on the motor, an adverse case of efficiency, as well as motor damage.  Technicians make calculations using fan laws to identify the working conditions of the air movement, to determine the proper fan and motor sizing while providing the proper static pressures and airflow throughout a duct system.  A combination of motor sizing and the proper selection of sheaves, can give the technician the ability to make minor adjustments to air flow.  Using the wrong combination can cause problems in a variety of ways, this is important, as most systems require a specific static pressure without overloading the fan motor.  It is important to note that changing air conditions will play a part in this determination too.  Learning these subjects will allow you to form a solid foundation for the next week’s session, V-Belt Drives & Fan Accessories.

Objectives:

On completion of this week’s assignments, the student should be able to:

- understand the four Fan Laws using the variables of CFM, RPM, Bhp, SP, and AD
- describe Law One, Airflow Varies in Direct Proportion to the Fan Speed (CFM)
- describe Law Two, Static Pressure Varies by The Square of the Fan Speed (SP)
- describe Law Three, Fan Power Varies by the Cube of The Fan Speed (RPM/Bhp)
- describe Law Four, Fan Airflow Volume Will Not Change With a Change in Density (AD)
- understand the effects of increasing the RPM in relationship to exceeding safe operational conditions of the motor
- identify the alterations in conditions which change the dynamics of a calculated airflow system and these effects on Fan Law calculations
- calculate to find a new CFM needed
- calculate to find a new RPM needed
- calculate to find the proper motor sheave diameter needed
- calculate to find the proper fan sheave diameter needed
- calculate to solve for finding new RPM using the square root of Static Pressure (SP)
- calculate to solve for the new Static Pressure (SP) using the square of RPM
- calculate to solve for the new CFM using the square root of Static Pressure (SP)
- calculate to solve for the new RPM using the cubed root of Bhp
- calculate to solve for the new Bhp using the cube of RPM
- understand the effects of air density change  in  relation to Static Pressure (SP) and Bhp

Key Vocabulary:

Brake Horsepower (Bhp)

Cube (Cubed) Root

Cubic Feet per Minute (CFM)

Diameter

Drive Sheave

Driven Sheave

Inverse Ratio

Inches of Water Column (“.w.c)

Fan Discharge

Fan Intake

Fan Laws

Pitch Diameter (PD)

Ratios and Proportions

Revolutions per Minute (RPM)

Square Root

Sheave

Static Pressure (SP)

V-Belt Drive