Flow Sensors - Blog Post 6

Flow Sensors are a critical part of any HVAC sensor system.  Flow sensors are used in ductwork to determine the flow rate of the air in the duct.  This can be done in two different ways either by differential pressure detection systems or thermal flow measurements.  The differential pressure detection either measures a pressure difference or a force.  Some common examples of these are pressure tubes or rotary measurement devices.  The rotary devices relies on blades being pushed by the flow and then calculating the flow rate from the speed at which the blades are rotating.  The thermal flow detection systems rely on the thermal properties of a material or the medium, which is flowing.  The idea behind the material based thermal flow sensors is it measures the cooling of a heated element and relates this cooling to the flow at which a fluid (air included) is moving to provide this cooling.  This relationship is non-linear and as the fluid is moving faster there is a greater cooling of the heated element.  The other form of thermal flow sensors uses an element to heat the passing fluid and measures the temperature of the fluid downstream from the heated element.  The difference between the initial temperature and temperature downstream of the element can be related to the rate at which the fluid is moving.  If the fluid is moving slowly it has more time in contact with the element and therefore is heated more and results in a higher temperature, where vice versa if it is moving rapidly the fluid is not heated as much and is cooler.

            Now how these various sensors work and their properties are good to know, but what are the applications of these sensors?  One application is in continuous monitoring of HVAC equipment.  Flow sensors can be placed down stream of large fans to measure if the fan is meeting its designed pressures.  If the flow sensor detects that the flow is dropping off it may mean that the fan element is going bad or experiencing wear.  This is important to the maintenance crew of a building as the efficiency of an HVAC system can experience a 20-30% decline with age due to failing parts.  If the crew can detect these failures and repair them money and time can be saved.  As sensors become more available and cheaper it will pave the way for more intelligent buildings and monitoring of its systems.

References:

·      B W Van Oudheusden, Silicon Thermal Flow Sensors,  Sensors and Actuators, p. 5-26, 1992

·      M. Ahauer H. Glosch F. Hedrich N. Hey H. Sandmaier W. Lang, Thermal Flow Sensors for Liquids and Gases Based on Combinations  of Two Principles, Sensors and Actuators, p. 7-13, 1999

·      Fu Xiao, Shengwei Wang, Progress and Methodologies of Lifecycle Commissioning of HVAC systems to enhance buildings sustainability, Renewable and Sustainable Energy Reviews, p. 1144-1149, 2009

Comments on other Blogs:

http://ae-510-ay13-14.blogspot.com/2014/02/flow-sensors.html

Very nice post lots of good information!  I honestly wouldn't have put flow sensors together with the measurement of electricity.  I think it is amazing how we can simulate flow around buildings in an urban setting.  Anyone who has walked by Ucross on a cold windy day sure wishes they had done this modeling for the building and figured out a solution.  Thanks for the post!

http://ae-510-ay13-14.blogspot.com/2014/02/week-7-humidity-sensors.html

That is amazing only $10! And only the size of a quarter!  It goes to show just how far technology has come in a few decades.  Moisture is one of the most damaging things to the built environment as we have learned from other classes and controlling it including the humidity is so important.  Really cool post!

http://ae-510-ay13-14.blogspot.com/2014/02/week-7-movement-sensors.html

Nice post Ian!  I had to laugh at your splashed comment.  There are a lot of applications out there for motion sensors.  I really didn't start seeing the motion light sensors until I cam to philly.  I assume this is when the became affordable to put in as light switches.  I have heard a few people complain about the grocery store ones though because if they have one of those speed radar monitors in their car it goes crazy when you pass one of these sensors.  Good post!