#plug resistant

Measuring the Air Flow in a duct can be difficult enough when the flow profile is not free from turbulence, disturbances, cyclonic flow, and reverse flow, but add to that particulate, ash, dust, or debris in the air, and it can almost be impossible.  So, how do you get accuracy when trying to measure the Air Flow in a duct that has particulate, ash, dust, or debris in the air stream? The traditional way is to use a blast purge or blow back system that will take high pressure compressed air and blow it back through the high and low ports of the air flow pitot or flow element so that any particles, ash, dust, or debris is blown out of the ports on the probe.  This not only uses a lot of compressed air, which needs to be completely dry to work properly, but it also requires a complex system of valves in order for this to be done automatically.  If the blast purge or blow back is not done on a regular basis, and if the particulate, ash, dust, or debris level is high enough in the duct, the pitot or air flow element or probe will plug very quickly causing the measurement to be inaccurate.  An automatic system is needed to perform this purge or blow out while the system is being used to measure the flow of the air in the duct.  This adds another level of complexity since the differential pressure transmitter or multivariable transmitter signal would need to be locked out or isolated while the purge or blow out is being performed so that the system does not think that the result of sending the high pressure compressed air through the pitot or flow element is changing or so that the transmitter does not get damaged from over pressurization.  As you can see this can be a very difficult thing to do and makes the cost of measurement of an air stream with particulate much more expensive. THE VAP³ PITOT A better way to deal with this would be to use a pitot or flow element or probe that is plug resistant, not allowing the particulate, ash, dust, or debris from plugging the ports on the probe.  The VAP³ Pitot is a perfect solution to this issue.  The VAP³ probe can be configured in what is called the PA mode, thus orienting the probe so that the probe becomes resistant to plugging.

VAP³ PA Pitot from Eastern Instruments

So, how does this work?  With the probe in the PA orientation, the High Port, which is typically on the upstream end of the probe, is located on the back side or downstream end.  Having the High Port on the back side prevents the particulate, ash, dust, or debris from impacting the port, plugging it and causing the differential pressure measurement to be altered, thus making the air flow measurement inaccurate.  This patented design can be used from 1,500 ft/min to 12,500 ft/min air flow velocities, making it perfect for most duct configurations.

VAP³ Pitot

The VAP³ Pitots are an Insertion Probe style or Direct Duct Mount style using the included Insertion Ports that would be welded or connected directly to any existing ductwork.  The probes are manufactured from 6061 Aluminum (Hard Coat Anodized) or 301 Stainless Steel, and are sized to fit the duct dimensions.  An array of probes is recommended based on the duct configuration or size to obtain the highest degree of air flow measurement accuracy.

Air / Gas Flow Application Guide

When it comes to measuring the flow rate of air or gas in a duct, there are many different devices used.  For example, there are thermal sensors, hot wire anemometers, rotating vane anemometers, static pressure drop devices (venturi, orifice plate), and differential pressure devices (pitot, annubar, primary element).  The most widely used and considered a true measurement of the flow is the differential pressure device.  The most widely used differential pressure device is a velocity averaging pitot tube.

Velocity Averaging Pitot Tube

The velocity averaging pitot tube is a device that when inserted into the air or gas flow stream, provides a high pressure measurement port and a low pressure measurement port.  The high pressure port measures the total pressure created when the air or gas stream impacts the port, where the low pressure port typically measure the static pressure in the duct.

VAP³ Velocity Averaging Pitot Tube from Eastern Instruments

The difference between the two pressures, called the dynamic pressure, can be used to calculate the flow in the duct using the Bernoulli principle and equation. This equation, in a simplified form when using a pitot tube, states that the velocity in a duct is equal to the square root of two times the differential pressure, divided by the density of the air or gas.

For the best results using a pitot tube, there should be an array of probes used to ensure that there is an adequate traversing across the duct cross-section.  In addition, the individual pitot tubes would have multiple ports, thus averaging the pressure measurement in any one probe and then in the array of probes.

Velocity Averaging Pitot Tubes in a Round or Square/Rectangular Duct

VAP³ Pitots in an Array in a Circular Duct

VAP³ Pitots in an Array in a Square/Rectangular Duct

Not all pitot tubes are created equally.  Most are just a cylindrical tube or set of tubes that have holes in them, or some modification of the cylindrical design.  The flow profile around these tubes can be distorted and even non-repeatable, causing them to have a calibration factor or "K" factor based on flow rate or velocity of the air or gas stream.

In order to get the best result and not have an affect based on the flow around the pitot tube, the pitot tube should have a cross-section such that the flow pattern is not disturbed.  Such is the case with the VAP³ Velocity Averaging Pitot Tube from Eastern Instruments.  Its patented design allows the flow profile to run parallel around it, before combining together behind the probe.  This not only allows the probe to get an accurate differential pressure measurement, but makes the pitot velocity independent, eliminating the need for the K factor.

While velocity averaging pitots can provide a very accurate way to obtain the flow rate in a duct, they are susceptible to particulate in the air or gas stream.  Most pitots can plug when the particulate is large or if it is heavily particulate laden. The high port will plug, causing an error in the differential pressure measurement.  The typical way to combat this issue is to use a high pressure purge or blow out, but these systems can be complicated and expensive to run.  The VAP³ PA Pitot from Eastern Instruments uses that same patented design that allows it to be independent of velocity yet also provides a plug resistant configuration where the high port is not in the direct impact of the air or gas stream, thus making it plug resistant.

VAP³ Velocity Averaging Pitot in the SA (Clean Air/Gas) and the PA (Particulate Laden/Plug Resistant)