# How to Measure Air Flow in a Large Tunnel at Various Different Points?

I am a student who is interning at a company this summer, and I've been put in charge of its first R&D project which will require me to collect a lot of data. The machine with which I am working is called a Tunnel Finisher. It is essentially a long tunnel through which clothes are pulled in order to dry them and remove wrinkles after a wash cycle. Hot air is blown down on the clothes through louvers from a shaft above the tunnel which leads from a fan that maintains the flow of air. What I would like to do is measure air flow, above the clothes, at the level of the clothes, and below the clothes, at multiple points in the tunnel. Because the air in the tunnel is hot and clothes move through it, this measurement cannot be done manually; I need sensors that can be placed in the tunnel, preferably stuck to the walls, that will provide data while the machine is operating normally. The goal of this endeavor is to reveal dead spots of significantly less air flow in the machine.

I've looked at some different kinds of sensors, but I don't really know what I'm looking for, and I assume what I end up using shouldn't be too expensive. I was hoping someone here could provide some insight as to which air flow sensors, or other type of sensor, I should be looking at for this purpose.

• By "hot", do you mean temperatures in the 300K-370K range? Will humidity present a corrosion problem to normal electronics? – Spencer Joplin May 31 '17 at 4:29
• I was told more in the 300*F range, so greater than 400 K as far as I know. I would have to ask about humidity... But I'm given to understand that the chamber in which the sensors would be placed would not be overly humid. The clothes going through it will be a bit damp, but the space inside is otherwise dry, with hot, dry air blowing down on the clothes. – Carly May 31 '17 at 12:06

I picked out several things of interest from your question.

1. You seem to be interested in measuring the airflow near the walls of the tunnel
2. You seem to be interested in finding areas of (relatively) low flow, but don't seem to be interested in the specific flow rate or velocity.

From these two items, I offer that you would be better served by a flow visualization technique, rather than a free stream velocity measurement technique. For your application, I would recommend the use of strings, as is commonly done in low speed wind tunnels.

These have several advantages. They are VERY low cost, show differences fairly easily, and can be placed anywhere on the walls of the tunnel and later removed. Unfortunately, they require visual monitoring (e.g. you have to see them or be able to take video) and they provide limited quantifiable data. However, they are a very simple and efficient way to visualize the flow field in the tunnel, and thereby determine where the areas of low flow may be.

How about a pitot tube that you can draw across the width of the tunnel - had an experiment where the velocity profile was measured before and after a fan - smaller scale (pipe was 6 inches in diameter) but you could even have one tube that moves or make a "comb" of 20 or 30 which would then be stationary.

• After looking into these, I'm unsure if these will suit my purpose. They sound like they're more for ducts, when what I'm working with is a tunnel large enough for a person to comfortably stand inside. It seems like it might be difficult to get the static pressure readings in such a large tunnel, and also that the pitot tube readings may be skewed because in normal circumstances there are clothes running through the machine. This may, however, be still useful for experimentation with no clothes. – Carly May 31 '17 at 13:01
• Nice to hear you looked - it was only a suggestion, best of luck. – Solar Mike May 31 '17 at 13:07

I can't give you a black and white answer to this without seeing it but a fair bet would be using ultrasonic sensors. These systems work by having pairs of ultrasonic transducers placed on the outside of an enclosure at angles, pointing towards eachother in the axis the airflow is travelling. One transducer will 'ping' the other, and the recieving one will record how long that ping took to reach it. It will then ping the first sensor, and that in turn records how long it took to receive. If you have air flow inside the enclosure, one ping will take longer than the other because one sensor is effectively travelling slower than the speed of sound (fighting upstream against the air flow) and the other faster (sailing downstream with the air flow). The time difference is directly propotional to the air speed.

As long as you have access to the outsides of the enclosure at your desired measurement points this could work, but they do have limits with regards to the diameter of the enclosure.

This link from E&H has a great video which demonstrates what i've written much more clearly, Their clamp-on sensors have a distance limit of 4m.

Another indirect way to find dead spots could maybe be to use humitidy sensors along the line (in known good spots if possible) and then place others in suspected dead spots. I'd assume the humidity would be higher in the dead-spots wouldn't you think?

You can measure evaporation directly: have smal glass vials with a (known) amont of water and a small opening on top, place these at various places and document evaporation over a cycle. If the tunnel is a dryer, you will have very moist air at some points and so no ecvaporation of water - then you should find something else, maybe an organic solvent with a not too high vapor pressure. In this case consider health and safety aspects - I imagine you use a total of 100ml or so at most, but think it through anyway.

You can also do thermal measurements: measure the current needed to maintain a sensor at a certain $\Delta T$ above air temp.The more energy needed, the more flow. Thisis available as propersensor for 2000 € or so (http://www.bindergroup.info/binder/en/products/flow/gasflowmeasure.php) but since you only need a qualitative statement you can maybe improvise something yourself.