I am assessing the performance of a

[Drogheda]

I am assessing the performance of air coolers and have encountered a difference of opinion about where the face velocity of air entering the tube bundle should be measured. The air coolers are induced draft type with relatively easy access to the lower tube rows. One view is that the anemometer should be placed as close as possible to the finned tubes. The other view is that the anemometer should be placed at least 150mm clear of the tubes. The two methods produce significantly different air velocities, making the difference between adequate or inadequate air flow. Can anyone shed any light on the correct location as intended by the design method, and if possible a reference to where this can be found as an authoritative source.

 

[chicopee]

At the location of the anemometer, you should know the air pressure and temperature to do calculations. Air mass can also be figured out at the exit point of the tube bundle. In all likelihood at 150mm of the tubes, the air velocity is probably close to zero. A course in thermodynamics coupled with heat transfer is needed to solve such problem. Fluid dynamics can also be used for compressible fluids for this analysis but you'll need to make assumptions such as is the problem to be treated as either an isentropic flow, isothermal or adiabic problem or a mixture of the three types of flows. Get the help of a graduate ME M

 

[dbill74]

Ideally, you would measure at the front face of the coils.

If you have to remove a panel to get to the coils and the panel exposes both sides of the coil, then any reading will be off due to the absence of the panel.

Perhaps a diagram or photo of the unit(s) will give us a better idea of what you are trying to assess and how. We can then better advise you on how to find what you are looking for.

 

[Drogheda]

I have attached a photo showing an example of the locations for air flow measurement. It is an induced draft arrangement, so we have access to the finned surface as shown. I hope the file attachment is accessible - I have not used this particular file sharing system previously.
Anemometer measurements show that there is a significant difference in air velocity between placing the anemometer in contact with the fins ("face of bundle") and 150mm from fins.
Measurements at 150mm from the fins indicates insufficient air flow, whereas when measured at the fins, the air flow IS adequate (better than design - that is, fans are pulling enough draft).
Since I first posted here, there have been efforts by a number of parties to find a definitive location for air flow measurements but to no avail. We are using HTRI software, and it's definition of face velocity is the velocity of the air as it "approaches the first row of tubes".
I believe that this definition originates from forced draft configuration where the air flow measurement is downstream from the fan, and distance from tubes is not so critical. For induced draft, the measurement is upstream of the fan, and I believe should be as close as possible to the fan - that is, anemometer in contact with the fins. Note, we measure at the fins NOT in the gap between tubes.
The distance from the finned surface to the fan centreline is about 2 metres. I have also attached a diagram to show the general arrangement.

 

[Drogheda]

The system seems to allow only one attachment per post, so attached below is a photo