measuring air flows across air cooled condensers

[2851971raj]

Hai
i work for a manufacturer of air cooled heat exchangers. we would like to do some air flow tests in our labe on our range of air cooled condnesers. can somebody suggest the best possible way of doing that using a hot wire anemometer probe or rotating vane anemometer, i.e points or rig to set up to get an accurate estimate of the air flow.
thanks

 

[quark]

Raj!

Hot wire anemometer has better accuracy. But your accuracy in result will depend upon how many readings you have taken to find out the average velocity.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[2851971raj]

hai quark
would you know how many points i need to measure (minimum)?

 

[quark]

Generally we used to take 16 readings for a filter area of 2'x2'. That depends upon size of ther radiator. If the velocity difference between the outer periphery and the middle of the radiator is more take more number of readings.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[2851971raj]

hai quark
the condenser i was measuring was a 800x762mm one. so i divided the area according to Log-Tebycheff (ASHRAE) rule and measured air flows at 30 points with a hot wire anemometer and took the average, will that be fine? althogh the rule is generally for ducts, i decided to use it here.
thanks
raj

 

[quark]

Raj!

30 readings are ok. Anyhow check the deviation of average value to the peripheral value. If there is not much difference it is ok.

Regards,
Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[ChasBean1]

2851971raj -

Is this ACCU a standard type of unit such as one that serves a home's central air system? If so, the cumulative error from hotwire or vane anemometer measurements I think would be high. Plus knowing the true net open area between the fins is almost unfathomable, unless thoroughly studied by the manufacturer.

If these are standard ACCUs with fans drawing air through the coils and blowing upward, I would rather see you capture the effluent fan flow. Depending on the size of the fan, this could possibly be done with a 2' x 2' flow hood. The flow hood (a.k.a., balometer) might, however, create too high a pressure drop with a standard propeller fan.

So as an alternative, a tall outlet tube (with diameter equal to the diameter of the outlet of the fan) should be constructed with 3/8" traverse holes drilled at 90° offset angles toward the top of the tube.

Seal the base of the tube to the fan shroud by some scientific means such as duct tape. Traverse readings can then be taken at the fan outlet, at several points perpendicularlly-angled, to obtain an average outlet flow rate.

 

[2851971raj]

hai Chasbean1

thanks for your input. i will try your method out.

another questions is with regards to testing different fans. could you give me some guidlines to build a fan rig to test different fans? what i did in the lab here was to mount the fan in front of 3 different coil blocks and measure the air flows using hotwire anemometer doing a traverse across the finned coils? can we use the tube technique for these tests as well?

 

[ChasBean1]

Most definitely. Your worst case scenario is the standard propellor fan, which generally produces a high air volume without the ability to handle pressure restriction. If you switch to any other fan like a centrifugal, you can probably go even easier by using an Alnor, Shortridge, or TSI flow hood. Reason is that these type of fans (ones which maintain flow at higher design differential pressures) will be less affected by any additional pressure drop applied by the hood.

The main goal is to obtain a decent flow measuring point without having the flow measuring apparatus create too much restriction. This can be a little bit of a battle as flow measuring instrumentation often, by its very nature, creates restriction. Use a wide tube and hot wire or pitot device to solve most of these problems.

You can construct standard rigs by hooking up with a sheetmetal company and having stock, 24-inch round, 10-foot high sheetmetal cylinders manufactured (plus other sizes as required to match various fan shroud diameters). For a 24-inch round, 10-foot tall piece of round duct, drill the hotwire traverse holes at about +7 feet up the tube. Drill the holes roughly proportionally for other duct diameters.

Best of luck, -CB