Hole size for PVC fire gear dryer

[teamz]

I'm constructing an air dryer for our fire department's PPE (turnout, bunker gear) using PVC pipe and a blower and need to determine the best hole size for all the PVC pipe and fittings (lots of holes throughout the PVC to supply air drying). The blower is a Zoom W4L 750W blower producing 700 CMF. To distribute the air, the base structure consists of a 2' x 8' box built using standard 2x6 lumber with a 3/4" plywood top and bottom, and 3" holes. Mounted to the top are eight capped 4 foot long 3" PVC pipes to dry 4 pairs of pants. For coats, there are four 4' long 3" PVC pipes each connecting to a 3" Tee which is reduced to 2" for the shoulder followed by 45 deg fitting that reduces to 18" of 1 -1/2" PVC pipe for the arms and then capped; imagine a stick figure. There will also be six capped 1-1/4" PVC pipes about a foot long for drying gloves and hoodies. The blower will be mounted in the middle facing down into the box to help distribute the air; perhaps with a cone mounted below it. The pipe mounting for pants and coats will most likely alternate with 2 coats and 2 pants on each side of the blower. I'm anticipating the need for about 700 or less holes. How do I calculate the best hole size to drill in the PVC pipe and fittings? I'm concerned that small holes could result in too fast of an air stream that might damage the PPE and/or cause too much back pressure and burn out the blower. And if the holes are too big, I'm concerned that not all PVC pipes will get sufficient airflow. I can always start with smaller holes and make them larger (700 more holes to drill out) but not conversely so I'm hoping to select the best size initially. I've attempted some calculations for 1/4", 5/16", 3/8", and 1/2" as I'm guessing these are most probable size have seen examples of "DIY PVC fire gear dryers" on the internet.; the results are velocity but not sure what makes sense. For simplicity I'm hoping to use the same hole size throughout all PVC pipes and fittings. Any suggestions would be appreciated, especially knowing how to calculate this seeing that my number of holes is just an estimate at this time. Thank you!

 

[dvd]

You might consider approaching you nearest engineering school and discussing this with a mechanical engineering professor. It might make a good project that the students could solve and apply some engineering theory to.

How did you decide on the blower size? It seems like you should start at the nozzle outlet flow and size back from there. Any thoughts on how you will deburr all of those drilled holes on the inside of the pipe?

Think about contacting a nozzle supplier. Spraying Systems sells air nozzles. A phone call with an applications engineer might provide some insights.

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[MintJulep]

too much back pressure and burn out the blower.

That's probably not a concern, but you need to know the blower type, or just do a simple test.

The way the gear is put on will probably far outweigh the effect of hole size.

 

[Snickster]

So you want to dry about 2 pair of pants and 2 coats at the same time, or do you expect all drying pipe assemblies would be used to dry 4 pants and 4 coats at the same time? Along with 4 pairs of gloves? I think you would want the flexibility to dry any number of pants or coats independently. In such case you would install a block valve at the base of each pipe where it connects to the distribution box to shut off flow to assemblies not in use.

I believe you might want to go with 4" PVC. For 3" PVC Sch 40 at 100 SCFM, the pressure drop per 100 feet is about 3 in. wg. and for 200 SCFM 10 in. wg. at 4000 feet per minute velocity which is very high. For 4" PVC at 100 SCFM pressure drop per 100 feet is about 0.7 in wg. and at 200 SCFM 2 in.wg at 2300 feet per minute velocity which is more in line with typical design. I would think you would want the flexibility to flow at least 200 SCFM through any one assembly if you are not drying too many pieces at one time and the flow is available.

Your blower I believe outputs about 4" wg at 700 SCFM if you assume a 50% mechanical efficiency for the 1 HP zoom blower.

I would consider making total area of holes in pipe equal to the flow area of the pipe itself as a minimum. This will give the same velocity in the holes that are in the pipe. For instance for 4" ID pipe flow area is Pi/4(D)^2= 3.14/4 (4)^2 = 12.6 square inches. Therefore number of holes N for given diameter of hole in 4" pipe:

(N) (3.14/4 (Dhole)^2) = 12.6

This will give a pressure drop of about 1 inch wg. through holes if velocity is 2300 feet per minute as indicated above for 4" pipe.

Without doing detailed calculations I believe then the total pressure loss at flow will not exceed 2" wg. in distribution box and pipes, which will leave about 2" wg available for drop through clothes. I assume when you dry the clothes you will block/tie off the ends of the pants /sleeves of coats to force the air through the fabric so you need some pressure available to do this.

If you have a definite configuration in mind I will check for you what kind of flows you get and what are pressure drops.