The Sheet Metal Shop .Com

Duct Construction and Uses - Good HVAC topic

pricer - May 19, 2004 - 03:36 AM
Post subject: Good HVAC topic

Air flow, hmmmmm. I think air flow is one of the most misunderstood aspects of our trade. Through the years I have meet people who could build many different fittings and had never taken in account the aspects of air flow. I myself have been guilty of this but after several bad experiences (learning experiences) with TAB personnel I began to learn. During that time I would study up of air flow. It started out slow for me, trying to understand what I could not see, air. I still learn more as I go and I am no expert by any means. When I design fittings and work out problems in the field the first thing I think of is air flow. That is what it is all about.

We had a problem one time with a set of heat strips that would burn out on a regular basis. By the time the problem had come back to me, I went to investigate. In a leaning process for me, the boss told me to take some Plexiglas and make an inspection access in the location of the strips. Once this was cut in we started the strips and you could clearly see hot spots where the strip had broken before. The strips were mounted within 5 feet of a radius elbow. With the inspection glass in place and looking at the glowing parts of the strip you could basically see the air. I then began experimenting with strips of metal, using them as temporary veins; I could redirect the air to get a more even flow across the strips. When a good vein position was found a permanent vein was installed, problem solved. From that day forward, I have been particular about the way elbows were made in my shop. The standard in my shop is a radius elbow with 1 ½ times throat radius or greater and at least one continuous turning vein. It makes a big difference. I have experimented with various elbow measuring pressure drop and the standard for us seems to be the best,

Thanks,

BobLuland - May 23, 2004 - 09:33 PM
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Pricer me boy, you picked a great subject to talk about! You have to get down to the root of the problem in our industry. Training? We are in a business that requires 110% trained personal and there’s only 50% to fill it. The trade schools teach antiquated practices to students that are coming out into a field were the technology changes buy the day not the year. Even with thirty years in the field as an industrial tech, I’m in school constantly. We have our own classroom in our faculty. We have no choice. What the tech schools are turning out are more confused when they graduate than when they entered. Keep in mind that these teacher these schools employ are not rocket scientists, if they were, they would be out in the field pulling down 100K a year. They don’t teach these kids anything about air flow other than if you don’t have 400 cfm per ton the coils going to freeze. I had a great job the other day at a Subway restaurant in the big lemon. The owner got me though a recommendation from Train Corporate. Their air handler has been pouring water down on them for two years. According to the owner they have had eight techs come and investigate this problem over the last two years. It took me three hours to resolve the problem (and that included driving around and Manhattan city block four times looking for parking. $125.00 per ticket isn’t the Big Apple Great, and if you just happen to be wondering, yes our offices were four blocks from the Trade Center). The problem was simple. Almost all air handlers come from the manufacture set to handle 1” static pressure. If you have a free flow return you will almost guarantee that the velocity of that air will pull the water off that coil and send it down stream. We have a standard at New England about these installations. We always install a mechanical damper in the return and use a metric-hela gauge to measure static and adjust before we end up ruining $10,000.00 worth the ceilings. These units were never designed to have their motor pulley’s opened far enough to facilitate a free flow return. The sheet metal industry is no different when it comes training. You ask one of my guys what the difference is between a regular fire damper and a dynamic damper is, they know. Why? Because we invested the money in training. There’s nothing worse than a kid that has been taught by a butcher for a year and then you’ve go a reprogram him or her. Airflow training is essential to this industry, weather service tech or sheet metal installer. As far as the duct heater problem you had. The duct was sized to big. If it had been sized properly the velocity of the air would have filled every cavity of that duct. I do not like radius elbows (1.5 radius. You guys in Alabama must have some room in thoses ceiling to play with). Give me square with turning vains. Faster to produce and tighter spaces. They are the greatest for dispersing air evenly. Training, Training, Training! Bob

Bud - May 23, 2004 - 10:00 PM
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There were turning vanes that were designed to tight (manufactured this way) I'm not sure of the specifics on this, but we pulled them out plugged about 50 percent shut with dirt and grime...I know that maintenance (lack of) was the problem and this isn't an isolated incident.

Do you fab your own turning vanes? or are they prebuilt racks and you just add the blades you need for the proper size?

Bud

pricer - May 24, 2004 - 01:04 PM
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If I use turning vanes for anything, I use bought vane rail and make my own vanes. I do not like the double thick vanes and only use them when it is an absolute. Here is a photo of what typically is made in my shop.

Thanks,

Photo will not load. I may have run out of space.

Bud - May 25, 2004 - 01:40 AM
Post subject:

pricer wrote:

If I use turning vanes for anything, I use bought vane rail and make my own vanes. I do not like the double thick vanes and only use them when it is an absolute. Here is a photo of what typically is made in my shop.

Thanks,

Photo will not load. I may have run out of space.

Pricer, you are only at about 30% of yoor allowed space:) Try to reduce the photo?

Bud

pricer - May 25, 2004 - 11:46 PM
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That was the problem. Thanks

bordontn - Jun 13, 2004 - 03:08 AM
Post subject: turning vanes

most engineers have finally discovered their airfoil vanes offer more resistance than a single thickness. We shear a 6 1/2 width/use our cleatbender/then our roller for the radius required to fit the vane rail. This 1/2"hem reinforces the vane. You can fab and install a set of vanes before you can layout and cut your radius ell.It takes minutes to layout a square 90. You only have 2 shear cuts & 2 hand cuts on a sq. 90. Most of this you can do in your head...No trammel/no compass...Most specs now ask for an extended trailing edge on your vane. Always sumpin',Huh? Supposed to stop your air from turning after it's done it's job...The idea with an airfoil vane....Like an airplane wing..The air moves faster over the long side..On a wing this causes the lift..Anyway, Idea

I opt for sq. w/ vanes.........bordontn 42 years of deticated service

BobLuland - Jun 13, 2004 - 01:00 PM
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Keep in mind also that with a radius elbow the air has a tendency to want to ride the heel. The vains solve this problem by evenly distributing the air forward (the electric duct heater problem). Although there are some draw backs with vains (most of all our stuff is internally lined and if required in the spec that you have to put a hat track under the rail it can become time consuming). As far as manufacturing your own vain, nuts. At an average cost of $.40 to .60 cents per foot for manufactured vain. There is no way you can justify making them. Sheetmetal Connecters Inc in Chicago make a really cool perforated lined vain for noise control. They also make a tool for their vain rail that that allows you to peen the vain and lock it to the rail in seconds. Want to speed things up even more? Take some scrap plywood (no not a full sheet. This two has tripled in price) and screw together until is about three to four inches thick. Lay out a pattern of vain holes in a straight line and cut with jigsaw. You have just created a super duper vain assembly tool for holding the little suckers that will never sell on E-bay. Bob

Bud - Jun 13, 2004 - 01:26 PM
Post subject:

BobLuland wrote:

Keep in mind also that with a radius elbow the air has a tendency to want to ride the heel. The vains solve this problem by evenly distributing the air forward (the electric duct heater problem). Although there are some draw backs with vains (most of all our stuff is internally lined and if required in the spec that you have to put a hat track under the rail it can become time consuming). As far as manufacturing your own vain, nuts. At an average cost of $.40 to .60 cents per foot for manufactured vain. There is no way you can justify making them. Sheetmetal Connecters Inc in Chicago make a really cool perforated lined vain for noise control. They also make a tool for their vain rail that that allows you to peen the vain and lock it to the rail in seconds. Want to speed things up even more? Take some scrap plywood (no not a full sheet. This two has tripled in price) and screw together until is about three to four inches thick. Lay out a pattern of vain holes in a straight line and cut with jigsaw. You have just created a super duper vain assembly tool for holding the little suckers that will never sell on E-bay. Bob

What is the distance apart the Vains should be? Is it always one distance of does it change with the velocity or the size of the elbow, also do you place vains in 45 degree elbows?

Bud

BobLuland - Jun 13, 2004 - 09:03 PM
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Bud, Vain rail comes spaced as per Smacna standard. Always at 45 degrees. If I have to make a 45-degree fitting it is always radius. If some one made a 22.5 degree vain. That would change to. Bob

Bud - Jun 14, 2004 - 01:27 PM
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What kind of turning vane do you use, I went to DuctMate to look at theirs and it seamss they are all double wall? The following information I found doesn't reccomend this

Elbow types range from mitered to long radius. Long radiused elbows are most efficient. An elbow with a centerline radius (r/D or r/W) of 1.5 is very efficient (C=0.19) and should be used in cases where duct air velocity is 13 m/s (2,500 fpm) or higher. A standard radiused elbow (r/D of 1.0) is more economical and only slightly less efficient (C=0.23); it is generally preferred. Sometimes only a mitered elbow (C=1.15) will fit into the space provided. In this case, a turning vane will reduce pressure losses if properly installed. However, a turning vane should not be used in a transitional (drop check) elbow or an elbow with an angle other than 90°. Accurate turning vane installation is critical to performance, so factory-made or carefully fabricated shop units are necessary. Improperly installed turning vanes can be counterproductive. A double thickness turning vane is more costly and its performance is much poorer than that of a single vane; however, a double thickness vane is required when spanning 0.9 m (3 ft) or more because of structural needs. Turning vanes have been known to become dislodged and have substantially blocked airflow because of seismic disturbance. There are almost always alternatives to the mitered elbow and vane combination. A very short radiused elbow with a single fully radiused vane (C=0.43) is often a good compromise. [Rozell, 1974]

Resource is from http://ateam.lbl.gov/Design-Guide/DGHtm/elbows.htm

JJ38 - Jul 07, 2004 - 03:43 AM
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bud is 100 percent right the long radis is the best also on the double thicks the reg vane rail the heavy duro dyne and make your own vanes 7 and and a eight long also we adjust center and then stack 2 smaller ones toget the size with no noise

pricer - Jul 07, 2004 - 03:52 AM
Post subject:

JJ38 wrote:

bud is 100 percent right the long radis is the best also on the double thicks the reg vane rail the heavy duro dyne and make your own vanes 7 and and a eight long also we adjust center and then stack 2 smaller ones toget the size with no noise

Thanks for the info and welcome to the"Shop".

JJ38 - Jul 19, 2004 - 02:23 AM
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pricer thank you for the welcome any way i can help any one just let me know

pricer - Jul 19, 2004 - 03:28 AM
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JJ38 wrote:

pricer thank you for the welcome any way i can help any one just let me know

After the summer I am sure things will begin heating up here. Look forward to hearing from you again, more air flow talk in store........

EBEBRATT - May 01, 2005 - 01:53 AM
Post subject: Electric duct heaters and water pulled off coils

Electric duct heaters are very specific with regard to the velocity of the air and the space around the heater. They should be installed a sufficient distance from any ob struction so that even air flow is obtained. The manufactures recomendations must be followed to avoid hot spots, overheating and other problems. They are usually ordered for a specific duct size-cant just stick any duct heater in any size duct.

Water pulled off of cooling coils-some manufacturers insist on trying to reinvent the wheel-it aint rocket science. Velocity across a dx or chilled water coil is 500fpm max-you can push it to 650-700 but must install moisture elimenators. This is the first step in air handler sizing. The air quantity needed at 500 fpm sizees the AHU "Box"-fan sizes, coil sizes all work off of this

Ed

meek - May 02, 2005 - 02:42 AM
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There are a couple of resources worth looking at. One is the back of ACCA manual D. It has the equvalent length of each fitting including with or without vanes in the appendix. I was just learning this stuff when the boss asked the install lead to put ductwork in her house. When he got done and it didn't work he said "I did it buy my ductulator and it isn't nearly 100'". I went over it and with the 90's and stuff it came to 420'. The other thing that I thought was really neat is the oldddd video that smacna has of bubbles in clear ductwork. Really helps visualize it. It is so old it's filmed in B&W. Finally I like to tell customers ductwork should look like a superhighway. would cars crash if they had to make that bend? Are there lane guides? Air is high speed particles, our fans give them motion, we try to keep them from stopping on the way to their destination. The fan is the pitching machine, how far is that baseball going to fly?

kodie - May 16, 2005 - 03:42 AM
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most of the contractors i have work for only put vanes on everyother tab on the rail . from what i understand you might as well leaveout turning vanes if you are going to do that . they think they are saving money

MechAcc - Sep 03, 2006 - 02:37 PM
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Good idea about the plexiglass window.

SMACNA has a video about air flow myths. They built plexiglass duct and fittings with a variable speed air handler. Helium injected bubble machine. Eye opening about fittings that you would think would get the best airflow turn out to be the worst. If you haven't had a chance to view the video try to get a copy.

Hazardousal - Nov 16, 2006 - 04:38 AM
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We use vane rail in our shop, its for double but only put single vanes in it.
Took duct work out of an animal hospital, hair in turning vanes, I dont know how they had any air flow. lol

There is a diffuser manufactuer web site that has some neat videos using smoke in the air to show the flow out of different diffusers.

Difuser placment, type, and install location can be quite a problem for your air flow as well

till then...

Al.

legacy - Nov 16, 2006 - 08:43 PM
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MechAcc-

I have seen that video, it is an eye opener

I am always amused to see the use of the square throat/radius heel elbow on a big commercial job. It is usually a dead giveaway that the the work was performed by a residential shop.

I had the pleasure of meeting a fellow from the ITI that had a lot of input into the making of that video. He had just retired and was moving from CA to somewhere in the Smokey Mountains. Being that he had worked his life in union sheet metal, he insisted on finding a union shop to install the duct in his new home. It took some doing, but he located a union shop to install the duct. One week, the fellow flew in to check on the progress of his new home only to find that the installer had used the worst of worst fittings. He made them rip them all out and redo the entire job using the correct fittings.

Dan

jw05450 - Feb 09, 2007 - 11:52 PM
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I was recently working for Boston air systems at wellsley college in wellsley mass, We put in some square throat and heal elbows and the company that they were purchased from had skipped every other vain.Long story short the engineer made us put in the extra vains that were left out.Kinda stinks putting them vains in after all the duct is installed.