Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. Excessively low or high conveying velocities can each cause undesirable consequences from dust falling out in the duct to excessive duct abrasion, and there are economical advantages to optimizing the air volume in the system to ensure the desired conveying velocity. Condensate systems also consist of pans and switches. It is a win all the way around. IVI can assist in the design of a new system or the redesign of an existing system. I still subscribe to larger ducts in the attic (500 600 fpm) and simply adding a few seconds travel time and a little bit more surface area. If it were, youd always try to get the lowest velocity possible by using the biggest ducts that fit the space without blowing the budget. Amer Conf of Governmental. Additional savings may be realized as you will also reduce the problems related to duct abrasion and/or undesirable dust fallout in your ducts. The Industrial Ventilation Manual1from ACGIH is the industry-recognized resource for determining recommended design practices in industrial ventilation, and it includes guidance on carrying (or conveying) velocities. The Invisible Problem With Duct Insulation. The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. Short branches to interior walls go a long way to minimize unwanted heat transfer in ducts routed through unconditioned attics. If you have an iPhone, subscribe to the podcast HERE, and if you have an Android phone, subscribe HERE. I have a subcontractor installing ductwork that he sealed with non UL listed tape and now I am examining everything very closely. HVAC Design Protocols NOTE: Comments are moderated. I would be curious to get a DB rating effect and Mikea opinion for the various velocities in unlined sheet metal. It implies that absolute zero [], 2021 - HVAC School | All rights reserved. You also have the option to opt-out of these cookies. Bruce, yes, thats why when the ducts are exposed in an attic and have the minimum allowed insulation (R-8 usually), you want to move the air faster. On the high-voltage side, the disconnect should be in a secure location, and it should be able to keep water out. Air is not like water where water takes its course of path. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected. Your comment will not appear below until approved. Including recorded tech training, interviews, diagnostics and general conversations about the trade. I am reconsidering unvented attics. The full chart covers duct sizes ranging from 4 to 18 and air flow rates from 0 to 1,200 cfm. He has installed insulation on the outside so I cannot tell if the seams are properly sealed. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); HVAC Design If I surround, at least my main rectangular ductwork with R-45+ foil-faced polyiso rigid insulation, am I at least minimizing the effects of my hot attic on my ductwork?
Thanks for writing it! Just hard to put it to practical use being that duct system are bigger & youll need a partner with a big enough thumb! Now more resistance is better? Industrial Ventilation Manual of Recommended Practice for Design. Its also worth noting that double-lugging is a poor practice that is against code. In summer, the cool air gains heat from the surroundings. We don't save this data. Does it have any specific qualities, such as high moisture content, abrasiveness, corrosiveness or combustibility. Reduce energy costs & improve air quality. If the duct does not remain free of debris, the duct or its supports may eventually fail due to the additional loads, or the accumulation of material may create other concerns such as combustibility or corrosion. I like your article, as itI like your article, as it gets across an important point about thermal versus fan energy issues. Some of our calculators and applications let you save application data to your local computer. When that conditioned air is moving through the ducts, the second law of thermodynamics comes into play because we have a temperature difference between the inside and outside of the ducts. The key to this is when youThe key to this is when you wrote If you put ducts in conditioned space, you can move the air as slowly as youd like. Get rid of ridge, soffit and gable vents. The ductwork of an industrial dust collection system connects the systems hoods, air cleaning device(s) and fan. When you put the ducts in an unconditioned attic and have the minimum insulation allowed, you want to move the air at a higher velocity, pushing it up near the maximum recommended by ACCA Manual D, 900 feet per minute (fpm) for supply ducts and 700 fpm for return ducts. Theres a lot of factors that play in my part. I just watched Joe LstiburekI just watched Joe Lstibureks presentation on YouTube To vent or not to Vent. (You can download the full chart by clicking on the image below or the link at the bottom of this article.) It is mandatory to procure user consent prior to running these cookies on your website. I see no air going upstairs. Lets start with Q=UxAxDT. A common flaw in this approach is not remembering to close the damper back down to return to design flow when installing new filters, which no longer have the resistance from a dust build-up. But thats not the end of the story. 
Calculating the size of the duct segments can be done using a series of calculations known as the Velocity Pressure Method consisting of two primary variables: Calculations for determining the appropriate airflow (Q) are provided in the Industrial Ventilation Manual of Recommended Practice for Design. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. Your source of knowledge about the world of building science, home energy efficiency, green jobs and more. Even the best tapes will eventually lose adhesion (faster if located in attic or crawl). Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features. If the fan has the static capacity to overcome the additional energy restriction related to a higher velocity, the section of the duct may experience increased abrasions as a result of the higher velocities.
IVIs engineering and design team has years of experience designing and sizing industrial ductwork. Our industry expertise, engineering and design, innovation and client focus is recognized around the world, demonstrated by our ever-expanding international network of industry partners and clients. This website uses cookies to give you the best browsing experience. And heres what I wrote about flex duct a couple of years ago: An excellent article andAn excellent article and chart to teach the science of velocity vs duct loads vs fitting losses, etc. These cookies do not store any personal information. So now you suggest a higher velocity in the attic is better? This device monitors the static pressure in the dust system just ahead of the collector and constantly sends feedback to a variable frequency drive on the fan to adjust fan speed. At worst, you size the upstairs ducts for a maximum of 900 FPM for single-zone calls. The type of contaminants being carried through the industrial ventilation system should be understood while determining the size of its ductwork. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. document.getElementById( "ak_js_2" ).setAttribute( "value", ( new Date() ).getTime() ); By submitting this form, you are consenting to receive marketing emails from: Energy Vanguard, 533 West Howard Avenue, Suite E, Decatur GA, 30030, http://energyvanguard.com. Theres another relevant fact, however, and ignoring it can lead to trouble. 
Im in north Texas, where the attic gets pretty hot, especially in a 1980s house with no radiant barrier, and lousy insulation. Necessary cookies are absolutely essential for the website to function properly. So check with your building inspector before doing anything else. Dan wrote Yes, zoning mayDan wrote Yes, zoning may help the comfort issue. If you put ducts in conditioned space, you can move the air as slowly as youd like. The humidity left for a couple hours, but later after 9pm returned for the rest of the week. The optimum conveying velocity may vary depending on the type of dust in the application, but there are several design practices and devices you can use to select and maintain the optimum air volume and resulting velocity. Is Low Velocity Bad for Air Flow in Ducts? I hate to be the bearer ofI hate to be the bearer of bad news but all seams (including boots) should have been sealed with duct mastic, not tape! By clicking Accept All, you consent to the use of ALL the cookies. 
Learn how your comment data is processed. My point is that hot air in the attic goes nowhere but down & all around inside the house, even if theres a cool breeze & clouds that came out of nowhere on which was a 95 degree day at around 4pm late afternoon in which the day was humid. The first thing to know about the velocity of air moving through ducts is that the slower you get the air moving, the better it is for air flow. (Figure 1). Yes, zoning may help the comfort issue, but now the downstairs stat satisfies and Im putting most of my air in a smaller duct to the upstairs, now my 900 fpm is up to Too much! We can think of the condensate assembly as its own system.
1 story with a attic and 3 gable openings & crawlspace. AddThis use cookies for handling links to social media. I dont see how you can condition a 3-level home WITHOUT zone control. Logical Fallacies False Cause & Strawman, Tips for Learning in HVAC/R w/ Trevor Matthews, Giving Kids The Tools For Any Job They Want, Refrigerant Changes Coming & The Helix w/ Emerson, Something to Watch For When Replacing a Thermostat, High air velocity at supply registers and return grilles resulting in air noise, Low velocity in certain ductsresulting in unnecessary gains and losses, Low velocity at supply registers resulting in poor throw and, therefore, room temperature control, High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal, High TESP (total external static pressure) due to high duct velocity, Single vs. multiple drains with other appliances, Drain pitch and if there could be too much fall. You also have the option to opt-out of these cookies. This error can lead to excessive air volume at the hoods, which may create problems such as collecting unwanted product. If you have ever sat in a theater, mall, or auditorium and been hit in the face with an airstream from a vent 20 feet away, you have experienced HIGH-designed velocity. Maintaining sufficient velocities to prevent dust from dropping in the duct is important, but there are negative impacts resulting from excessive duct velocities. And its not just time of contact thats responsible. There are a several methods for controlling conveying velocities the first being good initial duct design. The air duct velocity also known as air flow velocity is a measurement of the air that moves through your HVAC systems air supply and return grilles. In those cases, redundancy is desirable to prevent overflowing. That was the main point of my last article. 10 Building Science Lies to Watch out for, My Undersized Mini-Split Heat Pump in a Heat Wave, Make It a Mechanical Room, Not Just a Closet, Introduction to Whole-House Electricity Monitoring, Air Conditioner Sizing: Load Calculations vs Rules of Thumb, Five Fun Facts About Dew Point Temperature, Efficient Hot Water Delivery With a Simple Tool, Insulation level (as given by U, the heat transfer coefficient, or R, the resistance to heat flow). Thats horrible. At times the red brick is cooler than the concrete & the freeway concrete gives off an extremely hotter amount of heat, being surrounded by it in the big city. Upstairs has a chase to (sorry, the builder made me do it) the attic and the upstairs has insulated flex duct to the ceiling registers. Need to see it better, turn off lights light a candle, now watch your smoke as you blow it into the vent, some will leave into the vent but not all of it, as I pointed out before blowback will be seen more clearly with lights out, candle on & smoke source of your choice! Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website.
Sustainable Economics. (2013). This cookie is set by GDPR Cookie Consent plugin. These cookies will be stored in your browser only with your consent. For example, some jurisdictions require mastic instead of UL-181A-P tape on metal seams. path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/downflo-evolution, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/packaged-downflo-evolution, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/downflo-oval, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/downflo-workstation, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/vibra-shake, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/downdraft-bench, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/ambient-air-tubesheet, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/bin-vent, path: /content/donaldson/en-us/industrial-dust-fume-mist/equipment/dust-collectors/cartridge/td-series. Download Mike MacFarlands Duct Sizing Tool. Cleanouts and vents may be confused for each other, but cleanouts allow the technician to access and clean the drain.
Necessary cookies are absolutely essential for the website to function properly. Seal the attic space off and insulate under the roof deck. The duct selection process should ensure proper conveying velocity based on the design air volume. Furthermore, it requires much more energy to move air at a higher velocity. I asked him no to install the insulation before I inspected the seams and he packed up and left the job. This adjustment to fan speed can then maintain consistent air volume through the duct system, and, because air volume and duct velocity are proportional, the conveying velocity in the ducts remains stable and the system operates consistently at design conditions, ensuring effective dust collection. As for air residence time, that has no direct influence on duct heat gain or loss. Thomas, yes, the first priority should be to get the ducts into the conditioned space. Only emails and answers are saved in our archive. One of the challenges in practical applications is that systems may not operate at their ideal design air volume at all times. Heres the bottom part of the chart, covering air flow rates up to 300 cfm: If you need a duct to move 100 cfm, for example, youd go with a 7 duct if its in conditioned space, a 6 duct if its buried deeply in the attic insulation, and a 5 duct for exposed ducts in an unconditioned attic. As an example, lead dust, which is relatively heavy, typically requires a minimum conveying velocity of 4,500 feet per minute, whereas weld fume, which is much lighter, may require 2,000 2,500 feet per minute. Barring that, get them insulated as well as possible, by burying them in insulation if you can. I would however not say: If you put ducts in conditioned space, you can move the air as slowly as youd like. If you slow things down too much, the thermal losses defeat the purpose of the the ducts, which is to get the heating or cooling distributed to particular rooms. Your email address will not be published. Lighter particulate can be kept in motion with lower conveying velocities, which may help reduce your operating costs. What size of material is being transported? 1Industrial Ventilation: A Manual of Recommended Practice for Design (28th ed.). Like to know how I came to this very interesting point on which no one makes no mention of! You can target the Engineering ToolBox by using AdWords Managed Placements. It does not store any personal data.
Whether you are dealing with light or heavy particulate, you must design the ducts to prevent material from settling out of the air steam inside the duct. The other factor is how much air is involved in picking up each BTU. Thanks for the great article and helping my compatriots to understand air flow. Its to move heated air in winter and cooled air in summer. Measuring grille/registerface velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer. In fact, some jurisdictions now require equipment zoning or automatic air zoning in multilevel homes. On a side note, as a big hydronics junkie, the size of ducts in conditioned space at that slow of a velocity means huge ducts, and enforces the need to transferring the BTU through water pipe rather then air pipe. Of course that means lower velocity when more than one zone is calling, thus a bit more heat gain in attic runs during multi-zone calls. But that just tells you how many BTUs go into or out of the duct in an hour. Ducts can and will carry the right amounts of air to the required space if the design is accurate and based on design conditions. The important message here is that designers should be aware of all these trade-offs and know how to estimate the impacts as they work toward an optimal design. Duct velocity has many ramifications in a system, including: Duct FPM can be measured using a pitot tube and a sensitive manometer, induct vane anemometers like the Testo 416, or a hot wire anemometer like the Testo 425. Your point about velocity with air-zoned systems is manageable. But I dont really want a huge amount of noise from my ducts, plus that whole article about the effective length goes up as velocity increases. Required fields are marked *. (Figure 2). It may also increase abrasion rates in the duct system, as well as dramatically reduce the effective life of the filters, requiring more frequent replacement and servicing. Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. If you use closed cell foam under the deck, you get the added benefit of increased structural integrity by gluing the deck to the rafters/trusses. These cookies will be stored in your browser only with your consent.
How can I tell if a duct isHow can I tell if a duct is poorly made? Thus, the overall R-value of the duct (wall insulation plus surface convective resistances on each side) goes up with decreased air velocity, but this effect is small compared to the wall resistance if you have insulated ductwork. It is common to keep main duct velocity above 20 m/s (3940 ft/min). If ducts are inaccessible, theres a technique called AeroSeal that seals ducts from the inside, but its expensive. You can limit capital costs on the systems fan, and you can reduce the everyday operating expense for that fan by operating at appropriate and optimized conveying velocities. Air is also required to overcome the resistance of the duct and other parts of the industrial dust collection system. The cookie is used to store the user consent for the cookies in the category "Performance".
Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. Cleanouts are also capped when in use, but vents are not. However, the damper method is only effective if employees monitor the situation. When conveying velocities are not sufficient to keep the material moving through the duct, the potential exists for the dust to dropout and settle in the duct. These applications will - due to browser restrictions - send data between your browser and our server. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. More typically, the fan will not have sufficient static capacity, and, as a consequence, the total air volume will drop resulting in reduced capture performance at the hood and an overall reduction in the systems performance. Subscribe to the Energy Vanguard Newsletter. Other very important aspectOther very important aspect is to maintain a certain minimum velocity of airflow at discharge grilles and diffusers to provide good throw of the air jets and good mixing of air in the room (which is another great topic for article). Great article and very usefulGreat article and very useful guidelines for considering supply and return air velocities! If you decrease the air velocity, U also decreases. The insulation ratings also need to be appropriate. When it comes to dust collection, it is essential to understand the importance of conveying velocities and their impact on the proper design of a system. In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 500 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses. This can be measured in either feet- -per-minute (FPM) or meters-per-second (m/s). My larger ducts help so many airflow resistance challenges. I would be happy to share those if you would like. Overall, best practices include using proper grommets and ensuring that you have a solid connection. And the amount of heat that flows between the duct and its surroundings depends on three things: The equation that ties these things together is: Q here is the rate of heat flow and the units we use for it here in the US are British thermal units per hour (BTU/hr). Instant leak finder. Secondary drain pans should be large enough to overlap with the primary pan, especially in horizontal air handlers. The dominant term in the above equation is area (A) which is the duct surface area. Brian, thats a great question. Please, stop smoking and try a better approach to duct and airflow analysis. Data Collection I know, but getting your grandparents to understand what needs to be done is like teaching that old dog new tricks, some dogs dont even listen to you! Light a cigarette & blow into the vent fan. Watt's law is just as easy: Watts = Volts x Amps With this newfound knowledge, the student walks confidently into the real world with two equations and some elementary algebra [], When someone says there is no such thing as cold, only the absence of heat, you can point out that while cold is the absence of heat absolute zero is the definition of COLD. SUBSCRIBE TO THE ENERGY VANGUARD NEWSLETTER! Any cookies that may not be particularly necessary for the website to function and is used specifically to collect user personal data via analytics, ads, other embedded contents are termed as non-necessary cookies. Your human air ducts are infinitely more important than the ducts in your HVAC system. Likewise with your requested pre-insulation inspection. Please read AddThis Privacy for more information. 2022 Donaldson Company, Inc. All Rights Reserved, Understanding Conveying Velocities to Save Money on Dust Collection. Out of these cookies, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. A more reliable way to maintain design air volume in the system is by using anAirflow Controller. At times it gets so humid I feel the heat is coming from the ground, I understand where on a living organism thats needs to breathe too, especially when where suffocating it with concrete & asphalt all over this God given land. Moving air through ducts too slowly or too quickly can have significant consequences that are equally problematic. Buying a fan with more static capacity will increase the cost of initial capital and will increase energy costs every day the fan is running. I am currently analyzing ducts in conditioned spaces more carefully, but have done detailed experiments on the interactions between ducts and variable capacity heat pumps. The house does not cool till after 12am & all the hot air in the attic goes nowhere but down the side of the walls & out of every open surface available. Unless you put it in writing. In fact, if branch runouts are short enough, a duct can be increased by one size with minimal impact on surface conduction. That is why duct heat gain or loss in unconditioned spaces goes up, even if U goes down with higher duct air velocities. To me, this is the problem with most upstairs designs. When its humid the house is like a oven after 3pm. System Replacement Go inside your bathroom, close doors leave them open it doesnt matter. I achieve higher velocity with smaller ducts and therefore smaller fittings. Q = Airflow (sometimes referred to as volume), Fumes and metal smoke produced by welding, Light or fine dusts such as cotton or flour, Average dusts produced by grinding, buffing, material handling and foundries, Heavy dusts such as sawdust, sandblast dust, boring dust and lead dust, Heavy and moist dusts such as most cement, asbestos and lime dust. This 900 fpm design translates to even more resistance going upstairs than my normal 600 fpm design and my normal is still a challenge to cool the hot upstairs. Much appreciate this and theMuch appreciate this and the other arrivals you put together. As service technicians, we are often expected to understand a bit about design to diagnose a problem fully.
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