What is the difference between SMACNA patterned and regular fittings?

- SMACNA ( Sheet Metal and Air Conditioning Contractors National Association) has an over 60 year history of developing and instituting standards and recommendations. These standards and recommendations are utilized and recognized worldwide by construction and design communities. They have analyzed air flow characteristics for decades and found that an HVAC system with the smoothest flow patterns, least amount of restrictions and minimal internal turbulence tends to provide the most efficient and quiet end product.

- SMACNA patterned duct fittings are always the best option when it comes to air flow. It is generally accepted that a 90 degree elbow with a minimum 5 piece mitered construction and minimum centerline radius of 1.5 times the diameter will provide the smoothest airflow. The same goes for a 45 degree elbow with a minimum 3 piece mitered construction and 1.5 times centerline radius. The same is true when comparing a Tee to a Wye. The Wye will always be the better option because it offers the least amount of restriction and turbulence.

- The higher the Volume and Static Pressure (Water Gauge) ratings of a system, the more important it becomes to use SMACNA patterned fittings.

- Chemduct offers "standard" duct elbows which are formed construction and short centerline radius. We also offer 2 pc mitered 45 & 3 pc mitered 90 ells which have a short centerline radius and finally we offer the SMACNA recommended 3 pc mitered 45 & 5 pc mitered 90 degree ells. Spears Mfg. offers the different configurations to allow a system designer to tailor the ventilation system to their customers needs and budget.

What is the difference between a SMACNA patterned duct flange and a 150# pattern duct flange?

- SMACNA patterned duct flanges are designed around using 1/4-20 bolting. Bolt hole diameter is always 5/16" regardless of the pipe diameter. The number of bolt holes increases along with pipe diameter and the spacing allows for consistent stress along the entire flange.

- 150# patterned duct flanges have fewer bolt holes with increasing bolt sizes and varying space between bolts which means that the installers have to stock various sizes of bolting and the fewer quantity of holes tends to put more stress on the sections of the flange being bolted. This often results in breakage due to over Tightening, as installers are used to applying torque to a 150# flange.


18" 150# pattern duct flange has 16 bolt holes, 1-1/4' diameter to accept 1-1/8" diameter bolts so the installer will need to buy 1-1/8" diameter bolts with the correct length based on whatever they are connecting the duct flange to.

- 18" SMACNA pattern duct flange has 20 bolt holes, 5/16" diameter to accept 1/4-20 bolts. Usually the contractor only has to stock 1/4-20 bolts by 1" or 1-1/4" long to put any 2 SMACNA duct flanges together.

*** 1/4-20 bolts, nuts and washers are less expensive than the 150# equivalent bolting, are easier to carry around and usually easier/quicker to assemble when hanging duct or working on a ladder. The smaller bolt diameter and length also lends itself well to situations where a section of duct needs to be taken apart for inspection on a regular basis. ***

What is the difference between Static Pressure, Water Gauge and Water Column?

- SP (Static Pressure) - A measure of the resistance to movement of forced air through a system or installation, caused by ductwork, inlets, louvers, etc. Measured in inches of water gauge (W.G.); the height, in inches, to which the pressure will lift a column of water.

- For a given system, static pressure varies as the square of the flow rate. If the flow rate is doubled, system resistance or static pressure is increased four times.

- A Manometer is usually used to measure the static pressure in a system.

How does Static Pressure (S.P.) or Water Gauge (W.G.) relate to PSI or inches of Mercury?

- S.P. or W.G. ratings are a fraction of PSI or Inches of Mercury.

1 PSI = 27.6704523 inches of water (positive pressure)

1 inch of mercury = 13.5888977 inches of water (negative pressure)

- PVC/CPVC Duct pipe, elbows and other standard fittings are suitable for a maximum 10" W.G. (positive or negative) @ 100 degrees F according to SMACNA recommendations. That maximum is lowered considerably when dampers and other special fittings are added to a system.

- Taking the above into consideration, PVC/CPVC duct should not be the material of choice when the design parameters call for design pressures measured in PSI or Inches of Mercury.

What engineering specification section do we typically find these types of fume & duct products spec'd in?

- 233116 Nonmetal Ducts.

I have a drawing for a duct system, can you do a take-off and provide a quote?

- As a general rule, we do not like to do a take-off on a project. A take-off is a list of parts deemed necessary for the completion of a project. The contractor typically attends a walkthrough and is given a basic set of drawings to work with. The basic drawings usually include every system that is being installed in the building. It would take hours for our inside estimators to work their way through the drawings to separate out the duct and work up a list. We would also be working without the additional knowledge that the contractor picked up during the site walkthrough. We would be leaving ourselves open to an under bid error because of some missing piece of information or could over bid because we want to make sure we are covered. The best bet is for the contractor to submit his own take-off to everyone he is asking to quote the project so that he gets an apples to apples quote across the board.

What is the training procedure and what tools should be used for PVC/CPVC duct installation?

- Anyone wanting to be trained in the proper installation of our duct pipe and fittings up through 24" diameter should go through our ASME B31.3 training. The installation process for 24" diameter and below is the same as large diameter pipe installation detailed in B31.3. The only difference is that duct sockets are not as deep as a schedule 40 or 80 socket. That being the case, an installer can use a 4" long applicator/swab (Spears® p/n 4020, 4520 or 5020) for all sizes of PVC/CPVC duct from 6” diameter through 24" diameter.

What about cementing duct above 24" diameter?

- PVC and CPVC duct, 26" diameter and above is not available as an extruded product. The larger diameters are fabricated from sheet material and the end result is that the pipe is not as rigid as the extruded duct. The increased flexibility and larger dimensional tolerance of the fabricated duct pipe does not lend itself well to a primer/cement install. While it can be done, it is not recommended as there is a very good chance that the duct will not seal all the way around. The best way to assemble fabricated duct sections above 24" diameter is with "Hot Gas Filler Rod Welding". This work should be performed by a properly trained plastic welder. ChemDuct does not provide training in plastic welding.

What is our Thermal Welding or Hot Air Welding Procedure?

- As mentioned above, we do not offer training in PVC or CPVC welding. This is a highly specialized technique and while it is fairly easy to learn, it takes a lot of practice and work to master. There are several schools throughout the country that offer hands on training in plastic welding.

What is the best and safest way to clean the inside of a duct that has dust or chemical build-up?

- First off, any duct system that has the potential to accumulate dust, particles or chemicals should have access ports built in to facilitate periodic cleaning. That being said, the periodic cleaning of any duct system should be performed by a specialist who has the equipment and know how to do the job properly and safely.

How should you address concerns regarding static electricity build up in PVC/CPVC duct in cases where a static charge could cause a fire or explosion?

- The same way that it would be addressed in a metal duct system. The installer should make sure the system is properly grounded. The system designer should take this into account when choosing materials and should provide the installer with proper grounding techniques when it is deemed necessary.

- FYI, This situation usually presents itself in dust collection systems where the dust particles are highly flammable. PVC can and has been used for this type of system. The trick is to have the system properly designed by someone that knows what they are doing.

Can PVC/CPVC duct pipe and fittings be used in below slab and/or direct bury HVAC applications?

- Remember that PVC/CPVC duct pipe is a thin wall product and as such, does not lend itself well to load bearing situations.

- You can work around that one of 2 ways and there has been good success with both over the years.

A - If the pipe is going to be buried using standard ASTM pipe burial methods, switch to heavy wall (sched 40, 80 or an SDR pipe) for anything that will be underground then back to duct for the above ground pipe and fittings.

B - Build a chase-way or channel that allows the duct to be underground but not bearing any load. We have seen long channels that held the duct, and other service piping with either a slab or grating on top, and other type of underground encasements that held the pipe but protected it from any weight bearing type situation. In short, heavy wall pipe (load bearing) or a structure to contain and protect the duct pipe from external loads. Either way would work well in facilitating an underground duct system.

Please explain 90 degree elbows and tees that are "square throat with turning vanes"

- This terminology usually comes into play when we are building or discussing a square or rectangular PVC/CPVC exhaust system. Remember that a duct system needs to be designed to facilitate smooth flow with as little turbulence and flow restriction as possible. A rectangular or square duct system brings additional challenges that need to be overcome. SMACNA has dealt with those issues and put forth recommendations regarding how to facilitate smooth flow in a square or rectangular system. Since it is more expensive to build a mitered elbow in these systems, SMACNA recommends building a "square throat" (90 degree) elbow and building in "turning vanes" to gently turn the moving air, preventing it from hitting the rear wall of the ell and bouncing back. This limits the turbulence within the system. SMACNA Handbooks give clear guidelines as to the size and placement of the turning vanes within a square or rectangular elbow.

Is on site back welding allowed when repairing a leak in a PVC or CPVC duct system?

- When done properly, back welding is a viable way to repair a leak in a PVC or CPVC duct system. It must be done by an experienced plastic welder and the leaking joint must be properly prepared. All cement and primer must be scraped or ground away so that the welding is done on virgin pipe material. If the weld is attempted directly over primer and cement, the result will be a charred mess that will most likely continue to leak because the burnt material prevents a solid weld from taking place.

Is there a need for access points or access panels in a duct system? If so, how are they achieved?

- The need for access points is generally decided on by the designing engineer. They are most often seen in wet or hot processes where the fume being exhausted tends to condense as it is transported through the duct system. When it is determined that this could happen, the engineer will have access ports built into the system to allow for periodic cleaning of the duct interior. They may also design a built in spray down system. We can build whatever type of access or spray port drawn. All we need is a dimensioned drawing for quotation purposes. We have seen and made various types. Some allow access via a bolted door. Others provide the same access with a sliding panel that doesn t need to be bolted. SMACNA generally recommends bolted/gasketed access ports and provides sketches of acceptable construction methods in their duct construction manual.

What is the most common application for PVC/CPVC duct pipe? Most common sizes used?

- The most common application where PVC or CPVC duct is used would be in the plating or metal finishing industry. Most common sizes would be 6” through 24” diameter.

What are some of the more specialized installation markets?

- Plating and surface finishing

- Semiconductor fabrication facilities (usually CPVC due to smoke/flame concerns)

- Chemical transfer stations

- Research facilities & Laboratories

- Pool pump rooms

- Wastewater treatment plants

What applications would PVC be recommended or used over CPVC?

- PVC should always be the first choice as it usually falls in between sheet metal and stainless steel duct as far as price is concerned.

- CPVC is the best option when dealing with temperatures that PVC can t handle or when the smoke & flame and/or plenum rating comes into play.

Are the chemical resistance charts the same for our PVC/CPVC duct as for our fluid handling systems?

- Yes

Is the expansion contraction formula the same for duct pipe as it would be for sched 80/sched 40?

- Yes

Can ChemDuct provide CPVC Canopy Fume Hoods (wall mounted and/or suspended) with built in baffles? (these are available in metal - epoxy coated or stainless steel).

- Yes. We typically ask for a drawing that we can use to quote the hood & build it if we get the order

Are motorized butterfly dampers available?

- Yes, we can provide actuation on PVC & CPVC duct dampers. There are quite a few that have already been sold so while it may not show in our website, we can and do make them. Call and ask for more information.

Is opening a door or window a suitable way to bring make up air into a room that doesn't have a properly sized make up air system?

- We have seen that done in the past but that solution brings another set of problems when the outside air is too cold or too hot. Ideally, the system designer should allow for a tempered make up air system that is sized to bring in the same amount of air as is being exhausted and the placement of the make up air system should be planned to allow proper air currents throughout the room being exhausted.