HVAC Duct Design

Trane 800 numbers will help you faster

"The wide open static requirement pressure versus airflow curve is found in the performance data section of the catalog. To select an air valve, locate the required design cooling airflow for your terminal unit type and find its vertical intersection, with the smallest air valve size that has a pressure drop equal to or lower than the maximum wide open static pressure requirement"

http://www.hvacc.net/pdf/trane/Trane_VAVCatalog.pdf

http://www.manualslib.com/products/Trane-Varitrane-2968067.html

Ask the conductor.

He knows all about Tranes.

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Several years ago ducts were rectangular but now why they are circular?.

Because the're cheap, fast, lightweight, and work better....

Definitely cheap, fast, and lightweight but I disagree that all round duct works better.

The efficiency of conveying air when using flexible duct is significantly less than smooth metal duct due to the ribbed inner surface that presents added flow restriction(s) especially when the duct changes direction.

I agree.

Round, flexible AC ducting is ONLY used because it is cheaper, not because it is better.

Builders chase the bottom profit line like a hungry hound dog chasing bacon.

Flex duct is cheaper to buy, cheaper to transport, cheaper to install and cheaper to seal.

It is not better at the efficient flow of air.

It is more prone to higher inner turbulence (if not properly stretched and tensioned), easier deformation and easier damage and joint failure.

But it is cheaper.

A house built today only shares the shape with houses built 30+ years ago. They are junk compared to the houses built then.

YEP!

A short time ago I volunteered to help out in building a US HUD self-help home in my neighborhood.

The process is accomplished by the organization providing all materials, engineering, and supervision personnel.

I arrived at the site at 06:00 hours with my tool bag and saw ready to work.

I busted all of the bunks of lumber and was in the process of culling the bad lumber and marking the crown on all boards then staging the needed lumber sizes on the cement floor pad for framing the structure.

About 7:00 people started showing up at the site.

The one and only supervisor got out of his truck, walked up to me, and said; "I haven't seen anyone do that in over 20 years. We don't do that anymore, it takes too much time. Who are you?"

I introduced myself and explained that the owner of the new home was/is one of my best friend's daughters and that I would be continuing on with the process until I finished the needed tasks.

I finished while he proceeded to instruct the few others that showed up on how to use a hammer safely.

At first he was a bit indignant but he got over it and at the end of the project shook my hand expressing appreciation for my experience and help.

We shared some good stories and a laugh or two.

He told me it was good to work with someone that knew how to do carpentry work right and that needed a minimum of supervision.

I shudder to think what that house would have turned out to be if I had not ever showed up to help my friend.

My daughter's new home in the $350K range suffers greatly from poor craftsmanship and short cuts.

I do not think there is a square corner, plumb wall, or a flat surface (wall, floor, countertop) anywhere in that house and the HVAC system is way undersized with the flexible ducts hanging randomly from the trusses cris-crossing each other in every direction.

The difference between a good quality job and a poor one is about 15-20 minutes per task so I do not "get it".

My father, brother and I built two houses back in the early '60s. One with a full basement (Mo.) and one two story (Texas).

They sure don't build them the same way today.

The dimensionless parameter ratio "k" between Perimeter and cross sectional area between a circle and a square is the mean thing to consider

For circle k_c = p/√A = 2√¶

For square k_s = 4

then k_s>K_c, this would simply means circle has less lateral area than a square, a factor to consider for minimal heat loss on ducts. Therefore, circle is more advantageous to use, just by the geometry alone.

Because they blow?

You write that you use the VARITRANE software for VAV system duct sizing. We've been using this software for over thirty years for design of VAV systems.

Do you mean that you use the VARITRANE "Ductulator" for sizing the ducts? That's a later add-in applet for design of the LV ducts downstream of the VAV terminals, not for the HV ducts and VAV terminals. Don't design the HV ducts for equal friction, but rather use static regain method as recommended in the software manual.

Hope this helps.

Zvi

Thanks Zvi. The scenario is that we are using the VARITRANE software indeed and not the ductulator. The software does have the option of sizing by the Static Regain Method, but due to the higher initial cost the client is not going for it.

When we input the section by the equal friction method, it asks for the duct section data all the way till the diffusers, but the menu for inputting the data of VAV (such as insertion loss, etc.) becomes turned off.

If we do not opt for the diffusers and enter the data of the VAVs, the menu of the diffusers shows turned off.

This is the dilemma being faced.

We want to be able to complete both the VAV menu and the Diffusers Menu but they do not come on together, its either this or that.

Hi SolidSpaces,

The static regain method takes advantage of higher air velocities to obtain solutions with smaller duct sizes. The calculation employs repeated iterations that may result in over sizing some of the trunks at the far ends of the system. The VARITRANE software manual explains this and offers solutions.

Our experience has been that static regain design of a VAV system will always result in lower ductwork costs than the equal friction method and require equal or lower fan energy.

Can you explain what you meant by "higher initial costs" of the static regain design?

Best regards,

Zvi

HI Zvi,

By higher initial cost, I mean that the branches keep becoming larger in sizes so as to reduce the velocity to increase the static regain. This in turn will increase the initial cost, but it will repay for itself due to power savings and smaller fan static. The Owner however has his limitations about initial cost outlay and therefore this static regain idea was dropped.

The difficulty that I explained with Varitrane was with the equal friction method that we are running, so if you can share any ideas, it would be appreciated. Thanks.

Hi SolidSpaces,

I figured that was the problem. But VARITRANE allows you to overcome it by specifying a minimum velocity constraint. Set a constraint directly on the main tab by choosing from one of the three values listed, or specify your own velocity constraint. Also, Don't hesitate to start with higher velocities at the AHU.

Good VAV duct system design is an art. Do try several computer trial runs until you get what your looking for.

Remember that VARITRANES's "Ductulator" is a software version of Trane's famous vintage cardboard slide rule of the same name that we old timers worked with before computers. The static regain algorithm used in VARITRANE was created by Ray Dean of Tempmaster Corp., the post WWII pioneer of VAV systems. We used it on an IBM 360 main-frame with punch cards for input. That was years before Trane got into the VAV act in the late '70s.

Use the equal friction method design only for LV jobs which, of course, includes the ductwork between the VAV box outlet and the diffuusers. But, from the AHU to the boxes, design with HV ducts using with the static regain method.

Hello Zvi

In the ducts that are sized by the Static Regain Method, suppose if there are 3 diffusers on a branch duct, the size of the duct keeps increasing after each successive diffuser to achieve the same static pressure behind each diffuser and this will not require tedious air balancing at each diffuser. This is opposed to the Equal Friction Method in which the duct size becomes progressively smaller but this creates an unequal static pressure behind each diffuser. The initial ductwork cost of the static regain method is therefore more than the equal friction method. The owner has ruled out the static regain method.

Coming back to my difficulty in the capability of the Varitrane Duct Sizing Software that it does not calculate the duct sizes downstream of the VAVs. No place is seen in the menus to enter both the VAVs and the Diffusers. The Menus are either for VAV or Diffusers but not for both.

Thanks

Is kind of flow(laminar/turbulence), type of bend etc important in duct sizing

PNABAN

You have posed a good question, and you seem to know the answer also and have probably raised this point to enlighten the readers. A separate thread about laminar and turbulent flow can be started if you find it educative.

For the moment my concern is as follows which was also noted in the previous post(s): The difficulty in Varitrane Duct Sizing Software that there seems to be no provision in the software to size the ducts downstream of the VAVs. No place is seen in the menus to enter both the VAVs and the Diffusers. The Menus are either for VAV or Diffusers but not for both.

Hi SolidSpaces,

You're right that using Static Regain for short LV branches can result in what looks like a long plenum!!! Your owner is right that this just jacks up SM costs and not justified.

First design all the LV ducts and diffusers downstream of the VAV boxes using Equal Friction Method. That also gives you the SP that's required at the outlet of each box.

Afterwards, run the HV side of the system on VARITRANE, from AHU through the VAV boxes, using the Static Regain Method. Plug the excess SP calculated for LV ductwork into the input data for the VAV boxes.

If you do see some oversized ducts at the end of some of the long trunks, try the minimum velocity constraint option and do a second run. This may raise fan HP a bit, but it should solve the problem.

Use discretion through analyzing the diversity of your system. Is a zone's peak CFM only required for short periods of time? Undersizing ducts to that zone by raising velocity may not be as bad as it sounds since the extra HP will only be needed occasionally. VAV systems can be very dynamic and this should be factored into the design. It is an art; but that's what you're getting paid for!

Thanks a lot Zvi.

Regarding your response, I would like to add that the Client has ruled out sizing by the Static Regain Method. Can your idea be implemented if the entire duct run is sized by the Equal Friction Method? As a first step, as suggested by you, we can calculate the sizes of ducts downstream of the VAVs and then as a second step, input the sizes obtained from step 1 into the program together with the insertion loss of the VAV and thus size the ducts upstream of the VAV.

Another question (not related to the above matter of Varitrane Software) is that my Load Calculation HAP software asks for the ESP. If the ESP value is greater than the total tonnage of the equipment is more. I wonder how ESP raises the equipment capacity. Is it due to the heat gain from the higher velocity that is causing the ESP to be high?

SolidSpaces,

I think we've reached an impasse. Heart of the matter is that VARITRANE uses a minimum pressure method utilizing both static regain and sound power per CFM noise analysis to ensure that the static pressure available at the inlet of each terminal unit matches the minimum static pressure needed to deliver required air quantities at specified NC levels. It also corrects the air quantities to account for heat gain in duct travel.

To accomplish all this, Tempmaster Corp., a pioneer of VAV technology, developed a sophisticated algorithm involving multiple iterations back in the 70's. Trane acquired the program and later tacked on a PC version of their "Ductulator" slide rule for calculating duct sizes using equal friction. However, there's no connection between the two methods. Static regain works well for high velocity VAV systems, equal friction does not.

Yes, you can use large, oversized low velocity ductwork for a VAV system. But why would you want to do that?

As for ESP, energy imparted to the air by the AHU's fan has to be part of your cooling load.

Good luck,

Zvi

Zvi: I thank you for your time and valued input. good luck too!