CFMs from Load Calc Program

[swoosh172]

I'm hoping someone can direct me to the right answer. I've checked the users manual and there are no other ME's in the office to ask. I am using Trane Trace 700 and the outputs for airflow cfm's per room are always some odd number 93, 243, 432, etc. Is there a way for the program to round to the nearest 10, like 90 or 250 cfm since it is unlikely that the cfms will get balanced to 93 cfm? If not, how do I account for this, do I have to go back into every room tab and over-ride the program with the rounded number? With a large building, I would think the rounding could add up significantly (in the 100's of cfms) which would eventually affect the coil loads???

 

[tys90]

Correct me if I'm wrong, but won't your coil load stay the same assuming your amount of outdoor air stays the same and only your supply/return air amount gets rounded up?

Your room load isn't changing, your temperature set points aren't changing and the only other component is outdoor air load. I guess there could be a minuscule load from extra fan heat.

As far as keeping track of it, I've printed off the system components selection page and wrote down my rounded numbers next to the rooms on there. We use a 3rd party duct design program that keeps track of our totals for us so I haven't had to manually add them recently but I could.

 

[cdxx139]

tys90 what 3rd party software? Does it save a lot of time?

 

[swoosh172]

Wouldn't the mixed air temperature be different since you would have a larger percentage of return air coming into the system?? The equation cfm=q/1.08*DT leads me to believe that increasing the cfm across the coil will require a higher heat output from the coil.

 

[tys90]

cdxx139: We use DesignMaster. The product is relatively cheap and is geared towards the engineer/designer. We really like what it has to offer and they do free trials + a training session with you. We've been using it for almost 2 years now, it's just an add-on to Autocad. As far as drafting, it's roughly the same amount of time as using LISP routines once you are proficient with the DesignMaster method. The areas we found we save time are anytime you have to do modifications near the end of a design, design master already knows the CFMs, velocities, etc in each duct so you can just change a CFM and recalculate pressure and sizes all the way back to the AHU.

swoosh172: If you look at that equation, your mixed air DT required across the coil will decrease by the same proportion that your return CFM increases, or visa versa.

 

[DRWeig]

Swoosh,

I suggest rounding up for >= 5 and down for <= 4.

You've got enough safety factor in your inputs that it won't matter, and nobody can balance air to a high degree of accuracy.

You won't have a big effect on total airflow that way.

Just my experience!

Good on ya,

Goober Dave

 

[Wareagl487]

There is no way to get Trance Trace to round the numbers for you as far as I know. I typically just round up in increments of 25's for CFM's. Increments of 25 make it easier to ad numbers up in my head when reviewing the drawings. Keep you entering/leaving conditions the same when selecting equipment. Yes, you add a slight amount of tonnage to your over all system load, but the load is really just a best estimate anyway, right?