VAV Diversity in Fan Sizing

[BronYrAur]

Are there any rules of thumb on the amount of air flow diversity that can be expected in a a VAV system? In other words, how much below total cooling load design can the airflow be lowered? I have seen systems with a wide variety of percentages below design - anywhere from 0% to 25%

My thinking is that the only variable you can count on is the sun exposure throughout the day. Unless you know some internal diversity that is guaranteed, it seems to me that the sun is the only variable you can count on.

So I would think that the sun load on the east or west exposure (whichever is smaller) is the only thing that can be safely subtracted from the air flow.

Agree? Disagree? Any other thoughts?

 

[GMcD]

In the old days when I'd do the calcs by hand for office buildings, the diversity was figured from the building "block load" at the building's overall summer peak, at say 2 in the afternoon on August 15 (as an example), and then compare that to the "sum of the peaks", which are the peak loads on each exposure of the building at thier different times. For a fairly uniform exterior envelope of say 50% glass to opaque wall on all four sides of a square building plan, and no exterior sunshades, the diversity was around 75% to 80%. That can change easily depending on if the east/west exposure is larger than north/south (or vice-versa), and if the glass/opaque wall ratio is different on different exposures. The rule of thumb is that there is no rule of thumb. Every building is Model #1, Serial #1.

 

[AbbyNormal]

Well I use software to peak the building.

An interesting fact here with year round cooling is strong solar gain in December. My overall loads will be down as the humidity has dropped in the outside air, the ambinet dry bulb drops a bit, but the big difference is the lower dewpoint in december.

However, I need the highest airflow many times in December because of the solar gain especially in office with southern glass.

Up north it would be reduced run time on the heating system, here it is something to look out for in cooling.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[lilliput1]

You have to make a judgement based on your estimate. If the equipment & lighting load is relatively low then don't divesify to leave enough cushion for leakage, safety and future expansion. If the load is verh high, try to schedule hourly profile for each type occupancy & use load calculation programs such as Elite which factor in hourly schedules that can be assigned to lights, people & equipment.

 

[AbbyNormal]

I use Elite

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[atlas06]

With VFD's, you don't really pay a premium in designing at 100%. It only gives you a cushion for future expansion, leakage,etc.. as Liliput says, unless your next size fan HP is going from 60 to 75 HP. If you only go from 25 to 30 HP motor for an additional 20% load and you have VFD's, you get a quieter system and some room for expansion at no additionla cost to speak of.

for all these years guys, I've never heard of Elite. Goes to show that you learn soemthing everyday. So, what is Elite?

 

[PatBethea]

http://www.elitesoft.com/

 

[AbbyNormal]

i use the CHVAC program atlas, only been running cooling loads with it.

Tell the program it is VAV and will come up with the peak air flow to design around

It is considerably lower than the sum of the peaks option you can do with constant volume

Take the "V" out of HVAC and you are left with a HAC(k) job.