Carrier's Hap - Airflow rates in summary report

[Adnan86]

I need some clarification on following:
The 'Air system sizing summary report' generated from Hap software defines different airflow rates in 'central cooling coil data'. They are
a)Coil L/s at month/time
b)Max Block L/s at month/time
c)Sum of peak zone l/s.
The 'Supply Fan sizing data' also gives two airflow rates
d)Actual max L/s at month/time
e)Standard L/s
I have a VAV system and none of the above generated airflow matches with the sum of individual space airflow rates.I have gone through the explanations/definitions of above mentioned airflow rates from Hap's help menu. I need to understand clearly what is the difference between each of them and which one to be used for equipment selection. For VAV system Hap says to use Coil L/s for equipment selection. Can anyone put some light on this?

 

[RandomUserName]

Just a quick double check: in the air system properties, on the sizing data tab, in zone sizing, you have correctly set your zone airflow sizing method and space airflow sizing method?

 

[Adnan86]

Yes I did. For Zone airflow sizing, I have selected 'Sum of space airflow rates' and for space airflow sizing I have selected 'Individual peak space loads'. By doing so, I expect the coil L/s to be equal to sum of Individual peak space loads. But the results are different.

 

[Adnan86]

Waiting for experts thought on this.

 

[ZDR1985]

Did you mean a,b,c,d and e are not the same?

 

[Adnan86]

yes ZDR1985.They are not same.

 

[dbill74]

Adnan86,
Check each zone's peak time (month/time), with a VAV system you can expect each zone to peak at different times. I.E. east facing zones will have peak time earlier in the day than west or south facing zones.
HAP takes this into account and does 2 calculations. First for each zone individually to size VAV boxes. Then all zones together as a 'block' to determine AHU size.

For instance, you have 5 zones and each zone has a peak flow rate of 200 L/s (I'm making this up). Let's say 4 zones are external with one zone facing each cardinal direction, the 5th zone is an interior zone. It's not unrealistic for the north and east zone to peak around 1400, while the central, south and west peak at 1530. At neither time do you need 1,000 L/s of air from the central AHU to satisfy all 5 zones.

Hope this helps.

 

[Ziggypump]

Difference between Sum-of-Zone-Peaks, and System-Peak calculations here.

If done properly, the flow and load requirements when taken at each VAV box design should be more than the overall system size.

Think of it this way. A Zone facing East will have a maximum load during the morning, while the Zone facing West will have a much lower load. And during the evening, the Zone facing west will (should) be at maximum load, while the Zone facing East will have a lower load.