Multiple VAV Zones, sizing of the main heating coil

[ercegs]

Hi,

I was wondering if someone could help me a bit regarding sizing of the main heating coil inside AHU in VAV system, single duct, multiple zones...etc. So, I am aware of principle how VAVs with re-heating coils are operating (first, airflow is reduced until setpoint in the room is reached, and if temp is still dropping heater will start).

However, in this system (which is 100% fresh air), there are many VAV rooms and in addition some of them are CAV. In the main AHU there is a heater and humidifier. Re-heating coils (which are just at the room, after VAV) are sized based on temperature before it and after. This temperature before the re-heating coil is what is delivered by AHU.

Can someone please explain, if heating coil inside the AHU should be sized for minimium airflow in the system or for maximum? I size for example re-heaters based on the fact that temperature upstream re-heater is 15deg. But, main heater in central AHU how should get these 15deg, based on minimum airflow of the whole system or max?

If for example is considered minimum airflow, then since there are many zones and many CAV rooms it can happen that during winter if some VAVs are opened system will not be in minimum airflow and temperature after AHU will not be 15deg but it will be lower and then re-heater as well is going to be undersized.

Same question is applicable for humidifier which is also inside AHU.

Appreciate any feedback and reference to any literature.

Thank you

 

[DrRTU]

In a classical VAV system, central AHU heating requirement is calculated from the greater of 1. Supply air tempering requirement or 2. Morning warm-up heating requirements. These values are found in your load calculations.

 

[Drazen]

How could you size on any other basis but on basis of maximum possible air flow? What the heating coil would serve in any other scenario?

The purpose of you heating coil is to get planned temperature upstream of vavs in any regime. As simple as that.

what you don't mention, though is heat recovery. i doubt your 100% fresh air system can be planned without heat recovery. so it has to be be calculated for all scenarios: minimum outdoor temperature, maximum flow.