For FAHU load calculation, did we add the internal loads or not? why?

for FAHU we dont add internal load

we just add the full requirements of fresh air and add total fresh air load in FAHU thats y it is Fresh Air Handling Unit

In the design of Fresh Air Handling Unit (FAHU) sure we will consider the internal loads, area and occupants.

same as the AHU + the load of the fresh air required for this area.

 

we have a desired temp. to reach in this area and if we don't consider the internal load, we'll never reach this desired temp. specially when it is high load

Dear Sharif,

Your question a bit ambiguous for me.

Total building load (including internal loads from People, Equipment, Lighting and external loads from Sun, Air, Moisture) shall be balanced by building HVAC system to reach comfort condition (or any other specified condition in non-residential spaces). So, if you have to use FAHU, then you have no other way except considering above mentioned total load. But, if you utilize a complex system besides FAHU, then depends on your system it could be possible to balance internal loads by some other equipment and use FAHU just to supply conditioned fresh air. (in my opinion such a complex system is cost effective and not cost efficient)

Regards

In the design of Fresh Air Handling Unit (FAHU) we will not consider the internal loads instead we will consider the area and the expected occupancy of that particular zone.

For further details refer ASHRAE STANDARD Ventilation for Acceptable Indoor Air Quality.

 

 

www.ashrae.org

 

For load calculation we consider both sensible heat as well as latent heat when designing AHU or FAHU. As per ASHRAE , Q=4.5×cfm×∆h, Since, h=u+PV, ∆h= ∆u+p∆v+v∆p, This proves that internal load must be added. It can be understand by example, In case of FAHU totally 100% fresh air is taken ,if we assume Outdoor condition; 96°F db+68%Rh+h1=38, Indoor condition; 75°F db+55%Rh+h2=28, Then ∆h=38-28

The full form is the (FAHU) is fresh air handling unit, it's mean this unit no need to add the internal load because this unit taking total air from outside fresh air.

As I learnt from a leading consultant senior mechanical Engineer, a 100% fresh air handling unit is a heat recovery system plus reduce the humidity of incoming fresh air from outside. The FAHU is sized based on the indoor fresh air requirements in cfm or l/s referring Ashrae indoor air quality standards,

The sensible heat and upto the extent the latent heat shall also be removed from the incoming fresh air containing huge load, such that the supply air from the FAHU have a design criteria of 24 degree C and may be 70% RH by a cooling coil and a heat recovery systems.

A recommended and safe design is to size the cooling coil such that it alone can take care of the same load and provide the same DC as above, because if the heat recovery systems fail the FAHU still shall encounter the same huge load and provide proper results.

Concluding to answer the question: The FAHU is a machine which supplies fresh air 100%, in other words it takes in an X volume of fresh air and supplies the same X to fulfill the indoor requirements. Therefore the load shall be as I have mentioned in the previous paragraphs.

 

no its the ahu or fcu to be calculated for the same area but fresh air would only provide air without the internal load