AHU Sizing

[Zeroman]

Using basic heat transfer rules I have determined how much heat will enter a room on the "design day". I have also added the amount of heat rejected from equipment in the room to determine the cooling load on the air handling unit responsible for cooling the room to a predetermined temperature. Now let's say that my total heat dissipation is 100kW, why has the vendor come back to me with a unit that has a potential cooling load of 150kW? Is this because I have only analyzed the one system (the room) and they have taken the unit as a second system and determined how much cooling is required to cool the outside air? Can anyone explain this? Thank you for your help, it is always appreciated.

 

[lilliput1]

Did you include the following:
-Latent load from people
-Outdoor air sensible & latent load
-Heat gain from the supply air fan
-Heat gain from the return air fan
-Heat gain from supply air ductwork portion in unconditioned soace
- Heat gain from return air ductwork in unconditioned space
- Infiltration (sensible & latent) if any
- Heat gain from lights

The latent capacity of package units may be more than you require but you have to accept it to meet the sensible load.

 

[DouginMB]

Also, at what conditions is the unit rated for 150KW? It might be nominally rated for 150KW, but de-rated to 100KW for the actual space conditons and outside air temperature for your application.