iam want ask for air hand unit

its used in the central air condition
its considered as a heat exchanger
the air was cooled by the cold water which is cooled by the cheller
this air is flow through isolated ducts to cool the sites and places needed to cool
the air handling unit contains a three way valve to make a compensation by inserting fresh air and reject the old dirty air

pleas speaking more for air hand unit and if you hav the book im thanks for you