Air changes per hr (ACH / ACPH) simply describes how many times the quantity of air in a room (or structure) is completely replaced per hour.
If you have a 10’x10’x10′ room, the room contains 1000 cubic feet of air If the supply and return to the room is supplying a balanced 100 cfm (cubic feet per minute) of air to the room you would have 100 cfm x 60 minutes = 6,000 cfh (cubic feet per hour) which would equal 6 air changes per hour (6,000/1000 = 6).
Different spaces have different ventilation requirements based on occupancy level (how many people are in the room) and use type. This is a fairly simple thing to calculate based on the cubic feet of air in a room, this is NOT the heat gain /loss of the space, it is only the ventilation rate of the space. Generally, ACH is used as a guideline or reference for design and not as a basis for the design, a sort of solid rule of thumb used to check a ventilation design against the real world.
This ventilation rate also shouldn’t be confused with the outdoor air requirements which is a different, but related consideration and is based on ASHRAE 62.1 and 62.2.
In some cases when discussing ventilation for an entire structure ACH will be used as a means to describe “tightness” and is referenced in many codes as a standard for the structure. For example, when performing a blower door test a building test professional will use an air changes number to demonstrate whether a home or building is built to a tightness standard and how much supplemental outdoor air needs to be added to the space per hour.
Even more specifically a standard called ACH50 is used with a blower door to calculate the leak rate of a structure when the space is pulled into negative 50 pascals of pressure by the blower door and the leak rate calculated.
In most cases, a room/zone will be sized for airflow according to the heat gain/loss to include outdoor air requirements and then the ventilation requirements will dictate the need for additional mechanical ventilation and/or fan on position to provide the required air changes per hour.
The main point being, don’t simply provide a room with system airflow that is consistent with air change per hour rule of thumb and assume it will be properly conditioned and balanced… like with most things it isn’t that simple.