First, let's understand what each of these acronyms stands for and how they relate to each other in the context of heating, ventilation, and air conditioning (HVAC):

1. TR: This stands for Tons of Refrigeration. In the HVAC world, this is a unit used to measure the amount of heat a cooling system can remove in a certain period. A "ton" doesn't refer to the weight of the system but is rather an energy measure. One ton of refrigeration is approximately equal to the cooling power of one ton (2000 pounds) of ice melting in a 24-hour period. It's also equivalent to 12,000 BTUs per hour (British Thermal Units, another measure of heat).

2. CFM: This stands for Cubic Feet per Minute. It's a measurement of the volume of air an HVAC system can move or process in one minute.

The connection between the two is important for designing or sizing HVAC systems. In general, an HVAC system needs to be able to move a certain amount of air (CFM) to remove a specific amount of heat (TR) to maintain a comfortable environment.

Now, let's discuss the '1 TR = 400 CFM' rule.

This rule is a generalized rule of thumb in the HVAC industry. It suggests that to effectively remove one ton of heat (1 TR), an HVAC system should be able to move approximately 400 cubic feet of air per minute (400 CFM).

Here's a simple way to think about it: Imagine you're trying to cool down a big, hot room. If you blow a small, gentle breeze through the room, it's not going to have much of an effect. But if you blow a large amount of air quickly, it's much more likely to cool the room down. That's effectively what an HVAC system is doing when it moves 400 CFM to remove 1 TR of heat.

But please remember that this is a general rule and may not apply exactly in all situations because the actual CFM required can vary based on factors like the specific cooling needs of a space, the efficiency of the HVAC system, and local climate conditions. Therefore, in practice, HVAC system designers would often perform a more detailed calculation to determine the most appropriate system size and air flow rate.

Hopefully, this helps clarify the relationship between TR and CFM. Let me know if you have any further questions!