The HVAC industry is rife with rules of thumb. Just in case you
just joined our little group of engineers and were formally the lead designer
for cores of nuclear reactors, most HVAC calculations don’t have to be exact.
There isn’t enough time in a typical project to warrant the design to 6
decimal places. I always chuckle when I see a schedule that lists an
airflow with a number like 3456 cfm. There is no test and balance technician
on the planet that will measure the delivered airflow to that level of
accuracy, let alone a velometer, flow hood, or flow meter that can measure with
that level of precision. That said I feel that that level of accuracy in
precision in our delivered product lead to a great number of the rules of thumb
we deal with.
One of the first rules of thumb that most young HVAC design engineers,
technicians, and designers learn is that of nominal airflow. The nominal
airflow for a typical packaged rooftop unit is 400 cfm per ton of cooling
capacity. For example, a 4-ton rooftop would
nominally deliver 1600 cfm and a 25 ton rooftop unit would deliver 10,000 cfm
nominally. It really is that simple.
Another concept related to nominal airflow is cataloged airflow.
In the manufacturer’s catalog or technical guide is a list of approved
airflows for a specific size unit. These upper and lower bounds of
allowable airflows represent the spectrum of airflows that a specific unit will
properly operate across. This doesn’t mean that the equipment will not
properly operate outside this range, it just means that there might be problem
if airflow is permitted continuously outside this range for extended periods.
If a particular piece of equipment is expected to operate at higher than
catalogued airflows the following problems should be considered and evaluated:
Excessive pressure drop through filters; filters collapsing and being
drawn out of their rack; excessive moisture carryover from cooling coils; poor
latent cooling performance; high suction temperatures resulting in premature
compressor failure; and potentially several others. If the piece of
equipment operates at too low of an air flow, then the following risks need to
be considered: frosting of the evaporator coil; low suction pressure;
refrigerant slugging at the compressor; excessive temperature rise across the
heat exchanger; and potentially several others.
As a designer of customized equipment it is important for you to
remember that when you operate a piece of equipment’s airflow outside the
nominal band listed in the manufacturer’s catalogue you assume all the risk and
your design must account for this risk to protect the machine, building, and
its occupants.
