imagine for a moment a one lane road. (small duct size)
Speed limit on road is set for 60 miles per hour (velocity)
If you time the cars passing by for one minute, you count lets say 200 cars. (cars per minute... our cars are also only one foot tall, one foot wide, and one foot deep.... so each car is One Cubic Foot big...got it.... small cars muahahaha... so we are moving 200 Cubic Feet of cars per Minute on this road)
We double the size of the road.... its two lanes now... it can hold more cars... now we count how many cars go by in one minute(remember... the speed limit did not change, they are still traveling at 60 miles per hour)
We can now count 400 cars per minute.Velocity is how fast it goes measured in Feet Per Minute
In duct and airflow related terms.
FPM (Feet per Minute), the speed of the air (i.e. a car traveling at 60 miles per hour), Velocity
CFM (Cubic Feet per Minute): how many cubes of air pass by in one minute, Air Flow (not called air flow much, most just say CFM)
FPM and CFM are directly effected by:
the duct's resistance (Length and Size i.e. one lane road vrs 6 lane highway)
the 'umph' the blower has to overcome the resistance
and if you realy want to get to the niddy griddy and get really really technical, the weight of the air (i.e. its easier for you to throw a golf ball a long ways rather than a bowling ball)
"Weight?? What you mean weight?? How much can air possibly weight??"
Not much, but enough to make a difference if you are trying to calculate the exact airflow going into and out of a critical room such as an operating room.
Before I leave the weight issue... lemme throw this in to help clarify that and add a new issue to the mix.
Go make a peice of duct 12" x 12" x 12" with an end cap in the end (makes for a good loose screw can hehe)
Fill it up halfway with water.
That cube of water is now at 50% relative humidity
Fill it up all the way with water.
The cube is now at 100% relative humidity.
Which one weighs more?
Now that we know our cube of water can hold water.
Lets throw some gas in it.
Fill it up all the way to the top.
This represents HEAT.
Our cube of air is really really full of heat right now.
Empty it out.
Our cube has very little heat.
Keep in mind, air expands and contracts with the amount of heat it has. So the cube of air thats full will actually grow so to speak. the cube of air with less heat will shrink.
The air entering your return duct can be viewed as these little cubes (one cfm each) that hold Heat, Moisture, Odors, Dirt, etc etc
When it passes through the filter, the dirt is removed.
If it passes through a special filter like a charcoal filter, the odor is removed.
When it passes through a furnace, more heat is added
When it passes through an air cond, heat and moisture is removed.
there is a funny thing that happens thats a bit long to explain, but the air leaving an air conditioner actually has more 'Relative Humidity' than the air that entered it.
hehe as if this post aint already getting long.
Do a search and see if you can find a post I made on understanding heat... its a long post too, but full of info. There's a section in it talking about 'relative humidity'.
Lemme see if I can get back to the basics.
I'll see if I can shortin this up a bit...
It all starts with human comfort
We want the vents to blow hard enough to stir up the air in teh room but not so hard as to be noisy. Also, the faster the air comes out, the cooler it will feel. This is a major reason why heat pumps have suffered such a poor reputation. The heat pump is doing its job, the but air flow feels cold because its moving too fast. i.e. blow on the back of your hand real hard... feels cold, blow on the back of your hand slow, it feels warm.
To keep noise down and still stir up the air enough,
you want to stay somewhere around 600 to 700 FPM.
We calculate the CFM needed for a room through a load calculation.
If a room needs 120CFM. We can look at our data for the grilles we use to determine which grille size will deliver 120CFM in the 600-700 fpm range.
Welp... I'm finally getting tired.... so I'm going to cut myself off and give you the answer to your question.
You asked how fast you want the air going through a unit.
We don't adjust the air through the unit based on the speed of the air going through it... we base it on the amount of air going through it.
We want 350-400CFM Per Ton of Cooling. (most use 400 per ton)
So dont' worry about your furnace size.
Adjust your air flow (CFM) according to your cooling unit.
two ton unit 800 CFM
three ton unit 1200 CFM
and so on
If you'd like to learn more on airflow and duct design
Get an ACCA manual D. You will hear alot of people on here mention Manual J, that is another manual from ACCA that shows how to calculate the heat loss and heat gain on a house... also refered to as a Load Calculation
That would be a good place to start since you are into sheetmetal.
Extend to others the grace that God has given you.