HVAC Questions from a Chem E

[goodkd]

Being a Chem E with no HVAC experience I somehow ended up in a project requiring some basic HVAC design and operation knowledge. (Go figure)

Can anyone here help?

1. A typical, single AHU/fan can supply how much air to an office building (in units of volume/time)?

1a. Does this flow rate include outside air AND recirculated air?

1ai. If yes, what fraction of the total flow does the recirculated air typically make up?

1aii. If no, can you estimate the typical corresponding flow rate of recirculated air?


2. The typical, single AHU/fan from #1 is responsible for supplying air to how much of the building (in units of area or volume)?

2a. Is it common for more than one AHU/fan to be “ganged” together and operate as one large unit, supplying air to a larger area of the building? If so, how many units can be feasibly ganged together?


3. When developing/balancing an HVAC system for a building, do designers attempt to achieve a certain rate of air movement (e.g., 0.25 m/s at 1.5 m above floor)?

3a. For example, is a minimum flow desired to prevent the feeling of “stale” air while a maximum flow is set to avoid the feeling of draftiness?


Thanks!

KG

goodkd@yahoo.com

[core]

1. A typical, single AHU/fan can supply how much air to an office building (in units of volume/time)?
There are many types of fans in the HVAC world the amount of CFM (Cubic Feet per Minute of air) they supply is governed by fan size and RPM. Basically speaking there is no typical fan.

1a. Does this flow rate include outside air AND re-circulated air?
The fan just moves air, it could be a mixture of 50% outside air or 50% re-circulated air, 90% re-circulated and 10% outside air. This depends how you set the system up and if you have outside air ducts. Bottom line total output equals total input.

1ai. If yes, what fraction of the total flow does the re-circulated air typically make up?
This depends on the application of the system. Typical numbers for office systems are 90% re-circulated, 10% outside air. The actual amount of outside air will be governed by ventilation requirements and if any exhaust fans are in the space.

1aii. If no, can you estimate the typical corresponding flow rate of re-circulated air?
If no outside air is needed the return/re-circulated air is 100%.


2. The typical, single AHU/fan from #1 is responsible for supplying air to how much of the building (in units of area or volume)?
If a single story office building had only one HVAC unit this unit would supply air to the entire structure. Furthermore it would be required to supply outside air to the space to meet code requirements and make-up any exhausted air from bathroom exhaust systems or general exhaust systems.

2a. Is it common for more than one AHU/fan to be “ganged” together and operate as one large unit, supplying air to a larger area of the building? If so, how many units can be feasibly ganged together?
In smaller applications units at times are “Twinned” together. Never more then 2 small units twinned together.


3. When developing/balancing an HVAC system for a building, do designers attempt to achieve a certain rate of air movement (e.g., 0.25 m/s at 1.5 m above floor)?
Yes the term is “Air Changes” commercial application will require a certain amount of “Air Changes”
Example:
If you where supplying air to a Cafeteria the recommended air changes would be from 12 to 20.
Because this cafeteria is never busy we will use 15 Air Changes per Hour.

So the amount of CFM needed would be based on the structures volume and our designed air changes of 15.

CFM = Air Changes per Hr. x Volume of room / 60

If this cafeteria was 40 x 40 x 10 then the Volume would be 16000 cubic feet.
CFM = 15 x 16000 / 60
CFM = 4000
We would need to supply 4000 CFM to achieve 15 Air Changes per hour given this size room.

Example 2.

Given the same room volume of 16000 cubic feet if we had a known amount of air entering this room, say 3500 CFM we can calculate how many air changes this cafeteria will experience to see if it complies.

Air Changes per Hour = CFM x 60 / Volume
Air Changes = 3500 x 60 / 16000
Air Changes = 13.125 or just 13

An Air Change of 13 would still fall into the recommended air changes for a cafeteria with a moderate amount of people.

3a. For example, is a minimum flow desired to prevent the feeling of “stale” air while a maximum flow is set to avoid the feeling of draftiness?
Air traveling at a Velocity of 50 Feet Per Minute (FPM) or less is considered “Stagnant”


Core

[goodkd]

Thanks Core!

I appreciate the time it had to take to type that all up.

KG