Engineering room airflow may present a real challenge when balancing an HVAC system. Most calculations only use the heat loss or gain of a room to decide on required airflow and often don’t take into consideration required room ventilation needs. Let’s take a look at how an air change calculation may simplify this step in your air balancing.
What is an Air Change?
An air change is how many times the air enters and exits a room from the HVAC system in one hour. Or, how many times a room would fill up with the air from the supply registers in sixty minutes.
You can then compare the number of room air changes to the Required Air Changes Table below. If it’s in the range, you can proceed to design or balance the airflow and have an additional assurance that you’re doing the right thing. If it’s way out of range, you’d better take another look.
The Air Changes Formula
To calculate room air changes, measure the supply airflow into a room, multiply the CFM times 60 minutes per hour. Then divide by the volume of the room in cubic feet:
In plain English, we’re changing CFM into Cubic Feet per Hour (CFH). Then we calculate the volume of the room by multiplying the room height times the width times the length. Then we simply divide the CFH by the volume of the room.
Here’s an example of how a full formula works:
Now, compare 7.5 air changes per hour to the required air changes for that type of room on the Air Changes per Hour Table below. If it’s a lunch or break room that requires 7-8 air changes per hour, you’re right on target. If it’s a bar that needs 15-20 air changes per hour, it’s time to reconsider.
Room CFM Formula
Let’s look at this engineering formula differently. For example, what if the airflow is unknown and you need to calculate the required CFM for a room? Here is a four-step process on how to calculate the room CFM:
Step One – Use the above Air Changes per Hour Table to identify the required air changes needed for the use of the room. Let’s say it’s a conference room requiring 10 air changes per hour.
Step Two - Calculate the volume of the room (L’xW’xH’).
Step Three - Multiply the volume of the room by the required room air changes.
Step Four Divide the answer by 60 minutes per Hour to find the required room CFM:
Here’s an example of how to work the formula:
When designing or balancing a system requiring additional airflow for ventilation purposes, remember this room will normally demand constant fan operation when occupied. This may present a problem for other rooms on the same zone, so take that into consideration.
Many of these rooms may require a significant amount of outdoor air. The BTU content of this air has to be included in the heat gain or heat loss of the building when determining the size of the heating and cooling equipment.
Practice these calculations several times in the shop or office. Then do the calculations in the field several times over the next week to check airflow in rooms with uncommon ventilation requirements. Study the Air Changes per Hour Table to become familiar with the rooms that need more ventilation than the heating or cooling load requires.
ob “Doc” Falke serves the industry as president of National Comfort Institute, an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free Air Changes Calculation Procedure, contact Doc firstname.lastname@example.org or call him at 800-633-7058. Go to NCI’s website atnationalcomfortinstitute.com for free information, articles, and downloads.
Depending on your setup there are A variety of systems to set up for ventilation.
- Fan and window enter
- Hood with ventilation
When setting up your bench space the ventilation is an integral component to the studio. Build your bench around your ventilation
Fan in window method
For most of us this is the most practical set up.
But it also depends on your space. If you have a window installing a fan in the space is quite simple. One way is to remove the window and replace it with the frame and plywood with the fan mounted in it. One issue you can run into with this set up is overheating the fan motor so make sure your bench is a minimum of 3 feet from the wall and that your torch/flame is it pointing directly at the fan. Depending on your space building a box around your fan and space is ideal. If your space is small enough a box isn't necessary but a cover over the top of the fan is ideal. This way you can control the direction of the airflow containing it and leaving through your fan
Most high velocity floor fans are plenty adequate for this need all you need to do is disassembled the fan mounting the back half of the fan to your plywood saving the other side of the cage for the outside of the plywood. For example if your fan is 18 inches in diameter then cut your opening to 17 to 17 1/2 inches giving you space to mount the cage to the plywood giving the blades freedom to move without hitting the plywood. Another more practical option besides using the other half of the fans cage is to purchase a gable vents for the opening of the window or to cover the space of the opening of the fan. This will keep birds from flying into your fan as well as Bermance and whether out of the shop. As we all know an 18 inch fan traveling at a high rate of speed can be quite dangerous if not protected so cover your ass along with those around you. This type of set up is the most affordable only costing around $75-$100.
There is a link to a PDF checklist in the show notes
The second more efficient set up is a hood or HVAC system this set up is ideal for larger space, multiple torch set ups or any varmint without windows.
This can get expensive but on a budget you can accomplish in adequate set up. For starters you will need to build your bench and incorporate a hood or depending on your set up a box that your work inside of. Without getting into a larger HVAC system here is the basic way that you can create it on a budget.
First thing you want to do is build your space again like mounting a fan in the window your area that you're working in should be around 6 feet wide by 4 feet deep by 4 feet high. This will basically build yourself a box to work inside of that will give you the most adequate suction and airflow for your ventilation next you're going to want to look for a hood of some sort. Look on craigslist garage sales or other sales at your lake local home good stores and get a used or new kitchen hood. This will become the housing and space that your air will flow through. However you won't be using the fan in the hood just the housing so go basic. Next you'll need an in-line fan or a blower fan preferably 6 to 8 inches in diameter which will give you a round 300 CFM's or more and only cost around $85-$150 next you'll need your duct which will go from the top of your hood and then mount to the outside of your fan shroud. Typically the blower fans taper down to allow for the duck work to fit around it securely and then security even more so with a large hose clamp. Depending on where you are venting out your bad air you can either run it to a window that is near you or in front of your bench and then add a section of duct to the other side of the fan so that way your fan is basically sandwiched between two sections of ductwork. The outflow Side of the fan with its duck work attached candy run again through a window or out a door but remember that ideally you want to minimize the amount of turns and Ben's in your ductwork to minimize internal static pressure which slows down airflow especially when you deal with 90° bends so try to avoid a acute angle like a 90° band and arch is more ideal
The installation process is quite simple as long as you know some basic construction it's not that difficult to do. If you have already built your bench and your box which also includes a roof or ceiling in the box itself it'll be where you mount your hood. The top of the hood
Will have a taper collar which will be the diameter of the circle let you cut in the ceiling of your box allowing the vent hood to be secured onto the ceiling with that extension pushing through and then on the other side you will then attach the first section of ductwork which will then run to your fan and then for the fan you have another section of ductwork which that runs either out of A window or door but the goal again is to get this toxic air out of and away from your space. The breakdown on costs your fan will cost anywhere from $80-$150 the hood will cost anywhere from $30-$50 you can find 25 foot duck work that is 8 inches in diameter will cost a proximately $20 and then you can get a wall Which is very similar to ones attached to duck work for a dryer which will give you a clean look in the end of the process part of the consideration as well is whether you own or rent your house so make sure if you rent that you get permission from your landlord to do any and all of the above they do not want surprises and it will be a quick way for you to become evicted or have the police called.
Now that you have figured out your estimated CFM requirements for your space and the fan and box set up that you are required to have for this space you now I need to take into consideration how much and where you were getting your make up air. Your make up air is basically fresh air comes from the outside allowing your space to equalize The static pressures in the room and also continue to bring you and oxygen which is obviously viable for Long Jevity. This can come from an exterior open-door a window or garage door but the most important area to consider is where is the fresh air coming from and the direction it is blowing in the studio space. To ensure proper ventilation it is important to make sure the fresh air isn't blowing across your bench or interfering with your torch. If there isn't enough air coming you will be dealing with static air issues which means you're not refreshing air equally literally sucking the oxygen out of the room faster than you are replacing it. This will lead to an unhealthy environment and potential health issues in the future or worse if your studio is attached to your house you could potential he suck the air out of your house or in the winter your heat. If you run a gas furnace carbon monoxide poisoning could be an issue. I recommend buying a carbon monoxide detector her which is very fordable and in the end could save your life especially since carbon monoxide is known as the silent killer just like high blood pressure. If you do have a garage door I recommend either opening it all the way or at the least halfway giving you a good amount of fresh air coming in the studio. You could also get a Nother fan that would blow air in from the outside as well hoping to refresh your air quality in your space.
In conclusion the ventilation is the most important part of your studio set up. If you take the time and spend the money upfront you have a cleaner super studio giving you a healthy environment for years to come. Cutting corners in the long run can be deadly. I hope this is helped give you some clarity on Waze to set up your studio space especially your ventilation on a budget with the most adequate possible ventilation set up you could have. There are a few links in the show notes that will take you to a website that deals with HVAC systems along with ways to calculate how much CFM you are required based on your space that you are in I also have links to materials and products referred in this topic as well as a few diagrams or examples on the recommendations discussed in this episode
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