The Basics of Ventilation, Part Two: Doing the numbers

Ok. We’ve talked about the basics, now let’s take a look at some basic design issues.

Exhaust System Design

So, where do we start? Well, let’s talk first about a couple of important numbers and calculations that have to be made first.

CFM: Cubic Feet per Minute. The amount of air that a ventilation system can move. It is based on how much air a given fan can move against a given amount of pressure.

Velocity: The speed the air moves inside the duct. It is measured in Feet per Minute.

Velocity Pressure: The pressure created by trying to force air at a given Velocity through a given duct size.

SP: Static Pressure. The total pressure against which the fan moves air. SP increases as the size of the duct decreases, with the addition of bends, and with any amount of turbulence. As SP increases, the efficiency of the fan to move air goes down, or, to state it differently, the higher the SP, the lower the CFM from design.

Loss Factor: A multiplier, usually fractional, that is the amount of friction induced by ducts. This is number is a constant for specific duct types and is usually presented in a look up chart form. The chart we will be using in all these calculations can be found in here: https://mikeaurelius.files.wordpress.com/2007/12/table1.pdf  .

Each one of these numbers or calculations factors into the design of an exhaust system.

I will present several different designs to show how each affects the total design.

The first item that we need to know is what size fan (in CFM) we need for our design. The American Conference on Governmental Industrial Hygiene (ACGIH) published its Recommended Practices for Ventilation. In the 22nd edition, 1995, it states a variety of processes that require ventilation. Among those, the nearest to our specific process is Restaurant Hoods (over cook tops). The Practice states that for Wall Mounted hoods, the recommended air flow is 125 CFM per square foot of hood area (length times width). For Island type (or ceiling hung) hoods, the recommended air flow is also 125 CFM per square foot of hood area.

 Example: Ceiling mounted hood, 4 feet wide by 2 feet deep. Per the Recommended Practices, the CFM requirement for this hood would be 4 x 2 x 125, or 1000 CFM.

 Example: Wall or bench workstation mounted hood, 4 feet wide by 3 feet deep (or high – if you use a bench mounted workstation hood with sides that extend down to the table top, measure the height of the opening). 4 x 3 x 125, or 1500 CFM.

As you can see, this calculation does not take into account the type or size of the torch you may be using because it is not necessary, if you use the ACGIH recommendations. What drives the calculations, as you can see, is the size of your hood. This can be easily calculated by measuring how much space you want to cover with your hood.

You should also note that these numbers as shown are for standard air density and do need to be adjusted for such things as high temperature (90 degrees or more), high moisture content (high humidity areas), as well as elevation in excess of 1500 feet above sea level. Those calculations will be covered in Part Four of this primer.

Another important factor to designing a good ventilation system is the flow rate or Velocity. Low velocity air movement will not convey fumes or particulates. High velocity air movement is very noisy. Somewhere between the two is a delicate balance between moving fumes and particulates and noise.

ACGIH (as noted above) has another Recommended Practice that we need to look at: “Minimum Duct Velocities for Conveying Materials”.

  • Very fine light dust: 2500 to 3000 FPM (Feet per Minute)
  • Dry Dusts and Powders: 3000 to 4000 FPM
  • Average Industrial Dust: 3500 to 4000 FPM

For our purposes as glassworkers, we should be using duct velocities in the range of 2500 to 3000 FPM. If you do a substantial amount of work with frit or enamel, you should consider a system that moves between 3000 and 3500 FPM.

Design Criteria

The first thing you need to determine is what type of hood arrangement you are going to have. It is a very individual, studio-based decision, and it should be made on the basis of how much space you have and what your individual layout looks like. This will vary from studio to studio depending on window and door locations, and access to outside walls.

Next, we need the basic formulas, or what are known as the Fan Laws:

  • Formula 1: CFM = Area (of ductwork) multiplied times Velocity (of the moving air) multiplied by the Loss Factor.
  • We usually know CFM and ductwork size, so to solve for Velocity, divide CFM by Area of the duct converted to square feet: for round ducts use ( 3.14159 * (radius of the duct squared) ) / 144, for rectangular ducts, use width times height divided by 144.
  • Formula 2: VP (Velocity Pressure) = (Velocity / 4004) squared
  • Formula 3: Bend Loss Factor = Quantity times VP times 0.125 (for a 90 degree bend, for a 45 degree bend use 0.0625)
  • Formula 4: SP = VP times length of duct run times (Loss Factor (from Table 1) divided by 100) plus Bend Loss Factor

 Example 1:

CFM = 1300
Duct size = 6” round (or .196 square feet)
Total run = 6 feet
Number of Bends: 0
Loss Factor: 0.11 (from Table 1)

Solve for Velocity: 1300 / .196 = 6632 Feet per Minute
Solve for VP: (6632/4004) squared = 2.74
Solve for SP: 2.74 times 6 times .11 = 1.80 inches of pressure

 Example 2:

Fan Size: 1300 CFM
Duct Size: 8” (or .349 square feet)
Total Run: 6 feet
Number of Bends: 0
Loss Factor (from Table 1): 0.025

Solve for Velocity: 1300 / .349 = 3725 FPM
Solve for VP = (3725/4004) squared = .865
Solve for SP: .865 times 6 times 0.025 = 0.129 inches

Notice that the only thing that changed between Example 1 and Example 2 was the size of the duct. In Example 1, the duct size was 6 inches. This resulted in a static pressure of 1.80 inches and a velocity of 6632 FPM. In Example 2, the duct size was 8”, resulting in a static pressure of .129 inches and a velocity of 3725 inches.

These two examples show the importance of sizing your ducts to achieve the best performance of your exhaust system. By varying the size of the duct, you can lower static pressure and velocity in order to achieve as near as possible to your planned design. You can also adjust the system to meet the requirements of a particular fan.

Example 3:

Fan Size: 1300 CFM
Duct Size: 8” (or .349 square feet)
Total Run: 10 Feet
Total number of 90 degree bends: 3
Loss Factor: 0.025

The difference between Example 2 and Example 3 is the longer run and the 3 additional bends that have been added to the system.

Solve for Bend Loss Factor: 3 times .865 x 0.13 = .337
Solve for SP: .865 times 10 times .025 = .214 plus .337 = .551 inches

 Sample Exhaust System 1

For the purposes of this example, we will use a bench mounted hood with the following measurements: 36” wide by 20” deep (very similar to a kitchen exhaust hood). 36 x 20 = 720 square inches divided by 144 (to get square feet) is 5. The hood coverage area is 5 square feet. Multiply this times 125 (per the discussion above) and your result is 625 CFM. The hood is going to be 10 feet from the exhaust wall, and will need two 90 degree bends.

Using the formulas above, a 625 CFM fan in a 4” duct produces over 7100 FPM of velocity, way too much. A 6” duct provides 3188 FPM, and a 7” duct provides 2340 FPM. This last diameter is important because we need to find a reasonably priced fan that will work under the specified conditions.

Per the formulas above, the total SP for this particular system will be 0.43 inches.

If you grab your handy Grainger book and look at the blower (fans) section, you will see the following fan will meet the needed 625 CFM at .43 inches of static pressure: 12G805. In an axial design, we have to match the ducting to the fan, so in this case, a FanTech FDK8 moves 760 CFM at .25 inches of static pressure. This is larger than the design calls for, however, it also provides enough “overhead” if you decide to change your hood design.

This method of fan selection will guarantee that you will get the needed air movement for your situation.

Sample Exhaust System 2

For the purposes of this example, we will use a ceiling mounted hood with the following dimensions: 4 feet wide by 6 feet long. 4 x 6 = 24 square feet. Multiply this time 125 (per the discussion above) and your result is 3000 CFM. The total run from the center of the hood to the outside wall is 15 feet, there will be one 90 degree bend.

With this amount of air being moved, you have a couple of choices. You can use one single large fan, which will probably be mounted outside, or you can use two or three smaller sized fans. The advantage of using multiple smaller fans is that you can “zone” the hood, so that if you are only using one or two torches in one area, you only need to turn on the fan that is covering that particular area. You do have to be careful to design the hood so that it can be zoned, being sure that there are baffles in place to separate each section of the hood for each fan, otherwise the entire hood would act as a drawing area and the fan would lose suction efficiency for removing fumes.

For this particular example, I’ll design it with one single fan and with 3 individual fans.

Example A – one fan design.

For a single fan design, in order to get the static pressure to a reasonable number, a 14 inch diameter duct will have to be used. This will have to be supplied by a HVAC contractor. The numbers for this design are:

CFM: 3000
Velocity: 2800 FPM
Static Pressure: .22 Inches

From the Grainger catalog, there are a number of choices:

  • Axial direct drive downblast exhaust ventilators: 4YC52: 3109 at .25 inches, etc.
  • Centrifugal belt drive downblast exhaust ventilators: 7A561: 3040 at .25 inches, etc.
  • Centrifugal in-line duct blower: 7F678: 3395 at .25 inches, etc.
  • Belt drive tubeaxial fans: 7F939: 3045 at .25 inches, etc.

This type of exhaust system should be professionally installed, due to the size and weight of the individual components. I don’t advocate doing this particular design yourself!

Example B – three fan design.

For a three fan design, be sure that the hood is evenly divided with interior baffles. Each section should duct directly in the center of the hood section. The duct size will be 8”. The numbers are:

CFM: 1000
Velocity: 2800 FPM
Static Pressure: .37 Inches.

From the Grainger catalog, again a number of choices:

  • Permanent Split Capacitor Blower: 1TDU2: 1100 CFM at .50 inches
  • In-line duct blowers: 7H713: 1040 CFM at .37 inches

Remember, you will need 3 of each fan (they should be identical).

This particular example also presents us with a problem. When the system is running at full capacity, it will be exhausting 3000 plus cubic feet per minute of air. This is a huge amount of air, and it will have a very definite effect on the temperature of the air inside the studio, especially during the winter and summer months. Because, not only are we exhausting 3000 cubic feet per minute of air, but we are also bring in 3000 cubic feet per minute of fresh outside air. Let’s say the room is 20 feet by 20 feet with 10 foot high ceilings. This is a total space of 4000 cubic feet. We will be completely changing the air in the room every 1.33 minutes. If the temperature outside is -10 degrees, you can imagine that the room temperature is going to fall very quickly.

So, what do we do? There are a couple of options. First, we can temper, or pre-heat or pre-cool the air before it is emptied into the room. This can get very expensive very quickly. Basically, you need a furnace/air conditioner that has the BTU capacity to handle 3000 CFM at the temperature extremes for your location. The fresh air intake is routed through what would normally be used as the cold air return on a normal furnace and the heating/cooling ducts would dump out into the studio room. The furnace/air conditioner would be running full blast 100% of the time to keep up with the air flow requirements. Clearly, in the dead of winter or the height of a hot humid summer, this is not an approach I would advocate.

A far better solution is to direct the makeup air to where it is being used: the worktable. Run oversize, insulated ducting to underneath the worktable. Run individual ducts, as large as possible up into the tabletop. As the exhaust system removes air, it will be pulled directly up from the tabletop duct openings, creating a curtain of air that will further enhance the removal of fumes and particulates.

Part three of this primer will discuss the building of a overhead hood system. Part four will discuss adjusting the calculations to account for altitude and temperature.

NOTE: This document is copyright (C) 2006-2015 by Michael Aurelius. Permission is hereby given to readers to use and reproduce this document for their own use only. This document may not be reproduced on any other website or forum without express written permission by the author.

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