Principles of HVAC Duct Design in the Pharma Industry
How Does a Duct System Work
The duct, or air distribution, system used in cooling and heating your area is a collection of tubes that distributes the heated or cooled air to the different rooms in the Pharma Industry. This branching network of round or rectangular tubes—usually constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite—is found within your Area.
The duct system is designed to supply rooms with air that is “conditioned”—that is, heated or cooled by the heating, ventilation, and air conditioning (HVAC) equipment—and to circulate or return the same volume of air back to the HVAC equipment.
Typical air-duct systems lose 25 to 40% of the heating or cooling energy put out by the cooling and heating system. Leaks, one way in which conditioned air is lost in the duct system, make the HVAC system work harder, thus increasing your utility bill in Pharma Industry.
In addition, duct leakage can lessen comfort and endanger your health and safety. Your duct system has two main air-transfer systems—supply and return.
The supply side delivers the conditioned air to the home through the individual rooms.
The return side withdraws inside air and delivers it to the air handler of your central system. All of the air drawn into the return duct(s) is conditioned and should be delivered back through the supply
The process we use in designing duct systems, includes available static pressure, equivalent length, and choosing fittings.
Before duct design
Designing a duct system is important but there are a few critical steps that come first. Number one is the heating and cooling load calculation using a protocol. You’ve got to know how much heating and cooling you need for each room (in ACPH/hr) in Pharma Industry. Then those ACPH per hour requirements immediately translate to room-by-room airflow requirements in cubic feet per minute (cfm).
Once you know the ACPH/hr and cfm numbers for the building, you need to select the right equipment. a piece of equipment that meets the total heating and cooling loads for the home. You’ve got to make sure you adjust for the indoor and outdoor design conditions of the home.
Then you’re ready to start designing the duct system.
The weight of the air
The first thing you need to know is that air has weight. In the photo below, a man holds a 1 cubic foot block, which he says would weigh nearly 0.1 pounds if it were air. The actual number is 0.0807 lb at standard temperature and pressure.
If you have a 2.5-ton air conditioner, the nominal airflow would be 1,000 cfm. (The rule here is 400 cfm per ton.) That means the blower has to push about 81 pounds of air through the system each minute. It takes work to move weight around.
Well, actually, if you remember your introductory physics class, you know that’s not quite true. You can move weight for free if you move it horizontally and without any kind of resistance. It takes work to move it upward against gravity or to push it in any direction against friction.
The physics of airflow
If you take a fan out into your yard on a calm day and turn it on, you’ll get its maximum airflow. If you take that same fan and blow the air into a cardboard tube, it has to work against the pressure that builds up in that space. The more you reduce the size of that tube or make it longer or turn the air with it, the more static pressure builds up. And the more the airflow is reduced.
That’s the basic principle you have to work within duct design. I’ve written previously about the two factors involved in reducing airflow in ducts.
One is friction – As the air moves through a duct, it interacts with the surfaces. The smoother that inner surface is, the better it is for airflow. The rougher the surface, the more it slows down the air.
The second factor is turbulence – This generally arises when you move air through fittings, or when you turn the air. With rigid ducts, you turn the air with fittings, but unfortunately, that’s not always the case with flex ducts.
When air comes out of the air handler, several things happen to it. It gets sent to the various rooms in the area. As it travels through a trunk-and-branch duct system, the quantity keeps diminishing because some of it gets diverted down each branch on the way to the end.
Each section of duct, each fitting, and each turn of the air adds resistance to that airflow because of friction and turbulence. Grilles and registers, filters, and balancing dampers also add resistance. That resistance results in decreases in the static pressure, or pressure drops.
So, at the blower with high pressure. By the time the air comes out of the supply vents, that pressure has dropped to zero (relative to room pressure).
Available Static Pressure
Pressure drops all through the duct system in Pharma Industry. Whenever air encounters a filter, coil, heat exchanger, registers, grilles, balancing dampers, and the ducts themselves, it loses pressure.
The diagram below shows the components of our system. The AHU is the air handler (or handling) unit. That’s where the blower is. The air inside the home gets pulled back to the AHU through the return ducts. The air gets conditioned inside the AHU and then sent back into the room through the supply ducts.
When talking about pressures here, we’re not talking about absolute pressure. We’re talking about relative pressure. Our reference when we talk about pressures is the pressure inside the conditioned space. That’s our zero.
On the return side of the blower, the pressure will be negative. As the air moves from the room, into the return grille, and down to the AHU, the pressure gets more and more negative relative to the room.
On the supply side, the pressure is positive. As air moves from the AHU through the supply ducts and out into the rooms, the pressure gets less and less positive.
The maximum positive and negative pressures occur at the air handler. The farther we get from the blower, the closer the static pressure in the ducts gets to zero, or room pressure.
Blower capacity
To get a certain amount of airflow, a blower needs to operate against a certain pressure and at a certain blower speed setting. Here’s a table from one unit.
The blower speed is set by moving wires to different taps. In this case, there are 5 of them. The row of numbers across the top is the total external static pressure (TESP) the AHU is rated for. That’s the pressure change across the AHU when pushing and pulling air through the ducts.
You generally want to design a system to operate at medium speed (tap 3 in the table above). That way you have some room for adjustment when you commission the system. Also, most systems are rated to operate at a total external static pressure of 0.50 inches of water column (iwc).
For the system above, those parameters yield an airflow of 899 cfm. If that’s the number you need, you just have to make sure you design your system to operate at 0.5 iwc.
So, from the return (most negative) side of the AHU to the supply (most positive), we want a total pressure change of no more than 0.5 iwc. (That’s the typical number. Some air handlers are rated higher. Some are rated lower.) That’s the total pressure change across the AHU.
The actual pressure in the system will depend on the ducts and other components. As long as we’re at or below 0.5 iwc in this case, we’ll get good airflow.
Notice I said pressure change here, no pressure drop. The blower causes a pressure rise. It’s the force behind the airflow so from the negative side (return ducts) to the positive side (supply ducts), the pressure rises.
Finding the available static pressure (ASP)
What happens next is splitting up the two kinds of pressure drops in the duct system in the Pharma Industry.
First, we want all the external pressure drops of the components that are not ducts or fittings. Those things have to go into the duct system and generally have known pressure drops. We subtract them from the total external static pressure number (typically 0.5 iwc). What’s left is the available static pressure (ASP) for the ducts and fittings.
At the top is the total external static pressure. That gets entered automatically after you select equipment, but you can override the numbers here. In the table above, I’ve got different numbers for heating and cooling just to illustrate the effect on the bottom line, but usually those numbers are the same.
Second, you enter all the external pressure drops. The coil and heat exchanger are zero here because the coil is already included in the total external static pressure because it’s inside the AHU and there is no heat exchanger since it’s a heat pump. With a furnace, you’ll have a coil that’s outside the AHU and will need to add it. I don’t think we’ve ever had a project where the heat exchanger was external and needed to be added here.
The other numbers shown there are pretty standard numbers, but you want to enter the actual numbers if you have them.
For example, if you’re using wooden grilles, the pressure drops will be significantly higher. But .don’t use wooden grilles! They will make it very difficult to get good airflow.
Your duct budget
Once you’ve entered your external static pressure rating and all your external pressure drops, what’s left after subtracting the drops from the rated pressure is the available static pressure. That’s how much you have left to “spend” on your duct system.
To summarize where we’re at now:
- The blower creates a pressure rise to move air through the ducts.
- It’s rated for a certain amount of airflow at a specific total external static pressure.
- The ducts, fittings, and other components cause pressure drops.
- Subtracting the pressure drops for all the things that aren’t ducts or fittings from the total external static pressure yields the available static pressure.
- The available static pressure is the pressure drop budget you have to work with when designing the ducts.
We now go to the next step and design a duct system that will have a pressure drop of no more than the available static pressure. To do that, we size ducts and choose fittings using something called equivalent length.
Consequences of Poor Ductwork Design
According to the US Department of Energy, the average HVAC duct system is only about 60 percent efficient. That means air is not flowing through your space and your HVAC system as it should be, which leads to all kinds of undesirable consequences, including:
- Hot and cold spots, drafts, and stuffy air in your space due to impeded airflow.
- Extra wear and tear on your air conditioner, since it needs to run longer and work harder to compensate for flaws in the ductwork design, leading to more breakdowns and shorter equipment life.
- Poor air quality exposes the occupants of your space to increased levels of dust, pollutants, fumes, and even mold growth from too much humidity.
- Unbalanced air pressure causes odors to linger, doors to slam by themselves, and distracting levels of noise in your space.
Common Ductwork Design Mistakes
Proper ductwork design ensures the level of airflow that your HVAC system needs to operate efficiently and provide the comfort you want and expect in your renovated space. Here are some of the common ductwork design mistakes that impede the function of your air conditioning:
DUCTWORK DESIGN MISTAKE #1: Under sizing
Contractors can make the mistake of failing to consider the type of air conditioning system you have, the load requirements of different rooms, where ducts and equipment are located, and the materials used to construct them. All of these factors affect the proper sizing of your ducts, and getting it wrong often means your HVAC ductwork is undersized.
DUCTWORK DESIGN MISTAKE #2: Runs that are too long
When the location of the HVAC equipment and duct system and not optimized in the planning phase, the equipment may end up far away from the space to be cooled. That may require long runs of ductwork that make it hard for your HVAC system to move conditioned air to certain areas within the space.
DUCTWORK DESIGN MISTAKE #3: Sharp bends
Just like long runs impede airflow, bends in the ductwork that is too sharp or too numerous also decrease the amount of air that actually reaches the space to be cooled.
DUCTWORK DESIGN MISTAKE #4: Air leaks
Air conditioning ductwork that is incorrectly sealed or supported can end up leaking cooled air into the walls which it won’t do any good to the occupants of your space.
DUCTWORK DESIGN MISTAKE #5: Lack of returns
To maintain balanced air pressure and air movement, your duct system needs return vents for air in the room to be pulled back into the HVAC system. Not providing enough returns is a common ductwork design flaw that leads to comfort complaints.
Tips for Good Ductwork Design
To make sure your ductwork is properly designed, start by involving a knowledgeable HVAC design professional early in your renovation design process. An experienced pro will work with the architect and contractor to do the following:
1. CHOOSE THE BEST LOCATION FOR HVAC EQUIPMENT AND DUCTS. With proper planning, the HVAC equipment should be centrally located in the space to allow for the shortest possible duct runs. Ducts should be located in internal walls and ceilings to minimize the loss of conditioned air. Avoid installing ducts in attics and unconditioned crawl spaces for maximum efficiency.
2. A DETAILED LOAD CALCULATION. Especially when your space has different rooms or areas with varying heating and cooling requirements, it’s important that the load calculations be done individually for each room, rather than only for the space as a whole.
3. CONSIDER YOUR EQUIPMENT TYPE AND SUPPORTING SYSTEMS. Certain types of air conditioning systems, like heat pumps, require larger ducts. If your system includes air purifiers with activated charcoal filters, these also affect air flow and may require larger ducts and/or additional air returns.
4. USE THE RIGHT MATERIALS, FITTINGS, AND SUPPORTS. Ductwork materials can vary depending on the requirements and the budget, but make sure your installer uses the right materials for your needs. Flexible ducts (often called “flex”) made from reinforced plastic are easier and cheaper to install, but not as strong and durable as sheet metal.
If quiet operation and energy efficiency are very important to you, you might want to go with duct board, made from pressed fiberglass, which is more expensive but very quiet and efficient.
5. CHOOSE THE RIGHT DUCT SIZE AND LAYOUT. Once all the system variables have been decided, your HVAC design professional can determine the most efficient ductwork design layout and calculate the correct duct size.
6. ENSURE PROPER DUCTWORK SEALING. Did you know that as much as 20 percent of your conditioned air can be lost when duct joints are not correctly sealed? The problem is compounded with high-efficiency systems, which run longer at a lower capacity.
Air is in the ducts for a longer period of time and so more can escape through leaky joints. Make sure your duct joints are sealed with mastic gum or metal-backed tape to prevent leaks.
