Have you ever asked yourself what a manual volume damper (MVD) actually is?
I started asking hundreds of people in the HVAC/R industry, including air-balance technicians, this question. I received the same answer from 99 percent of the people I asked: “A manual volume damper is a device installed in the duct to damper down the “air flow” to a specific CFM (cubic feet per minute) to either a single outlet or inlet, or a specific zone or branch line.”
This is true, but that is only one use of an MVD. This article will explain some other ways to use an MVD that you might not have thought of.
When designing an HVAC system, one of the important tasks for the mechanical engineer is to calculate how much air flow is required, and how much static pressure the system must overcome to deliver the proper amount of air to each given area.
The fan manufacturer has already calculated the static pressure losses through the filters and coils in a packaged unit in their laboratory condition. Static pressure (force of the air pushing outwards against the duct) is the biggest thief of energy in an HVAC system. The engineer will try to eliminate as much of this as he can in designing the system. He will try to properly size the duct according to the volume of air, and the distance it must travel to deliver to the furthest point in the system. There will be turning vanes inserted in the elbow sections, 45-degree take-offs instead of 90 degrees, low static-pressure drop boxes, etc. There are many facets of statics that he must overcome.
I don’t want to bore you with all of them. I’m sure you get the picture.
Let’s take a typical floor of a high-rise building VAV system that has one fan and 52 VAV boxes (30 perimeter zones with re-heat coils and 22 interior zones). The VAV box farthest from the fan is approximately 250 feet from the fan discharge. At the fan, the discharge static pressure in the duct is 2.40″ (all the pressures stated are measured in water column and are approximations). The static pressure at the first VAV box closest to the fan is 2.10″, and the VAV box farthest away from the fan is 0.80″. The static-pressure sensor for the fan is located two-thirds downstream in the duct from the fan; its static pressure is 1.55″.
Now we start setting each VAV box for total air for its heating and cooling modes and proportion the outlets served by each VAV box. As we’re doing this, we notice that all the VAV boxes closest to the fan (approximately 15 to 25 of them) are almost closed in the full cooling mode, and creating a lot of noise. Why?
Because the static pressure that is closest to the fan is very high (in the range of 2.00″) due to the fact that you need that much in the main to be able to deliver the proper amount of air to the farthest VAV box.
Here is an example of how much 2.00″ of static pressure is. When you’re at the AC unit and you want to inspect the fan, you have to open the door on the suction side. It takes just about all the strength I have to open that door. So you can imagine how much noise some of these VAV boxes are inducing when they’re barely opened and delivering somewhere in the neighborhood of 200 CFM for a 6″ VAV box to 3,000 CFM for a 16″ VAV box. (This is in the cooling mode. Imagine the heating mode, when only about half the amount of air is needed.)
Most of your perimeter VAV boxes that serve the areas along the perimeter of the floor are usually approximately 20 feet from the main duct and the interior boxes are approximately 4 feet from the main duct. In addition to the high static pressure in the main duct, additional static pressure is traveling through every one of the branches that serve the boxes, wasting more of the fan’s efficiency. That puts additional pressure on the damper motor in the VAV boxes. In heating mode, the dampers in the VAV boxes are almost completely closed, causing a jet velocity and an uneven airflow across the reheat coils, thus causing the reheat coil to be very inefficient in its heat transfer mode.
Solution: Manual Volume Damper (MVD)
Here’s what the MVD achieves in this application: While reducing the static pressure in these branches, the airflow instantly gets diverted downstream with no wasted static pressure filling all the ducts prior to the VAV boxes with unnecessary amounts of air pressure. This reduces the speed of the fan, in turn saving 15 to 25 percent on the total energy consumption of the unit.
You start closing down the MVD until each VAV box damper (in its full cooling mode) is just about completely open 100 percent, yet still modulating in control. Your velocity now decreases in the VAV box, yet you still have the volume of air required by design. Now the reheat coils in their heating mode will have a much wider airflow across them, allowing them to have a very even and efficient heat transfer.
Last, but not least, the noise factor: By reducing the static pressure at the branch coming off the main duct, we allow the damper in the VAV box to open further, yet deliver the same quantity of air. This eliminates the jet velocity, which reduces the noise off the damper blade in the VAV box.
In conclusion, not only does MVD restrict airflow, saving hundreds, even thousands, of dollars on the energy bill, it also dampers static pressure, total pressure, velocity pressure, and noise.
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