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What Is A Manual Volume Damper?
Have you ever wondered what a Manual Volume Attenuator (MVD) actually is?
I started asking this question to hundreds of people in the HVAC/R industry, including air balance technicians. I got the same answer from 99 percent of the people I asked: “A manual volume damper is a device installed in a duct to dampen the ‘flow of air’ at a specific CFM (cubic feet per minute) into a single opening or inlet or a specific area or offshoot.”
This is true, but it is only one use of MVD. This article will explain some other ways to use MVD that you may not have thought of.
When designing an HVAC system, one of the important tasks of a 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 any given area.
The fan manufacturer has already calculated the static pressure losses through the filters and coils in the packaged unit in their laboratory. Static pressure (the force of air pushing outward against the duct) is the biggest energy thief in the HVAC system. The engineer will try to eliminate as much of this as possible when designing the system. It will attempt to size the duct appropriately based on the volume of air and the distance it must travel to reach the furthest point in the system. Turning vanes will be inserted in the elbows, 45 degree takeoffs instead of 90 degrees, low static drop boxes, etc. There are many aspects of statics that he has to overcome.
I don’t want to bore you with all of them. I’m sure you get the picture.
Consider a typical floor of a high-rise building’s VAV system, which has one fan and 52 VAV boxes (30 zones with reheat coils and 22 indoor zones). The VAV box furthest from the fan is approximately 250 feet from the fan discharge. At the fan, the outlet static pressure in the duct is 2.40″ (all pressures listed are measured in the water column and are approximations). The static pressure at the first VAV enclosure closest to the fan is 2.10″, and the VAV enclosure that is the most distant. from the fan is 0.80″. The static pressure sensor for the fan is located two-thirds of the way down the duct from the fan; its static pressure is 1.55″.
Now we start setting up each VAV common air cabinet for its heating and cooling modes and proportion the outputs each VAV cabinet makes. When we do this, we notice that all the VAV boxes closest to the fan (there are about 15 to 25 of them) are almost closed in full cooling mode and make a lot of noise. Why?
Because the static pressure closest to the fan is very high (in the 2.00″ range) due to the fact that you need that much in the main to deliver the proper amount of air to the furthest VAV box.
Here’s an example of what 2.00″ of static pressure is. When you’re at the AC unit and want to inspect the fan, you have to open the door on the intake side. It takes almost all the strength I have to open that door. So you can imagine the noise cause some of these VAV enclosures to barely open and provide about 200 CFM for a 6″ VAV enclosure to 3,000 CFM for a 16″ VAV enclosure. (This is in cooling mode. Imagine heating mode when only about half the amount of air is needed .)
Most of your perimeter VAV cabinets serving areas along the perimeter of the floor are usually about 20 feet from the main duct, and interior cabinets are about 4 feet from the main duct. In addition to the high static pressure in the main duct, additional static pressure travels through all the branches serving the boxes, resulting in greater fan efficiency. This puts extra pressure on the damper motor in VAV boxes. In heating mode, the dampers in VAV enclosures are almost completely closed, causing jet velocity and uneven airflow across the reheat coils, making the reheat coil very inefficient in heat transfer mode.
Solution: Manual Volume Damper (MVD)
Here is what MVD achieves in this application: While reducing the static pressure in these branches, the air flow is immediately diverted downwards without any lost static pressure filling all ducts in front of the VAV boxes with unnecessary amounts of air pressure. This reduces the fan speed and saves 15 to 25 percent of the unit’s total energy consumption.
You start closing the MVD until each VAV box damper (in full cooling mode) is almost fully open to 100 percent but still modulating control. Your velocity is now reduced in the VAV box, but you still have the amount of air required by the plan. Now the reheat coils in heating mode will have a much wider airflow, allowing them to transfer heat very evenly and efficiently.
Last but not least, the noise factor: By reducing the static pressure on the branch coming from the main duct, we allow the damper in the VAV box to open further, but supply the same amount of air. This eliminates the jet velocity, which reduces damper flap noise in the VAV box.
In short, MVD not only restricts airflow, saving hundreds, even thousands, of dollars on your energy bill, but also suppresses static pressure, total pressure, velocity pressure, and noise.
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