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What Is A Manual Volume Damper?
Have you ever asked yourself what a Manual Volume Damper (MVD) is?
That’s the question I started asking 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 or inlet, or a specific area or branch line, to reduce the “air flow” to a certain CFM (cubic feet per minute).”
This is true, but MVD has only one use. This article will explain some other ways to use MVD that you may not have thought of.
When designing an HVAC system, an important task for a mechanical engineer is to calculate how much air flow is required and how much static pressure the system must overcome to distribute the correct amount of air to each given area.
The fan manufacturer has already calculated the static pressure loss through the filter and coil in their laboratory condition packaged unit. Static pressure (the force of air pushing outward against the duct) is the biggest energy thief in an HVAC system. An engineer will try to eliminate as much of this as possible while designing the system. It will attempt to size the duct appropriately for the volume of air it must travel to reach the farthest point in the system. Turning vanes will be inserted in the elbow section, 45-degree take-off instead of 90 degrees, less static-pressure drop box, etc. There are many aspects of statics that he must overcome.
I don’t want to bore you with all this. I’m sure you get the picture.
Let’s take a typical floor of a high-rise building’s VAV system that has one fan and 52 VAV boxes (30 perimeter zones and 22 interior zones with re-heat coils). 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 pressures stated are measured in the water column and are approximate) the static pressure at the first VAV box closest to the fan is 2.10″ and the VAV box furthest away. away from the fan is 0.80. The static-pressure sensor for the fan is located two-thirds down the fan duct; Its static pressure is 1.55″.
Now we start setting up each VAV box for total air for its heating and cooling modes and the amount of outlet provided by each VAV box. While we were doing this, we noticed that all the VAV boxes closest to the fan (approximately 15 to 25 of them) were almost closed in full cooling mode and making a lot of noise. why
Because the static pressure closest to the fan is very high (in the 2.00″ range) you need that much in the main to be able to deliver the correct amount of air to the furthest VAV box.
Here’s an example of what a static pressure of 2.00″ is. When you’re in an AC unit and you want to check the fan, you have to open the door on the suction side. It takes all the strength I have to open that door. So you can imagine Some of these VAV boxes are quiet when opened and deliver anywhere from 200 CFM for a 6″ VAV box to 3,000 CFM for a 16″ VAV box. (This is in cooling mode. Imagine heating mode, when only half the amount of air is needed.)
Most VAV boxes in your perimeter that run along the perimeter of the floor are usually approximately 20 feet from the main duct, and 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 each of the branches serving the box, wasting more of the fan’s efficiency. This puts extra pressure on the damper motor in the VAV box. In heating mode, the dampers in the VAV box are almost completely closed, causing jet velocities and uneven airflow in the reheat coils, making the reheat coil very inefficient in its heat transfer mode.
Solution: Manual Volume Damper (MVD)
Here’s what MVD accomplishes in this application: When reducing the static pressure in these branches, the airflow is quickly diverted downward, without the wasted static pressure filling all the ducts before the VAV box with unnecessary air pressure. This reduces fan speed, resulting in a 15 to 25 percent savings on the unit’s overall energy consumption.
You will start closing the MVD until each VAV box damper (in its full cooling mode) is almost 100 percent fully open, still improving control. In the VAV box you are now slowing down, but you still have the air volume required by design. Now the reheat coils in their heating mode will have a much wider airflow, allowing them to have a very even and efficient heat transfer.
Last, but not least, the noise factor: By reducing the static pressure in the branch exiting the main duct, we allow the damper in the VAV box to open further, still delivering the same amount of air. This reduces the jet velocity, which reduces the noise of the damper blades in the VAV box.
Finally, MVD not only restricts airflow, saving hundreds, even thousands of dollars on energy bills, it also reduces static pressure, total pressure, velocity pressure, and noise.
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