What Is A Handbook Quantity Damper?

Have you ever asked you what a guide quantity damper (MVD) actually is?

I started out asking hundreds of individuals in the HVAC/R market, such as air-balance specialists, this query. I acquired the similar answer from 99 % of the individuals I asked: “A manual volume damper is a product mounted in the duct to damper down the “air circulation” to a distinct CFM (cubic ft for each moment) to possibly a single outlet or inlet, or a unique zone or department line.”

This is correct, but that is only just one use of an MVD. This short article will describe some other methods to use an MVD that you could possibly not have thought of.

When creating an HVAC program, one particular of the critical jobs for the mechanical engineer is to estimate how substantially air circulation is essential, and how a great deal static tension the procedure must prevail over to provide the correct amount of air to every supplied location.

The admirer producer has by now calculated the static force losses via the filters and coils in a packaged unit in their laboratory affliction. Static pressure (force of the air pushing outwards in opposition to the duct) is the largest thief of vitality in an HVAC technique. The engineer will try to eliminate as a great deal of this as he can in developing the technique. He will try to properly size the duct in accordance to the volume of air, and the distance it will have to vacation to deliver to the furthest point in the program. There will be turning vanes inserted in the elbow sections, 45-diploma get-offs instead of 90 degrees, minimal static-tension drop packing containers, and so on. There are numerous aspects of statics that he have to conquer.

I don’t want to bore you with all of them. I am guaranteed you get the image.

Illustration #1
Let us get a regular ground of a significant-increase building VAV program that has a person admirer and 52 VAV bins (30 perimeter zones with re-heat coils and 22 inside zones). The VAV box farthest from the lover is about 250 feet from the enthusiast discharge. At the admirer, the discharge static force in the duct is 2.40″ (all the pressures stated are calculated in h2o column and are approximations). The static strain at the initially VAV box closest to the lover is 2.10″, and the VAV box farthest away from the enthusiast is .80″. The static-strain sensor for the admirer is positioned two-thirds downstream in the duct from the supporter its static strain is 1.55″.

Troubles Come up
Now we start out location each and every VAV box for whole air for its heating and cooling modes and proportion the retailers served by each individual VAV box. As we are carrying out this, we observe that all the VAV bins closest to the enthusiast (around 15 to 25 of them) are pretty much closed in the complete cooling manner, and building a lot of sounds. Why?

Due to the fact the static tension that is closest to the supporter is extremely large (in the range of 2.00″) because of to the actuality that you need that much in the principal to be capable to deliver the appropriate sum of air to the farthest VAV box.

Below is an illustration of how considerably 2.00″ of static pressure is. When you’re at the AC device and you want to inspect the fan, you have to open the door on the suction aspect. It takes just about all the toughness I have to open up that doorway. So you can picture how much noise some of these VAV packing containers are inducing when they are barely opened and offering 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 method. Visualize the heating mode, when only about 50 percent the total of air is desired.)

Most of your perimeter VAV packing containers that serve the areas together the perimeter of the ground are commonly close to 20 feet from the most important duct and the interior boxes are roughly 4 toes from the main duct. In addition to the large static force in the major duct, further static tension is touring via each and every one particular of the branches that serve the containers, losing additional of the fan’s effectiveness. That puts supplemental strain on the damper motor in the VAV packing containers. In heating manner, the dampers in the VAV containers are pretty much totally closed, creating a jet velocity and an uneven airflow throughout the reheat coils, as a result producing the reheat coil to be pretty inefficient in its heat transfer manner.

Resolution: Handbook Quantity Damper (MVD)
Here’s what the MVD achieves in this software: Though lowering the static strain in these branches, the airflow quickly gets diverted downstream with no wasted static strain filling all the ducts prior to the VAV bins with unneeded amounts of air pressure. This cuts down the velocity of the enthusiast, in turn preserving 15 to 25 percent on the complete energy usage of the unit.

You commence closing down the MVD right up until every single VAV box damper (in its complete cooling mode) is just about totally open up 100 %, yet continue to modulating in manage. Your velocity now decreases in the VAV box, still you continue to have the quantity of air essential by design. Now the reheat coils in their heating manner will have a a great deal wider airflow across them, allowing for them to have a pretty even and effective warmth transfer.

Final, but not least, the sound component: By reducing the static pressure at the branch coming off the most important duct, we allow the damper in the VAV box to open additional, yet supply the same quantity of air. This gets rid of the jet velocity, which lowers the sound off the damper blade in the VAV box.

In summary, not only does MVD limit airflow, preserving hundreds, even thousands, of pounds on the power monthly bill, it also dampers static pressure, complete strain, velocity stress, and sounds.