Monday, April 11, 2016

Using Manual D Speed Sheet

The free ACCA Manual D speed-sheet makes properly sizing residential duct work really easy. However, you need to complete three important steps BEFORE you are ready to use the Manual D speed-sheet. You must first do a Manual J room by room load study, select the specific equipment you will be installing, and draw out your duct system in stick form. The speed-sheet will help you size your ducts according to the required heat load in each, the unit output, the unit airflow, and the external static pressure requirements of the unit. There are three basic steps: determining the total effective length of the duct system, determining the design friction rate, and finally sizing the duct.

Effective Length
Manual D looks for the worst case duct run and bases the design friction rate on that longest run. The idea is that if the blower can move the air through the longest run, it can easily push the air through the other ducts. The assumption is made that each run will have a balancing damper, and that the balancing dampers will be used to balance the system airflow once the system is installed. The speed sheet gives you four columns to use for determining the longest effective length. You don’t have to use all of them if the worst case run is obvious. Note that you are NOT entering data for every run, just looking for the longest run.

There are three rows labeled Trunk. They are there for systems which have multiple branching trunks. Most systems will only use one.  Enter the length of the trunk duct from the plenum to the branch takeoff in one of the Trunk rows. Then enter the length of the branch run beside Runout Length. The second set of rows on this tab are for entering the equivalent length of all the fittings. They are arranged in groups of fittings with similar functions. Click on a group to go to the tab showing the different fittings. Choose a fitting that best matches the fittings you will use. You will need to remember it, or jot it down. Click return to return to the Effective Length tab. You will probably NOT have a fitting for every group. Just leave spaces blank which do not apply to your system. Repeat the process for the return. Note that the group numbers change bit because the equivalent length of return air fittings varies from supply air fittings.

Friction Rate
The second tab is for determining the design friction rate. You need to know the specific unit for this part because you will be entering the unit airflow and external static pressure. The idea is pretty simple. All things that the air moves across cause a pressure drop. You list the pressure drop in wc for all the air components. This is totaled and subtracted from the system external static pressure, and what remains is available static to be used for moving air through the ducts.

Since friction charts are based on 100 feet, the friction rate, or wc friction drop per 100 feet, needs to be determined. For example, if your available static was 0.12 and your total effective length is 200 feet, the design friction rate would be 0.06. A duct which would cause a pressure drop of 0.06 per 100 feet would create a total pressure drop of 0.12 by the time the air traveled 200 feet. The speedsheet does this for you based on the total effective length calculated on the Effective Length tab.

Duct Sizing
You need to have a Manual J calculation of the heating and cooling loads for each room before using the final tab, Duct Sizing, Simply list the room name, heating BTUs and cooling BTUs and the speed sheet calculates the duct size based on the friction rate and CFM from the Friction Rate tab. Note that it re-sizes the ducts every time you enter more data – so don’t be alarmed if it tells you the first room you enter requires a 16 inch run. The sizes are not accurate until you have all the room information in.

Trunk sizing is as easy as clicking a box for each branch duct that t trunk feeds. Note there are probably more rows for trunk ducts than you will need.  Returns are also sized the same way. You click the box of each supply run which you believe will be served by that return. This is obviously not an exact science. However,  it is important that each supply run is selected in a return. If you have return trunks, you size them by selecting the return branches which feed into the return trunk.

Creative Application of Manual D Speed Sheet
You can use the Manual D Speed Sheet as a teaching/learning tool by varying some of the entries. For example, play with different equivalent length fittings to see the effect between best case and worst case fittings. Try different external static pressures and airflows to see the effect on duct sizing. Thi is a great way to see the effect different design decisions can have on the end result.

No comments:

Post a Comment