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QUESTION
I am trying to learn more about residential HVAC design, load calculation guidelines, duct run design, etc. Is there one good resource for information?
For instance, which is better, one larger duct feeding several registers that are somewhat distant from the plenum, or three smaller diameter ducts that run all the way from the plenum to the registers? Intuition says that to minimize losses both to flow and duct wall conductivity minimize surface area by using a larger duct to feed the registers. Is this the case, or do other factors weigh more heavily?
REPLIES
quark
I concur with your basic idea.
Cosidering a duct velocity of 600 fpm, as suggested by friartuck, if you have to size the duct for 3000cfm(say), the duct cross sectional area will be 5sq.ft. This will give you an equivalent diameter of 2.52ft and surface area will be 7.91sq.ft/linear foot.
Split it into 3 ducts of 1.45ft dia. 3 ducts to carry 1000 cfm each, and the surface area becomes 4.56*3 = 13.68sq.ft/linear foot. You have to cough up extra 5.77sq.ft of sheet metal per linear foot.
As the frictional loss is proportional to v2/d, frictional losses with one large duct will be lower(for a fixed velocity)
Further, heat losses will also be high. Increase in internal convective coefficient due to increase in Reynold’s number will be offset by constant conductive and outside convective coefficients and increase in surface area.
MintJulep
The “Standard Handbook of Engineering Calculations” by Hicks, published by McGraw-Hill has example calculations for heating load and duct sizing (and many other things)
It’s a pretty good general reference book, not just HVAC.