[Equest-users] question about inefficiency of duct

[Varkie C Thomas]

See page 14 to 17 "Thermal Analysis"
http://bepan.info/class-notes/e6_-_loads-ducts-pipes 
AES-Ductwork-System-Design
When the cooling DT (Room-Suppy Temp) is 20 degs, every degree temp rise from the cooling coil affects the supply air by 5%.  For a draw-thru system with a supply static of 8" to 9" the heat gain across the fan, the temp rise can be significant. The rule of thumb in the old days, when no one could be bothered doing these calculations manually, the temp-rise was assumed to be = 0.5 x SP.   
The velocity upstream of the terminal box from the AHU through the unconditioned shafts of high-rise bldgs is typically about 4,000 fpm which results in low heat gain due to the high velocity. The terminal box and the low velocity ductwork is above conditioned space (typically RA plenum).  If the ductwork is insulated on both sides of the TB, the rule of thumb for a temp rise through the ducts is about 1 - 2 degs which requires about a 10% increase in supply air.
Before anyone replies to this message saying that it has wandered off the point from the issue being discussed,  I will admit the reply is an excuse to for mentioning the website. I am thinking of closing it down if there is no interest from others in developing building energy analysis course material from a full design perspective and not just using computer program.  The emphasis would be on teaching Energy Efficient Building Design with project case studies.  It does not quite fit in with BEMBook The objectives are in the Introduction http://bepan.info/introduction
Varkie
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Michael:

"if the air in duct is the same air as the air being supplied to the space, 
there is no net effect"?  What do you mean by net effect?

The duct heat loss/gain depends on the difference of the temperature of the air 
in the duct and the temperature of the air surrounding the outside of the duct.  
It is highly unlikely that these temperature will be the same.

Paul Diglio




________________________________
From: "McArdle, Mike" <Mike.McAardle at state.vt.us>
To: xiaoyang shi <xs2144 at gmail.com>; "equest-users at lists.onebuilding.org" 
<equest-users at lists.onebuilding.org>
Sent: Tue, March 29, 2011 5:39:40 PM
Subject: Re: [Equest-users] question about inefficiency of duct


Dear xiaoyang shi
 
It Depends on the temperature differentials between air in duct and space and 
insulation thickness 

 
But more importantly , if the air  in duct is the same air as the air being 
supplied to the space there is no net effect 

 
 
Thank you,  Sincerely,
 
Michael J. McArdle , P.E.
Buildings Engineer II, Project Manager 
State of Vermont, 
Buildings and General Services (BGS)
2 Governor Aiken Avenue
Montpelier, Vt. 05633
 
Office: 802-828-5650;  cell: 802-272-4828
 
From:equest-users-bounces at lists.onebuilding.org 
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of xiaoyang shi
Sent: Tuesday, March 29, 2011 5:37 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] question about inefficiency of duct
 
Hi folks,

In the winter time, some heat loss in HVAC is from duct.

What is the percentage of heat loss through the duct to the total building heat 
load, due to the inefficiencies of duct delivery system?

Thank you guys.
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