1、OR-05-2- 1 Duct Loop Systems- Savings and Performance G.J. Levermore, PhD, CEng Member ASHRAE ABSTRACT Duct loops are often used in VAV air systems to save energy, ensure a balanced air delivery, and simplify fabrica- tion and construction. Duct loop performance is more compli- cated than that for a
2、 standard radial tree network, and simulation methods are required. Such simulation is examined. Part-load and asymmetric duct loops are also analyzed where numerical methods are required rather than dividing the symmetrical duct in two for easy calculation. The numerical method is discussed. The fu
3、ll andpart-load simulation ofduct loops and comparable networks are then considered. Signij- cant savings in fan power arepossible compared to radial tree and two-branched networks, but the non-duct pressure losses (coils, jlters, etc.) will determine the actual system saving in each case. However,
4、duct loops do require a larger amount of sheet metal in most cases for which the costs will count against the duct loops. However, economics are not considered in any depth in this paper: INTRODUCTION When air is distributed in ducts to a number of outlets there are many different distribution metho
5、ds. One form is the duct loop (Figurela). A small duct loop serving four variable air volume (VAV) terminal boxes is shown for ease of under- standing. This may be connected to a riser duct that serves a number of floors in a building with a loop on each floor. Figure Ib shows such a riser with a nu
6、mber of branches forming a ladder of basic radial networks. Duct loop, radial tree (see Figure Id), or two-branch (see Figure lc) networks could replace the basic radial networks in the ladder. This paper compares these different networks, concentrating on the duct loops. VA b-k/ terminal box U Figu
7、re la A duct loop. Y I# I Figure Zb A ladder network. G.J. Levermore is a professor of the built environment in the Manchester Centre for Civil and Constniction Engineering, UMIST, Manchester, England. 02005 ASHRAE. 507 f = friction factor, dimensionless; I = length of duct, m (ft); p = density, kg
8、m-3 (lb R3); v = velocity, ms- (fi s-); and Dh = hydraulic diameter of the duct, mm (in.). tion l becomes In terms of the volume flow of air V(m3s-, ft3 s-) Equa- 4000flp v“ 2nDh 5. Mr = The friction factor f is given by Colebrooks equation (ASHRAE 2001), - I = -2105 “-+-I, 2.5 1 4- 3.7Dh Re. Applic
9、ation to low-energy HVAC and natural ven- tilation control. E Part II: Cal- culation procedure and economic analysis. ASHRAE Transactions 94(2):90-111. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engi- neers, Inc. Tsal, R.J., H.F. Behls, and R. Mangel. 1990. T-method duct system design, Part III: Simulation. ASHRAE Transac- fions 96(2):3-33. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc. 51 4 ASHRAE Transactions: Symposia
