![]() ![]() This is the key driver for the application of multizone VAV systems in buildings that have multiple zone loads that peak at different times of the day. The power, P (W), required to move the air through a ducted air system with a total pressure drop of ∆ p T may be determined from P = q ∆ p T and since ∆ p ∝ q 2, significant savings are achievable as the required air power (and so, fan power) will reduce by the cube of any reduction in volume flow. Since ∆ p ∝ q 2, then for any particular flowrate ∆ p 1 = R q 1 2, where R is the characteristic resistance of the ducted system, R may be established from the design pressure drop and flowrate and can be applied to discover the system pressure drop at other air flowrates. The relative roughness will only alter for a particular duct as the surface characteristics change – for example, as the duct ages or becomes contaminated.įor turbulent flow, the implicit Colebrook-White equation, or one of the many explicit alternative equations such as the Haaland equation (as used below), is applied to determine the friction factor whereįigure 1: The pressures in a duct with flowing air When turbulent, the friction factor will be influenced both by variations in Re (that in a particular duct is proportional to the velocity) and, to a lesser extent, the ‘relative roughness’ – – where k is the surface roughness of the duct (for example, for new galvanised steel, k = 0.15mm) and D is the hydraulic duct diameter (measured in a unit consistent to that of k). However, the velocities in HVAC ductwork will inevitably produce turbulent flow. When flow is laminar, the friction factor is given by the Poisseuille equation, λ = 64/Re. s is commonly used as a representative value.įlow is considered as laminar (streamlined) when Re 3,000. Air viscosity does not alter significantly across the range of typical conditions in HVAC ductwork, and the viscosity for air at 25☌, 0.18 × 10 -4Pa. s) – characterises the flow regime of air in a duct. The Reynolds number – Re = ρ c d / μ, where μ is the dynamic viscosity of the fluid (Pa. Reynolds number, Re, is applied to determine the friction factor, λ. Where λ = friction factor L = length of conduit (m) D = hydraulic diameter (= 4 x area/perimeter) of conduit (m) c = velocity of fluid (m. (In a ducted air system, the velocity pressure of the air may be determined using a pitot-static tube traverse or, more practically, in an operating system through a fixed measuring device such as velocity tubes or a pitot grid.)Īs explained fully in CIBSE Guide C1, the ‘Darcy equation’ is used to provide a relationship between the parameters of a conduit (such as a pipe or duct) and the pressure drop (because of frictional resistance) in the fluid (water or air) flowing in that conduit: m -3 is used (this varies +/- 4% across the range of typical HVAC conditions) so p V is commonly noted as being 0.6c 2. Typically for air systems, a density, ρ, of 1.2kg. s -1), may be determined from the average velocity of the air, c, multiplied by the duct area, A (m 2). P T = p S + p V, and where velocity pressure, p V = 0.5 x fluid density x (air velocity) 2 = 0.5 ρ c 2, as shown in Figure 1.Ĭonsidering the flow of air through a duct (as in Figure 1), the volume flow, q (m 3. In building services engineering, when considering flow of air through ducts, the pressures are such that the air is considered as being ‘incompressible’ – the volume does not change with pressure (although the specific volume will alter as the air temperature varies).Īs air flows along a duct, it is convenient to consider it in terms of static pressure, p S (Pa), velocity pressure, p V (Pa), and total pressure, p T (Pa) where This preceded the 1970s energy crises (that subsequently heightened the interest in VAV) but, as noted by Shepherd in his seminal work, 1 it was quickly adopted as ‘enlightened engineers and building clients appreciated the potential of VAV techniques’. Variable air volume (VAV) is well established in the global air conditioning market, having been embraced in the US more than half a century ago.
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