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| AIR CONDITIONING does not alter significantly across the range of typical conditions in HVAC ductwork, and the viscosity for air at 25C, 0.18 10-4Pa.s is commonly used as a representative value. Flow is considered as laminar (streamlined) when Re < 2,300 and turbulent when Re > 3,000. When flow is laminar, the friction factor is given by the Poisseuille equation, = 64/Re. However, the velocities in HVAC ductwork will inevitably produce turbulent flow. 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 k lesser extent, the relative roughness ( D ) 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). The relative roughness will only alter for a particular duct as the surface characteristics change for example, as the duct ages or becomes contaminated. For turbulent flow, the implicit ColebrookWhite equation, or one of the many explicit alternative equations such as the Haaland equation (as used below), is applied to determine the friction factor where 1/ = -1.8log [ + ( 6.9 Re ) ( 3.7 ) k D 1.11 Yellow shaded area indicative of potential (and idealised) reduction in air supply over a day East + West load Cooling load CPD PROGRAMME West zone load Figure 2: Idealised and simplified representation of reduction in air supply volume over a day for two zones with VAV compared to VAV cooling coil outdoor air intake D1 economiser D2 dampers D3 heating coil to other zones variable flow supply fan terminal C units C room 1 room 2 variable flow exhaust and recirculation fan exhaust air discharge ] It is generally considered that the friction factor alters only slightly for a particular duct system, as the flowrate varies through the duct within a range of typical air velocities. (In reality, this may be an oversimplification as, at lower velocities of approximately < 5m.s-1 as volume flowrates and velocities vary, the effect of the change in Re will significantly alter the friction factor.) Assuming a constant friction factor, as the air velocity (and hence volume flow, q) changes and assuming all other parameters in the Darcy equation are constant the ductpressure drop is proportional to the square of the fluid velocity, and to the square of the volume flowrate, p q2. This applies tothe whole duct flow system including fittings as long as the geometry stays constant(for example, damper settings remain unaltered ). Since p q2, then for any particular flowrate p1= R q12, 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 airflowrates. The power, P (W), required to move the air through a ducted air system with a total pressure drop of pT may be determined from P = q pT and since p q2, significant savings are achievable as the required air power (and East zone load from other zones Figure 3: A simplified basic VAV system so, fan power) will reduce by the cube of any reduction in volume flow. 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. In a constant air volume multizone system (CAV), the cumulative total of the peak airflows to supply the zonal design (peak) loads must be supplied continuously, whereas VAV needs to supply air only to meet the concurrent zone loads that, as shown in Figure 2, offer the potential for significant fan energy savings. Early VAV installations which revolutionised the air conditioning marketplace of the 1960s were controlled using, in todays terms, relatively basic pneumatic or electrical controllers, but they were still able to provide systems that delivered a step change in reducing energy consumption compared with constant volume multizone air conditioning. The prevalent pneumatic controls were effective in controlling zone temperatures and system static pressure, but provided little or no data that could be used by the operator or for overall systems management. The mechanical control techniques of the time that were used for varying the volume flow delivered by fans similarly lacked the connectivity, flexibility and performance of todays digitally controlled variable speed motors. There are many variants of VAV systems. The simple VAV system, shown in Figure3, provides the characteristic elements where each of the terminal units receives primary air from the central air handling unit (AHU) at the same temperature. The flow through the supply fan is typically modulated to maintain the supply air static pressure so that it is sufficient to supply the required air though every terminal, while attempting to keep duct static pressure as low as possible. The exhaust fan flow will be varied to meet the needs of building pressurisation and the supply air flowrate. The central plant is able to use an economiser cycle (mixed air), but controlled so that zones supplying minimum airflow are still providing an appropriate proportion of outdoor air. Traditionally, the primary supply has been at a fixed temperature and is often at about 13C, since the original role of the VAV system was to provide only cooling to core areas in buildings. Some systems have evolved to include seasonal (or continuous) reset in the supply air temperature so as to meet space 60 April 2019 www.cibsejournal.com CIBSE Apr19 pp60-62 CPD v2.indd 60 22/03/2019 17:05