Energy savings in the distribution systems are often very cost effective because they reduce the demand for fan power and HVAC unit capacity. Optimising the distribution system reduces running costs and improves indoor comfort. Poor distribution system design can reduce supply flow-rate, which can contribute to coil icing and reduced comfort, and can reduce air conditioning efficiency by about 10%.1
There are several considerations2 in selecting and integrating distribution system components:
- An increase in the air duct’s cross-sectional area by 15% will increase the duct cost by around 15%; however, this will reduce the friction rate by 50%, reduce pressure drop contributed by ducts by 50%, reduce overall pressure drop in the distribution system to the order of 40% (when diffuser losses are included) and reduce fan energy consumption in the order of 15–20%.
- Appropriate air duct layout and the use of efficient fittings help to minimise pressure loss and cost. Consider that:
- straight ducts have lower pressure losses and lower cost than duct turns
- round spiral ducts have lower pressure loss and cost less, and are easier to install than ducts with longitudinal joints
- smooth wye (Y-shaped) branch fittings have lower entrance pressure losses than right angle fittings
- turns that are immediately before a supply or return-air register, increase pressure loss and decrease diffuser performance.
- Sealing ducts to prevent air leaks, saves energy by reducing ventilation capacity, reducing HVAC unit capacity, and preventing undesirable heat transfer.3
- Insulating ducts also prevents undesirable heat transfer. Installing insulation to at least 50 mm, R1.5 is cost-effective for ducts outside of the air-conditioned space and is required for HVAC systems under the Building Code of Australia (Section J). Duct wrap insulation is preferable to lined duct, which increases friction losses.
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