What is the most efficient use of sheet metal in ductwork construction?
Answer:
Most efficient use is to size your rectangular ductwork to the common sheet metal sizes so there is less waste, remember an allowance for joints. Might have to be in touch with the sheet metal contractors so see what is readily availabe.
Square duct will have less of a perimeter since you will need to keep the cross sectional area the same.
Circular ducting (ducts with a circular cross-section) use the LEAST sheet metal per unit of airflow. They also have the least heat loss/gain per unit of airflow as a result.
however, circular ducting is more difficult to machine, and joins and turns are much more difficult than square ducting, so unless the ducting needs to be flexible, or unless it can be very straight and long, most ducting is not actually circular but square or rectangular.
Ducting is designed to allow the correct airflow through the sytem and should not be designed with material costs in mind. A good metal worker will use the drawing and order the correct size sheets beforehand and plan to efficiently mark out the patterns. There should be very little waste as ducting comes in all sizes so on another job the offcuts will be useful.
The most wasteful aspect is using to thick a sheet for the job.. There is no point using 16 gauge when 22 gauge sheet will do. eg 150mm ducting.
Use of sheet metal in ducting has started becoming obsolete due to high fabrication & labour costs and wastage ; also loss of pressure en-route, if formation of bends and reducers is not right. It is far easier to use circular tubes / pipes made of variety of materials - PVC is most common, both for rigid as well as flexible applications. Since PVC is not a good conductor, the insulation costs are also low. Also, due to ease of joinery - elbows, tees, bends etc - and availability in variety of sizes, erection is much faster at very low cost. Unlike sheet metal, PVC doest not corrode and hence gives a very long life without any leakages.
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