Are We Unknowingly Wrong-Sizing Ducts?
Our fan/duct assembly is schematically illustrated in Figure 3. By adjusting dampers, it can operate in three ways: as a continuously, closed, recirculating air system; as an open, positive pressure supply system discharging to atmosphere; or as an open, negative pressure exhaust system also discharging to atmosphere.
The centrifugal fan is controlled by a variable-speed controller. Airflow cfm is measured with a duct-mounted airflow monitor with a digital output display. Duct pressures are measured with an oil-filled inclined manometer.
ON TO PRESSURE LOSS
The first test was to determine the pressure loss rate (in. of water/ 100 linear ft of duct) in the straight length of duct. These test results are compared with the ASHRAE duct-sizing chart (Figure 4), highlighting the problem: as we see the data, it turns out the conventional 60-yr-old duct design criteria can be more than 50% in error.
The second test was to determine whether a positive pressure duct supply system’s pressure loss (in. of water) is the same as a negative pressure duct exhaust system’s pressure loss when both are the same size duct experiencing same quantity of airflow, and when that airflow is moving in the same direction. Testing would probably be a waste of time; after all, it has always been written and preached by seasoned engineers that the pressure losses will always be the same.
However, knowing that air is compressible and will react differently when being pushed rather than when being pulled, it didn’t seem logical that the pressure loss would be the same for both systems. One clue that supports this view is the certified airflow performance data (Figure 5) for louvers. You can see that the pressure drop differs by 20%, depending on whether the air is fan forced (pushed) or fan forced (pulled) through the louver. Interesting, but inconclusive.
With our supply duct test data and exhaust duct test data, Figure 6 compares the two sets of results. The conclusion is that there will always be a difference of 16% between the two systems with the same duct size and same airflow (flowing in the same direction).
Since 1950, the only authority of HVAC duct design has been ASHRAE. All of its universally accepted duct-sizing charts, plus the popular Ductulator, are based on airflow formulas by Daniel Bernoulli (d 1782), Henry Darcy (d 1858), Julius Weisbach (d 1871), and C.F. Colebrook (d 1939). Their formulas, expressed here:
… have been confirmed to produce the ASHRAE chart. Our team member Dorothy Schroeder, math department head at Middleton High School, confirmed this. Unfortunately, to our knowledge, there had been no hands-on airflow testing to authenticate the popular and exclusively used duct sizing method until now.
Based on the results of the first test, duct sizing will require starting over with laboratory airflow testing with repeatable results. The flow coefficients, Cp, based on these formulas for determining pressure loss for duct fittings (elbows, tees, etc.), also have to also be airflow tested.
These duct test results indicate the following problems:
•The operational system pressure loss is greater than the calculated losses, by up to 140%.
•The design fan motor amp draw will be less than operational.
•Fan sheaves have to be frequently changed to overcome higher pressure drops.
The benefits of correct duct sizes are that systems work correctly, they are easier to troubleshoot, testing and balancing are simpler, and good IAQ can be achieved.
Based on the results of the second test, duct sizing for both airflow being pushed and airflow being pulled through duct will require laboratory testing with repeatable results.
This shows that the exhaust fans must be located at the point of air exhaust, which results in reduced pressure drop, reduced brake horsepower, and reduced energy, Also, the pressure drop calculations will be different for supply duct and exhaust (return) duct with the same duct size, same airflow, and flowing in the same direction.
And finally, a nationally recognized air testing and balancing organization (the Associated Air Balance Council, www.aabc.com/) and others, witnessed and participated in this testing. They are considering a comprehensive airflow test to produce duct-sizing criteria to be included in their national standards handbook for the HVAC engineering design community. ES