Page 60 - Build 152

Ventilation
25 20 15 10
-5 -10 -15 -20 -25
Figure 1: Measurement errors of a non-powered or passive ow hood. The shaded area indicates the acceptable accuracy range.
25 20 15 10 55 00
±2.5 l/s,10%accuracy Inlet ows
Outlet ows
0 10 20 30 40 50 60 70 80 90 100 110 Reference ow (l/s)
-5 -10 -15 -20 -25
Figure 2: Measurement errors of a powered or active ow hood. The shaded area indicates the acceptable accuracy range.
58 — February/March 2016 — Build 152
±2.5 l/s, 10% accuracy Inlet ows
Outlet ows
0 10 20 30 40 50 60 70 80 90 100 110 Reference ow (l/s)
Ventilation systems are low-pressure systems, and their ductwork characteristics and fan working points are altered when devices are, even temporarily, attached to them. Adding a flow hood causes the pressure in the ventilation system to drop, leading to a lower reading for the ow rate. This pressure drop occurs on both supply and extraction vents. The severity of this e ect depends on ventilation system param- eters such as ductwork type, fan type and pressure drop in the ductwork.
Active ow hoods more accurate
There are two main categories of ow hoods – powered (active) and non-powered (passive). Passive flow hoods measure the flow that goes through the hood passively. In contrast, active ow hoods track the static pressure and control a fan so that ow through the ow hood matches the ow through the vent, bringing the pressure di erence to zero.
Using an active flow hood avoids the systematic ow measurement error caused
by the pressure drop the ow hood intro- duces. For this reason, passive ow hoods should not be used.
The accuracy of the ow measurement also depends to a large degree on the uniformity of the ow that enters the ow hood.
Errors from non-uniform ow
Figures 1 and 2 show lab results obtained under controlled airflow conditions by research teams at Lawrence Berkeley National Lab in the US.
Non-uniformity of incoming flow caused greater measurement errors in a passive ow hood (see Figure 1) than the errors with an active ow hood (see Figure 2).
Non-uniformity can be caused by the duct system itself and by the vent (di usor/grille). Placement of the ow hood over the vent has also been found to be critical.
Position of ow hood is critical
The biggest measurement errors are caused by o -centre placement of the ow hood
over the vent. Often, the operator has little choice as vents are placed too close to the intersection of ceilings and walls or other obstacles.
Some of these problems can be addressed by making the flow that enters the flow hood more uniform with grids and meshes. Devices with these additional components deliver considerably more accurate results.
Measure/adjust during commissioning
When commissioning a ventilation system in a residential building, the performance of the system should be measured and, if necessary, adjusted for an optimal ow rate. Vents placed in the ceiling or walls during installation should be placed in a way that ensures a ow hood can be placed centrally over them.
Given the sensitivity of the air ow meas- urements using ow hoods, only active ow hoods should be used for measuring either air handler ow or the ow of individual vents.
FEATURE SECTION
Di erence from reference (l/s)
Di erence from reference (l/s)