CPD PROGRAMME
This relationship, although relatively straightforward, is simplifi ed in its application to operational HVAC ductwork, as most of the parameters are fi xed or vary little in the normal range of air temperatures (and pressures). So a measuring device (such as an orifi ce plate) will be calibrated by the manufacturer and will normally be supplied with a single coeffi cient that, when applied to the pressure drop, will provide a direct value of volume fl ow rate. The same fl uid fl ow principles apply to venturi
devices. There have been many developments of the simple venturi since the original devices, which were relatively long and expensive. Modern devices used in ducted air systems (as in the example shown in Figure 5) are robust and very low maintenance, and have a length similar to the outside diameter of the ductwork. The pressure drop as seen by the system
(adding to the power demands of the fan and known as ‘permanent loss’) will be somewhat less than the measured (p1
- p2 ) across the
device, as there will be a recovery of (static) pressure as the air velocity reduces following the constriction. For an orifi ce plate, this may be in the order of 70% recovery (worsening with decreasing value of β), and a venturi will recover at least 90% (as there will less turbulence) – so, in practice, the permanent losses will be relatively small with a venturi-based device.
Pitot tube devices
A classic pitot-static tube, as shown in Figure 6, is made of two concentric steel tubes with an outside diameter of approximately 10mm. It measures two pressures – the static pressure, ps
, that is measured at right angles
to the direction of fl ow through the outer tube (preferably in an area of low turbulence) and the total pressure, pt
(also known as the impact
pressure), measured through the ‘nose’ of the inner tube that is open ended and facing directly into the oncoming air. Since the total pressure at a point in a fl uid is the sum of the velocity pressure, pv pv
, and the static pressure, so = pt - ps and from this, the air velocity may be obtained as pv = 0.5·ρ·c2 If pitot-static tubes are made to appropriate
ISO standards, they will not require any calibration coeffi cient, and so the velocity of the air may be directly obtained at a single point in the duct from c = √[2 (pt
- ps)/ρ] However, as noted earlier, the velocity
varies across the cross-section of the duct so to determine an average velocity, multiple measurements need to be taken. The required positions for measurement have been established in numerous standards and, specifi cally for commissioning purposes, are clearly illustrated in BSRIA Guide to Commissioning Air Systems3
www.cibsejournal.com Flow
Reynolds calculation Determining the Reynolds number for 0.2 m³·s-1
air at 20°C in an example
rectangular duct 400 mm wide x 200 mm high.
Purpose-drilled hole in the duct wall
Output to controller
Figure 6: Basic pitot-static tube measurement of airfl ow
Air kinematic viscosity at 20°C (from tables) = 1.51x10-5 m2·s-1
(this is the
dynamic viscosity/air density) Hydraulic diameter of duct = 4 x (fl ow area/wetted perimeter) = 4 x ([0.4x0.2]/ [0.4+0.2+0.4+0.2]) = 0.267 Average velocity = volume fl ow rate/duct area = 0.2/(0.4x0.2) = 2.5 m·s-1
– a velocity
that would be typical in a fi nal or branch distribution duct Reynolds number, Re = (velocity x diameter)/kinematic viscosity = (2.5 x 0.267)/1.51x10-5
= 44,205
Since turbulent air is considered as having an Re value of greater than 4,000, this is clearly turbulent fl ow.
Figure 7: A commercial velocity grid (Source: CMR Controls)
To enable this in a permanent application, frameworks of measuring points have been developed (known as velocity or fl ow measurement grids) that take measurements across the whole face of the ductwork, as shown in Figure 7. The measuring points are distributed to ensure a representative average velocity sample. Such grids are likely to measure total pressure and static pressure separately, using appropriately orientated holes drilled into sets of sampling tubes. Velocity grids will be supplied with their own calibration factors to convert the measured pressure into velocity or volume fl ow rates. They have a signifi cant advantage over single measurements, as they can produce a much higher pressure signal, providing a greater resolution and so feasibly less error – giving measuring accuracies of ±0.2% of full range. A similar concept is the averaging tube
for a ‘pitot traverse’.
that can be added to an existing duct or new installation. This is a simple tube, fl anged at both ends to fi x securely into the duct walls, and specifi cally drilled with sampling points at locations designed to provide a representative measure of the velocity profi le across the duct. Two or more of these are normally installed to provide a reasonable sample of velocity pressures over the cross-section. Properly applied, these can provide a similar resolution and accuracies to the velocity grid.
Figure 8: A signal processor that takes the high and low pressure connections (red and blue) and is calibrated to provide direct readings of air fl ow parameters (Source: CMR Controls)
The pressure signal is typically processed
in a local device (such as that shown in Figure 8) that may have a local readout of velocity,
pressure difference and volume fl ow, having been commissioned with the duct dimensions and calibration factor for the measuring device, known as the ‘K factor’ – the airfl ow required to produce a set value of pressure difference. This is then able to transmit information to the building management system. © Tim Dwyer, 2014.
References 1
Building retuning training guide: AHU minimum outdoor air operation, PNNL–SA–88958,
buildingretuning.pnnl. gov/documents/pnnl_sa_88958.pdf , accessed 4 January 2014.
2 Legg, R.C., Air-conditioning systems – designing commissioning and maintenance, Batsford, 1991.
3 Parsloe, C., BSRIA BG49/2013 Guide to Commissioning Air Systems, BSRIA, 2013.
Turn over page to complete module February 2014 CIBSE Journal 67
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