The air flow (Q) at the diffusers can be measured by channelling the air and measuring the air discharge velocity with a calibrated anemometer [4]. The blower opening can be channelled by means of a circular cross-section duct, the length of which is ten times the diameter of the duct (D).
The air flow through the opening i is defined by the following relationship:
Vi
Qi D 4
× 2
= π (C.7)
where
D is the diameter of the duct in metres (m);
--`,,```,,,,````-`-`,,`,,`,`,,`---
Vi is the mean velocity of the air measured at a distance of 0,242 × 2
D in metres per second (mãs-1) from the wall of the duct;
Vi is equal to the discharge velocity at this distance, in metres per second (m.s-1).
The air flow is then obtained by totalling the flow rates determined for each opening: =∑
i
Qi
Q .
The disadvantage of this method is a possible imbalance in the flows due to a loss of pressure in the duct used for the measurement. Therefore, rather than measuring the flow by channelling it through each opening, it is preferable to design a collector to include all the blower openings and measure the discharge velocity in the collector duct.
C.3.2 Measurement of the flow rate of new air by tracing
The flow rate of the new air (Qn) is measured by means of the gas tracing technique. The method involves injecting a gas tracer (helium) at a constant, known mass flow rate into the air inlet of the cab and measuring the steady state concentration of the tracer inside the cab (Cit∞).
Figure C.2 shows the typical changes in internal tracer concentration.
Key
Cit Internal concentration of the tracer Cr Residual concentration of the tracer
Figure C.2 — Changes in internal tracer concentration — Determination of steady state concentration (Cit∞∞∞∞) The flow rate is obtained using the following relationship:
--`,,```,,,,````-`-`,,`,,`,`,,`---
23
( ) ε
× ×
=
∞ n
it n
V C M
Q q (C.8)
where
q is the mass flow rate of the tracer in kilograms per second (kgãs-1);
Cit∞ is the steady state concentration (àg/g);
Cr is the residual concentration of the tracer (àg/g);
M is the molar mass of the tracer in kilograms (kg);
Vn is the molar volume in cubic metres (m3) under normal conditions (T = 0 °C and P = 1,013 x 105 Pa);
ε is a correction coefficient defined by:
P T
×
×
= ×
273 10 013 ,
1 5
ε (C.9)
where
T is the temperature of the air in Kelvins (K);
P is the pressure in Pascals (Pa).
C.3.3 Measurement of the flow rate of new air by thermal anemometry
C.3.3.1 Measuring apparatus C.3.3.1.1 General
The flow rate is measured using a hot-wire cylindrical probe anemometer with a maximum diameter of 8 mm and accurate to ± 3 % of the values observed, in compliance with EN 14387.
C.3.3.1.2 Characteristics:
measurement range: 0 mãs-1 to 30 mãs-1;
resolution: 0,01 mãs-1 < 3 mãs-1;
operating temperature: 0 °C to + 50 °C ;
accuracy: ± 3 % of the measurement to ± 3 mãs-1. C.3.3.2 Measurement points
See Figure C.3. The air flow velocity is measured at the front upstream of the air delivery and filtration system.
The measurement lines are divided into abscissas and ordinates of the open area of the air delivery and filtration system.
The measurement lines on the perimeter of the open area are 15 mm from the edge of the closed area corresponding to the frame of the air delivery and filtration system. The measurement lines in the same
--`,,```,,,,````-`-`,,`,,`,`,,`---
orientation axis are positioned at even intervals. These intervals shall be at least 30 mm and not exceed 50 mm.
NOTE This measurement method, which is partly taken from Clause 10 of ISO 3966:2008, is based on the velocity field analysis according to the "log-tchebycheff" method. Several aspects have been modified to take account of the configuration of the ducts in the re-circulation systems of the cabs of agricultural vehicles, when present, which cannot be compared with the configuration of the ducts mentioned in ISO 3966.
Dimensions in millimetres
Key
X Measurement lines in abscissa Y Measurement lines in ordinate 1 Closed area
2 Open area
3 Measurement point
Figure C.3 — Measurement points C.3.3.3 Measurement conditions
C.3.3.3.1 Air delivery and filtration system equipment
The measurements are taken in the original equipment configuration of the air delivery and filtration system with all the accessories upstream and downstream of the air delivery and filtration system, such as guards, ductwork, grilles, flaps and covers in place.
C.3.3.3.2 Positioning the probe
The minimum distance between the sensitive element of the probe in the measuring device (hot wire) and the front part of the open area of the air delivery and filtration system shall not exceed 15 mm. The probe shall be positioned at an angle that makes it possible to obtain the highest resultant velocity.
--`,,```,,,,````-`-`,,`,,`,`,,`---
25 C.3.3.4 Determination of the flow rate of new air
C.3.3.4.1 Recording the measurements
Each measurement is recorded on a table indicating the different speeds noted at the measurement points.
Table C.1 — Recording speeds at the different measurement points Speeds in metres per second (mãs-1)
A B C D E
1 1,320 1,350 1,380 1,440 1,340
2 1,330 1,330 1,350 1,420 1,360
3 1,320 1,350 1,370 1,440 1,340
4 1,340 1,340 1,380 1,420 1,330
5 1,310 1,350 1,360 1,450 1,360
C.3.3.4.2 Calculation of the flow rate of new air
The flow rate of the new air (Qn) is calculated from the following formula:
600
×3
×
=S V
Qn (C.10)
where
Qn is the flow rate of new air in cubic metres per hour (m3ãh-1);
S is the open front area of the air delivery and filtration system in square metres (m2);
V is the mean velocity calculated from the different measurement points in metres per second (mãs-1).