Method and apparatus for determining the total flow rate in a ventilation installation

A method and apparatus for determining the total flow rate of air flowing through a ventilation installation with a free suction fan (15) is described. The flow rate or a corresponding pressure is determined by measurement of the partial flow rate or a corresponding pressure at at least one measurement point in the vicinity of the inlet opening of the fan. A measurement tube (20) open at both ends is suitably used, and oriented with its longitudinal axis in the inflow direction, flow sensors being arranged inside the measurement tube. In this way, there is obtained good measurement accuracy within a large flow range.

FIELD OF THE INVENTION 
The invention relates to a method and apparatus for determining the total 
flow rate in a ventilation installation. 
BACKGROUND OF THE INVENTION 
According to a primary aspect, the invention more particularly relates to 
an installation provided with a free suction fan, the inlet(s) of the fan 
being at least partially defined peripherally by an annular inlet wall 
portion that tapers in the inlet direction, and the flow rate 
being-measured on the suction side of the fan in the vicinity of the inlet 
opening(s) thereof. Such a method, as well as an apparatus for carrying 
out the method is known from EP-A1-0419798 (Gebhardt Ventilatoren GmbH). 
The performance of ventilation installations is dependent to an essential 
degree on the total flow rate. A given minimum flow rate is thus required 
in any ventilation installation for achieving the desired indoor climate, 
particularly with respect to low pollution percentages in the room air and 
desired room temperature by regulated supply of heat or cooling with the 
air. 
The greater requirements made on the indoor climate, the more important it 
is to be able to measure, monitor and regulate the total and partial flows 
in the ventilation installation. If the total flow rate from a central 
unit decreases by 10%, the partial flows to each room will also decrease 
by 10%. If monitoring of the total flow rate is enabled, the partial flows 
in the entire ventilation installation can thus be indirectly monitored as 
well. 
Several methods are known for flow measurement, particularly partial flows, 
but these methods either require an extra pressure drop with accompanying 
energy increase, generation of noise and increased operational costs, or 
they require high flow rates for achieving sufficient measurement 
accuracy. Such high flow rates are not normally present in ventilation 
ducts, and in addition there are often obstacles, e.g. in the form of 
bends close to the measuring point. Therefore, it is generally not 
possible to achieve sufficient measurement accuracy with certain simple 
flow meters, such as so-called Prandtl tubes (which measure dynamic 
pressure, i.e. the difference between total pressure and static pressure) 
or temperature-responsive electrical components (e.g. a resistor, the 
resistance of which depends on the temperature and thus also on the flow 
rate of the cooling air). For satisfactory measurement accuracy within a 
large flow range there is further required that the flow meter is placed 
in a straight duct section with a distance of about 5 duct diameters 
before and about 3 duct diameters after the measurement point. 
In accordance with prior art, and as indicated above, the flow rate in a 
ventilation installation may be determined on the basis of pressure-drop 
measurements which can be made at different places in the ventilation 
installation, e.g. in a duct system connected to a central unit, as is 
proposed in the published Swedish patent application SE-A-8704163-8 (AB 
Bahco Ventilation). A pressure difference across a component in the duct 
system is measured here with the aid of pressure measurement outlets, the 
pressure drop across the component then being proportional to the square 
of the flow. The pressure measurement outlets are connected via hoses to a 
pressure-sensing means in a meter with a pointer for visual indication of 
the flow. This measurement method is however burdened with the 
disadvantage that comparatively poor measurement accuracy is obtained, 
partly due to the comperatively low flow rate in the ducts and partly due 
to practical installation difficulties. 
Another method of flow determination is described in the Swedish patent 
specification SE-C-455 442 (AB Bahco Ventilation). In this case a filter 
in a central unit is exchanged for two perforated plates serving as 
constriction means, pressure sensors then being used to measure the 
different pressure drops which occur with the filter in place on the one 
hand, and the constriction plates on the other. On the basis of the flows 
which have already been measured with the constriction plates located in a 
similar unit, the flow rate is interpolated or extrapolated when the 
filter is in place, e.g. graphically with the aid of a diagram. 
This method also gives comparatively poor measurement accuracy, and it 
cannot be used for continuous measurement during operation of the 
installation, at least not without considerable complications and work 
from personel. 
Yet another known method is described in the published Swedish patent 
application SE-A-8701663-0 (Flakt AB), the pressure drop measurement being 
carried out on the suction side of a suction fan in a ventilation 
installation. 
The fan is placed in an apparatus housing and on its pressure side is 
connected to a duct system. A constriction means is arranged in the inlet 
portion of the housing on the suction side of the fan and has two pressure 
tappings connected to a differential pressure measurement device for 
determining the pressure drop and the flow rate. 
The constriction means, e.g. in the form of adjustable baffles is 
adjustable between a completely open position and a maximum constriction 
position, i.e. the measurement position, which enables determination of an 
empirical graph of the relationship between the measured pressure 
difference in the measurement position and the corresponding flow rate. 
This known apparatus thus requires a considerable constriction of the total 
flow rate during measurement while the apparatus is in operation, which 
results in increased need of energy, increased operational costs and 
disturbing noise. 
Both in measurement of the total flow rate in or in connection with an 
air-conditioning unit (apparatus housing) and in measurement of the 
partial flows in ducts it has therefore been necessary to compromise 
energy demand and measurement accuracy. No suitable method for readily 
determining the total flow rate in a ventilation installation has been 
provided so far. An attempt has been made to measure the pressure drop in 
a measurement chamber provided with constrictions and situated in the 
outlet part of a fan (PCT/FI88/00149, publication No. W89/02581-Imatera) 
but even this method has turned out to be unsuitable, due to complicated 
apparatus and comparatively high costs. 
Attempts have also been made to measure the local flow rate in the vicinity 
of the inlet opening of a fan. See EP-A1-0419798 (Gebhardt Ventilation 
GmbH, mentioned in the opening paragraph) and JP-A-59 13 1116 (Nihon 
Furooseru K.K.) In both these cases, the measurement is carried out by 
means of a tapping hole, made in the annular inlet wall portion, and an 
adjoining pipe or hose for sensing the static pressure at the surface of 
the inlet wall portion of the fan. However, in the region closest to the 
surface of the inlet wall portion, the air flow is somewhat irregular and 
possibly subject to frictional disturbances, which depend on the exact 
geometrical configuration and the smoothness of the surface and the flow 
rate. Therefore, the static pressure measured in such a way is generally 
not representative of the total flow rate, in particular when using damper 
control of the flow at a constant rotational speed of the fan. Moreover, 
of course, such a method can only be used upon modifying the inside 
structure of ordinary fans. There is also risk of clogging of the tapping 
holes being freely exposed in the inlet. 
SUMMARY OF THE INVENTION 
Against this background, the present invention has the object of providing 
a method and apparatus enabling measurement of the total flow rate in a 
fan-driven ventilation installation without using special constriction 
means, which would affect the total flow rate to an essential degree, or 
using specially modified fans with an especially designed inlet portion 
and internal measurement devices, while at the same time securing reliable 
measurement results within a large flow range, irrespective of what 
disturbances or constrictions that may be present in different parts of 
the ventilation installation. The method and apparatus shall thus be 
generally useable in fan-driven ventilation installations and give good 
measurement results, even in existing installations by simple and 
inexpensive means. 
Further objects are to enable reliable flow measurement with simple means 
to low cost, both in installation and in operation and service of the 
ventilation installation. In addition, it shall be possible to use the 
method and apparatus without the requirement of exacting work by personel, 
e.g. for regular supervision of the installation. 
These objects are achieved in accordance with the invention by determining 
the total flow rate on the basis of a representative partial flow rate 
being measured in a stable air flow region. 
The invention is based on the understanding that the inflow conditions are 
very stable for free suction fans, irrespective of whether it is a 
question of centrifugal fans (radial flow fans), axial flow fans or other 
types of fans, e.g. mixed flow fans. Free suction fans thus have a conical 
or otherwise inwardly tapering inlet wall portion, either formed as a part 
of a fan casing or in the form of a separate, annular element, so that the 
air flowing towards the fan wheel is guided smoothly into an inflow region 
leading directly to the fan wheel. This inflow region with particularly 
stable air flow is restricted radially inside and at a distance from the 
above-mentioned annular inlet wall portion and axially outwardly by an 
outer radial plane, which is located axially somewhat outside the inlet 
wall portion, approximately at a distance of about 30% of the least 
diameter of the annular inlet wall portion. In this stable inflow region, 
the flow pattern of the air is maintained intact irrespective of large 
variations in the flow rate. At the same time, the flow rate is very high 
in the major portion of the inflow region, which gives substantially 
better measurement conditions than in remaining parts of the ventilation 
installation connected to the fan. 
The method and apparatus in accordance with the invention have been found 
in practical tests to give, inter alia, the following advantages: 
negligible reduction of the total flow rate (less than 1% drop of the total 
pressure achieved by the fan); 
simple installation, even for existing plants; 
low installation and operational costs; 
high versatility; 
very high and stable measurement values (pressure values) with 
advantageous, particularly square flow characteristic; 
good measurement accuracy (measurement error less than .+-.5%) both with 
damper control on the pressure or suction side of the fan (at constant 
rotational speed) as with rotational speed control of the fan; 
no noise generation. 
The practical tests have been carried out with different fan sizes (inlet 
from 110 mm to 350 mm) and with different types of fan wheels (forwardly 
as well as backwardly angled blades on centrifugal fans), all with good 
results. Moreover, it appeared that various disturbances on the inlet or 
outlet side of the fan had no effect on the measurement results, e.g. 
normal belt drive arrangements or very heavy constrictions on the outlet 
side. A bend directly connected to the fan outlet has thus affected the 
measurement accuracy by merely .+-.0.5% 
The greatest measurement pressure values are obtained if the flow 
measurement means is placed axially in the vicinity of the annular inlet 
wall portion of the fan, the flow characteristic then being practically 
quadratic. However, it is also possible to place the measurement means 
axially inside or slightly outside the region of the inlet wall portion. 
The characteristic is then changed to an exponential value m (according to 
the formula: .DELTA.p=k.multidot.q.sup.m) up to about 3 or down to about 
1.7, while at the same time the measurement pressure values (or the 
pressure difference .DELTA.p in pressure drop measurement, explained in 
more detail below) will be lower. The flow conditions are still stable, 
however, and consequently it is quite possible to determine the total flow 
rate with good accuracy after calibration. 
The best results have been obtained, if a measurement tube open at both 
ends is placed in the immediate vicinity of the fan inlet opening, with 
the longitudinal axis of the tube oriented substantially in the flow 
direction and with the outlet end of the tube situated in the axially 
central portion of the stable inflow region of the fan. Measurement is 
then carried out by sensing of the partial flow axially through the 
measurement tube, this partial flow rate being substantially proportional 
to the total flow rate. 
In this manner, there are thus ensured good measurement conditions by 
sensing with a special measurement tube, in which the axial flow is 
protected and alike for different flow rates. Also, the sensing means, 
e.g. the pressure tapping holes, are well protected inside the tube. In 
check measurements, it has been found that the partial flow rate through 
the measurement tube is substantially proportional to the total flow rate 
through the fan. The partial flow rate through the measurement tube is 
accordingly representative and can be utilized as a measure of the total 
flow rate within a large flow range, i.e. the measurement will be correct 
irrespective of what disturbances or constrictions that have occured in 
the ventilation installation. 
Such a measurement tube suitably has small dimensions, and a small diameter 
in particular in relation to the area of the fan inlet openings, so that 
the total flow through the inlet opening is only affected insignificantly 
by the measurement tube. The diameter of the tube is preferably only a 
fraction, preferably at most about 1/10 of the fan inlet opening diameter, 
and is suitably 10-20 mm. 
In the above mentioned inflow region at the fan inlet, particularly in the 
axially inward portion of this region, the air flow rate increases 
compared with the flow rate axially outside this inflow region, thus 
causing a greater static subpressure in the inflow region. The measurement 
tube is then suitably placed with its inlet end axially outside the 
axially central portion of the inflow area, so that a driving pressure 
occurs between the ends of the tube. High flow rates through the tube and 
extremely good measurement conditions are thus ensured. 
In principle, the partial flow through the measurement tube can be measured 
with the aid of a flow sensor, e.g. in the form of a transducer, such as a 
temperature-responsive electrical component, e.g. a resistor, the 
operational temperature of which is an unambiguous function of the cooling 
effect obtained in response to the flow rate. However, even in this case, 
it is also possible to sense some other physical parameter which is 
dependent on the flow. 
A practically simple method of sensing the partial flow rate through the 
measurement tube is to sense the air pressure with the aid of at least one 
pressure sensing means placed adjacent to a constriction situated inside 
the measurement tube and at a distance from each end. A pressure 
difference can here be measured between two places, one immediately 
upstream and the other immediately downstream of the constriction (which 
gives a particularly high pressure difference namely up to about 600 Pa or 
even higher, and thereby very good measurement accuracy) or by the 
difference in pressure immediately downstream of the constriction on the 
one hand and a reference point outside and upstream of the measurement 
tube on the other hand. 
In the practical tests which have been made using measurement tubes of the 
kind described above, it has been found that the dimensions of the tube 
and the configuration of the constriction can vary somewhat without 
substantial alterations in the measurement values, i.e. the manufacturing 
tolerances are not critical and do not cause any great cost. 
The exact positioning of the measurement tube is not critical either, and 
mounting it can therefore be carried out without any special, costly 
control arrangement. 
The air flow rate is normally greater radially outside the central axis of 
the fan inlet opening than in the vicinity of its central axis. The 
measurement tube (or corresponding measurement means) is therefore 
suitably placed at a radial distance from this central axis, preferably at 
a radial distance amounting to about 20-90% of the least radius of the 
annular inlet wall portion. 
In radial flow fans, the inflow conditions are different in the peripheral 
direction as a result of the normally spirally shaped expanding 
configuration of the fan casing. The best measurement result is obtained 
here if the measurement tube or corresponding measurement means is placed 
in the vicinity of the part of the fan inlet opening which is located 
substantially opposite to the fan outlet (the outlet connection of the fan 
casing) as seen in a peripheral direction. 
Generally, the inventive concept also includes an apparatus for determining 
the total flow rate of air flowing through a ventilation installation, the 
apparatus being characterized by the combination of a flow measurement 
tube and a bracket for mounting the tube.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1 there is schematically illustrated a unit 1 with a heat exchanger 
and duct connections 2,3,4,5 included in a ventilation installation. The 
unit 1 is situated in a housing 6 with intermediate walls 7,8 so that four 
separate chambers 9,10,11,12 are formed. The chamber 9 is connected to the 
duct connection 2 for outside air and accomodates a filter 13 for 
filtering this air, which is caused to flow to the chamber 12 via a plate 
heat exchanger 14 centrally placed in the housing 6. The flow is achieved 
with the aid of a supply air fan 15, and the supply air is blown out by 
the duct connection 4 to an unillustrated duct system with supply air 
means (unillustrated) for supplying the supply air in a building. From 
similarly unillustrated exhaust air means the air is sucked out from the 
building via a duct system to the duct connection 3, from which this air 
flows through an exhaust air filter 16, the chamber 10, the plate heat 
exchanger 14, chamber 11 and out via the duct connection 5 by the action 
of an exhaust air fan 17 arranged in the chamber 11. 
The described ventilation installation is conventional, and only 
constitutes an example. A problem present with most such equipment is to 
maintain, upon installation and during operation, the desired flow rates, 
as discussed in the introduction. 
In accordance with the present invention, the total flow rate, (i.e. the 
supply air flow rate and/or the exhaust air flow rate) in the ventilation 
installation is measured with the aid of a flow meter means, preferably in 
the form of a special measurement tube, which is placed in immediate 
association with the inlet opening of a free suction fan. 
In the embodiment example illustrated on the drawing (cf. FIG. 2 and FIG. 3 
also), a measurement tube 20 is mounted on a bracket 21 adjacent the inlet 
opening of the supply air fan 15. The fan 15 is a conventional centrifugal 
fan with double inlets and a fan wheel 22 provided with blades and formed 
as a double drum. Both parts 22a,22b (FIG. 3) of the drum are connected by 
a common hub plate 22c in the centre of which is fixed the shaft 22d (FIG. 
2) of the drive motor. A casing 23 surrounds the fan wheel 22 and forms in 
a manner known per se a spirally shaped, peripherally expanding outlet 
duct, leading to a fan outlet 24. This outlet is connected in an 
unillustrated manner to the duct connection 4 in FIG. 1. 
On either side of the fan wheel (seen in an axial direction) the casing 23 
forms an axially inwardly tapering, rounded wall portion 25 (only the 
right hand wall portion in FIG. 3 is visible in FIGS. 2 and 3), which 
radially defines an inflow region I. In an axial direction the inflow 
region I is defined by the lines denoted by A and D, the line D lying 
axially outside a line C (the radial plane through the outer end of the 
wall portion 25) at a distance a corresponding to 30% of the least 
diameter d (at the line B) of the opening. 
The bracket 21 comprises an angled plate 21a fastened to the side wall 26 
of the fan casing 23, with a vertically dependent holder plate 21b for 
carrying the measurement tube 20. If so desired, the plate 21a and/or the 
holder plate 21b can be adjustable for altering the position of the 
measurement tube 20. 
The measurement tube 20 is fitted with its longitudinal axis oriented 
parallel to the central axis L of the inlet opening and the fan wheel 22, 
and has its outlet end 20a situated in the axially central portion I' 
(between the lines B and C) of the inflow region I relatively close to the 
inlet wall portion 25, and substantially opposite to (in the peripheral 
direction) the fan outlet 24. The inlet end 20b of the tube 20 is 
approximately conically expanding like a funnel. The diameter of the 
substantially circular-cylindrical measurement tube 20 is at most about 
10% of the diameter d of the inlet opening, and the measurement tube thus 
takes up a very small part (about 1%) of the inlet opening area. The 
length of the tube is preferably 5-10 greater than its diameter. 
Inside the measurement tube 20 (see FIG. 4) there is a throttle plate 27 
with a central, circular through flow hole 28, the plate being inserted 
approximately halfway between the ends 20a, 20b. On either side of the 
throttle plate 27, immediately upstream and downstream of it, two small 
holes 29,30 are bored in the cylindrical wall of the tube for pressure 
measurement. Pipe studs 31,32 for-connecting hoses 33,34, cf. FIG. 2 also, 
are fixed round the respective holes 29,30. 
When fan 15 is in operation, air is sucked in through the central inlet 
opening. The flow has its greatest speed in the radially outward portion 
of the inflow area I. The measurement tube 20 is preferably arranged in 
this outer portion, namely at a radial distance r from the central axis L 
corresponding to 50-90%, e.g. 70% of the radius (d/2) of the opening. 
However, when the measurement pressure is very high the measurement point 
can be placed closer to the central axis L, e.g. at a distance of 20-50%. 
The flow direction of the inflowing air coincides substantially with the 
longitudinal axis of the measurement tube. A small portion of the total 
flow flows through the measurement tube 20, and the flow rate in this 
partial flow is at a maximum when the measurement tube is placed in 
accordance with the drawing figures. 
With the aid of the pressure measurement tapping holes 29,30 and via the 
hoses 33,34, an unillustrated differential pressure measurement device of 
any suitable kind can sense the pressure drop across the throttle plate 
27, and with this as a basis, both the partial flow rate and the total 
flow rate proportional to it can be determined. The pressure drop will be 
relatively large since the sensed pressure upstream of the throttle plate 
27 in the pressure tapping hole 31 will be somewhat greater than the 
static pressure at a greater distance upstream of the throttle plate 27 
inside the tube, e.g. at the point P1 in FIG. 4, and the sensed pressure 
downstream of the throttle plate 27 in the pressure tapping hole 30 is 
somewhat lower than the static pressure at a greater distance downstream 
of the throttle plate 27 inside the tube, e.g. at the point P2 in FIG. 4. 
This pressure drop increase at a throttle plate inside a tube is known per 
se, but is particularly advantageous to utilize in connection with the 
present invention. Accordingly, there can be obtained very good 
measurement results within a large flow range. 
The invention can be applied in many ways. By "free suction fan" there is 
intended any fan, e.g. a centrifugal fan, an axial flow or a mixed-flow 
fan, which has a free inlet on the suction side and which is thus not 
directly connected to a duct on the suction side. On the other hand, the 
fan can naturally be accommodated in a chamber, which in turn is connected 
to a duct on the suction side, as is the case in FIG. 1. 
By the expression "ventilation installation" is intended any apparatus or 
system which is connected to the fan in question. It may be a question of 
a very simple arrangement, e.g. merely a free-blowing fan, or a more 
complicated system of ducts on both the suction and pressure sides, with 
associated terminal devices and other equipment. 
By "inlet opening" is intended any inlet opening through which air is 
sucked into the fan. Centrifugal fans often have two opposing inlet 
openings, as in the embodiment example, and in such cases a flow 
measurement means can be placed adjacent one or both inlet openings. It is 
just as well possible, of course, to arrange more than one flow 
measurement means at the same inlet opening. If two or more flow 
measurement means are used, a mean value of the measured magnitudes can 
suitably be formed. 
When a measurement tube is used, it does not need to be cylindrical, but 
may be conical and/or have an expanded portion. In addition, it can be 
somewhat curved or comprise different mutually connected parts with 
somewhat different orientation. What is essential is that the measurement 
tube permits the air to flow through it in such a way that the partial 
flow rate will be substantially proportional to the total flow rate. 
Instead of mounting the measurement tube or corresponding flow measurements 
means on a bracket, radial holding arms arranged like spokes may be used. 
Alternatively, the flow measurement means in certain cases may be 
suspended at a shaft mounting for the fan wheel a, e.g. with belt driven 
fans. 
The definition of the inflow region I (inside the line D in FIG. 3) has 
been made for practical reasons. The radial plane D does not correspond to 
any actual, critical threshold effect, but serves as a practical outward 
limit for placing the flow measurement means, e.g. the outlet end of the 
measurement tube, with retained measurement accuracy.