SE512894C2 - Measuring total flow rate in ventilation installation - Google Patents

Abstract

The method determines flow rate by measurement of the local pressure at a measurement point in the vicinity of an inlet opening of a free suction fan (15). A measurement probe (20), open at one end, is used as a measurement device and oriented with its longitudinal axis in the inflow direction. The measurement point (20a) is situated axially inside an outer radial plane (B), which is located at an axial distance (a) outside an inner radial plane (A). The measurement point is located adjacent the inner radial plane (A), though always in the narrowest section of the fan inlet.

Description

20 25 512 894 s.k. Prandtl pipes (which measure dynamic pressure, ie the difference between total pressure and static pressure) or temperature-dependent electrical components (eg a resistor, the resistance of which is temperature-dependent and thus also dependent on the cooling air flow rate). For satisfactory measuring accuracy within a large flow range, it is further required that the flow sensor is placed in a straight duct section with approx. 5 duct diameters before and approx. 3 duct diameters after the measuring point.

According to the prior art, as indicated above, flows in ventilation systems can be determined on the basis of measurements of pressure drops, which can be performed at different places in the ventilation system, e.g. in a duct system connected to a central unit, as proposed in the published Swedish patent application SE-A-8704163-8 (AB Bahco Ventilation). In this case, a pressure difference across a component in the duct system is measured by means of pressure measuring sockets, the pressure drop across the component being proportional to the flow squared. The pressure measuring sockets are connected via hose lines to a pressure sensing device in a meter with a pointer for visual indication of the flow. However, this measurement method has the disadvantage that relatively poor measurement accuracy is obtained, partly due to the relatively low flow rate in the ducts, and partly due to practical installation difficulties.

Another method for flow determination is described in the Swedish patent specification SE-C-455 442 (AB Bahco Ventilation), whereby an existing filter in a central unit (apparatus housing) is temporarily replaced with two different perforated plates serving as throttling means and measuring by means of pressure sensors. the different pressure drops that occur with the filter inserted resp. with the throttle plates. Based on the flows previously measured with the throttle plates in a similar unit, the flow size is interpolated or extrapolated with the filter inserted, e.g. graphically using a diagram. 1 1 UI 0 5 25 3 0,1 C (TI 512 894 This measurement method also gives relatively poor measurement accuracy, and it cannot be used for continuous measurement during the operation of the ventilation system, at least not without considerable complications and personnel efforts.

Another known method is described in the initially mentioned published Swedish patent application SE-A-8701663-0 (Fläkt AB), wherein the pressure drop measurement is performed on the suction side of a free-sucking fan in a ventilation system.

The fan is located in an apparatus housing and connects on the pressure side to a duct system, whereby on the suction side of the fan in the inlet part of the apparatus housing a throttling device with two pressure measuring probes and a connected differential pressure measuring device for determining pressure drop and flow is arranged.

The throttling device, e.g. in the form of adjustable slats, is adjustable between a fully open position and a maximum throttling position (measuring position), which makes it possible to determine an empirical curve of the relationship between the measured pressure difference in the measuring position and the corresponding flow.

This known device thus presupposes a significant restriction of the total flow through measurement during operation, which entails increased energy consumption, increased operating costs and disturbing noise.

Both when measuring the total flow in or in connection with a climate unit (apparatus housing) and when measuring partial flows in ducts, it has thus far been necessary to compromise between energy consumption and measurement accuracy. No suitable method for convenient determination of the total flow in a ventilation system has yet been found. An attempt has been made to measure pressure drop in a measuring chamber provided with chokes in the outlet part of a fan (PCT / FI88 / 00149, publ. No. W89 / 02581 - Imaterä), but this method (51 10 15 20 30 512 894 method has also shown unsuitable due to complicated equipment and relatively high costs.

Against this background, the present invention aims to provide a method and a device which enables the measurement of the flow rate in a fan-driven ventilation system without the use of special throttling means, which significantly affects the size of the flow rate, while the measurement result must be reliable regardless of which disturbances or throttles that may be present in different parts of the ventilation system. The method and device must thus be able to be used in general in ventilation systems with free-sucking fans and give good measurement results.

Additional purposes are to enable reliable flow measurement with simple means and at low costs, both during installation and during operation and maintenance of the ventilation system. Furthermore, the method and device must be able to be used without demanding personnel efforts, e.g. for regular monitoring of the facility.

These objects are achieved according to the invention by directly or indirectly measuring the local flow rate of the air flow at at least one measuring point located in immediate connection with the inlet opening of the fan, as further specified in claim 1.

Additional advantageous features of the method resp. the device according to the invention is stated in claims 2 - 24.

The invention is based on the insight that the inflow conditions are very stable in free-sucking fans, whether they are radial fans, axial fans or other types (eg semi-axial fans). Thus, exhaust fans have a conical or otherwise tapered inlet wall portion, either formed as part of a fan housing or in the form of a separate, annular body, so that the air flowing towards the fan wheel begins softly in an inflow area in 1012 894 direct connection to the fan wheel. This inflow area with particularly stable air flow is limited in radial direction by said annular inlet wall portion and in axial direction by an outer radial plane which lies axially slightly outside the outer end of the inlet wall portion, namely at a distance of about 30% of the smallest diameter of the annular inlet wall portion. In this stable inflow area, the air flow pattern is maintained intact regardless of large variations in the size of the flow. At the same time, the flow rate is very high in most of the inflow area, which provides significantly better measurement conditions than in other parts of the ventilation system connected to the fan.

The way resp. the device according to the invention has in practical tests been shown to give e.g. the following advantages: - negligible extra pressure drop (less than 1% of the total pressure set of the fan; - easy installation, even at existing systems; - low installation and operating costs; - versatile usability; - very high and stable measuring pressures with square flow characteristics; - good measuring accuracy (measurement error less than 15%) both with throttle control on the pressure or suction side of the fan (at constant speed) as well as with speed control of the fan; ~ no sound stringing.

The practical tests have been carried out with different fan sizes (inlets from 110 mm to 350 mm) and with different types of fan wheels (forward or rearward-curved blades for radial fans), all with good results. Nor do various disturbances on the inlet or outlet side of the fan seem to affect the measurement results, e.g. normal belt drive devices or very strong throttles on the outlet side. A bend directly connected to the fan outlet has thus affected the measuring accuracy by only 10.51.

(D 10 15 20 3C 512 894 The measurement of the local flow rate at the measuring point can be done directly with a flow sensor, but the flow rate can also be measured with indirect methods.

The best results have been obtained if (compare claims 7 and 13-21) a measuring tube open at both ends is placed in immediate connection with the inlet opening of the release fan with the longitudinal axis of the measuring tube oriented substantially in the direction of flow and with the outlet end of the measuring tube located at the place, where the measurement is performed by direct or indirect sensing of the partial flow axially flowing through the measuring tube, which is substantially proportional to the total flow flow.

In this way, particularly good measuring conditions are ensured by the sensing taking place in a special measuring tube, in which the axial flow is protected and similar for different large flows. During control measurements, it has been established that the partial flow through the measuring tube is substantially proportional to the total flow through the fan. The partial flow through the measuring tube is thus representative and can be used as a measure of the total flow within a large flow area, ie. the measurement will be correct regardless of what disturbances or throttles have occurred in the ventilation system.

Such a measuring tube suitably has small dimensions, in particular small diameter, in relation to the area of the inlet opening of the fan, so that the total flow through the inlet opening is only insignificantly affected by the measuring tube. Preferably, the diameter of the measuring tube constitutes only a fraction, preferably at most about 1/10, of the diameter of the fan inlet opening. A suitable diameter for the measuring tube is 10 - 20 mm.

In the above-mentioned inflow area at the inlet of the fan, especially in the axially inner part of this area, the air flow rate increases compared with the flow axially outside this inflow area, whereby a stronger static negative pressure arises in the inflow area. Suitably, the measuring tube with the inlet end is placed axially outside the axially inner part of the inflow area, so that a driving pressure arises between the ends of the measuring tube. This ensures high flow velocities in the measuring tube and extremely good measuring conditions.

In principle, the partial flow through the measuring tube can be measured directly by means of a flow sensor, e.g. in the form of an electrical sensor, such as a temperature-dependent electrical component, e.g. a resistor, the operating temperature of which is an unambiguous function of the cooling effect achieved depending on the magnitude of the flow. Even in this case, however, it is possible to sense any other physical quantity which is dependent on the flow.

A practically simple way of indirectly sensing the partial flow through the measuring tube is that the air pressure is sensed by means of at least one pressure sensing means located in connection with a throttle located inside the measuring tube at a distance from its ends. In this case, a pressure difference in two places can be measured immediately upstream resp. immediately downstream of the throttle (which gives a particularly large pressure difference, namely up to about 600 Pa, and thus very good measuring accuracy) or the difference in pressure on the one hand immediately downstream of the throttle and on the other hand in a reference point outside and upstream of the measuring tube (in such a reference point the flow rate is lower and the static pressure consequently higher).

In the practical tests carried out with measuring tubes of the type described above, it has been shown that the dimensions of the tube and the design of the choke can be varied somewhat without significant changes in the measured values, ie. the manufacturing tolerances are not critical and do not cause any major cost.

The exact location of the measuring tube is also not critical, and the installation of the measuring tube can therefore be carried out without any special, costly control measures. 10 20 25 30 512 894 Normally the air flow velocity is greater in the vicinity of the radially outer parts of the fan inlet opening than in the vicinity of the center axis line of the inlet opening. The feed pipe (or corresponding measuring means) is therefore suitably placed at a radial distance from this center axis, preferably at a radial distance amounting to 50-90%, preferably about 70% of the smallest radius of the annular inlet wall portion.

In the case of radial fans, the inflow conditions are different in circumferential direction due to the normally helically widened design of the fan cover. The best measurement results are obtained if you place the measuring tube or corresponding feed means in the vicinity of the part of the fan's inlet opening which, calculated in circumference, is substantially in the middle of the fan outlet (the fan connection's outlet connection).

The invention, its features and advantages are explained in more detail below with reference to the accompanying drawings, which illustrate a preferred embodiment.

Fig. 1 schematically shows a unit included in a ventilation system with fans, filters and a heat exchanger arranged in a housing; Fig. 2 shows in perspective view a fan included in the assembly according to Fig. 1 with a meter arranged in connection with the inlet opening; Fig. 3 shows an axial partial section of the fan according to Fig. 2 with associated measuring tubes; and Fig. 4 shows an axial section through the measuring tube on a larger scale.

Fig. 1 schematically shows a unit 1 with a heat exchanger and duct connections 2,3,4,5 included in a ventilation system. The unit 1 is housed in a housing 6 with partitions 7.8, so that four separate chambers 9.1Û, 11.12 are formed. The chamber 9 is connected to the duct connection 2 for outdoor air and houses 10 p: G1 20 3 | J 512 894 a filter 13 for filtering the outdoor air, which is caused to flow through a plate heat exchanger 14 arranged centrally in the housing 6 to the chamber 12. The flow is effected by means of a supply air fan 15, and the supply air is blown out via the duct connection 4 to a duct system (not shown) with supply air diffusers (not shown) for supplying supply air in a building. From similarly not shown exhaust air, air is extracted from the building via a duct system to the duct connection 3, from where the exhaust air flows through an exhaust air filter 16, the chamber 10, the plate heat exchanger 14, the chamber 11 and out via the duct connection 5 under a fan 17 air supply.

The described ventilation system is conventional and is only an example. A problem that exists in most ventilation systems is to maintain the desired flow rates during installation and operation of the system, as discussed in the introduction.

According to the present invention, the flow-through flow (eg the supply air flow and / or the exhaust air flow) in the ventilation system is measured by means of a flow measuring device, preferably in the form of a special measuring tube, which is placed in immediate connection with the inlet opening of a suction fan.

In the exemplary embodiment shown in the drawing (compare also Figs. 2 and 3), a measuring tube 20 is thus arranged on a holding device 21 next to the inlet opening of the supply air fan 15. The fan 15 consists of a conventional, so-called double-suction radial fan with a vane-mounted fan wheel 22 designed as a double drum.

Both parts 22a, 22b of the drum (Fig. 3) are joined to a common hub plate 22c, in the center of which the shaft 22d (Fig. 2) of the drive motor is fixed. A surrounding cover 23 connects to the fan wheel 22 and forms, in a manner known per se, a helical, circumferentially widening outflow channel, which opens into a fan outlet 24. The fan outlet 24 is in a manner not shown tightly connected to the duct connection 4 in fig. 1. 10 20 25 30 512 894 10 The cover 23 forms on either side of the fan wheel (seen in axial direction) an axially inwardly tapering, rounded wall portion 25 (only the right wall portion in Fig. 3 is visible in Figs. 2 and 3), which in radial direction delimits an inflow area I. In axial direction the inflow area I is delimited by the lines denoted by A and C in Fig. 3, the line C lying axially outside a line B (the radial plane through the outer end of the wall portion 25) on a distance a corresponding to 36% of the minimum diameter d of the opening (at line A).

The holding device 21 consists of a bracket 2la attached to the side wall 28 of the fan cover 23 with a perpendicular (downwardly projecting) holding arm 21b, which supports the measuring tube 20. If desired, the bracket 21a and / or the holding arm 2lb can be adjustable to adjust the position of the measuring tube 20.

The measuring tube 20 is set with its longitudinal axis oriented parallel to the center axis line L of the suction opening and the fan wheel 22 and with the outlet end 20a located in the axially inner part I '(between lines A and B) of the inflow area I relatively close to the inlet wall portion approx. 24. The inlet end 20b of the measuring tube 20 is approximately conically widened like a funnel, the diameter of the substantially circular-cylindrical measuring tube 20 is at most about 10% of the diameter d of the inlet opening, and the tube thus occupies a very small part (about 1%) of the inlet opening area. The length of the measuring tube is preferably 5 to 10 times larger than its diameter, inside the measuring tube 20 (see Fig. 4) a throttling disc 27 with a central, circular flow hole 28 is inserted approximately midway between the ends 20a, 20b.On either side of the throttling disc 27, immediately upstream and downstream thereof, two pressure measuring sockets 29,30 are arranged in the form of small slides in the cylindrical wall of the measuring tube. these pressure measuring sockets 29,30 are fixedly connected to UI k »91 25 512 894 11 fitted pipe fittings 31,32 and hoses 33,34 connected thereto (cf. also Fig. 2).

As the fan 15 operates, air is drawn in through the central inlet opening. The flow has its greatest velocity in the radially outer part of the inflow area I. Preferably, the measuring tube 20 is arranged in this outer part, namely at a radial distance r from the center axis line L corresponding to 50-90% of the radius of the opening (df2). However, as the measuring pressures are very high, the measuring point can be placed closer to the center axis line L, e.g. at a distance of 20-50%. The flow direction of the inflowing air essentially coincides with the longitudinal axis of the measuring tube. A small part of the total flow flows through the measuring tube 20, and the flow rate in this partial flow is maximum when the measuring tube is placed in accordance with the drawing figures.

By means of the pressure measuring terminals 29,30 and a differential pressure measuring device connected via the hoses 33,34 (not shown, can be of any kind), the pressure drop across the throttle 27 can be sensed and on this basis both the partial flow and the proportional total flow determined. The pressure drop becomes relatively large in that the sensed pressure upstream of the choke 29 in the pressure outlet 31 becomes slightly higher than the static pressure at a greater distance upstream of the choke 29 inside the pipe (eg at point P1 in Fig. 4) and the sensed pressure downstream the choke 29 in the pressure outlet 30 is slightly lower than the static pressure at a greater distance downstream of the choke 29 inside the pipe (eg at point P2 in Fig. 4). This pressure drop increase at a throttle flange inside a pipe is known per se per se but is particularly advantageous to use in connection with the present invention.

Thus, one can thereby obtain very good measurement results within a large flow range.

The invention can be applied in many ways within the scope of the appended claims. By "suction fan" is meant an arbitrary fan, e.g. radial fan, axial fan or semi-axial fan ~ G1 10 15 20 512 894 12 fan, which has a free inlet on the suction side and which is thus not directly connected to a channel on the suction side. On the other hand, the fan can of course be housed in a chamber, which in turn is connected to a duct on the suction side (as is the case according to Fig. 1).

The term "ventilation system" means any device or system connected to the fan in question. It may be a question of a very simple device, e.g. only a free-blowing fan, or a complicated system of ducts on both the suction and pressure side, with associated devices and other equipment.

By "inlet opening" is meant any inlet opening through which air is drawn into the fan. Radial fans often have two opposite inlet openings (as in the exemplary embodiment), and in such cases a flow measuring device can be placed next to one inlet opening or next to each inlet opening. It is also of course possible to arrange more than one flow measuring device at one and the same inlet opening. If two or more flow measuring devices are used, it is convenient to form an average value of the measured quantities.

When a measuring tube is used, this does not have to be cylindrical but can be conical and / or have a widened portion. It can furthermore be slightly bent or consist of different, mutually joined parts with a slightly different orientation. The important thing is that the measuring tube enables flow through in such a way that the partial flow becomes substantially proportional to the total flow.

Instead of mounting the measuring tube or corresponding flow measuring device in a bracket-like holding device, radial, such as spokes, holding holding arms can be used. Alternatively, the flow measuring device can, if applicable, be suspended during a shaft bearing to the fan wheel, e.g. for belt-driven fans. 512 894 13 The definition of the inflow area I (between lines A and C in Fig. 3) has been made for practical reasons. The radial plane C does not correspond to any real, critical threshold effect but serves as a practical external limit for the location of the flow measuring device (eg the outlet end of the measuring tube) while maintaining measuring accuracy.

The expression "substantially in the inflow area (I) which lies radially inside and axially in the vicinity of said inlet wall portion <25)" in claims 1 and 8 shall be read against this background.

Claims (6)

10 15 20 25 512 894 Uf PATENTKRAV Device for determining the flow rate in a ventilation system driven by a fan (15), comprising a measuring tube (20) open at both ends, which is placed with its longitudinal axis line oriented substantially in the flow direction, characterized in that - the measuring tube (20) is supported by a bracket (21) so that the measuring tube is held freely at a measuring point located in the inlet of said fan (15), where the flow is stable and representative of the total flow through; - that the measuring tube (20) is substantially circular-cylindrical with a diameter amounting to only a fraction, preferably at most 1/10, of the diameter of the fan inlet (d), and - that a choke (27, 28) is arranged internally in the measuring tube (20) between its ends, at least one pressure sensing means (29,30) being located adjacent to the throttle. Device according to claim 1, characterized in that two pressure sensing means (29, 30) are arranged immediately upstream and downstream of the throttle, respectively. Device according to claim 2, characterized in that said pressure sensing means placed in connection with the restriction consists of one or two pressure measuring sockets (29, 30) which are connected to a differential pressure measuring device. Device according to one of Claims 1 to 3, characterized in that the throttle consists of an annular throttle flange (27) with a central through-hole (28). 21810. requirements; 99-12-20 512 894 .i 153 Device according to Claim 4, characterized in that the flow-through hole (28) is circular. Device according to one of Claims 1 to 5, characterized in that the inlet end portion (20b) of the measuring tube is conically widened. document2: 99-12-20