A DEVICE FOR MEASURING THE VOLUME FLOW OF A FAN

Abstract: A measuring device for measuring a pressure difference ( DELTA p) and thereby the volume flow (qv) of a fan by carrying out the measuring of the pressure difference ( DELTA p) in a two-chamber measuring part (2) located in the pressure aperture or its continuation in the fan, each chamber having small-diameter apertures (5), that inner surface (8) of the measuring-device housing (1) against which or close to which the measuring part (2) is located being the tongue (10) or tongue plate (9) of the fan mantle, or a continuation of these. The measuring part (2) is installed inside the housing (1) in such a manner that the direction (F) of the flow to be measured is substantially perpendicular to the length (C) of the measuring part.

Description: A device for measuring the volume flow of a fan The invention relates to a measuring device for measuring the volume flow of a fan in an air-conditioning plant or in some other air-treatment system. The air volume flow in an air-conditioning plant is gener¬ ally measured according to the multi-point methods pre¬ sented in Directive 2:1984 issued by the Finniεn National Board of Public Construction. In multi-point methods the volume flow of air is obtained as the product of the mean air velocity and the cross-sectional area of the flow in the duct. The mean velocity of the air is determined by measuring the air velocity at mathematically predetermined points of the cross section of the duct and by calculating the mean of these velocities. Depending on the size of the duct, the measuring points number 5 - 24. Another method for measuring the air volume flow in an air- conditioning plant is to measure in a measuring device fixedly installed in the duct system the pressure differ¬ ence proportional to the air volume flow. In connection with such a device there is always throttling means, which may be, for example, a throttling flange, a nozzle, a ven- turi tube, a Dall tube or a segment flange, for producing the pressure difference. The method for determining the volume flow of a fan in an air-conditioning plant is based on the measuring of the total pressure and rotative velocity of the fan and the electric power taken by the fan motor. The volume flow of the fan is obtained with the aid of these from the charac¬ teristic curve charts prepared by the fan manufacturer. Another method for determining the volume flow of a fan is in use in the fans used in civil defense shelters in Fin¬ land. A by-pass via a rotameter is obtained by using air¬ flow throttling means located in the pressure aperture of the fan. The rotameter is calibrated to indicate the air volume flow of the fan. The multi-point system for volume-flow determination is cumbersome. In. practice it is also susceptible to errors, since complicated air-conditioning ducts seldom have a disturbance-free flow profile required by reliabl measur¬ ing. Owing to the throttling means, the permanent pressure loss caused by measuring devices fixedly installed in a duct system is usually significant, and the lower limit of the measuring range is limited to correspond to relatively high volume flows. The method for determining the volume flow in a fan with the aid of a characteristic curve chart of the fan is very- approximate. The throttling means used in fans in civil defense shelters causes a permanent pressure loss. Owing to its tendency to become soiled, a rotameter for its part is not suitable for use on a continual basis in conventional air-conditioning plants. With the device according to the invention, a crucial im¬ provement is achieved with respect to the disadvantages presented above. In order to realize this, the device ac¬ cording to the invention is characterized in what is dis¬ closed in the characterizing clause of Claim 1. It can be regarded as the most important advantage of the invention that the total air volume flow in an air-condi- tioning plant can be obtained easily and reliably with one measurement and one measuring tool. It is a further advan¬ tage that the volume flow is obtained as a continuous value, in which case, for example, disturbances are visible immediately and the control of the plant is effective, since the feedback time constant is low. The invention is described below in detail with reference to the accompanying drawings. Figure 1 depicts a measuring , device according to the inven¬ tion, installed in a fan. Figure 2 depicts one embodiment of the measuring part of the measuring device according to the invention. Figure 3 depicts other embodiments of the measuring part of the measuring device according to the invention. The measuring device according to the invention comprises the housing 1 of the device and a measuring part 2. The housing of the measuring device 1 is connected to the pres¬ sure aperture of the fan 3 in such a way that the inner surface 8 of the housing on the same side as the measuring part forms a continuation of the tongue plate 9 of the fan in the flow direction F. The pressure difference measured in the measuring part 2, which is located immediately adja¬ cent to the inner surface 8 of the housing 1, or close to the inner surface 8, and transversely to the flow F and which extends over its length c substantially across the whole housing from its one side 11 to its other side 11, is proportional to the volume flow of the fan. Advantageously the length c of the measuring part 2, transverse to the housing, is perpendicular to the flow direction F. The housing 1 of the device is a tubular mantle which has the size and shape of the pressure aperture of the fan and does not throttle the flow. The measuring part 2 is lo¬ cated on that inner surface 8 of the housing or close to that inner surface 8 of the housing which constitutes the tongue 10 or tongue plate 9 of the fan mantle, or a contin¬ uation of them, but at a distance of at minimum E = 0.5 x b from the real or theoretical tongue 10 of the mantle in the flow direction F. If the distance E of the measuring part 2 from the mantle tongue 10 or some other point causing flow disturbance or throttling is clearly more than 1 x b, the measuring part can be located even further away from the inner surface 8 on the same side as the tongue plate in the cross section of the housing 1. When the above conditions are fulfilled, the measuring device functions reliably within the entire wide normal operating range of the air- conditioning plant even if the operation of the plant is regulated by throttling the flow even drastically. Figure 1 also depicts the flow field in the pressure aperture of the fan and in its vicinity, when the flow is strongly throt¬ tled by non-depicted means, for example by using control dampers, in some other part of the pipe system. It can be seen from the figure that in the measuring device according to the invention the measuring part remains in an even pressure field in spite of the turbulence caused by the throttling. If the throttling is less vigorous, the flow field is more even, i.e. less turbulent. The length of the housing 1 of the measuring device in the direction of the flow must be such that the distance E of the measuring part from the trailing edge 12 of the housing is at minimum 0.3 x b, to avoid the turbulence, etc., caused by the trailing edge or other similar discontinuity point, and to maintain reliability. Figure 2 depicts the measuring part 2 of the measuring device. The pressure caused by the gas flow F is trans¬ mitted via the pressure-measuring apertures 5 and chambers 6 to the pressure-measuring tubes 4 and further to the differential pressure gauge 13. A partition 7 separates the pressure-measuring chambers 6 from one another. The differ¬ ential pressure gauge 13 may be structurally of any known suitable type. The measuring part of the type depicted in Figure 2 is suitable for use in such a way that the straight or somewhat curved surface between the pressure- measuring apertures 5 is either pressed tightly against the inner surface 8 of the housing or is at a small distance x from the inner surface 8 of the housing. Figures 3a and b depict certain other embodiments of the measuring part 2 of the measuring device; these embodiments are suitable for being positioned either tightly against the inner surface 8 or at a small distance x from the inner surface 8. Figure 3c further depicts one embodiment of the measuring part 2 which is suitable for being located at a small distance y from the surface 8. All these embodiments have pressure-measuring apertures 5 and chambers 6, pressure-measuring tubes 4 and partitions 7. In addition, each of the measuring parts in Figure 3 has an external protrusion 14 to increase the pressure difference. The protrusion 14, as well as the entire measuring part 2, is dimensionally as small as possible compared with the cross sectional dimension b of the housing 1 and at the same time of the pressure aperture of the fan, in order to avoid a throttling effect in the flow. By using a measuring part of the type described above (Fig¬ ures 2 and 3) it is possible to measure directly not only the pressure difference but also the absolute pressure p prevailing inside the housing, i.e. in the measuring duct. Since, furthermore, in the measuring part there is located a temperature sensor for measuring the temperature T of the gas, which measuring can be carried out in several simple ways, all the values necessary for the determination of the volume flow and for compensating for the reading in all various operating situations are available. In this case, an output of the volume flow g v of the fan can be obtained directly by a simple arrangement, since it is an unambig¬ uous function of the quantities measured, i.e. q v = f (Δp, P, T). The measuring part 2 of the measuring device can, depending on its design, be installed inside the housing 1 also so as to turn about its longitudinal axis, and especially so if the measuring part 2 is designed to be of a blade structure type in order to produce a pressure difference.

Claims: Claims 1. A measuring device for measuring a pressure difference (Δp) and, thereby, the volume flow (q v ) of a fan by car¬ rying out the measuring of the pressure difference (Δp) in a two-chamber measuring part (2) located in the pressure aperture of the fan, or its continuation, each chamber having small-diameter apertures (5) and/or a corresponding gap or corresponding gaps which open substantially in par¬ allel or perpendicular to the principal flow direction (F), characterized in that the inner surface (8) of that side of the measuring-device housing (1) against which or close to which the measuring part (2) is located is the fan mantle tongue (10) or tongue plate (9), or a continuation of the same, and that the measuring part (2) is installed inside the housing (1) in such a manner that the principal direc¬ tion (F) of the flow to be measured is substantially per¬ pendicular to the length (C) of the measuring part. 2. A device according to Claim 1, characterized in that the length (C) of the measuring part (2) is substantially equal to the width (a) of the housing (1) inner surface (8) transverse to the flow (F), and the height (d) of the meas¬ uring part (2) in the direction perpendicular to the inner surface (8) is substantially small compared with the hous¬ ing (1) height (b) perpendicular to the inner surface (8). 3. A device according to Claim 1, characterized in that the measuring part (2) is on the surface (8) or in its vicinity at such a distance from the tongue (10) as is at least equal to one-half of the height (b) of the housing (1), and preferably equal to the housing height (b), and that the trailing edge (12) of the housing (1) is at such a distance from the measuring part (2) as is at least equal to 0.3 times the housing height (b). 4. A device according to Claim 2 or 3, characterized in that the measuring part (2) is installed tightly against the housing (1) in such a way that the flow to be measured passes on only one side of the measuring part. 5. A device according to Claim 2 or 3, characterized in that the measuring part (2) is installed in the housing (1) at a small distance (x, y) from its inner surface (8) in such a manner that the flow to be measured passes on both sides of the measuring part. 6. A device according to Claim 4 or 3, characterized in that the walls of the pressure-measuring chambers (6) in the measuring part (2) have small-diameter pressure- measuring apertures (5) and/or a corresponding gap or cor¬ responding gaps which open in relation to the flow substan¬ tially in parallel to the direction (F) of the flow. 7. A device according to Claim 4 or 5, characterized in that the height (d) of the measuring part (2) in the area between its pressure-measuring apertures or gaps (5) is greater than in the area of the pressure-measuring chambers (6), the pressure-measuring apertures (5) or pressure- measuring gaps. 8. A device according to Claim 4 or 7, characterized in that the walls of the pressure-measuring chambers (6) of the measuring part (2) have small-diameter apertures (5) and/or a corresponding gap or corresponding gaps which open into the flow substantially perpendicularly to the direc¬ tion (F) of the flow on that side of the pressure-measuring chamber on which the flow is to be measured. 9. A device according to Claim 5 or 7, characterized in that the walls of the pressure-measuring chambers (6) of the measuring part (2) have small-diameter apertures (5) and/or a corresponding gap or corresponding gaps which open into the flow substantially perpendicularly to the direc¬ tion of the flow in a direction transverse to the two oppo¬ site sides (F) of the pressure-measuring chambers (6). 10. A device according to Claim 1, characterized in that the measuring part (2) is provided with a temperature- measuring sensor.

Inventors: LAINE JUHANI, NYMAN MIKKO, PAANANEN TAPIO, LAINE, JUHANI, NYMAN, MIKKO, PAANANEN, TAPIO
Applicants: ILMATERAE OY
Application year: 1988
Main classification: G01F1
IPC codes: F04D27, F24F11, G01F1