METHOD AND APPARATUS FOR MEASUREMENT OF TOTAL FLOW RATE IN VENTILATION INSTALLATIONS

Abstract: A method and apparatus for determining the total flow rate in a ventilation installation with a free suction fan (15) are described. The flow rate is determined by measurement of the local pressure at at least one measurement point in the vicinity of the inlet opening of the free suction fan. A measurement probe (20), open at one end only, is used as measurement means, 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 to said inner radial plane (A), though always in the narrowest section of the fan inlet. In this way there is obtained good measurement accuracy within a large flow range.

Description: Method and apparatus for measurementof total flow rate in ventilation installations. Description The invention relates to a method and apparatus for deter¬ mining the total flow rate in a ventilation installation , 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 determined by measurement on the suction side of the fan. Such a method, as well as an apparatus for carrying out the method, is known from SE-A-8791663-0. 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 re¬ spect 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 in¬ directly monitored or controlled as well. Several methods are known for flow measurement in ventilation installations, 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 compo- nents (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. Another known method is described in the initially mentioned published Swedish patent application SE-A-8701663-0, the pres- sure 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 measuring probes con¬ nected to a differential pressure measurement device for de- terming the pressure drop and the flow rate. The constriction means, e.g. in the form of adjustable baff¬ les, is adjustable between a completely open position and a maximum constriction position (measurement position) , which enables determination of an empirical graph of the relation¬ ship between the measured pressure difference in the measure- ent 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. In case measurement means or parts thereof are placed in the flow path of a flowing medium, such measurement means or parts will cause a certain air resistance which must be compensated for by an increased energy consumption. Moreover, even when said measurement means or parts thereof adjoin the side limi¬ tations of a medium duct, there will be turbulences and other phenomena which have an adverse effect on the measurement results. Finally, the previously known measurement devices are often relatively complicated and expensive, whereby increased costs will be incurred. Against this background, the present invention has the object of providing a method and apparatus avoiding the above- mentioned drawbacks and 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, while at the same time securing reliable measurement results, irrespective of what disturban¬ ces or constrictions that may be present in different parts of the ventilation installation. The method and apparatus shall thus be generally useable in ventilation installations with free suction fans and give good measurement results. Further objects are to enable reliable flow measurement with simple means to low cost, both in installation and in opera¬ tion 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 a method of the kind referred to initially and having the features defined in claim 1. Further advantageous features of the apparatus and the method according to the invention are stated in the other claims. 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 radial flow fans, axial flow fans or other types of fans (e.g. mixed flow fans) . Free suc¬ tion 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 in¬ flow region leading directly to the fan wheel. The method and apparatus in accordance with the invention have been found in practicle tests to give, inter alia, the following advantages: - negligible extra pressure drop (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 pressure values with 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 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 distur¬ bances 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 direct- ly connected to the fan outlet has thus affected the measure¬ ment accuracy by merely ±0.5%. The exact positioning of the measurement probe is not critical either, and mounting it can therefore be carried out without any special, costly control arrangement. 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 probe or corresponding measurement means is placed in the vicinity of the part of the fan inlet opening which is located substan¬ tially towards the fan outlet (the outlet connection of the fan casing) as seen in a peripheral direction. The invention, its characterizing features and its advantages will now be explained in more detail below with reference to the accompanying drawings, which illustrate a preferred embo¬ diment example. Fig. 1 schematically illustrates a unit in a ventilation in- stallation, the unit including a housing in which are arranged fans, filters and a heat exchanger; Fig. 2 is a perspective view of a fan included in the unit according to Fig. 1, the fan being provided with a measurement probe in association with its inlet opening; Fig. 3 is an axial, partial section of the fan according to Fig. 2 with its associated measurement probe; and Figs. 4a)-d) illustrate various kinds of measurement probes. In Fig. 1 there is schematically illustrated a unit 1 with a heat exchanger and duct connections 2,3,4,5 included in a ven¬ tilation 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 (unillustrat¬ ed) 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, the 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 opera¬ tion, the desired flow rates, as discussed in the introduc¬ tion. 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 probe, 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 probe 20 is mounted on a holding device 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. The two 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 (planes) denoted by A and B in Fig. 3. The refe- rence character a denotes the narrowest section of the fan in¬ let, which is located in the plane A. The widest section is located in the plane B. The holding device 21 comprises a bracket 21a fastened to the side wall 26 of the fan casing 23, with a vertically dependent holder arm 21b for carrying the measurement probe 20. If so desired, the bracket 21a and/or the holder arm 21b can be ad¬ justable for altering the position of the measurement probe 20. The measurement probe 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 A and B) of the inflow region I relatively close to the centre line L on the same side as the fan outlet 24. The inlet end 20a of the measuring probe 20 is straight, as shown in Fig. 4c) , or pro¬ vided with a bend, a plate or a ball, as shown in Figs. 4a) , 4d) and 4b) , respectively, and the measuring probe will thus take up a very small part (>1%, preferably approximately 0.1% only) of the inlet opening area. The length of the measuring probe is adapted to the distance a and to the width and positioning of the holder arm. In Figs. 4a) - 4d) the air flow pattern is illustrated with some principal lines. When fan 15 is in operation, air is sucked in through the cen¬ tral inlet opening. The flow has its greatest speed in the radially outward portion of the inflow area I. The measurement probe is preferably arranged at a distance from the wall por- tion 25, preferably on or close to the central axis , e.g. at a distance of 0 - 90%, preferably 30 - 50%, alternatively 30 - 40%, of the smallest radius (d/2) of the annular inlet portion (25) . With the aid of a pressure measurement tapping hole 29 and via a hose 30, an unillustrated differential pressure measurement device of any suitable kind can sense the pressure drop relative to the surrounding, and with this as a basis, the total flow rate can be determined. The pressure of the sur- rounding can be measured at a point 31 at a corner portion of the chamber 12 or, of course, at any other suitable spot. The invention can be applied in many ways within the scope of the following claims. 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. Then, the chamber is regarded as the surrounding. The invention can be applied in many ways within the scope of the followig claims. 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. Then, the chamber is regarded as the surrounding. 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. Instead of mounting the measurement probe or corresponding flow measurements means on a bracket-like holding device, radial holding arms arranged like spokes may be used. Alter¬ natively, the flow measurement means in certain cases may be suspended at a shaft mounting for the fan wheel, e.g. with belt driven fans.

Claims: CLAIMS 1. Method of determining the total flow rate in a ventila¬ tion installation with a free suction fan (15) having its in- let opening(s) at least partially defined peripherally by an annular inlet wall portion (25) tapering in the inlet direc¬ tion, the flow rate being determined by measurement on the suction side of the fan, c h a r a c t e r i z e d in that, in order to determine the total air flow through the fan, one measures the pressure at a measuring point (20a) in the inlet opening of the fan in the inflow region (I) , which is located radially inside said inlet wall portion (25) , either in the middle of the fan inlet or within a region half way between the middle and the periphery of the fan inlet, and in that the measuring is carried out on the lee side of the media flow only, although not on the lee side of a body possibly disposed in the fan inlet, such as a hub, a belt wheel, spokes or the like. 2. Method as claimed in claim 1, c h a r a c t e r i z e d in that the measuring point (20a) is located inside an outer radial plane (B) constituting the widest section of the fan inlet, said widest section being located at an axial distance (a) outside an inner radial plane (A) , which constitutes the narrowest section of the fan inlet, and in that the measuring point is situated in or adjacent to said inner radial plane (A) , e.g. between 0 and 20% of the distance between the two planes (A,B) from the inner plane (A) . 3. Method as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the measurement is carried out directly by means of at least one pressure sensor and/or in that the measurement is carried out by sensing a physical parameter which varies with the flow rate, and/or in that the measurement is carried out by sensing the pressure drop adjacent to a fan inlet. 4. Method as claimed in any one of claims 1 - 3, c h a r a c t e r i z e d in that a measurement probe (20) , which is open at its downstream end only, is placed in imme¬ diate association with the inlet opening and with its longi¬ tudinal axis oriented substantially in the flow direction, and with its outlet end (20a) located at said measurement point, wherein measurement is carried out by direct and indirect sensing of an axially measured pressure, which is being sub¬ stantially proportional to the total flow rate. 5. Apparatus for determining the total flow rate in a ventilation installation with a free suction fan (15) having its inlet opening(s) at least partially defined peripherally by an annular inlet wall portion (25) tapering in the inlet direction, and including a flow measurement means (20) with a measurement point (20a) for measuring the flow rate on the suction side of the fan, c h a r a c t e r i z e d in that said measurement point (20a) is placed in the inlet opening of the fan in an inflow region (I) of the fan, which is located radially inside said inlet wall portion (25) , either in the middle of the fan inlet or within a region half way between the middle and the periphery of the fan inlet, and in that the measurement point is constituted by the only open end of a measurement probe, said open end opening with the medium flow, i.e. on its lee side, although not being located on the lee side of a body possibly disposed in the fan inlet, such as a hub, a belt wheel, spokes or the like. 6. Apparatus as claimed in claim 5, c h a r a c - t e r i z e d in that the measurement point (20a) is located axially inside an outer radial plane (B) constituting the widest section of the fan inlet, said widest portion being located at an axial distance (a) outside an inner radial plane (A) constituting the narrowest section of the fan inlet, and in that the measurement point is situated in or adjacent to said inner radial plane (B) , e.g. between 0 and 20% of the distance between the two planes (A,B) from the inner plane (A). 7. Apparatus as claimed in claim 5 or 6, c h a r a c t e r i z e d in that the measurement means com¬ prises a flow sensor for direct measurement of the flow rate, and/or in that the flow measurement means comprises at least one sensing means (20) for indirect measurement of the flow rate by sensing a parameter varying with the flow rate. 8. Apparatus as claimed in any one of claims 5 - 7, c h a r a c t e r i z e d in that the flow measurement means comprises an open measurement probe (20) , which is placed with its longitudinal axis oriented substantially in the flow di- rection and with its outlet end (20a) located at said measure¬ ment point in said inflow region (I) , and in that the measurement probe measures a pressure, which is substantially proportional to the total flow rate. 9. Apparatus as claimed in claim 8, c h a r a c ¬ t e r i z e d in that the measurement point (20a) is situated on or at a radial distance (r) from the central axis (L) of the fan inlet opening, and in that said radial distance (r) is 0 - 90%, preferably 30 - 50%, alternatively 30 - 40%, of the smallest radius (d/2) of the annular inlet wall portion (25). 10. Apparatus as claimed in claim 5, the free suction fan being a centrifugal fan (15) with a spirally shaped fan casing (23) radially outwards of a fan wheel, and with a fan outlet (24) , c h a r a c t e r i z e d in that said measurement point (20) is situated in the vicinity of the portion of the fan inlet opening which is located substantially towards the same side as the fan outlet (24) , as seen peripherally.

Inventors: HOLMBERG ROY, SJOEQVIST LENNART, HOLMBERG, ROY, SJOEQVIST, LENNART
Applicants: FLAEKT AB, ABB FLAEKT AKTIEBOLAG
Application year: 1992
Main classification: F24F11
IPC codes: F04D27, F24F11, G01F1, G01P5