Question

Describe two of the three methods commonly used for flow control in large centrifugal fans (as for example those installed in large boiler plants). Show in head versus fow diagrams (graphs) for fan and system how the operating point (flow) can be changed in each case and explain why the change has occurred

Answer 1

2. The Rotational Speed Control General As already discussed in work sheet 2 "The characteristic curves, each rotational speed relates to a different Q- transmission flow is subject to more or less significant characteristic. All QH characteristic curves of a rotary fluctuations. Fig. 10 illustrates a plant that, in order to pump are equal within the validity range of Affinity Laws maintain simplicity, consists of two equally sized Related points of the various Q-characteristics are customers in parallel order. If both of them have to be located on parabolas, i.e. the parabola P,0 in Fig. 11 supplied with water, then the plant adjusts itself to Al intersection points of this parabola with the Q operational point Bi and the pump head pressure Hi of caracteristics for the various rotational speeds are the pump is equal to the plant pump head pressure HA characterised by geodesically similar speed relations parabolas are designated as parabolas of equal impact condition depending on approach onto the In many pump plants due to various reasons the impeller and inlet vanes в' Bi Pa P1 Charad curve one of the two customers is in use than the operational point of the pump characteristic curve moves to B, the point of intersection of the system characteristic curve 2 ithin a limited rotational speed range the pump with the QH curve. This customer receives transmission flow that is too large due to the increased constant. Fluctuations arise because the power pump head pressure. The control must reduce the pump head pressure H2 of the pump to the plant pump head change according to other laws. In a speed regulated pressure Ha2 This will result in the desired operation pump plant the efficiency will only then remain constant point B2, which when neglecting the changed pipe friction when the plant pump head pressure is exclusively of a losses Hy in the common supply line assures for the dynamic type. Only then the plant characteristic curve customer a flow rate equal to that previously in point B1 efficiency along such a parabola remains nearly consumption of the rotary pump includes factors that will equals the parabola P,0. The pump initially operates in point B, the point of intersection of the system characteristic curve with the QH curve for the rotational speed n. It has a transmission flow Q, and a 1. The Throttling Control pump head The excessive pump head pressure at Q2 is reduced by pressure H1. If the rotational speed is lowered from n, to partially closing the sliding valve in the pressure line. The n2 then the operation point on the plant characteristic throttling control is a simple but uneconomic way of curve changes from B to B2 with the transmission flow control. Its inefficiency increases with the increase in loss of pump head pressure due to adjustment. A p a flat QH characteristic curve should be preferred. The characteristic for n2, pump and plant pump head throttling control is not suitable for side channel pumps pressure and axial rotary pumps whose power consumption increases with a decreasing transmission flow Q2 and the pump head pressure H2. Since B2 is the point of intersection of the system characteristic curve with the Q- ump with H are equal. The decrease of the transmission flow from Q, to Q2 has been accomplished without a pump head pressure loss

The transition from the parabola P,0 to P20, however, This is essetil for pump systems with low power represents a change in efficiency. If the operation point pfrmances, at small consumption fluctuation, or when B1 is located at the optimal transmission flow or to the left, then the process results in a loss in efficiency, which large geodesic values. A typical example is the pressure is even more significant the greater the degree of static ncrease system in a high-rise building. Here, the pump head pressure and the flatter the dynamic pressure must be maintained even at the highest drain characteristic curve i the control range is limited to tight tolerances due to point, for the slightest water use. The possible decrease in rotational speed resulting thereof is only minimal and This type of pump control requires a variable speed the rotational speed regulation would only be justifiable if drive. The machines used in the past, such as direct current motors, asynchronous motors with slip ring rotors, motor-generator combinations or geared motors have significantly lost their importance, and frequency inverters have become more popular. This also applies to pole switchable three-phase motors, which are, however, only suitable in extreme situations due to their the demand-controlled sequence circuit of one or more great steps in rotational speed. Less affected from this smaller capacity pumps operating in parallel offers a development are voltage regulated small type motors uite economical alternative. The application of rotary that still allow an economical solution of control problems greater pressure fluctuations were occurring within the supply system. 3. The Pump Sequence Circuit Although it is not a pump control in the original sense pumps with a flat characteristic curve and the appropriate distribution of the water supply not only guarantees a very economical system but also a practically constant Variable speed drives combustion in combination withsupply pressure combustion engines or steam turbines will naturally retain their purpose in special areas 4. A solution predominantly found in side channel pumps is the bypass control. In this case a partial stream is redirected to the suction side by means of a secondary line and slide valve fitted after the pressure fitting. The operating point moves to the end of the characteristic curve, which can easily cause motor overload in radial pumps. Independent of the pump type the moving of the operating point to greater transmission capacities leads to a more unfavourable NPSH. If critical suction conditions exist then cavitation must be expected. Other Control Types The frequency inverter without doubt offers the ideal solution to rotational speed adjustment since it operates almost without any loss and virtually maintenance free. Its greatest advantage, however, is the possibility of being able to use a standard three-phase motor. Also of advantage is the high performance factor, which, depending on manufacturer, can reach values of up to cos φ = 0.95 and remains practically constant throughout its entire control range. In practical operation this motor has a surprisingly low power consumption. However, it must be noted that the motor will become warmer than if it is fed by a sinus shaped current. The c design types, are the impeller vane adjustment resulting power reduction amounts, depending on the and pre- spin control. The former is being used manufacturer of inverter and motor, to up to 15%. An pumps. It is equally advantageous as a rotational speed also available increase in rotational speed in excess of the characteristic rotational speed of the motor should be limited to 20% in serial type motors Additional control types, although only applicable for adjustment and allows the maintenance of a positive efficiency over a wide transmission range. The pre-spin control is also limited to pumps with a high specific rotational speed, preferably half-axial pumps. It depends The high purchasing costs suggest that a test inon the use of adjustable inlet vanes that are positioned economy is performed versus other control types. before the entrance to the impeller vane. An equalized or opposite spin is generated by adjusting these vanes which decreases and/or increases the transmission flow and pump head pressure