Systemic resistance

Fig. 4. Selection of fan size where the soHd line represents a typical setting and the dashed lines the operating extremes, (a) Desirable sizing. The system resistance curve intersects the fan curve near its maximum efficiency. Changes in system resistance from a flow-control element also intersect the fan curve at desirable points for good flow control. The dashed curves also intersect system resistance curves at desirable locations, (b) A fan essentially too large for the system. The intersection of the system curve near the peak of the fan curve results in poor system flow control and perhaps surging.

For air-flow control, the system may contain a control valve or damper that automatically or manually modulates system pressure drop. The dotted curves in Figure 4a on each side of the system resistance curve might represent operating extremes of the system resistance as the control valve is varied from maximum to minimum opening. These curves also intersect the fan curve at desirable operating portions of its range both for efficiency and flow control. 

Fig. 7. Control of fan performance with inlet vane control. SoHd lines marked A and N show normal performance without vanes (vanes wide open). As vanes are progressively closed, static and power curves are modified as indicated by dashed lines. Intersection ( - ) of the system resistance curve with these reduced pressure curves at points B, C, D, and E shows how imparting more spin to the inlet air reduces flow. Projecting points A to E vertically downward to the corresponding power curve locates fan power points A through E7 Power savings achieved over throttling control can be estimated by projecting points B through E vertically downward to the A power curve and comparing the value with that from the proper reduced power curve. To...

The rated discharge is at a static head of /7j and a motor h.p. P. In the process of controlling the discharge from Q to and Qy, the valve is throttled, which increases the head loss of the system (or system resistance) from A/ji to and H,yy respectively. The operating point on the curve now shifts from point A[ to /It... 

Consider Figure 6.42 with typical Q-Hj curves at different speeds and different system resistances, introduced by the throttle. Point A refers to the rated discharge at rated speed and head when the throttle valve is fully open. Lets us consider the condition when the discharge is to be reduced to say, 0.67 0. ... 

The system resistance increases and discharge reduces at the same rated speed This condition refers to point B, to which the earlier point A, has now shifted. The system now operates at a higher head f/j2. whereas the actual head has not increased. This condition has occurred due to higher system resistance offered by the throttle. The pump and the prime-mover efficieticy will now reduce to 73 7r from its original 8.3%. 

The discharge pressure developed hy the compressor must he equal to the process gas s total system resistance, of control valves, hand valves, orifices, heat exchangers, and any other process-related devices through which the discharge gas from the compressor must flow. As this resistance changes, the gas flow through the compressor will automatically adjust itself to equal the new resistance. ... 

Normal expected static pressure based on system resistance at... 

Before a fan selection can be completed either by the owner s engineer or the manufacturer, a system resistance calculation must be made at several selected fan volume flow rates. This will be discussed in the section, Summary of Fan System Calculations, Figure 12-128... 

Outlet Damper mth Constant Fan Speed. The system resistance is varied with this damper. The volume of gas delivered from the fan is changed as a function of the movement of the damper. It is low in first cost and simple to operate but does require more horsepower than other methods of control. 

Figure 12-138A, B, C, D. Centrifugal fan system control methods. Note the effect of air distribution zone control (A for VAV) terminal on sudden jump in system resistance to fan operating static pressure curve. (Used by permission Haines, R. W. Heating/Piping/Air Conditioning, p. 107, Aug. 1983. Penton Media, Inc. All rights reserved.)...

Note that this less dense air requires less horsepower, and also that the fan can produce only 1.66 in. static as compared to 2 in. with standard air. The system resistance must he adjusted to accommodate this lower static pressure otherwise, the fan will follow its characteristic curve hy reducing its flow until it discharges at the static pressure of 1.66 in. 

A fen can operate only along its characteristic curve, but after that fan is placed in a fixed system, it can operate only at the one point where pressure-volume conditions match the pressure-volume system curve calculated based on the system resistance, see Figure 12-134. Thus, if the fan characteristic curve is superimposed on the plot of the system, the point of intersection will be the point of operation. To change this point requires changing at least one condition on the fan or the system. 

In order to analyze the total system resistance and its relation to fen performance. Figure 12-140 is used. Without defining what comprises the system resistance, but representing it by Curve A-A, this system is to flow 13,000 cfrn of air at 1.1 in. static pressure. A fan has been selected that operates at 600 rpm and is represented by its static pressure Curve C-C. The intersection of these two curves. Point 1, is the only point of... 

The fan will now have a system resistance Curve A-B and operate at Point 2. As an alternate approach to securing a system balance at the point required, the motor speed can be changed by a suitable means. If the fan speed is reduced by 13,000/13,600, the new speed should be (0.889) (600) = 573 rpm. A new fan Curve E-E will go through the desired point conditions. The new horsepower for this operation will be (3.87) (573/600)" = 3.35 hp. 

Usually fans are placed in series to obtain more pressure than any reasonable single fan will produce. They may be required to boost pressures as dictated by system resistance changes. In this latter case, it is important to pick fans near peak efficiency to allow for deviations in operations without paying a premium in horsepower. It is important to keep in mind that the addition of fans in series does not increase the capacity of flow by the additive values of the individual fans. 

Figure 12-143 shows the individual static pressure curve Pf and total pressure curve Pff If pressure losses between the two fans are neglected (and they should he very low for good design), the combined total pressure curve is twice the value of curve Pft, 2 Pff The new operating static pressure also should be twice the individual total pressure value minus the velocity pressure, 2 p — p for identical fans, the new operating static pressure is equal to 2 p + Pf. The operation of the series fans will be along the system resistance curve, and the resultant point of operation will be at the intersection of the system curve with the curve for (2 pa — Pfv). 

Figure 12-143 shows the curves for identical fans. When unlike fens are placed in series, the individual static and total pressure curves are placed on the graph. The individual curves for assumed fans No. 1 and No. 2 and the system resistance curve define the system. The combined total pressure curve is the sum of the individual values at the same capacity. Then total combined pressure curve = Pft (No. 1) + Pft (No. 2). The new combined operating static pressure curve is the sum of the individual static pressure valves at the same capacity and exists only at the oudet of the second fan then, the total combined static pressure = Pfti + Pftg — Ps. Any losses in connections between the fans will reduce the values of the total static and system total pressures. 

Fans are used in parallel to obtain increased capacity in preference to a single large installation, to increase capacity at constant pressure, and for low-resistance systems requiring large capacities. It is important to study the effect of the addition or removal of fens on the system. This is done using the system resistance and the fan characteristics. 

Note that the total static pressure curve of Figure 12-145 is limited by the lowest output pressure of the multifan system. The limit curve is established using the fan curve (No. 1 in this example) having the smallest volume increment to the system resistance curve. In this situation fen No. 2 cannot add to the system until its pressure-volume relation reaches the peak point on its curve. 

To have the fan represent the best possible selection considering the particular circumstances and requirements, it is important to study the fan type curves and to recognize whether a small change in system resistance would be easily handled by a particular fan, whether speed variations and the resulting volume and pressure changes are acceptable, and whether the fan can be protected against corrosion, etc. References 19, 31, and 38 will be helpful. Specifications should be submitted to several manufecturers for their recommendations. In this way full advantage is received from... 

The system resistance must be calculated in the usual manner and at the actual operating conditions of the fan. Corrections are then applied to convert this condition to standard for use in reading the rating tables. 

Toroidal System Resistivity (after Gearhart-Halliburton). The system uses one toroidal transmitter operating at 1 kHz and a pair of toroidal receiver coils mounted on the drill collars. Figure 4-277 shows a sketch of a toroid. 

To operate effectively, the flue has to apply a pressure differential sufficient to overcome the system resistance and enable the products of combustion to flow from the combustion chamber to the terminal. This pressure differential can be mechanical (by forced or induced draft or a combination of the two) or thermal, possibly combined with mechanical. 

Forced-draft flues The above design parameters are relevant to natural-draft flues. With forced-draft flues, it is possible by choice of a fan - either forced or induced draft - to overcome system resistance so that the flue will still clear the products. A cmde mle-of-thumb is to allow 1 mm of flue area for each 2.2-3.7kW for natural draft and for 4.5-13.6kW for each forced draft. 

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