Series Fan Connection

In this case the outlet of the first fan is connected to the inlet of the second fan. With this arrangement, the total head developed at a given volume is equal to the sum of the total heads developed by the individual fans. Sometimes more than one fan is used in a duct arrangement. In the series connection (Fig. 9.49), the flow through the fans has the same volume flow, and the leaving flow from the first fan is connected to the suction side of the second fan.

For the series connection, we indicate by 1 and 2 the suction and leaving sides of the first fan, respectively, and by 3 the leaving side of the second fan. The common total pressure difference is Rot — P3 + JPcJ — Pi ~ pcl — p3 + 7PC3 -p2~ pc2 Z z z z (9.134) ] 1 1 1 P2 + 2PCl ~P1 ~ 2PCl = ^totl + ^Ptot2 ~ + fc<tv2)1

Where qv is the volume flow through both fans and f(qv) = Apml is the total — pressure-dtfference volume flow dependency of the first fan. The points (qv, fi(q„)) are the characteristic curve of the first fan, and the points are the char­acteristic curve of the second fan.

Equation (9.128) gives

APt ot = A P = Aptoti(<7^) + Aptot2(<7,/2)- (9.135)

The network and fan combination operating point can be obtained from Eq. (9.134).

In a Ap — qv chart, the characteristic curve of the ducts is drawn. In the same chart is drawn the characteristic curve of both fans. At each volume flow qv, the total pressure of each fan is added. In this way, we get a new Aptot — q„ curve. The intersection point of this new curve and the characteristic curve of the ducts is the operating point (Fig. 9.50).

Must be adjusted or changed, this will happen by changing the fan characteris­tic curve or the network impedance. The fan and network then settle in a new operating point, and the volume flow changes.

The impedance of the network can be influenced by

• the change of the minor loss resistance number of a damper.

When the minor loss resistance changes, the impedance of the network and hence the fan and network common operating point also change. The choking regulation with a damper is a bad regulation mode from the point of view of fan energy consumption. The fan is chosen so that in normal conditions the fan is running at the design point. At this point, air absolute velocity, and cir­cumference velocities of the impeller and blade angles are such that the veloc­ity triangles are in the correct form, that is, the flow relative velocity is parallel to the blades, so shock losses are not generated and efficiency is at its highest value.

When choking, neither circumference velocity nor blade angles change. The direction of the flowing gas’s absolute velocity toward the impeller does not change, but the magnitude changes. The velocity triangles are now such that the relative speed is not parallel to the blade, and shock losses are gener­ated. This causes a decrease in the efficiency.

FIGURE 9.51 Leading blades’ influence on the inlet velocity triangle of a centrifugal fan. Solid line: velocity triangle without leading blades. Dashed line: velocity triangle with leading blades.

A change in the fan characteristic curve can be made by

• adjustable leading blades (in front of suction opening of centrifugal fans),

• regulation of blade angles of impeller (axial fans), or

• regulation of rotational velocity of impeller.

With adjustable leading blades for a centrifugal fan, the volume flow changes by changing the velocity triangles’ form at the impeller inlet. By giving the coming airflow a tangential velocity component, the inlet velocity triangle can be changed in such a way that the absolute velocity radial component changes and the relative velocity remains parallel to the blade. Then the fan ef­ficiency is high. The leading blades change the coming flow’s absolute velocity in such a way that the tangential velocity component is in the same parallel di­rection as the circumference velocity. Leading blades are generally used if the blades are backward curved (Fig. 9.51).

The regulation of axial fan blade angle also influences the inlet and exit velocity triangles in such a way that the axial velocity and thus the volume flow change. When the relative velocity remains parallel to the blade, the effi­ciency remains high (Fig. 9.52).

Recently, the regulation of impeller rotational velocity has become a pop­ular regulation mode for volume flow. Electric-motor rotational velocity is regulated by a frequency changer, and its price has dropped lately. Changing the rotational speed also affects the circumference velocity of the impeller. The volume flow can be changed by the same ratio as rotational speed. The form of the velocity triangles and the efficiency remain the same.

Consider the fan characteristic curves for two different rotational veloci­ties nl and n2. Select the operating point for the characteristic curve nx as (A/>totl, qvl). The corresponding point is (Aptot2, qv2) for characteristic curve n2.

I N 2

Based on Eqs. (9.112) and (9.116),

(9.136)

From Eq. (9.136), the characteristic curve of n2 can be drawn when the char­acteristic curve of nx is known.

Denoting (A/7tot, qv) as the corresponding point of the operating point (Apmtl, qvl) and the corresponding rotational velocity n ^ n, Eq. (9.136) gives /

Afi Tot = Aptoti

Qvl Tfv 1

Or Aptot = ■ q2v2. (9.137)

CJv

The corresponding points to (Aptotl, qvl) compose a parabola, the affinity parabola in the Aptot — qv chart. The parabola passes through the origin and point (APwuqvi). _

At the corresponding points of the parabola, the rotational velocity and volume flow change by the same ratio. If the efficiency is high at the point (Aptotl, qlA), it is also high at the corresponding point. The fan is connected to a network where there is no static height, so the network (Apmt, qv) curve is a pa­rabola which passes through the origin. The fan operating point is the intersec­tion point of the characteristic curve and the network (Aptot, qv) curve. Both parabolas have two common points; the origin and the operating points. If the volume flow is changed, increasing or decreasing the rotational velocity, the new operadng point will be on the affinity parabola. If the efficiency is high for the original operating point, then it will also be high for the new one ^Fig. 9.53). Hence, rotational speed regulation is a good mode for energy consumption.

1v

FIGURE 9.53 Fan characteristic curve for two rotational velocities, n, and n2, and three fan affinity parabolas.

Example 6

A fan delivers air to a ventilating system at total pressure difference 500 Pa, The fan is running at 10 rev s_1, and the shaft power needed in these conditions is 7.46 kW. Determine the volume flow, total pressure dif­ference, and shaft power if the fan speed is increased to 12.5 rev s~J.

Solution. Denote qvl = 1.55 m3 s”1, Apwtl = 500 Pa, nx = 10 rev s_1

-l’

And Pal = 7.46 kW. The new fan speed is n2 = 12.5 rev s’

The new operating point is on the affinity parabola through Qlfl = 1.55 m3s ‘, and Aptotl = 500 Pa. From Eq. (9.136) we have for corresponding points on the affinity parabola

— rh — qv

^ • 1.55 = 1.937 m3 s’1,

12.5 10
^Pxotl	Apt,	500 = 781 Pa.

Finally,

_ Aptotl ‘ 4vZ _

Aptoti

( n-,

Qvl kPx

‘Jvl

Y

V 12.5 10

V / «2 NL V V

3

= 14.6 kW.

= 7.46 ■

= p

A ‘

[1] In a laminar boundary layer, no mixing takes place and the flow is parallel. In this case the heat transfer occurs mainly by conduction through the boundary layer.

[2] High or medium initial cost. The energy cost for ozone synthesis is about 12-15 kW h (kg 03)_1 consumed (1999), while 10-15 years ago it was in the 25-30 kW h (kg 03)_1 range.

• Limitations on water temperature and quality.

[3] Is also a human carcinogen? Unknown

— Does not cause any effect

Source: Modified from Savolainen and Vahakangas.1 S1

Humans. Humans are exposed to aflatoxins in hot and humid regions in Africa and Asia where peanuts and grain have to be stored in inappropriate condi­tions which favor the growth of fungi. In these regions, hepatitis is also com­mon, and these two factors may, in fact, act synergistically to promote the formation of liver cancer.5’150 Historically, workers involved in the production of polyvinylchloride (PVC) polymers (plastics, elastomers) were exposed to high concentrations of vinyl chloride. In these workers, the incidence of liver angiosarcoma increased dramatically, and the incidence of brain tumors has also been reported to be higher than the incidence in control workers.36 Ethyl alcohol can increase the risk of liver carcinoma. It is not primarily considered a very potent liver carcinogen, but nonetheless is important because the doses of ethyl alcohol to which humans are exposed are so high.

Kidney Toxicity

The integrity of mammalian kidneys is vital to body homeostasis, because the kidneys play the principal role in the excretion of metabolic wastes and the regulation of extracellular fluid volume, electrolyte balance, and acid-base

[4] In experimental animals

[5] Zones located within the same room on different levels. These zones have different air temperatures and/or contaminant concentrations (Fig. 7.107 e).

Air and contaminant movement between different zones may be caused by one or several of the following mechanisms:

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