Fans in parallel and series

A pair of identical fans connected in series handles the same volume as a single fan but develops twice the fan total pressure. Thus, the characteristic data for a single fan running at 8 rev s”1 in example 15.14 would become, for two in series:

0 0.5 1.0 2.0 2.5 3.0 3.5

720 770 820 866 848 800 686

Variable position disc

Forward-curved fan impeller

Fig. 15.30 (a) Characteristic curves for: a single fan, 2 fans in series and 2 fans in parallel. (b) Section through a forward-curved centrifugal fan showing a movable position disc. If the fan is regarded as having a characteristic like that of 2 fans in parallel then its capacity can be reduced by moving the disc towards the inlet eye. This is seen by the broken line in Sub-figure (a).

(a)

When identical fans are connected in parallel they handled twice the volume of a single fan at the same fan total pressure. This normally causes no problems but, occasionally, with forward curved fans operating in the vicinity of their point of inflection (see Figure 15.20(a)), the performance can hunt between the three possible airflow rates for a given total system resistance. The solution is to increase the system resistance a little in order to rotate the system characteristic out of the area of instability, with the loss of some airflow rate.

Exercises

1. A system of duct and plant has a total pressure loss of 600 Pa when 1300 litres s’1 of air flows. A fan running at 11 rev s“1 is coupled to the system and has the following characteristic:

Litres s 1 0 100 200 300 400 500 600 700 800 900 1000 1100

Pa 450 457 462 465 461 445 417 376 327 266 194 100

(a) Determine the actual duty achieved and the speed at which the fan must run to get the design duty.

(b) If two of these fans are connected in series what will be the duty for the system mentioned if they each run at 11 rev s“4?

Answer

(a) 883 litres s“[4] at 275 Pa, 16.2 rev s-1, (b) 1015 litres s-1 at 360 Pa.

2. The total pressure loss through an air handling plant and supply duct index run has been wrongly calculated as 250 Pa for a design flow rate of 5 m[5] s_1. A fan having the following characteristic performance when running at 7 rev s-1 has been selected and installed:

M3 s’1	0	1	2	3	4	5
Pa	235	272	284	284	273	250
Efficiency %	—	58	63	67	69	70

During commissioning the measured duty was 3.45 m3 s 1 at a fan total pressure of 280 Pa.

(a) Determine the correct pressure through the plant and system for the design airflow rate and the speed at which the fan should run to deliver this.

(b) If the maximum speed at which the fan may safely run is 9 rev s-1 and if the pulleys are changed to obtain this speed, what will then be the airflow rate and the absorbed fan power?

(c) If the original filter in the plant had a pressure loss of 200 Pa when passing 3.45 m3 s-1 and if, in an attempt to increase the duty at the new fan speed, it is replaced by a different type of filter with a pressure drop of 150 Pa when passing 5 m3 s“1, what should the airflow rate and absorbed fan power then be? Assume that the pressure loss through all parts of the plant and system is proportional to the square of the volumetric flow rate.

Answer

(a) 588 Pa, 10 rev s_1, (b) 4.44 m3 s-1, 3.02 kW, (c) 4.53 m3 s_1, 1.70 kW.

3. (a) State three fan laws that describe the behaviour of a fan for: a given fan size, a

Given duct system and a fixed air density.

(b) State three additional laws for the case of: a given fan, a given duct system, a given speed and varying density.

(c) A fan has a pressure-volume characteristic at 10 rev s-1 as follows:

M3 s_1 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Pa 400 435 460 487 483 474 450 393

Notation

Symbol Description Unit

TOC o "1-5" h z A cross-sectional area of a duct or an air jet m2

A’ reduced cross-sectional area at a vena-contracta m2

Af0 area across the flanges at fan outlet m

A area, or overall dimension of a flat oval duct m2 or mm

B overall dimension of a flat oval duct mm

C duct pressure drop constant or the curve ratio of a bend —

Ca coefficient of area —

CE coefficient of entry or flow coefficient —

Cv coefficient of velocity —

D diameter of a fan inlet eye m

Dt equivalent duct diameter m or mm

Dh mean hydraulic diameter = 4AIP m or mm

D internal duct diameter m or mm

/ dimensionless coefficient of duct friction (Fanning) —

/c coefficient of friction for a circular duct —

F coefficient of friction for a rectangular duct —

/’ coefficient for determining an approximate value off —

G acceleration due to gravity m s-2

H head lost in metres of fluid flowing m

H height of a duct or a turning vane m or mm

K ratio of throat to heel radius for a bend —

Ks absolute roughness of a duct wall material m or mm

L length of straight duct m

LID number of equivalent duct diameters —

I length of straight duct m

m mass kg

N3 constant = n3/(32p) = 0.308 42p-1 m3 kg-1

N4 constant = 1.255Ti|i/4p = 0.985 67|xp_1 m2 s“1

N number of turning vanes, dimensionless constant or

Fan speed — or rev s’1

P perimeter of a duct or a section of a jet of air m or mm

P pressure Pa

Pat atmospheric pressure Pa

Ps static pressure Pa

PsF fan static pressure Pa

Pso static pressure at fan outlet Pa

Pt total pressure Pa

PtF fan total pressure Pa

Pti total pressure at fan inlet Pa

Plo total pressure at fan outlet Pa

Pv velocity pressure Pa

Pvo velocity pressure at fan outlet Pa

Ap pressure loss or rate of pressure loss Pa or Pa nr1

Aps change of static pressure Pa

Apt total pressure loss or rate of total pressure loss Pa or Pa m“1

Q	Airflow rate	M3 s 1
Q’	Theoretical airflow rate without loss at a suction opening	M3 s“1
(Re)	Reynolds number	—
Rc	Centre-line radius of bend	M or mm
R0	Throat radius of a bend	M or mm
Rt	Throat radius of a bend	M or mm
Rn+1	Heel radius of a bend	M or mm
R	Fraction of static regain
S	Space between turning vanes	Mm
T	Dry-bulb temperature	°C
V	Mean velocity of airflow	M s"1
V	Theoretical velocity without loss at a suction opening	M s“1
Vfo	Notional mean air velocity at fan outlet	M s"1
W	Width of a duct	Mm
	Air power	KW
Wf	Fan power	KW
Y	Duct length between supply air branches	M
8«	Small quantity of air	3 M
8,	Small quantity of time	S
M	Total fan efficiency	%
S	Pressure loss coefficient, applied to a velocity pressure	—
Ca	Pressure loss coefficient for an abrupt expander	—
Cg	Pressure loss coefficient for a gradual expander	—
0	Included angle of a duct transition piece	O
	Absolute viscosity	Kg m“1 s-1
V	Kinematic viscosity	M2s-1
P	Density	Kg m“3
References

Posted in Engineering Fifth Edition