Water Pump Impeller Theory

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Transcript of Water Pump Impeller Theory

In most cases the differential pressure across the pump Dptot is measured and the head H is calculated by the following formula: H = where

ptot/ /g

(m) total pressure difference density of fluid in kg/cu.m free fall acceleration 9.8 sq.m/s

ptot g

flow Q n INLET INLET AREA CALCULATED AS for radial impeller A1 = 2(PI)*r1*b1 sqm r1 = b1 =

0.0025 cu m 4000 rpm

radial position of impeller's inlet edge (m) the blade height at the inlet (m)

for semi axial impeller A1 = 2*(pi)*(r1 hub+ r1 shroud)/2*b1 (sq.m)

The entire flow must pass through this ring area. C1m is then calculated C1m = Q impeller/A1 (m/s)

The tangential velocity U1 equals the product of radius and angular frequency: U1 = 2*pi*r1*n/60 = r1* (m/s)

n = rpm = angular velocity When the velocity triangle has been drawn, see figure 4.4, based on 1, C1m and U1, the relative flow angle can be calculated. Without inlet rotation (C1=Cm)this becomes: tan 1 = 1 = OUTLET C1m/U1 Inv tan 1

:

Outlet area calculated as A2 = 2*pi*r2*b2 (sq.m)

for semi axial impeller A2 = 2*(pi)*(r2 hub+ r2 shroud)/2*b2 (sq.m)

C2m is then calculated C2m = Q impeller/A2 (m/s)

The tangential velocity U2 equals the product of radius and angular frequency: U2 = 2*pi*r2*n/60 = r2* (m/s)

In the bignig of design phase ,2 is assumed to have the same value as the blade angle The relative velocity can be calculated from: W2 = C2m/sin 2 (m/S)

and C2u as: C2u= U2 - C2m/ tan 2 (m/s)

4.2 Eulers pump equation Eulers pump equation is the most important equation in connection with pump design. The equation can be derived in many different ways. The method described here includes a contr moment of momentum equation which describes flow forces and velocity triangles at inlet and outlet.

A control volume is an imaginary limited volume which is used for setting up equilibrium equations. The equilibrium equation can be set up for torques, energy and other flow quantities w of momentum equation is one such equilibrium equation, linking mass flow and velocities with impeller diameter. A control volume between 1 and 2, as shown in figure 4.6, is often used for an impeller. The balance which we are interested in is a torque balance. The torque (T) from the drive shaft corresponds to the torque originating from the fluids flow through the impeller with mass flow m=Q: T= m*(r2*C2u - r1 * Ciu) (Nm)

By multiplying the torque by the angular velocity, an expression for the

shaft power (P2) is found. At the same time, radius multiplied by the angular velocity equals the tangential velocity, r2 w = U2 . This results in: P2 = = = = = T* (Watt) m**(r2* C2u - r1 * C1u) m*(*r2* C2u - *r1 * C1u) m (U2* C2u - U1 * Ciu) Q* * (U2* C2u - U1 * Ciu)

According to the energy equation, the hydraulic power added to the fluid can be written as the increase in pressure ptot across the impeller multiplied by the flow Q Phyd = ptot *Q (Watt)

The Head is defined as : H = ptot / * g (m)

and the expression for hydraulic power can therefore be transcribed to:

Phyd = =

Q*H* * g m * H *g

(Watt)

If the flow is assumed to be loss free, then the hydraulic and mechanical power can be equate: Phyd = P2

m * H *g = m (U2* C2u - U1 * Ciu) H= (U2* C2u - U1 * Ciu) / g

This is the equation known as Eulers equation, and it expresses the impel lers head at tangential and absolute velocities in inlet and outlet. If the cosine relations are applied to the velocity triangles, Eulers pump equation can be written as the sum of the three contributions: Static head as consequence of the centrifugal force Static head as consequence of the velocity change through the impeller Dynamic head H= ((U2 sq - U1 sq) / 2/g) + (W1 sq- W 2 sq)/2/g) + (C2 sq- C1 sq)/2/g (m)

Static head as consequence of the Centrifugal force

Static head as consequence of the Velocity change

Dynamic head

If there is no flow through the impeller and it is assumed that there is no inlet rotation, then the head is only determined by the tangential velocity based on (4.17) where C2u=U2 H0 = U2 sq/g (m)

When designing a pump, it is often assumed that there is no inlet rotation meaning that C1u equeals zero H= (U2 * C2u)/g (m)

4.8 Specific speed of a pump As described in chapter 1, pumps are classified in many different ways for example by usage or flange size. Seen from a fluid mechanical point of view, this is, however, not very practical because it makes it almost impossible to compare pumps which are designed and used differently.

A model number, the specific speed (nq) Specific speed is given in different units nq = Where nd Qd = Hd = nd *sqrt (Qd)/ Hd power (3/4)

is therefore used to classify pump In Europe the following form iscommonly used:

= rotational speed in the design point [rpm) = Flow at the design point [m3/s] = Head at the design point [m]

The expression for nq can be derived from equation (4.22) and (4.23) as the speed which yields a head of 1 m at a flow of 1 m3/s

The impeller and the shape of the pump curves can be predicted based on the specific speed, see figur 4.17. Pumps with low specific speed, so-called low nq pumps, have a radial outlet with large outlet diameter compared to inlet diameter. The head curves are relatively flat, and the power curve has a positive slope in the entire flow area

On the contrary, pumps with high specific speed, so-called high nq pumps. have an increasingly axial outlet, with small outlet diameter compared to the width. Head curves are typically descending and have a tendency to create saddle points. Performance curves decreases when flow increases. Different pump sizes and pump types have different maximum efficiency

kp/kg

A1 = r1 = b1 =

0.00285 sqm 0.03 m 0.01512 m

p1-p2

C1m =

0.877177 m/sec

U1 =

12.56637 m/sec

is becomes: tan 1 = 1 = 0.069804 3.992969 deg

A2 =

0.002262 sq m

C2m =

1.105243 m/sec

U2 = tan 2 = 2 =

18.84956 m/sec 0.058635 3.355693 deg

W2 =

18.88193 m/sec

C2u=

0 m/sec

cribed here includes a control volume which limits the impeller, the

and other flow quantities which are of interest. The moment

ultiplied by the flow Q

mmonly used:

he head curves