Xylem Lowara Scuba Series, Manual

Page: 17 / 32

17
TECHNICAL APPENDIX
CALCULATING THE SPEED OF THE FLUID
THAT FLOWS AROUND A SUBMERGED MOTOR
AND SIZING OF THE COOLING SLEEVE
The following formula is used to verify whether the speed of
the fluid that flows around the motor of a submersible pump
is high enough to guarantee the proper cooling of the motor:
Where: Q in [m
3
/s] is the operating flow rate of the electric pump; only half of this flow is taken into account,
because the fluid which is sucked into the area of the filter (2), comes from the motor side (3)
as well as from the pump side (1);
D in [m] corresponds to the diameter of the well;
d in [m] corresponds to the diameter of the motor (3);
v in [m/s] is the calculated speed of the fluid that flows around the motor.
Now, compare the speed thus calculated (v) with the minimum speed required for correct cooling of the motor
(v
m
): if v
≥
v
m
it means that the motor is properly cooled, if v < v
m
will be necessary to mount a cooling sleeve (4).
Example:
An electric pump OZ630/12 (motor diameter d = 0.144 m) operates in an 8” well (well diameter D = 0.203 m)
with flow rate
Q = 20 m
3
/h = 0.0055 m
3
/s.
Speed of fluid v = (0.0055/2) / { π ·[(0.203)
2
/4 – (0.144)
2
/4]} = 0.17 m/s.
The minimum speed required for proper motor cooling is v
m
= 0.20 m/s.
Because v < v
m
, it will be necessary to mount a cooling sleeve.
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The following formula is used to determine the maximum
diameter of a cooling sleeve to be mounted on a submersible motor:
Where: Q in [m
3
/s] is the operating flow rate of the electric pump; the entire flow is taken into account
because the fluid comes from the motor side (3) only;
D in [m] corresponds to the diameter of the cooling sleeve (4);
d in [m] corresponds to the diameter of the motors(3);
v
m
in [m/s] is the minimum speed of the fluid that flows around the motor.
If the electric pump operates at different flow rate, the minimum flow rate must be taken into account for
calculating the diameter of the cooling sleeve.
Example:
A motor coupled to the electric pump OZ615/24 (motor diameter d = 0.144 m), which operates with flow rate
Q = 15 m
3
/h = 0.0042 m
3
/s, requires a minimum speed of the fluid of v
m
= 0.20 m/s.
Cooling sleeve diameter D = {4·[(0.0042/(0.2· π )+(0.144)
2
/4] }
0.5
= 0.217 m.
)
4 4
(
2
2
2 d D
Q
v
− ⋅
=
π






+
⋅
⋅ =
4
4
2 d
v
Q
D
π