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English (US)
10. Technical data
10.1 Operating conditions
The operating envelope of a refrigerant pump is generally limited
by a number of different factors related to the operation close to
the boiling point of the refrigerant and the specific properties of
the refrigerant.
Avoid dry running at all times. Even a few seconds of dry running
may damage the bearings. Avoid also operation at conditions with
constant cavitation.
Fig. 15
General operating envelope for CRN-H MAGdrive
pumps
The individual limits are described in the table below:
Maximum operating pressure: 362 psi
Refrigerant temperature range: -40 °F to 248 °F
Viscosity: 0.15 cP to 300 cP.
10.1.1 NPSH (Net Positive Suction Head)
If the pressure in a hydraulic circuit drops below the vapor
pressure of the circulating liquid, boiling will occur, typically at the
pump inlet, resulting in vapor bubbles appearing in the flow.
When the mixture of liquid and vapor bubbles enters the first
pump impeller, the pressure will increase, and the bubbles will
collapse, resulting in noise, vibrations and sometimes also
damage to the impeller and bearings. The process of bubbles
collapsing in a liquid is called cavitation.
Generally, the higher the flow rate, the higher the required NPSH.
In closed hydraulic circuits where a risk of cavitation is present, it
is possible to reduce or prevent cavitation as follows:
• Shorten the pipe from the separator or receiver to the pump
inlet, and hereby reduce friction losses.
• Increase the cross-section of the pipe from the separator or
receiver to the pump inlet to reduce the velocity of the liquid,
and hereby reduce friction losses.
• Minimize the number of pipe bends, and use pipe bends that
have a bending radius of more than 1.5 times the pipe
diameter. The number of other fittings, for example for a
change in diameter, must be minimized.
• Lower the liquid temperature to reduce the saturated vapor
10.1.2 NPSH curve chart
The NPSH required (NPSHr) is defined as the NPSH at which the
pump total head has decreased by 3 % due to low suction head.
The result is cavitation within the pump.
The NPSHr for one of the CRN-H MAGdrive pumps is shown in
fig.
.
Fig. 16
Required NPSH
The margin in which the NPSH available (NPSHa) is to exceed
the NPSHr is 2 ft. The 2 ft. margin is needed to protect the pump
from damage caused by cavitation.
Example
In an application where the maximum flow rate is 23 gpm (see
fig.
above), the NPSHa must exceed 2 ft. plus a 2 ft. margin
which equals 4 ft.
The NPSHa is to exceed 4 ft.
NPSHa is typically defined as the refrigerant liquid height above
the centerline of the pump inlet.
TM
05
0
616
12
11
CAUTION
The limits below must always be observed and
respected. Operation outside the envelope will
invalidate the warranty.
Abbreviation
Description
Q
min.
The minimum flow rate is 10 % of the nominal
flow rate of the pump.
n
max.
Too high speed for mechanical design. If n
max.
is exceeded, vibrations will occur, wear of
bearings will increase, and the power
consumption of the motor will be too high.
Maximum speed 60 hz.
n
min.
Too low bearing surface speed. If n
min.
is
exceeded, the lubrication of bearings will be
inadequate. Minimum speed 30 hz.
∆
H
min.
The minimum differential pressure is to be 7 psi
below the setpoint differential pressure. The
absolute minimum differential pressure is 20 %
of the closed-valve pressure at 60 hz.
H
30
35
40 45
50
55
60 Hz
Accepted
Not Accepted
Rated duty
n
max.
Q
min.
n
min.
Δ
H
min.
T
M
06
84
70
02
17
QH 1750 rpm
NPSH
Summary of Contents for CRN-H MAGdrive
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