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I have done some more refinement on my earlier curves of water pump output.
The description below and the accompaning figures provide some insite in how
the system will work.
First, some changes from my earlier curves that were posted, please throw
away those curves. I only had one pressure guage at the outlet of the pump,
and a considerable length of corrugated tubing to connect the pump to the
water source. As a result, the results were interesting but not as useful as
those attached to this email. Also, I had not calibrated the bucket used for
the measurements.
THE SETUP
I placed a 55 gallon barrel on a workbench next to the engine on my "mock
firewall", and filled it with water until the level was just a bit above the
outlet of the pump. An 18 foot length of 1-1/4" sump pump hose (the cheap
plastic corrugated kind) ran from the barrel, up over the edge, down to the
floor, and up to the intake of the pump. A 6 foot length of the same hose
was run from the outlet of the pump back to the barrel. A 1-1/4" plastic
ball
valve was installed at the outlet of the pump, before the hose. For the
final
test I installed two pressure guages, one on the outlet of the pump, and the
other on the inlet of the pump. This way, I could remove the effect of
pressure drop from sucking water through the 18' plastic hose.
My 1-HP electric saw motor was mounted on the firewall such that the pump
pulley could be driven by a long V-belt. I used 3 different pulley sizes to
give different pump speeds. The pulley diameters were ~2-5/8, 4", and 6".
The pump pully measured 3.7" diameter. This gave me rough pump speeds of
5594, 3730, and 2448 rpm.
DATA COLLECTION
I would float the bucket on the surface of the water in the barrel, turn on
the pump, and when I diverted the water into the bucket, start a timer. The
bucket would sink, displacing the water in the barrel and essentially
holding
it constant (no head change). By changing the setting of the valve, I could
change the flow rate. The bucket (filled to overflowing) held 5.8 gallons,
and all data points were duplicated.
For each run, I would measure the pressure on the pump outlet (affected by
the valve setting), and the pressure on the inlet to the pump, (affected by
the flow rate). The data, plotted as pressure vs flow rate is shown on
Figure 1, Water_pump_data.jpg. There are 5 curves, two for the 5594 rpm
case
(taken on two separate days), one each for 3730 and 2448 rpm, and a new one
for the pressure at the pump inlet. Note that it starts at ~zero and goes
negative with increasing flow rate. This 8 psi drop is due to the flow
through the corrugated tubing at ~29 GPM. In an actual installation, one
would not want to use 18 feet of tubing, so I wanted to adjust the data to
allow compensating for that pressure loss.
DATA ANALYSIS
Flow through pipes causes a pressure drop, that is proportional to the
SQUARE
of the velocity of the flow, therefore, if I plotted the data from Figure 1
vs the square of the flow rate, I should get a straight line. Combining the
two 5594 rpm runs, the data as pressure drop vs the square of the flow rate
is shown in Figure 2, Pump_fit_Data.jpg. There is some scatter, but the
basic shape of all the curve is linear, and the equations for each of the
lines is shown on the graph.
At 5594 rpm, Pressure (psi) = 18.77 - 0.02(flow^2)
At 3730 rpm, Pressure (psi) = 8.09 - 0.018(flow^2)
At 2448 rpm, Pressure (psi) = 4.29 - 0.0226(flow^2)
Pressure loss in inlet tubing = - 0.3339 - 0.0103 (flow^2)
Now, for any given flow rate, I can get the performance of the pump as it
would be installed in the airplane, (not having 18 feet of tubing to suck
through), by just subtracting inlet pressure drop from the flow curves from
the pump curves. However, there is one other piece of information that is
needed to know what the flow in the system is, and that is the pressure drop
across the heat exchanger.
HEAT EXCHANGER PRESSURE DROP
Next installment, more data to be take tonight on the heat exchangers.
PUMP CURVES
Using the equations above, the pressure available for the system to force
the water through the heat exchanger cores, is shown on Figure 3
Pump_smooth.jpg . Please note that the points beyond 29gpm are
extrapolations based on the linear fit to the velocity squared.
Bill Schertz
KIS Cruiser #4045
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