pokrovskii - fluid mechanics review

8/6/2019 POKROVSKII - Fluid Mechanics Review

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Fluid Mechanics ReviewA pump

with infinitepower

Pool of Water

Lets say you want to pump water up through a pipe

vertically as shown in the diagram to the left. The body ofwater that is being pumped from is at sea level and

exposed to atmospheric pressure. How high could you

pump the water if the pump has infinite power?

(The pump is not the limiting factor)

If you tried this same experiment in Lake Tahoe would youget a taller, shorter or same height column of water?

Gravity

Atmospheric pressure at sea level is around 14.7 PSI

h

Please Read thisQuestion:


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Fluid Mechanics Review

A pump with infinite power

A pump works by creating a smaller pressure, relative to

atmospheric pressure, at the inlet of the pump. So

theoretically the smallest pressure the pump can create

would be 0 PSIA (lb/in^2 absolute) or in otherworld's avacuum. Our pump with infinite power can do just that.

Lets pretend the pump is working full throttle; creating a

vacuum at its inlet. So now we have a tube standing

vertically with 14.7 PSIA at the bottom and 0 PSIA at the

top. The atmospheric pressure literally pushes the fluid up

the pipe. But how far will it go?

Gravity

Pool of Water


0 PSIA


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A pump with infinite power

As the water travels up the pipe more and more of it gets

above the pool water level, increasing its potential energy.

Here is a relationship that describes the pressure of a water

column at a given height. g h = pressure

The water will equalize when the pressure it produces at

the bottom due to its height equals the atmospheric

pressure. So reworking the equation to solve for this height:

h = atmospheric pressure / g

h = 33.9 ft

Gravity

Pool of Water


0 PSIA


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Now the second portion of this question. Would the water

column be larger if this experiment was done in Lake Tahoe

(Elevation: 6225 feet above sea level)

We know that at higher elevations the atmosphericpressure is less. The water column will be less high

because the atmospheric pressure is less.

Pool of Water

Lake

Tahoe

27.0 ft

San Jose (sea level)

33.9 ft

What does this mean?

When you design for fluid pumps at height elevationsyou must take the elevation into account. The lower

atmospheric pressure will play a negative role on

NPSH. (We will talk about this soon)

IMPELER PUMPS WILL NOT AND DO NOT WORK

IN SPACE!


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ASI NPSH Calculator Application

for Designing HVAC HydronicOpen Systems

Air Systems Inc.

Dimitrii PokrovskiiAugust 10th, 2007


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Presentation Overview

Basic fluid mechanics overview

Possible Applications of the ASI Fluid Flow

Calculator Define NPSH and why it is important in

hydronic system design

Perform a example calculation using theASI Fluid Flow Calculator


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2 NPSH Terms to Know

NPSHA Net Positive Suction Head Available

Determined by the fluid system configuration

NPSHR Net Positive Suction Head Required

This depends on the type of pump and flow rate

The NPSHR value is taken from the pump

performance curve


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The definition of NPSHA (Net Positive Suction Head

Available):

NPSHA = (1) Static head + (2) surface pressure head (not

zero for open systems) (3) the vapor pressure of your

product (4) the friction losses in the piping valves and

fittings (down stream of the pump)

liquidlevel

(2) Surface pressure

(1) Static head

(3) Liquid vapor pressure (depends on temperature and pressure)

(4) the friction losses in the piping valves and fittings


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NPSHR Net Positive Suction Head Required

This is the minimum pressure allowed at the inlet of the pump to preventcavitation.


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When the pressure drops lower then the vapor pressure

of the liquid causing it to boil. (Change phase from

Liquid to Gas) Its exactly like boiling water on the stove

except you achieve boiling by bring the pressure down

instead of raising the temperature.

Cavitation


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Cooling Water

Tower 1

Cooling Water

Tower 3

Cooling Water

Tower 2

Cooling Water

Tower 4

800 gpm

800 gpm 800 gpm

800 gpm10

10

10

10

25 20

20

20

Tee90 Ell

90 Ell

Pump

1600 gpm 3200 gpm

The pump is 5

ft above the

water level in

the cooling

towers


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Cooling Water

Tower 1

Cooling Water

Tower 3

Cooling Water

Tower 2

Cooling Water

Tower 4

800 gpm

800 gpm 800 gpm

800 gpm10

10

10

10

25 20

20

20

Tee90 Ell

90 Ell

Pump

1600 gpm 3200 gpmThe pump is 5

ft above the

water level in

the cooling

towers

1

2

3

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