Page 24 - Boiler_Operators_Handbook,_Second_Edition

Operating Wisely 9

Volume is a measure of space. A building’s volume space, then asked you to stand on the scale and tell us
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is described as cubic feet, abbreviated ft , meaning we what it reads, what would your answer be? Zero! You
multiply the width times the length times the height. don’t weigh anything in space, but you’re still the same
One cubic foot is space that is one foot wide by one foot amount of mass that we weighed at sea level. There is
long by one foot high. a difference in weight as we go higher. You will weigh
I’ll ignore references to the metric system because less in Denver, Colorado, because it’s a mile higher,
that’s what American society appears to have decided but for all practical purposes the small difference isn’t
to do. It’s regrettable because the metric system is easier important to boiler operators. Once you accept the fact
to use and there’s little need to convert from one to the that mass and weight are the same thing with some ad-
other after we’ve accepted it. After all, there’s adequate justment required for precision at higher elevations you
confusion and variation generated by our English can accept a pound mass weighs a pound and let it go at
system to keep us confused. When it comes to linear that.
measurements we have inches and yards, one twelfth of Volume and mass aren’t consistently related. A
a foot and three feet respectively. Measures of area are pound mass is a pound mass despite its temperature or
usually expressed in multiples of one of the linear mea- the pressure applied to it. One cubic foot of something
sures (don’t expect an area defined as feet times inches can contain more or less mass depending on the tem-
however). For volumetric measurements we also have perature of the material and the pressure it is exposed
the gallon, it takes 7.48 of them to make a cubic foot. to. Materials expand when heated and contract when
Note that the volumetric measure of gallons cooled (except for ice which does just the opposite).
doesn’t relate to any linear or area measure, it’s only We can put a fluid like water on a scale to deter-
used to measure volumes. That’s some help because mine its mass but the weight will depend on how much
many trades use unit labels that are understood by them we put on the scale. If we put a one gallon container of
to mean area or volume when we couldn’t tell the differ- 32° water on the scale, it will weigh 8.33 pounds. If we
ence if we didn’t know who’s talking. A painter will say put a cubic foot of that water on the scale, it will weigh
he has another thousand feet to do. He’s not painting a 62.4 pounds.
straight line. He means one thousand square feet. We’ve Density is the mass per unit volume of a substance,
already mentioned the cement hauler that uses the word in our case, pounds per cubic foot. So, water must have a
“yards” when he means cubic yards. Always make sure density of 62.4 pounds per cubic foot. Ah, that the world
you understand what the other guy is talking about. should be so simple! Pure clean water weighs that. Sea
When talking, or even describing measurements water weighs in at about 64 pounds per cubic foot. Heat
we will use descriptions of direction to aid in explain- water up and it becomes less dense. When it’s necessary
ing them. While most people understand north, south, to be precise, you can use the steam tables (page 353) to
east and west plus up and down other terms require determine the density of water at a given temperature
some clarification. Perpendicular is the same as per- but keep in mind that its density will also vary with the
fectly square. When we look for a measurement per- amount of material dissolved in it.
pendicular to something it’s as if we set a square on it In many cases water is the reference. You’ll hear
so the distance we’re measuring is along the edge of the the term specific gravity or specific weight. In those
square. An axial measurement is one that is parallel to cases it’s the comparison of the weight of the liquid to
the central axis or the center of rotation of something. water (unless it’s a gas when the reference is air) Know-
On a pump or fan it’s measured in the same direction as ing the specific gravity of a substance allows you to
the shaft. Radial is measured from the center out; on a calculate its density by simply multiplying the specific
pump or fan it’s from the centerline of the shaft to what- gravity by the typical weight of water (or air if it’s a gas).
ever you are measuring. When we say tangentially or One quick look at the number gives you a feel for it. If
tangent to we’re describing a measurement to the edge the gravity is less than one it’s lighter than water (or air)
of something round at the point where a radial line is and if it’s greater than one it will sink.
perpendicular to the line we’re measuring along. Gases, such as air, can be compressed. We can pack
Another measure that confuses operators is mass. more and more pounds of air into a compressed air stor-
Mass is what you weigh at sea level. If we put you on age tank. As the air is packed in, the pressure increases.
a scale while standing on the beach, we would be able When the compressor is off and air is consumed, the
to record your mass. If we then sent you to Cape Ken- tank pressure drops as the air in the tank expands to
nedy, loaded you into the space shuttle, sent you up in replace what leaves. The compressor tends to heat the