Page 33 - Boiler Operator’s Handbook

18 Boiler Operator’s Handbook

because there are some simple shortcuts described here happen to be a mile high, like Denver, you’ll have to
that can help you. subtract about 3 psi from the table data. Any steam table
Water is the basis for heat energy measurement. used by an engineer will relate the temperatures to ab-
Our measure of heat energy, the British thermal unit solute pressure.
(Btu for short) is defined as the amount of heat required What is absolute pressure? If you must ask you
to raise the temperature of water one degree Fahrenheit. missed it in the part on measurements, flip back a few
We engineers know that’s not precisely true at every pages.
condition of water temperature but it’s good enough for Provided the temperature of water is always less
the boiler operator. As for the energy in steam, well it than the saturation temperature that matches the pres-
depends on the pressure and temperature of the steam sure the water is exposed to, the water will remain a
but, for all practical purposes it takes 1,000 Btu to make liquid and you can estimate the enthalpy of the water
a pound of steam and we get it back when the steam by subtracting 32 from the temperature in degrees Fahr-
condenses. enheit. For example, boiler feedwater at 182°F would
If you want to be more precise, you can use the have an enthalpy of 150 Btu. It takes 970 Btu to convert
steam tables (Page 353) A few words on using those one pound of water at 212°F to steam at the same tem-
steam tables is appropriate. Engineers use the word perature so you’re reasonably accurate if you assume
“enthalpy” to describe the amount of heat in a pound of steam at one atmosphere has an enthalpy of 1,150 Btu
water or steam. We needed a reference where the energy (212–32+970). If we sent the 182°F feedwater to a boiler
is zero and chose the temperature of ice water, 32°F. That to convert it to steam, we would add 1,000 Btu to each
water has no enthalpy even though it has energy and en- pound. Just remembering 32°F water has zero Btu and
ergy could be removed from it by converting it to ice. So, it takes 970 Btu to convert water to steam from and at
the enthalpy of water or steam is the amount of energy 212°F is about all it takes to handle the math of saturated
required to get a pound of water at freezing temperature steam problems.
up to the temperature of the water or steam at the outlet We do have other measures of energy that’s unique
of the boiler. Since we use freezing water as a reference to our industry. One is the Boiler Horsepower (BHP).
point, the difference in enthalpy is always equal to the With 1,000 Btu to make a pound of steam and the
amount of heat required to get one pound of water from ability to generate several hundred pounds of it the
one condition to the other. numbers get large and cumbersome, so the term Boiler
Did I forget to mention that steam is really water? Horsepower was standardized to equal 34.5 pounds of
Some of you are going to wonder about my sanity in steam per hour from and at 212°F. Since we know that
making such a simple statement but I’ve run into boiler one pound requires 970 Btu at those conditions a boiler
operators that couldn’t accept the concept that the water horsepower is also about 33,465 Btu per hour (34.5 ×
going in leaves as steam. Steam is water in the form of 970), more precisely it’s 33,472. It’s important here to
gas. It’s the same H O molecules which have absorbed note the distinction that a Boiler Horsepower is a rate
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so much energy, heated up, that they’re bouncing value (quantity per hour) and Btu’s are quantities. We
around so frantically that they now look like a gas. The abbreviate Btu’s per hour “Btuh” to identify the number
form of the water changes as heat is added, it gets hotter as representing a rate of flow of energy.
until it reaches saturation temperature. Then it converts Another measure of energy unique to our industry,
to steam with no change in temperature and finally but not used much anymore, is Sq. Ft. E.D.R. mean-
superheats. There is, for each pressure, a temperature ing square feet of equivalent direct radiation. It’s also
where both water and steam can exist and that’s what a rate value. It was used to determine boiler load by
we call the saturation point or saturation condition. calculating the heating surface of all the radiators and
Most of us are raised to know that water boils at baseboards in a building. There are two relative values
212°F. That’s only true at sea level. In Denver, Colorado, of Sq. Ft. E.D.R. depending on whether the radiators are
it boils at about 203°F. Under a nearly pure vacuum, operating on steam or hot water. It’s 240 Btuh for steam
29.75 inches of mercury, it boils at 40°F. The steam tables and 150 Btuh for water. There are rare occasions when
list the relationships of temperature and pressure for you will encounter the measure but its better use is to
saturated conditions. Since a boiler operator doesn’t relate what happens with heating surface. If a steam in-
need to be concerned with the small differences in at- stallation were converted to hot water, it would need an
mospheric pressure the table shows temperatures for additional 60% (240/150 = 1.6) of heating surface to heat
inches of mercury vacuum and gage pressure. If you the same as the steam. Flooded radiators can’t produce