Let’s get back to
drycleaning basics
Continuing with the “Back to
Basics” series, this month we will cover the properties
of steam and definitions of steam-related items and terms.
Some of the following definitions were
discussed in my article of August, 2001 (Steam Traps), but they
are included herein for your convenience with additions
pertaining to manufactured steam by a boiler. Types of boilers
and their operation and maintenance will be discussed in
subsequent articles.
Boiler: A closed vessel that contains
water. There are two basic types of boilers: hot water
production and steam production.
One type of hot-water boiler will heat
water to temperatures close to 225° F without making steam
as long as the amount of “latent heat,” or
“heat of evaporation,” of the particular boiler
pressure is not present.
Latent heat or heat of evaporation: The amount of heat (expressed in BTUs)
required to change a pound of boiling water to a pound of
steam. The same amount of heat is released when a pound of
steam is condensed back into a pound of water.
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British Thermal Unit (BTU): The amount of heat energy required to raise the
temperature of one pound of cold water by 1 degree F. Also, A
BTU is the amount of heat energy given off by one pound of
water in cooling (Example: from 70° F to 69° F.)
Saturated steam: Pure steam at the temperature that corresponds
to the boiling temperature of water at the existing pressure.
Absolute pressure and gage pressure: Absolute pressure is pressure in pounds per
square inch (psia) above a perfect vacuum.
Gage pressure is pressure in pounds per
square inch above atmospheric pressure, which is 14.7 pounds
per square inch absolute. Gage pressure (psig) plus 14.7 equals
absolute pressure. Or, absolute pressure minus 14.7 equals gage
pressure.
Pressure-temperature relationship: For every pressure of pure steam there is a
corresponding temperature. Example: The temperature of 100 psig
pure steam is always 337.9° F.
Heat of saturated liquid: The amount of heat required to raise the
temperature of a pound of water (1 gallon = 8.33 pounds) from
32° F to the boiling point at the pressure and temperature
in BTUs per pound of return condensate.
Total heat of steam: The sum of the heat of saturated liquid and
latent heat in BTUs. It is the total heat in steam above
32° F.
Flash steam: When
hot condensate or boiler water, under pressure, is released to
a lower pressure, part of it is re-evaporated to become what is
known as “flash steam.”
Flash steam is important because it
contains heat units that can be utilized for economy of plant
operation that would be otherwise wasted. Example: Utilizing
the heat in the return condensate and its flash steam to pass
over the coil of a heat exchanger through which the cold city
water flows, thus pre-heating the cold water from about 55°
F to about 115° F. This would save a considerable amount of
boiler horsepower (bhp) and BTUs, thus saving a considerable
number of fuel dollars.
Flash steam is formed when water is heated
at atmospheric pressure (14.7 pounds) and its temperature
rises until it reaches 212° F, the
highest temperature at which water can exist at this pressure.
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Further addition of heat does not raise
the temperature, but converts the water to steam which, itself,
will increase pressure.
The heat absorbed by the water in raising
its temperature to boiling point is called “sensible
heat.” The heat required to convert water at boiling
point to steam at the same temperature is called “latent
heat.”
If water is heated under pressure,
however, the boiling point is higher than 212° F, so the
sensible heat required is greater. As explained above, the
higher the pressure, the higher the boiling temperature and the
higher the heat content.
If pressure is reduced, heat content is
reduced and water temperature falls to the boiling temperature
at the new pressure.
This means that a certain amount of
sensible heat is released. This excess heat will be absorbed in
the form of latent heat, causing part of the water to
“flash” into steam.
Basic steam concept
Steam is the invisible gas that is
generated by adding heat energy to water in a boiler. Enough
energy must be added to raise the temperature of the water to
the boiling point. Then, more energy must be added to cause the
water to change to steam without any further increase in
temperature.
Additional BTUs are required to make
boiling water change to steam. These BTUs have not been lost.
They are stored in the steam, ready to be released to heat air
and water or to press and dry garments, etc. As stated before,
this heat required to cause boiling water to change to steam is
called latent heat or heat of vaporization.
It takes 1 BTU for every 1° F increase
in temperature up to the boiling point, but it takes a lot more
BTUs to change water at 212° F to steam at 212° F.
Fact: 1
pound of water at 70° F plus 142 BTUs = 1 pound of water at
212° F plus 970 BTUs = 1 pound of steam at 212° F.
Fact: 1 pound
of water at 70° F (0 psig) plus 270 BTUs = 1 pound of water
at 337.9° F (100 psig) plus 880 BTUs = 1 pound of steam at
337.9° F (100 psig). Note the extra heat and higher
temperature required to make water boil at 100 psig than at
atmospheric pressure.
Note, also, the lesser amount of heat
required to change water to steam at the higher temperature as
the gage steam pressure rises.
Boiler efficiency
This is normally expressed as a percentage
with 100 percent being complete efficiency. Any efficiency
rating is simply a ratio of the output of the equipment against
the amount of energy needed to produce this output.
For a boiler, the output is the steam
produced, and the input is the heat content of the fuel used to
supply the heat. The BTU is used in these calculations.
Normally, the output is expressed as the
number of pounds of water evaporated or boiler horsepower. One
boiler horsepower (BHP) means that 34.5 pounds of water at
212° F have been evaporated and converted to steam at
212° F in one hour. One boiler horsepower is equivalent to
33,500 BTUs per hour.
The input of a boiler is the potential
heat of the fuel that is available for combustion. The heat
content of various fuels is as follows:
Natural gas: 1,000
BTUs per cubic foot
No. 2 fuel oil: 139,000 BTUs per gallon
Propane liquid gas: 98,000 BTUs per gallon
The following formula is used to calculate
boiler efficiency:
Output of boiler (BTUs of steam) divided
by input of boiler (BTUs of fuel) X 100 = boiler efficiency.
Example: To compute the efficiency of a
boiler using five gallons of No. 2 fuel oil to evaporate 552
pounds of water per hour, the following steps are necessary.
First, convert the pounds of water
evaporated into boiler horsepower by dividing by 34.5 (552
divided by 34.5 = 16 BHP).
Second, change the 16 BHP into BTUs by
multiplying by 33,500 (16 X 33,500 = 536,000 BTUs). This is the
output of the boiler.
Third, one gallon of No. 2 fuel oil
contains 139,000 BTUs.
Therefore, five gallons would contain
695,000 BTUs. This is the output of the boiler.
Fourth, divide the boiler output in BTUs
(536,000) by the boiler input in BTUs (695,000). Take this
answer times 100 to obtain the boiler efficiency, which, in
this example, is 77 percent.
New boilers can maintain a continuous
operating efficiency of 80 to 85 percent, but most older
boilers will operate at 70 percent to 75 percent efficiency.
Of course, efficiency depends chiefly on
how well the boiler is maintained and operated.
Boiler rating
There are three ways of rating a boiler:
1. Square feet
of heating surface.
2.. Amount of
water evaporated per square foot of heating surface per hour.
3. Heat output
in steam, expressed in BTUs per hour.
When computing boiler horsepower from a
manufacturer’s plate affixed to the boiler, apply the
following factors:
• 34.5 pounds of water (at 212°
F) have been evaporated and converted into steam (at 212°
F) in one hour = 1 BHP. (all boilers)
• 33,500 BTUs per hour = 1 BHP. (all
boilers)
• Fire Tube, Scotch Marine boiler: 5
square feet of heating surface = 1 BHP.
• Water Tube boiler: 8 square feet of
heating surface = 1 BHP.
• Horizontal Return Tubular (HRT)
boiler: 7 to 10 square feet of heating surface = 1 BHP.
Note: One manufacturer averages 7.5 square
feet per BHP and another manufacturer is a constant 10 square
feet per BHP.
• Tubeless boiler: 4 square feet to
5.2 square feet of heating surface = 1 BHP.
Note: My spotting video, “The Caplan
Method of Stain Removal,” which includes my comprehensive
text and the handy spotting board reference, is available in
English, Korean (video only) and Spanish (video only) from the
Golomb Group, c/o Dennis McCrory, 7664 Plaza Court,
Willowbrook, IL 60521, phone (800) 679-5856. A lecture and
demonstration are presented similar to my classes over the
years at IFI and SDA. Also available from the Golomb Group, in
English and Spanish, is my video on step-by-step- shirt
finishing which includes my comprehensive text in loose-leaf
form (English only) outlining each procedure for a
single-operator and two-operator cabinet shirt unit using a
cabinet sleever. Proper forming of the collar using heated
collar formers is demonstrated. Each lay is demonstrated
for top quality with very little effort by the operators.
Attractive detailing and packaging of the hangered shirt,
padding, steam pressure and timing are all discussed. A unique
wash formula to give whiter whites, brighter colors and total
removal of grease and body oils is included in the loose-leaf
text book.
Stan Caplan has over 35 years experience
in his own high volume drycleaning, laundry and tailoring plant
and two package plants with adjoining coin-operated laundry and
drycleaning. He is former chief instructor at the International
Fabricare Institute, the Southwest Drycleaners Association and
various other trade association-sponsored schools. He has
taught numerous short courses in all areas of the fabric care
industry throughout the US, Canada, Singapore, Hong Kong,
Mexico and South Africa. Stan offers consulting, training and
engineering services in all areas from customer service area to
the boiler room. His complete system (Total Quality Management)
encompasses maximum efficiency, economy and product excellent
quality. Stan can be reached at 3601 Clarks Lane, Suite 307,
Baltimore, MD 21215, phone/fax (410) 358-0870. His e-mail
address is stancap100@aol.com.
