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Armstrong Machine Works presents.... [Laughter]
Psst! Yeah, you. Come here. You do know that the energy used in industrial processes comes
from 3 basic types of fuel; solid, liquid and gas? Oh, you know that? Then you probably
know that with fuel getting costlier, the cost of manufacturing is also going up. Good,
you know that, too. I'll bet you also know that much of the fuel used in industry goes
for the generation of steam used in various steam systems, and that industrial people,
in an effort to save energy and get the most out of their fuel dollars, have taken steps
to eliminate obvious steam losses; leaks to atmosphere, uninsulated pipes, dirt in steam
equipment, faulty components in steam traps. Huh, I figured you knew that. But here's one
you might not know. There are barriers to heat transfer that are very often overlooked;
condensate and air layers in heat exchangers, and burned product or other films on heat
exchange surfaces, barriers that waste energy. Did you know that? That's what I thought.
Well, here's Mr. Otho Ulrich, director of field engineering for Armstrong machine works.
Let's hear what he has to say about all this.
When water is boiled to produce steam, certain amounts of non-condensible gases are released.
These gases include oxygen and carbon dioxide, and are generally referred to as air. Some
of the gases enter the boiler dissolved in the feed water, while oxygen, is of course
part of the water itself. In addition to the air released when water boils, there's always
air in the system during equipment startup. When the supply steam to any equipment is
turned off, the residual steam in the space collapses, causing a vacuum. Air enters the
steam space through the seals, the joints, traps, glands, and through vacuum breakers.
Before the equipment can be heated to operating temperature again, this air must be evacuated
and replaced by steam. Conventional drainage systems are not capable of quickly discharging
large quantities of air. Let's consider the behavior of air and steam during the startup
period. There are two factors that basically determine the steam flow and distribution
within a heat exchange; the shape of the steam space, and the location of the steam inlet
and condensate drain. In this heat exchanger, steam enters through
the inlet at the left, and pushes the air in front of it to the drain at the right.
If the steam trap in the drain line is capable of venting a sizable quantity of air, the
steam will quickly fill the steam space, producing a fast and uniform temperature rise. The same
steam trap arrangement could be effective if the steam inlet were at the top of a vertical
heat exchanger, and the drain were at the bottom. In this type of heat exchanger, the
steam inlet and condensate drain are on the same end of the steam space. The steam pushes
the air to the right side of the space, where it becomes trapped, forming a cold spot on
the heat exchange surface. Some of the air mixes with the steam, and the rest forms a
cold pocket, resulting in uneven heating. To remove the air from the space, a thermostatic
air vent should be installed at the end of the heat exchanger, opposite the steam inlet
and condensate drain. We will discuss the operation of air-handling steam traps and
air vents after we consider the effects of air in steam.
The pressure of the supply steam to any heat exchanger is determined by the temperature
pressure curve of pure steam. For example, steam at 50 psig has a temperature of 297
degrees Fahrenheit. If a process requires a temperature of 297 degrees Fahrenheit, the
operator sets his control valve for 50 pounds gauge pressure. Air-steam mixtures are cooler
than pure steam, as shown in this table. Dalton's law of partial pressures states that in a
mixture of gases and vapors, the total pressure of the mixture equals the sum of the partial
pressures of all of the gases or vapors. Thus, a mixture of 3 part steam and one part air,
at 100 pounds absolute pressure, produces the equivalent temperature of saturated steam
at 75 pounds absolute, or 307 degrees Fahrenheit. The temperature of saturated steam at 100
pounds absolute pressure is nearly 328 degrees Fahrenheit, a difference of over 20 degrees.
Now if this loss of a few degrees were the only effect of air in the steam, most processes
could easily compensate by raising the supply pressure, if the operator were aware of the
low temperature. Non-condensible gases are carried with the steam towards the heat exchange
surface. Unable to condense, these gases form an insulating layer between the steam and
the heat exchange surface. To overcome the insulating effect of this layer of air, steam
pressures must be increased, or process time is lengthened. Both of these alternatives
are costly.
Air is one of the best heat insulators in use today. Insulating material merely confines
air to minute pockets to restrict its natural convections. The heat conductivity of air
is 0.2, compared with 5 for water, and 340 for cast iron. The iron is 1700 times as good
a heat transfer agent as air. Therefore, a film of air, one one-hundredth of an inch
thick, provides the same barrier to heat transfer as a wall of cast iron 17 inches thick. In
fact, as little as one-half percent by volume of non-condensible gas in the steam can reduce
the heat transfer efficiency by as much as 50 percent.
This illustration shows a cross section of a heat exchange surface for a process requiring
a product temperature of 210 degrees Fahrenheit. As steam reaches the surface, it gives up
its latent heat and condenses, leaving a film of condensate. Air in the steam collects with
the condensate on the inside of the heat exchange surface. If the heat exchanger is immersed
in the product, a layer of water or burned product may form on the outside of the heat
exchange surface. To overcome the heat transfer barriers, and obtain a product temperature
of 210 degrees Fahrenheit, the supply steam must be set at 15 pounds gauge, or 250 degrees
Fahrenheit. There is a 40 degree temperature loss that results from the insulating layers
on the heat exchange surface. If it were possible to reduce the thickness of the condensate
and air on the inside of the surface by one-half, the steam pressure could be lowered from 15
to 10 pounds gauge, or 240 degrees Fahrenheit.
This figure shows an autoclave used for sterilizing hospital instruments. The process requires
that a temperature of 228 degrees Fahrenheit be maintained for a predetermined length of
time. Normally, the required temperature can be reached by applying 20 pounds absolute
to the steam inlet. However, if the steam is mixed with 25 percent air, the temperature
would only reach 213 degrees Fahrenheit, and sterilization would not be assured. Many industries
can benefit from installing special air-handling devices, and other evacuating components,
on their equipment.
In the textile industry, such devices were installed on stacks of dry cans. Initial warmup
time was immediately reduced from an average of 90 minutes to less than 10 minutes. The
improved performance resulted in a saving of 4,800,000 BTUs per day in a 2 dry can stack.
This saving is the equivalent of 32 gallons of fuel oil, or 5,330 cubic feet of natural
gas per day.
In the food processing industry, one of the most consistent problems is removing air from
jacketed kettles and pans. One small pan was fitted with an air removal device. The result
was to reduce the cooking time by 2 minutes per batch, and to save approximately 59,500
BTUs per batch. When a large jacketed pan was similarly fitted, the cooking time was
reduced by more than 10 minutes per batch, saving 18 pounds of steam for each batch processed.
Because of the reduced process time, production was increased by 8 batches per day.
One of the major factors in removing air from steam spaces is the location of the air vents.
The air must be evacuated quickly to prevent mixing with the incoming steam, and to achieve
uniform temperature, and obviously, the place to install the air vent is where the air collects.
Usually, the air vent will be elevated above the highest part of the steam space to minimize
the liquid carryover through the vent.
Air removal devices use thermostatic elements, like those used in thermostatic steam traps.
They operate on the balanced pressure principle, and follow the steam temperature-pressure
curve. Air in the steam reduces the temperature of the space, surrounding the thermostatic
element, causing the valve to open. As the steam-air mixture is evacuated, the temperature
at the thermostatic element rises, causing the valve to close. Now the amount of steam
that's lost in this charge is insignificant when compared to the resultant temperature
rise in the steam system. Thermostatic air vents may be separate units, as shown here;
or may be incorporated into a steam trap, as in the float and thermostatic trap, as
shown here. Now the right unit, for a particular heat exchanger, depends on the shape of the
steam space and the flow pattern from the steam inlet to the drain. Now, although air
venting is often overlooked by many engineers, the proper application of air removal devices
can provide substantial savings of both steam and time, through quicker startups and more
efficient equipment operation.
Your Armstrong representative can provide specific information on venting air from your
production equipment.