Posts Tagged ‘temperature’

Desuperheating in the Steam Turbine

Monday, September 2nd, 2013

      Last time we learned that the addition of a superheater to the electric utility power plant steam cycle provides a ready supply of high temperature steam, laden with heat energy, to the turbine, which in turn powers the generator.   But this isn’t its only job.   One of the superheater’s most important functions is to regulate the ongoing process of desuperheating that takes place as the turbine consumes heat energy.   To understand this, let’s see what takes place if the superheater were to be removed from its position between the boiler and turbine.

Steam Turbine Engineering Expert

Figure 1

 

      Without the superheater, the only available remaining source of sensible heat energy to the turbine would come from the meager amount present in phase C steam as shown in Figure 1.   If you’ll recall from a past blog, the sensible heat energy contained in superheated steam is the best source of energy for a steam turbine, because it’s able to keep it operating most efficiently.

      As the turbine consumes the heat energy in phase C, starting at point 3 and continuing to point 2, the steam it’s consuming is in the process of desuperheating, as evidenced by the downward slope between the two points.   Desuperheating is an engineering term which means that as sensible heat energy is removed from the steam due to its use by the turbine, there will be a resulting drop in steam temperature.   And if this process were to continue without the compensatory function provided by the addition of a superheater to the steam cycle, the steam’s temperature would eventually return to mere boiling point, at point 2.   This is an undesirable thing.

      With the steam’s temperature at boiling point, the only remaining source of heat energy to the turbine is the latent heat energy of phase B.   This heat energy will lead to an undesirable circumstance for the operation of our power hungry turbine as we will see next week.

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Forms of Heat Energy – Latent

Monday, July 15th, 2013

      If you took high school chemistry, you learned that water is created when two gases, hydrogen and oxygen are combined.   You may have even been lucky enough to have a teacher who was able to perform this magical transformation live during class.

      Depending primarily on the amount of heat energy absorbed, water exists in one of the three states of matter, gas, liquid, or solid.   Its states also depend on surrounding atmospheric pressure, but more about that later.    For our discussion, the water will reside at the atmospheric pressure present at sea level, which is around 14.7 pounds per square inch.

      Last time we learned that the heat energy absorbed by water before it begins to boil inside our example tea kettle is known as sensible heat within the field of thermodynamics.   The more sensible heat that’s applied, the more the water temperature rises, but only up to a point.

      The boiling point of water is 212°F.    In fact this is the maximum temperature it will achieve, no matter how much heat energy is applied to it.   That’s because once this temperature is reached water begins to change its state of matter so that it becomes steam.   At this point the energy absorbed by the water is said to become the latent heat of vaporization, that is, the energy absorbed by the water becomes latent, or masked to the naked eye, because it is working behind the scenes to transform the water into steam.

      As the water in a tea kettle is transformed into steam, it expands and escapes through the spout, producing that familiar shrill whistle.   But what if we prevented the steam from dispersing into the environment and continued to add heat energy?   Ironically enough, under these conditions temperature would continue to rise, upwards of 1500°F, if the stove’s burner were powerful enough.   This process is known as superheating.   Now hold your hats on, because even more ironically, the heat added to this superheated steam is also said to be sensible heat.

      Confused?    Let’s take a look at the graph below to clear things up.

power plant engineering

      Sensible heat is heat energy that’s added to water, H2O, in its liquid state.   It’s also the term used to describe the heat energy added to steam that’s held within a captive environment, such as takes place during superheating.    On the other hand, the latent heat of vaporization, that is the heat energy that’s applied to water once it’s reached boiling point, does not lead to a further rise in temperature, as least as measured by a thermometer.

      Next time we’ll see how surrounding air pressure affects water’s transition from liquid to steam.

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Forms of Heat Energy – Sensible

Sunday, July 7th, 2013
      In our house the whistle of a tea kettle is heard throughout the day, no matter the temp outside.  So what produces that familiar high pitched sound?

sensible heat power plant boiler

      When a tea kettle filled with room temperature water, say about 70°F, is heating on the stove top, the heat energy from the burner flame will transfer to the water in the kettle and its temperature will steadily rise.  This heat energy that is absorbed by the water before it begins to boil is known as sensible heat in thermodynamics.  To read more about thermodynamics, click on this hyperlink to one of my previous blog articles on the topic.

      So, why is it called sensible heat? It’s so named because it seems to make sense.  The term was first used in the early 19th Century by some of the first engineers who were working on the development of boilers and steam engines to power factories and railways.  Simply stated, it’s sensible to assume that the more heat you add to the water in the kettle, the more its temperature will rise.

      So how high will the temperature rise?  Is there a point when it will cease to rise?  Good questions.  We’ll answer them next week, along with a discussion on another form of heat energy known as the latent heat of vaporization.

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Food Manufacturing Challenges – HACCP Design Principle No. 3

Sunday, October 30th, 2011
     How do parents make life safer and healthier for their kids?  One of the ways is to  impose limits on things like roaming distance within the neighborhood, curfews, and insisting that you eat your vegetables.  Just common sense, right?  Let’s take a look at some more of it.

     Limits are also necessary within the food manufacturing industry.  Let’s take a look at Hazard Analysis and Critical Control Point (HACCP) Principle No. 3 to see how they’re established and why.

     Principle 3: Establish critical limits for each critical control point. – You can think of a critical limit as a boundary of safety for each critical control point (CCP).  So how do you determine that boundary of safety?  It’s difficult to generalize, but if you’ve ever watched the TV show Hoarders, you have an excellent example of one that has not only been breeched, but torn asunder.   

     In order to prevent things in the commercial food industry from getting anywhere near Hoarders bad, maximum and minimum values are set in place, representing safeguards to physical, biological, and chemical parameters at play within the industry.  Critical limits can be obtained from regulatory standards and guidelines, scientific literature, experimental studies, as well as information provided by consultants.  These critical limits come into play with issues as varied as machine design, raw material temperatures, and overall safe processing times.

     How could the hoarders let things get so bad?  If you listen carefully, you’ll hear bits of information that provide a clue.  They’ll say it started with a few things falling to the floor which they didn’t feel like picking up and it escalated from there. 

     Now all of us live within environments which differ as to their cleanliness, but by and large we live within space where we feel comfortable and consider to be reasonably clean.  We don’t all habitually move stoves and refrigerators to clean, for example.  But if we were so inclined, refrigerators do come with front access panels that are easily removed.  Trouble is the space they provide access to often isn’t large enough to accommodate hands and a vacuum cleaner nozzle comfortably.  You can imagine how frustrating and potentially dangerous it would be to public health to have commercial machinery that provided such limited access for cleaning.    

     To cope with this problem design engineers institute minimum and maximum parameters, such as in the critical limit dimensions of a removable cover.  Their guideline would ensure that enough space is provided so that personnel can fully access all aspects of machinery with tools for cleaning.  That same cover can also have established maximum critical limits, so that dimensions aren’t too large and heavy to be manipulated by hand.  Human nature being what it is, something that is too difficult to remove may be “forgotten” and parts of the machine may never get cleaned.

     Raw meats and many produce can contain hazards like salmonella, E. coli, and other nasty critters that are dangerous to human health.  One of the ways the commercial food industry works to ensure that these contaminants aren’t unleashed on the public is to install programmable control systems into processing machinery that essentially cooks the meat at an established minimum temperature for a minimum amount of time.  Utilizing this type of temperature control in conjunction with an established maximum cooking parameter for temperature and time will virtually eliminate the possibility of overcooked or burnt food products.  When you buy that frozen dinner in most cases it’s completely cooked, but it’s a rarity to find it’s been burned.  

     Another situation in which critical limits are utilized is in the maintenance of machinery, such as when they limit the number of hours a machine can be operated before it is shut down for servicing.

     Next week we’ll move on to Principle No. 4 and see how it establishes monitoring requirements for each CCP.  ____________________________________________