|
Did you know that water droplets traveling at high velocity can take on the force of bullets? It can happen, particularly within steam turbines at a power plant during the process of condensation, where steam transforms back into water. The last couple of weeks in this blog series we’ve been talking about the steam and water cycle within electric utility power plants, how heat energy is added to water during the boiling process, and how turbines run on the sensible heat energy that lies within the superheated steam vapor supplied by boilers and superheaters. We learned that without a superheater there is a very real possibility that the steam’s temperature can fall to mere boiling point. When steam returns to boiling point temperature an undesirable situation is created. The steam begins to condense into water within the turbine. To understand how this happens, let’s return to our graph from last week. It illustrates the situation when there’s no superheater present in the power plant’s steam cycle.
Figure 1
After consuming all the sensible heat energy in phase C in Figure 1, the only heat energy which remains available to the turbine is the latent heat energy in phase B. If you recall from past blog articles, latent heat energy is the energy added to the boiler water to initiate the building of steam. As the turbine consumes this final source of heat energy, the steam begins a process of condensation while it flows through the turbine. You can think of condensing as a process that is opposite to boiling. During condensation, steam changes back into water as latent heat energy is consumed by the turbine. When the condensing process is in progress, the temperature in phase B remains at boiling point, but instead of pure steam flowing through the turbine, the steam will now include water droplets, a dangerous mixture. As steam flows through the progressive chambers of turbine blades, more of its latent heat energy is consumed and increasingly more steam turns back into water as the number of water droplets increases.
Figure 2 – Water Droplets Forming in the Turbine
The danger comes in when you consider that the steam/water droplet mixture is flying through the turbine at hundreds of miles per hour. At these high speeds water droplets take on the force of machine gun bullets. That’s because they act more like a solid than a liquid due to their incompressible state. In other words, under great pressure and at high speed water droplets don’t just harmlessly splash around. They hit hard and cause damage to rapidly spinning turbine blades. Without a working turbine, the generator will grind to a halt. So how do we supply the energy hungry turbine with the energy contained within high temperature superheated steam in sufficient quantities to keep things going? We’ll talk more about the superheater, its function and construction, next week.
________________________________________ |
Posts Tagged ‘condensation’
Condensation Inside the Steam Turbine
Sunday, September 8th, 2013
Industrial Ventilation – Local Exhaust Ventilation Filters and Air Cleaners III
Sunday, May 15th, 2011| I was out in the garage today spray painting, a job I would have preferred to have done outdoors, but alas, it was raining. It wasn’t a big job, and I probably didn’t spend more than about an hour doing it, but by the time I was done I was all too aware of how noxious the chemical fumes were that were put out by my aerosol spray can. I had thought that with all the garage doors open and a good cross breeze going through I’d be spared the unpleasant smell. Now imagine this all on a much larger scale, say an industrial setting, where massive spray painters are used all day long.
We’ve been talking for awhile now about filtration, from fabric filters to cyclones, and how they are most effective when integrated into a local exhaust ventilation system. These filtration devices are great for the removal of airborne particles like dust, but they don’t do a good job removing chemical vapors like paint fumes, much in the same way as a dust mask wouldn’t have made my spray painting job any less smelly. This week we’ll focus on filtration capable of addressing the special challenges presented by chemical vapors in the air. Chemical vapor contaminants can be separated from good air trapped in a local exhaust ventilation system by way of an air cleaner in a process known as absorption. In this instance, just like with our smelly goldfish tank, the media can consist of activated carbon, a carbon created by intense heating of substances like bituminous coal, wood, or coconut shell. The heat removes everything except carbon and creates myriads of tiny pores throughout. These pores give activated carbon tremendous surface area, meaning lots of nooks and crannies for chemical molecules to get lodged in. And when I say “lots” of nooks and crannies, I mean it. One pound of granular activated carbon has enough pores to give it a surface area of 125 acres! As the air-vapor mixture passes over the huge surface area, chemical vapors are absorbed by combining chemically with the carbon. Jamb packing surface area into a small space, as activated carbon does, creates a media capable of absorbing vast amounts of chemical molecules for a long time. As effective as this system is, the carbon pores will eventually become saturated with contaminants, and when it does, it is easily addressed. Simply replace the media with fresh carbon. Another means of removing harmful vapors from the air is through the use of an air cleaner employing temperature as its means of filtration. I’ll bet you’re asking how that works, and here’s an example you can relate to. It’s a hot, humid day, and the only thing standing between you and total discomfort is a glass of ice water. As you eagerly lift the glass to your lips, you notice the glass is wet on the outside, so wet that it’s actually dripping. In the stupor caused by your heat exhaustion you may for a moment think that the glass is actually leaking, but you soon realize that the water has accumulated on the outside of the glass because the hot, humid air that is making you so uncomfortable has also come into contact with the cold surface of the glass. When the water vapor in the atmosphere hits the cool of the glass filled with ice, it condenses into droplets. This condensation process stops when the glass temperature equalizes to that of the temperature in the surrounding air. Air cleaners can make use of the same phenomenon to filter contaminants. In their case the contaminated air mixture is cooled to the point where the humidity and chemical vapors present condense together to form a liquid, and the liquid is then drained out for proper disposal. That’s it for our look at filters and air cleaners. To sum things up, remember that there are a variety of factors that have to be considered when selecting filters and air cleaning devices. These include the volume of air flowing through the system, the concentration of contaminants in the air, the chemical and physical properties of the contaminants, the hazards associated with the contaminants, and the emissions standards established by federal, state, and local environmental regulations. Next time we’ll explore the workhorse of a local exhaust ventilation system, its fan. _____________________________________________ |
