Tuesday, January 29, 2013
Tuesday, May 17, 2011
Understanding Oil Return Systems In Unitary Systems
Ed Keuper, a GEA colleague, has presented three (3) very informative postings concerning oil return in refrigeration systems. His emphasis was on chillers and other refrigeration systems that use flooded evaporators and depend on oil separators at compressor discharge as well as a means for returning lubricant from the system to the compressor sump. His key points are (1) that the compressor must always have enough lubricant in the sump and (2) oil must not be present in the heat exchangers, mainly the evaporator, to the extent that heat transfer is significantly degraded.
This sequence deals with direct expansion unitary systems that are designed to move lubricant around the system and back to the compressor sump by momentum, in the case of refrigerant gas transport (Suction and discharge), and solubility where refrigerant is in liquid form. Another way of stating the design objective for this type of refrigeration system is that if, say, 1 % (oil in refrigerant) of lubricant is discharged from the compressor during stable operation, that ratio must be present anywhere in the system at any given time so that the 1 % is safely returned to the sump. One can view this arrangement as two (2) fluid streams - oil and refrigerant - traveling side by side from the compressor, through the condenser and evaporator, and back to the compressor sump after reaching equilibrium.
This series is presented as an overview at best. Many details for piping design to provide oil entrainment and circulation and to prevent oil drainage into places while a system is dormant can be found in the 2010 ASHRAE Handbook of Refrigeration, Chapter 1 - Halocarbon Refrigeration Systems.
Equilibrium and Minimum Run Time
It is important that a system as discussed here - or any system for that matter - be allowed sufficient run time to approach equilibrium after startup. What is equilibrium and how long to reach it? If a compressor is started, transients will occur for a period of time before the lubricant and refrigerant streams settle to a constant, or near constant pace. To be certain that equilibrium is obtained, the very minimum run time after startup can be calculated by dividing the system refrigerant charge by the refrigerant flow rate. For example, if we have a system with refrigerant charge of 20 lb and a flow rate of 900 lb per hr (typical for 5 ton R22 system), the very minimum run time should be 20/900, or 0.022 hours (1.32 minutes). A safe minimum run time in this case might be 2 or 3 minutes. Minimum run time is usually determined by the unitary system manufacturer and programmed into the control system; but, if not, this is one way of calculating it. Run time is usually ensured by a timed on control or some space thermostats. Note that the time required will be system dependent. Most unitary systems apply scroll compressors. Run time information and many other subjects are treated in Copeland Bulletin AE4-1331 R3.
Maintaining an Adequate Lubricant Level
Copeland Bulletin AE4-1331 R3 covers the need to ensure that adequate lubricant always remains in the compressor sump. For instance, they recommend the addition of 1 oz oil for every five (5) lb of refrigerant over 20 lb of refrigerant charge.
A scroll compressor is equipped with about 65 oz lubricant charge. One can quickly calculate the lubricant residing in a system having, say, and 1% lubricant in refrigerant. If the system has 20 lb of refrigerant, the oil in continuous circulation (away from the compressor) would be 20*.01 = 0.2 lb, or 3.2 oz. A system charged with, say, 50 lb refrigerant would have 8 oz lubricant in circulation. This is a substantial portion of the oil charge (12.3%) that cannot be in the sump. Six (6) oz. additional oil, as recommended by the compressor manufacturer, is needed in the system. Note, too, that these calculations are for steady state operation. Many times during transient operation, the circulation rate can be higher, which emphasizes the need for sufficient run time after the compressor is started.
Minimum Velocities in Suction and Discharge Gas Risers
We have to accept the fact that pressure drop of any sort degrades performance. A good rule of thumb for a cooling cycle (looking at the compressor only) is that the compressor COP will drop 2.4% for every degree F drop in evaporating temperature and 1 1/2% for every degree F rise in condensing temperature. Designing tube sizes so that refrigerant velocities are adequate for oil entrainment in suction and discharge risers while minimizing the system efficiency penalties is a delicate balance.
Each low side component (evaporator and suction line) should have a maximum saturation drop of 2F. Each high side component (condenser/receiver and discharge line) should have a maximum of 2 F saturated temperature drop. The liquid line is not as critical as long as you have a liquid feed to the TXV (for subcooling).
Without going through detailed calculations, the industry has successfully applied some rules of thumb for velocities for decades. These rules are to use minimum refrigerant velocities of 1000 ft/minute for vertical vapor risers and 750 ft/minute for horizontal vapor lines. This requirement also applies to individual evaporator and condenser circuits that require the transport of gas vertically.
Vapor lines can pretty easily meet these criteria for single capacity systems. However, if a system is designed to provide more than one level of capacity, the designer must be certain that the required velocities are operative at the system's lowest capacity. This means, of course, that the system at full capacity might have to have velocities considerably higher than 1000 ft/minute. If a designer wants to be more precise so that multiple capacity systems can operate as efficiently as possible, refer to 2010 ASHRAE Handbook of Refrigeration, Chapter 1 - Halocarbon Refrigeration Systems.
Local MBA Program VS Online MBA Course: What is Right for You?
Do you think you are better suited to a local MBA program? Or would you prefer to take an MBA course through the Internet? You can find legitimate opportunities to secure your MBA either way, but you have to seriously determine what will best fit your life. You have to keep all of these things in mind as you make this decision:
? Your Lifestyle
? Other Obligations & Responsibilities
? Available Finances
? Available Time
There are advantages and disadvantages to both options, so let's take a closer look.
Local MBA Programs
A local MBA program will be close enough to your home to travel and to and from on a daily or weekly basis for classes. Most institutions will offer a wide variety of program options, including full time and part time MBA course opportunities.
The advantage to this type of MBA course is you can participate in on campus activities and learn next to other students with the same career goals you have right now. You may also have direct access to professors or teachers for extra instruction or for questions after class.
The disadvantage to this type of MBA program is that you have to travel to and from the campus and will have to be present for each class at a set time every day or week. You will have to make sure that the class times fit well with your schedule so you can be there every time.
Local MBA courses are best for students who have a relatively open schedule and reliable transportation to and from the MBA program class site. If there are evening classes or other flexible time options, then this can be a great pick for parents and those with day jobs that they can't quit.
Online MBA Courses
An online MBA program will offer courses through the Internet to people all around the world. Lessons are presented through the computer so there is a lot more flexibility as to what time of day the work is completed. Since study is completed in your own home you don't have to worry about traveling to and from the MBA program class site.
The flexibility of an online MBA course is attractive to many people, but this is not the best form of study for every student. You have to be extremely disciplined to make sure that you do the required work every week or every day. There won't be scheduled class times or other students beside you to make sure you pay attention and stay on top of your studies.
It can also be difficult for some students to really focus at home, especially with a lot of distractions like children and pets. Students trying to study around an active working schedule can easily fall behind in studies without those dedicated class times that would be required with a local MBA program.
An online MBA course can be an excellent option for those with limited time to make it to local MBA class times. You just have to make sure you are disciplined and dedicated enough to stay on top of your studies on a daily and weekly basis. Once you fall behind it is very easy to find yourself flunking out of an online MBA course.
Adult Education Can Open New Earning Opportunities
Sometimes people do not get a good education when they are young, and adult education classes can help them to achieve their goals, no matter what stage of life they are currently in. There are many reasons for not getting a high school diploma or learning computer skills when one is young, but that does not have to mean that the door is closed to them forever.
Adult education classes can often be attended in the work place. High schools and community learning centers also provide basic education classes. These classes can teach reading, writing, basic math skills, computer skills and many qualify for high school education credits. GED programs are especially popular, helping to remove a huge block from advancement at work.
A high school equivalency can offer new, better paying work opportunities for many adults. Adults can also learn other skills that will make them a valuable employee and qualify them for advancement in the work place. Many of these students go on to college and are able to earn an associates degree, bachelors degree or an even higher degree of learning.
Adult education classes can also be found online. Correspondence courses are available. If you prefer to learn in a classroom setting with an instructor, there are resources online to help you find a class that will be near and convenient for you to attend.
Most of these classes are held at night and on the weekends so that people who are working full time can still participate in them. If you work swing shifts or odd hours, you may be able to find morning classes or the online or correspondence courses might be a good option for you.
Whatever your reasons are for wanting to continue your education, remember that it is never too late. In five years, you will be five years older. But, you can be five years older and have accomplished your goals. If you do not pursue your goals, the time will still continue to come and go.
Many adults choose to get their education even if they have no intention of going back to work or climbing the latter at their current job. For many, it is a personal goal worth pursuing. Adults have advantages over students when it comes to education. Adults are doing it voluntarily, so they are much more motivated to learn. They have also acquired knowledge during their years of raising families, working and serving in the military. It is easy to build on this knowledge and meet your educational goals.
How To Pass Exams
A young student was terrified that he would fail his exams. The more he thought about it, the more he was convinced that he was bound to fail and so, eventually, he went to see his lecturer for some advice. The lecturer asked him, "What seems to be the trouble?"
"I don't want to fail my exams."
"OK, but what is it you want?"
"I don't want to fail my exams."
"Yes, I understand that, but what do you want?"
The student was quite confused by this and so he thought for a while and then, rather questioningly, he repeated, "I don't want to fail my exams".
The lecturer took a long, slow deep breath. He looked him straight in the eye and said, "That's what you don't want. What I want to know is what you do want".
Gradually it dawned on the student what the lecturer meant and he said, "I want to pass my exams".
"And what do you need to do to pass your exams?"
"I need to study and work hard."
"And will you do that?"
"Yes."
"Then there is little for you to worry about, but if you are having difficulty with a specific subject, tell me and I will help you."
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Notice how this has turned everything completely around. At first the student was only concerned with what he didn't want. No doubt there were very many things that he didn't want (the list is probably endless), but failing his exams was the only one he was worrying about. The lecturer got him instead to focus on what he did want, and once he had done that the course of action was obvious. Yet while the student was focussed only on what he did not want, it was difficult - even impossible - for him to know what to do.
Many people live their lives like that; they constantly attempt to avoid what they don't want. They are motivated only by moving away from problems or fears. They bounce around aimlessly in a lifetime of worry and fear, only concerning themselves with how to protect themselves from this danger or that. In reality the dangers that they see everywhere don't exist and if they would only let their guard down, they would see it. A lot of people seem to always find something to worry about and if they can't find something then they invent things to worry about instead. I think that Mark Twain put it best when he said: "I've known a great many troubles, and most of them never happened." Most worries never do happen; and even if they do, they're usually not as bad as we imagined. It is clear that much of our worrying results from excessively thinking about our fears. Instead of wasting all that energy on worries and fears, why not put it to good effect instead?
Fear of failure will often bring with it paralysis and inaction because of that fear. Worse, the inaction will be in precisely the areas that you are afraid of, which are exactly the same places where you need to take action. In other words, fear of failure will probably bring you the failure that you are afraid of. To overcome the failure you fear, you must take positive action and as you do, the fear will fade.
So let's see how our student can be a bit more positive about his desire to pass his exams. As the lecturer pointed out, if he identifies weak areas in his knowledge, then the lecturer will help him to strengthen those areas. So the first thing is to identify his weak areas and his strong areas. He needs to know what he knows and to compare that to what he needs to know. That way he can work out a plan of action.
Many schools, perhaps most, imply that the only way to pass an exam is to know everything that there is to know about the subject. The idea is that if you know everything, then you can answer any question that you might get and so you are certain to pass. But then there is the saying, 'Jack of all trades, master of none'. If you attempt to learn everything in the time available, you will perhaps end up mediocre at everything. Wouldn't it be better to concentrate on some questions and therefore to know them really well? The only problem with this approach is the possibility that the areas you know well might not turn up in the exam. So what can you do?
When I was at college, I had the luxury of knowing that my lecturers were the people who set the questions. So during the year, I listened carefully to what they said. Most of them gave tips that might have been, "You should make sure you know this". They usually only said that once or twice while they were teaching the subject concerned and they would never say it again. When I heard that, I would make a note that there would be a question on that subject in the exam.
Even if the questions were set independently, you can still get a good idea of the likely questions if you can get hold of past exam papers. Get hold of as many as you can and analyse which questions came up and when. Look for patterns. If a question comes up every year, then you can be fairly certain that it will come up this year as well. If a question comes up every other year, then you can decide whether it will come up this year or not. In short, by analysing past exam papers you can get a very good idea of the likely questions that you will face when you take the exam. Use every piece of information available to you to determine which questions are likely to be in your exam paper and which ones will not. Of course there is no certainty, but you can determine the probability nevertheless.
Once you are as certain as you can be about the likely questions, start looking at your notes. Discard any notes on questions that you think are unlikely to be in the paper as well as any notes on any area where you think you are so weak that you would struggle to answer a question on it. What you are left with are those areas where a question is likely and you think you have a chance to get a good mark. Study those areas of your notes exclusively. Get someone to test you on them if you can.
As the exam draws near, make notes about your notes! Get a separate piece of paper and write down bullet points of the important items. By now, you should know the subject well enough so that you only need a reminder of a basic fact for you to be able to remember the whole subject. For example, in mathematics, I might write down a formula. So long as I knew the formula, I could answer the question. In history, I might write down salient dates, people and related facts. In this way, I got every subject down to a single side of one piece of paper. Instead of being overloaded with an endless amount of information, I only worked with what I needed to know. Keep your notes with you at all times and keep reading them. Take them to the exam with you. Don't use them to cheat because if you are caught you will fail. Just read them up until you go into the exam room. If they give you a piece of paper for notes, as soon as you can write notes about formulae or dates or whatever you need for the exam. Do not copy your notes, which should be hidden away by now. Write down only what you can remember and make it obvious that this is what you are doing.
Finally, in the exam itself you should be aware of how many marks you get for each question. Spend time at the beginning of the exam going through all of the questions. If you think you can't do a question, don't waste time attempting it. Work out where you can gain the most marks and do that first. Do the questions in whatever order will give you the most marks. Work out how long you can spend on each question and when you reach your time limit, stop and move on. If you continue, you are not gaining marks. Always work where you can get as many marks as possible in the shortest time. Do that and you will hugely increase the chances that you will pass the exam.
Important Information About Bullying In Schools And How You Can Stop It
Bullying in schools has been a long time problem all over the world. Bullying has now become a major problem in schools and has severely affected the lives of many children. Bullying in schools can cause a child to live in fear. It is not surprising that a child who is being bullied will lose the desire to go to school. School becomes a place of torture for the child because of the bullies. In schools it must be stopped so that children can go to school happily without the constant fear of bullies.
This thing in schools has lead to more serious effects such as suicide. There have been incidents of children committing suicide after being tormented by school. This can take many forms other than physical bullying. There is also such a thing as verbal bullying. Bullies can spread vicious lies about your child which can cause your child to feel like an outcast in school. Bullying in schools commonly leads to depression. There are times when we do not recognize bullying when we see it. You need to be able to recognize that your child is a victim of bullying in schools to be able to do something about it.
If you are a teacher, talk to your students about bullying. Teach them about the bad effects by role playing. Have one child play and another child play the victim then have them switch roles. It is always best to teach by example This is to show a bully student of yours what it feels like to be bullied. They may learn to be more tolerant once they feel what it is like to be tormented.
Parents should be greatly involved in the effort to stop bullying. The values that we teach our children at home shape them as a person. Teach your children that each person is special and unique. Just because one of their classmates is different that does not make him weird. Teach your children to accept the uniqueness of each person. As a parent, you should also set a good example for your children. It is always best to teach by example. It not only happens to small kids. Some adults are also bullies among their peers. Your child may have learned it in school by watching you. Before you teach your child that bullying in schools is bad, make sure that you are not a bully yourself.
The Modern World
The United States of America: The United States of America, after the Civil War, made tremendous progress in the field of science, technology, agriculture, industry, transport and communication. By the beginning of 19th century, The U.S.A emerged as a powerful and industrialized nation. The U.S.A entered the First World War after the British ship Lusitania was sunk by the Germans. The U.S.'s help was crucial for the victory of Britain and France. The 'Great Depression' which began in 1929, was a severe blow to its economic growth. The prices of commodities fell and income of people also decreased. In 1933, Franklin D. Roosevelt was elected as the president of the United States of America. He introduced the economic reform called "The New Deal". According to this policy, the industries reduced the number of working hours and increased the pay for workers, compensation was paid to the farmers and the unemployed. Within a decade, the U.S.A. once again emerged and became an economic power.
Russian Revolution 1917 and Soviet Union- Socialism: In the 19th century Russia was ruled by an absolute and autocratic emperor called the Czar. His regime was repressive and cruel. People had no rights. The condition of the peasantry was miserable. The feudal lords exploited the peasantry. The situation worsened when Russia was defeated by Japan in 1905. There were wider spread revolts in every part of Russia. The Czar suppressed the revolts. It was during this time that the Communist leader Lenin organized a band of revolutionaries. They were influenced by ideology of Karl Marx. Marx advocated the dictatorship of proletariat as a solution to all feudal problems. They set up revolutionary communities throughout Russia. They overthrew Czar's regime in 1917 and Russia became the Soviet Union. This is called the Great Russian Revolution. The government took over the land and all other means of production. Peasants and workers became government servants. It was dictatorship of a single party. Lenin who brought about the revolution was succeeded by Stalin in 1924. Through his year plans, Stalin expanded the industries. Stalin ended private farming and introduced collective farming. Stalin also developed military power and made the Soviet Union as powerful as the United States. During the Second World War, initially the Soviet union was on the side of Germany. When Hitler attacked the Soviet Union, the latter joined hands with England. Stalin became a leader of Allied Powers together with Franklin Roosevelt and Winston Churchill. In the victory of the Allies, the Soviet Union played a major part. After the war the Soviet Union made rapid progress in scientific research and became a prosperous nation. It entered space research when it launched space satellite 'Sputnik' in 1957. It produced atom bombs and competed with the United States in arms race. But within four decades, after the end of the Second World War, the Soviet Union lost the great influence that it had in Eastern Europe. The rule behind the 'iron curtain' of the Soviet Union ended and today the Soviet Union is once again called Russia and it is a vibrant democracy.
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