But surprisingly, the second-worst was the two variable pump setup that pretty much everyone uses. Configurations with three pumps or for very small systems, two pumps, and a hydropneumatics tank proved to be better than two variable speed pumps.
All the three pump configurations were fairly close to one another in cost. The reason they were so good was that their best efficiency point could be matched fairly well with the most common demands with only one pump running.
For those cases, where demands were high, simply turn on the second pump until the demand drops. A lot depends on the actual flow distribution that the pump station will encounter. If the peak flow is gpm and the average flow is gpm, then any configuration will work well. However, if the peak flow is gpm and the average flow is gpm, then it will be virtually impossible to make the two-pump designers work efficiently. In an earlier blog Abusing Affinity, 4 March , I mentioned how the calculations of head and efficiency for variable speed pumps are governed by the pump affinity laws, and sometimes those folks selling VFDs misrepresent the affinity laws to make their product look more beneficial than it really is.
Another issue is the fact that the VFDs themselves introduce a certain amount of inefficiency. They work by breaking up the smooth sine wave of alternating current into pieces and reassembling those pieces in a sine wave with a lower frequency, thus varying the pump speed. How to Make Your Decision. How do you decide on the best configuration for your pump station?
What you need to do as a conscientious engineer is try many designs to arrive at the best pump selection. The most tedious step in a good evaluation is calculating the life-cycle energy cost because the pumps must work over a range of conditions.
In my early days, I did these calculations manually, and you needed to make a lot of simplifications to make the effort feasible. Walski, T. Blog Infrastructure Insights. If at an instant in time the distance traversed is divided by the arbitrarily small traversal time, the instantaneous speed is obtained. It can be written as a derivative. The instantaneous speed of an object can be calculated by using limits.
Suppose one decides to drop a ball out of his second-floor window. There is a formula that tells how far it will fall after a given numbers of seconds ignoring the air resistance.
So the average speed may be calculated as the distance divided by the time i. The instantaneous speed of an object can also be determined through calculating the average speed over a short distance and time.
In daily life, it can be said that the faster object has a faster speed. Constant speed neither increases nor decreases over time but remains constant. Earth revolves at a constant speed around the sun in a certain orbit. Satellites also revolve at constant speeds in a certain path. Instantaneous speed is a certain speed at a specific instant in time. At a certain instant, what the speedometer reads is the instantaneous speed.
Difference Between Constant and Instantaneous Speed. Difference Between Similar Terms and Objects. MLA 8 Kaushik, Nimisha. Talking about runaway diesels, inert gas CO2 injection is also starting to become a more popular method than slamming the intake air shut-off closed on heavy duty hazardous environment diesels, as it avoids some of the issues with sudden vacuums damaging seals, etc. By injection, I mean injection into the intake manifold to stall the engine.
There are a lot of sound reasons to maintain constant speed RPM. My favorite used to be the C Hercules aircraft - considered by many to be the world's most successful cargo aircraft.
During operation, the four turbojets run constant speed no matter what activity if performed. To adjust power output, the pitch of the blades is moderated, and fuel consumption rate is adjusted to present specific available power to the blades. Four very strong engines on an airframe are very easy to coordinate. Vibration quotients throughout the craft are more stable, and the engine life is elongated through constant speed. Ancillary systems are tuned to that RPM, and the system is more efficient.
For as much as any power system may be "notched" into a specific portion of its power curve - never leaving that notch, constant RPM can serve as an endurance factor.
If the notch is selected in the most favorable portion of the power curve for application, you can expect the maximum-available power at peak load, and moderate economy during light load. Idling is a loss factor, unless the controls 'step' from true idle to power. That's an excellent point about turboshaft engines or turbine engines in general. A recip diesel engine can readily operate at a wide range of speeds, but they are often optimized to operate at a specific speed when used to drive a synchronous generator or pump.
If you operate the engine much outside of this speed range, its combustion can become unstable. With turboshaft engine helicopters, the engine operating speed range becomes even more restricted due to dynamic issues with the rotor system. This is true, in that these applications tend to run at a constant speed for an extended period of time. However, the speed setting may be changed from time to time in order to adjust to changing operating conditions or external demand.
That's not what I meant - there are actual constant speed diesels for mechanical drive applications which have a non-adjustable fixed operating speed rpm in one example. These engines can only operate at low idle for warmup and shutdown and the rated speed. The engine software and legal requirements emissions certifications prevent operation at any other speed. Lest you go away thinking that emissions certifications were the only thing that drove creation of these engines, the application existed before the emissions-certified engine was developed or it wouldn't have been.
A constant-speed engine can be tailored to give better transient load response at the desired operating speed, which helps in gensets and also in some mechanical drive applications. Red Flag This Post Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework. Learn methods and guidelines for using stereolithography SLA 3D printed molds in the injection molding process to lower costs and lead time.
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Difference between constant speed and variable speed engines Difference between constant speed and variable speed engines Hello everyone, I wanted to what exactly is the difference between the constant speed and the variable speed engines? RE: Difference between constant speed and variable speed engines The throttle position. RE: Difference between constant speed and variable speed engines Thank you RE: Difference between constant speed and variable speed engines Most constant speed engines are running generators where it is very important to keep the speed constant to produce the desired 60 or 50 Hz AC waveform.
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