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When you compress a gas, is 100% (assuming a perfectly efficient motor doing the compression etc) of the energy you expend during compression transferred into extra heat energy in the gas?
russ_watters said:Actually, losses are generally all heat, but in this case the answer is so obvious that I'm not sure the question is worded correctly: If all the energy expended went into heat, there'd be no compression. Assuming that's not what was meant, some of the energy expended goes into compressing the gas and some is lost as heat by the motor/pressure efficiency. In addition, any heat generated by the compression (via the ideal gas law) may be lost, depending on the specifics of the situation.
I'll attempt to re-phrase. Let's say you have a syringe blocked off at the needle end, fully let out, filled with air. If you put 1000 J of energy into the syringe , by applying a force of 1000N over 1 meter (arbitrary numbers, average force) would the internal heat energy of the air inside the syringe increase by 1000 J? I'm just trying to make sure there isn't anything I'm forgetting.
russ_watters said:Actually, losses are generally all heat, but in this case the answer is so obvious that I'm not sure the question is worded correctly: If all the energy expended went into heat, there'd be no compression. Assuming that's not what was meant, some of the energy expended goes into compressing the gas and some is lost as heat by the motor/pressure efficiency. In addition, any heat generated by the compression (via the ideal gas law) may be lost, depending on the specifics of the situation.
russ_watters said:Actually, losses are generally all heat, but in this case the answer is so obvious that I'm not sure the question is worded correctly: If all the energy expended went into heat, there'd be no compression. Assuming that's not what was meant, some of the energy expended goes into compressing the gas and some is lost as heat by the motor/pressure efficiency. In addition, any heat generated by the compression (via the ideal gas law) may be lost, depending on the specifics of the situation.
Q_Goest said:Great question, the answer is a bit counter intuitive. But first you need to make some assumptions.
Assume:
1) 100% isentropic efficiency
2) Heat Added = heat required to reduce discharged gas back to inlet temp.
3) No mechanical losses in compressor
4) No unrecoverable pressure losses in any portion of the flow stream
Needless to say, those are some rather broad and unrealistic assumptions, but what it says is that, with the exception of losses due to friction or other mechanical or fluid losses, we will compare the power needed to compress a gas isentropically to the energy removed to return the flow to the original temperature after compression.
I checked the results of this using a computer program that I use all the time where frictional and other losses can be added. I simply removed all losses to calculate the following.
Depending on the gas, the heat removed may be higher, equal to, or lower than the power required to compress the gas. They will be roughly the same.
In real life, losses typically result in more power being needed than is removed.
If you make different assumptions, the answer will also change.
You can start with the ideal gas law: pv=nrt. Or you can read the energy numbers directly from a table of those gas properties. Here's one:BlueWaterGuy said:Are there any formulas to actually calculate to know if the energy in a cylinder of compressed gas/s (air) contains more energy or less energy than was used to compress it. doe is depend on nature of gas being compressed. A cylinder of compressed O2 by itself clearly would have a higher energy content than the energy consumer in compressing it, right. But I don't get the sense, and can't calculate the energy in a cylinder of compressed air, a mixture of gases. Any Ideas or formulas out there?
I'm not exactly sure what you mean by that. Are you looking for the fraction of energy dissipated as heat and the fraction that is stored in the compression? Or are you trying to compare one gas to another?I'm trying to calculate if there is consistently, in all cases, less energy in the compressed volume then energy expended and lost in the performance of the compression.
The energy stored when compressing a gas refers to the potential energy that is stored in a gas when it is under compression. This energy is a result of the work done on the gas to compress it into a smaller volume.
The energy stored when compressing a gas can be calculated using the equation: E = P * V, where E is the energy stored, P is the pressure of the gas, and V is the volume of the gas.
The energy stored when compressing a gas is affected by the pressure and volume of the gas. The higher the pressure and the smaller the volume, the more energy will be stored in the gas.
When the compressed gas is released, the potential energy stored in the gas is converted into kinetic energy as the gas expands and does work on its surroundings. This can be seen in the form of the gas performing mechanical work or generating heat.
The energy stored when compressing a gas has many practical applications. It is used in engines to produce mechanical work, in refrigeration systems to cool substances, and in pneumatic tools to provide power. It is also commonly used in gas storage systems for energy storage and transportation.