# Efficiency of compressors

chhitiz
for a given volume of gas,is it more efficient to compress smaller volumes of gas more no. of times or compress large volumes of gas less no. of times?

Homework Helper
Gold Member
Hi chhitiz,
I'm not sure how you mean that. Consider for example, the compression of a gas from a lower pressure, P1 to a higher pressure, P2. Let's consider a single stage of compression that has a given iesntropic efficiency. Regardless of how much gas (how much mass) is compressed, the states the gas goes through, including how much energy is needed per unit mass, is the same.

The other way your question might be interpreted is if we have the same compression from P1 to P2, but we compress it in steps and then cool the gas off before compressing a bit more. In this case you'll find that this 'gradual' compression in which heat is being constantly removed during the compression process, is more efficient than the compression of the gas in a single stage. In other words, compressing a gas while removing heat requires less energy per unit mass than compressing the gas without removing heat. Compressing gas isothermally is the most efficient, though it isn't realisitic to see that ideal type of compression.

chhitiz
Hi chhitiz,
I'm not sure how you mean that. Consider for example, the compression of a gas from a lower pressure, P1 to a higher pressure, P2. Let's consider a single stage of compression that has a given iesntropic efficiency. Regardless of how much gas (how much mass) is compressed, the states the gas goes through, including how much energy is needed per unit mass, is the same.
i mean, that if we have V volume of gas to be compressed from p1 to p2, is it better to compress V/2 volumes 2 times or V/4 volumes 4 times. i mean, of course, the energy needed per unit mass will be same for a process, but how will efficiency of the compressor be affected?
like, for a reciprocating compressor, will it be better to compress smaller volumes, so there is lesser temperature rise and quick dissipation of heat or compress large volumes, as the energy lost in friction is less.

Homework Helper
Gold Member
If the energy needed per unit mass is the same for the two processes, then the efficiency is the same. Regarding recips, that's a good question. I don't know... There are some generalizations you can make such as:
1. Piston ring friction per unit mass compressed will decrease as volume displaced increases.
2. Heat transfer out of the gas (during the discharge stroke) per unit mass compressed will decrease as volume displaced increases.
3. Heat transfer into the gas (during the suction stroke) per unit mass compressed will increase as volume displaced increases.
4. Piston ring leakage per unit mass compressed will decrease as volume displaced increases.

There are other factors such as reciprocating speed which generally decreases as volume per displaced increases, so in the end, I suspect there won't be a big difference in isentropic efficiency when comparing small machines to larger ones.

chhitiz
no, i guess not. i was just wondering how they decide the initial volume of gas to be compressed in an ic engine. is it for max. efficiency, or that much fuel burned pushes the piston for the required stroke?

Gold Member
no, i guess not. i was just wondering how they decide the initial volume of gas to be compressed in an ic engine. is it for max. efficiency, or that much fuel burned pushes the piston for the required stroke?

The basic consideration is the desired power characteristic. Given an approximate air fuel ratio, it's pretty easy to calculate the mass of air required for a certain amount of torque.

chhitiz
The basic consideration is the desired power characteristic. Given an approximate air fuel ratio, it's pretty easy to calculate the mass of air required for a certain amount of torque.

could you tell me the exact calculations that show how much torque is generated for a given mass of air-fuel mixture of particular composition?

blessie
An engine is used to lift a 2700 kg truck to a height of 3.0 m at constant speed. In the lifting process, the engine received 3.3x105 J of heat from the fuel burned in its interior. What is the efficiency of the engine?

chhitiz
2700x3x9.8/3.3x105=229=impossible