2 replies to this topic

[Alexandru^]

dear all,

 

I meet some difficulties in understanding what happens inside a multistage reciprocating compressor.

The problem starts from the fact that the compressor's mass flow rate aspired (value measured by a mass flow meter) is over 30% higher than the mass flow rate discharged by compressor (value provided by the manufacturer, and also calculated by having the amount of gas inside the tank in which  the air is stored (PV=mRT -ideal gas equation) and measuring the time to reach from the state 1 to the state 2 we can calculate an average mass flow rate,  values that are proving to be the same as those offered by manufacturer)

Normally I expect this mass flow rate being higher because the compressor is equipped with an air filter and a dryer system in order to evacuate the water condense resulting from air cooling using intercoolers, but how much? with 2-4% not 30% .

The content of water vapor in the air means almost nothing as long as in ambient conditions 1 kg of dry air contain about 10-15 grams of water vapors.

Another thing that I want to know is how could I estimate the evolution of the air temperature inside the storage vessel? considering two hypothesis: 1) the air leaves the last cylinder at a constant pressure and cooled near to the ambient temperature.

2) the discharge valve of the last  cylinder works on differential pressure, and also in this case the air is cooled  near to the ambient temperature.

 

regards,

[Art Montemayor]

Alexandru:

 

Your post is very hard to understand.  You should furnish a detailed flow diagram of what you are proposing with the flow rates, temperatures and pressures all indicated in the related flow lines.  show how your reciprocating compressor is connected with the tank you mention.  It is very difficult to follow your flow description without a detailed flow diagram.

 

A reciprocating compressor is designed and rated using VOLUMETRIC flow rates - not mass flow rates.  So you should forget about any mass flow rates related to the machine.  A reciprocating compressor is a POSITIVE DISPLACEMENT machine.  That means that it displaces volume, not mass.  You should be basing your calculations on a volumetric flow through the machine.  Because it is a positive displacement machine, WHATEVER GOES INTO THE FIRST STAGE OF COMPRESSOR MUST ALSO EXIT THE LAST STAGE.  Therefore, what you are stating about the suction flow being greater than the discharge flow cannot be true in a positive displacement machine.  Your sketch should clearly show this fact.

 

Await your quick reply.

 

P.S.  Please stop making multiple posting of the same topic or thread.  I deleted your prior thread on the same topic.

[Alexandru^]

Art,

sorry for double post, that was not my intention.

 

Now, what I want to do is to store energy in a form of compressed air. I have read in plenty of articles that 70% efficiency can be reached for an advanced adiabatic compressed air energy storage system. 

For this reason I try to understand how my compressor (http://www.bauergrou...us/TMV3-1_0.pdf) works. So far, few days ago I was sure that the pressure discharged by compressor is all the time the same (constant pressure) and in this case I have concluded that using such a compressor the efficiency of a CAES system will be very low.  

I will never succeed to maintain a constant pressure into the tank during expansion process, or maybe I could if the volume of my storage vessel is variable (but this would involve a hard work of research).

 

Why to discharge the compressed air from a high pressure into the tank  when thisl is empty? For a higher efficiency the cylinder's discharge valve should operate function of differential pressure in order to reduce the energy consumed during compression process.

Measuring the power consumed by the compressor to fulfill the tank I noticed that the curve of power shows a constant character for a period of time and after that it shows a upward slope, which makes me think that only the discharge valve of the last cylinder operates with differential pressure.  (I have tried to understand this also from attached file)

 

Talking about flow rate , I don't know where I am wrong, and why that difference.

 

Ps I have tried to attach two charts to this post but I haven't seen the attached bottom.

 

Kind regards,

Alexandru.