5 replies to this topic

[MarianaB]

Hello,
I have some doubts regarding the performance of a reciprocating compressor

Propane vapors are being compressed in the cylinder.

Suction pressure: 14.9 psia
Discharge pressure: 49.7 psia

Suction temperature: -40°F

Piston displacement: 2730 cfm
Volumetric efficiency: 63.4%
Capacity: 1731 acfm

 

Since reciprocating compressors displace the same volume, my question is the following: what would happen if the flow rate decreases to 500 cfm? for example. 

I've read in other topics that, since PV = mRT, the suction pressure would decrease to compensate for the lower mass available. The compression ratio would increase and so would the discharge temperature.

 

I've also read that there are different methods to control the capacity of a cylinder, such as the use of pockets. I can regulate the capacity in 3 steps: 0%, 50% and 100% . However, the minimum capacity I can obtain would be 866 cfm (50%), it's still higher than 500 cfm. 

 

Would another regulation method be needed? 

Is there a way to estimate the suction pressure or the compression ratio? discharge temperature?

Is it very unsafe to use this machine, with a capacity of 866 acfm, to compress 500 cfm?

 

English isn't my first language, so please excuse any mistakes.

 

Thanks in advance.

 

 

[Bobby Strain]

There is a valve to recycle the hot discharge to the suction scrubber to control the suction pressure. Since the discharge vapor is superheated, the low pressure recycle gas is cooled by injecting liquid propane from the accumulator into the flow line to maintain the desired suction temperature. The recycle valve is always active. The unloaders and clearance pockets can be automated or manually activated.

 

The suction pressure should always be maintained above atmospheric pressure to avoid vacuum conditions and ingress of air through connections and packing.

 

If the lower flow is to be the normal flow, the gears in the gearbox could be changed to run the compressor at lower speed. Or, if the compressor is motor driven the motor can be equipped with a variable frequency controller to reduce the motor speed.

 

If the flow reduction is only occasional and the normal higher flow is needed, then you would simply operate the compressor with 50% unloading, pockets open as available (or add one), and recycle.

 

If you search the web with Google, you will find lots of tutorials on reciprocating compressor operation. That's what I suggest you do. And you should find some calculators on the web, too. Don't trust them to be without flaws, however. But the calculations are at least indicative of operations. Start here: http://checalc.com/calc/compress.html. There is another site that provides all the equations used in the calculations, but the URL eludes me.

 

And, your communication is excellent. PV = ZNRT where Z is compressibility and N is the molar quantity = M/MW where M is mass quantity.

 

Bobby

Edited by Bobby Strain, 11 December 2015 - 12:36 PM.

[S.AHMAD]

Dear Mariana B,

What you need to do is a mass balance and the application of the gas law in real life. I know you have learnt the mass balance and the gas law at the university class, but now you need to apply the knowledge in the real industrial environment.

1. My suggestion is that you do the calculations required using excel spreadsheet and attached for other members to review and comments as required. Let me just refresh and hopefully my comments open up your mind.

 

1. The compressor capacity is 866 acfm. and the flow rate available is only 500 acfm based on suction pressure of 14.9 psia  Under the situations where the flow rate is the same or more than the compressor capacity, then initial compreaaing pressure shall be 14.9 psia. However under the current conditions, the flow rate of 500 acm is at lower than compressor capacity, then what shall be the initial compressing pressure?. Use the gas law P1V₁ = P2V₂ to determine the new initial pressure. Please take note the discharge pressure can be the system requirement at 49.7 psia. However if there is significant frictional pressure drop of the compressor discharge piping you need to determine the new pressure accordingly. In this case I assume is the same at 49.7 psia. Initially the compression ratio is 49.7/P₁ but now the compression ratio change to 49.7/P₂. I forgot to mentioned that V₁ = 500 and V₂ - 866 acf. With these information then you can determine what is the new compressor discharge pressure using PVⁿ = constant.The value of n can be determined from existing data since you know suction pressure and temperature and discharge pressure and temperature ten can be calculated.

 

Hope the above short comments open-up your mind.

 

Good day.

 

S.Ahmad

[S.AHMAD]

Sorry... the last sentence should be " The value of  n then can be calculated"

[S.AHMAD]

can determine what is the new compressor discharge pressure temperature using PVⁿ = constan and PV/T = connstant.

 

Sorry for the mistakes.

[Art Montemayor]

Mariana:

 

After many years process designing, purchasing, installing, and operating many types and sizes of reciprocating compressors, I have submitted many recommendations and comments on our Forum on the topic.  If you use our Search Engine, you will find hundreds of threads on the general topic and lots of responses on the specific topic of capacity controls – how and when to use them.

 

It is very important to be specific about what you are dealing with when it comes to reciprocating compressors.  If you want specific answers, you must furnish specific data: what make, type of compressor do you have?  What is the driver?  What is the application?  Do you require INSTANTANEOUS or momentary capacity control, or do you need intermittent capacity reduction from time to time that will last for a reasonable period?  If this is a propane refrigeration system, it could be either.  Is the suction feed controlled or is downstream target the flow setting criteria?  In other words, does reducing the capacity cause a backup or pressure increase in your suction source?  What type of cylinders do you have – single or double-acting?  Are the cylinders balanced-opposed?  Are you in contact with your compressor’s manufacturer and have you this resource for loading information and recommendations?

 

Now to respond to your specific queries:
Since reciprocating compressors displace the same volume, my question is the following: what would happen if the flow rate decreases to 500 cfm?
Assuming you maintain the net discharge flow rate and pressure constant, the horsepower will decrease if you use clearance pockets or plug valves.  The power will remain the same if you use recycle valve controls (but with aftercooling of the recycle to match the suction temperature).  Admittedly, it will be difficult to maintain the suction of -40 oF, but that’s why I insist you tell us the application and how it is controlled.  Raising the suction temperature will decrease the suction specific density and increase the discharge temperature.  I would not control capacity this way.

 

the minimum capacity I can obtain (with clearance pockets) would be 866 cfm (50%), it's still higher than 500 cfm.
How do you determine the minimum capacity with clearance pockets?  Is this information from your manufacturer?  Depending on the application, you may have capability of decreasing the volumetric capacity of certain cylinders.  Anything affecting the cylinder and piston rod loads should always be approved by the manufacturer.  If you are not intimately familiar with reciprocating machines, the mechanical design, and the thermodynamics involved in their design, then you should get someone who is or become an expert yourself before modifying any operating features of the machine.  I have amassed a lot of information on reciprocating compressors and I have been working on putting a Tutorial for some years.  It is still “work in progress”.  If you request specific information, I may have it to share.

 

Would another regulation method be needed?
Again, this can’t be answered with the data you’ve furnished (or failed to furnish).  Read the many documents I have uploaded in prior threads that discuss and help determine the type of capacity controls available and recommended.

 

Is there a way to estimate the suction pressure or the compression ratio? discharge temperature?
Yes there is.  Refer to the attached typical plant process calculations I did for some hydrogen compressors many years ago and you will have your answer and an instructional way of how to estimate the operational capacities of a reciprocating compressor.  Nothing has changed in the last 75 years; calculations are still basic and done the same way.  Note that the operation of a reciprocating compressor is an ADIABATIC process (as opposed to Polytropic).  Therefore, there is no polytropic “n” involved in defining the operation.  The adiabatic “k” is what is still used because the operation is as close to an adiabatic one as it can get.  Every major compressor manufacturer I’ve dealt with in the past has attested to this.

 

Is it very unsafe to use this machine, with a capacity of 866 acfm, to compress 500 cfm?
Again, knowing that the cylinder and piston rod loadings are affected, it should be common sense that dictates that you should always be mindful of your compressor manufacturer’s basic mechanical design and OBEY WHAT IS SPECIFIED for your machine as maximum and minimum operating characteristics.  This has already been mentioned above.  It should be your job as an engineer to also understand the "whys" and consequences of straying from the recommendations.  For example, it is common sense to expect a compressor capacity and power demand decrease by simply reducing the machine's speed (i.e., when  a gas engine is the driver); however, the speed reduction also reduces the lubrication pump capacity because this pump is normally driven off of the compressor's crankshaft.  This could cause a major lubrication failure.  This should highlight the importance I give to an engineer knowing the intimate details of the mechanical machine he is modifying or proposing to modify.  In my opinion, being a chemical engineer does not absolve us of learning the skills involved in other engineering disciplines - mechanical design, instrumentation, electrical power, hydraulics, etc.

 

I would also add one further comment on the subject of reciprocating compressor capacity controls: don't forget that you are dealing with a POSITIVE DISPLACEMENT machine.  This type of machinery requires many times that the machine be started up and shut down in the UNLOADED condition.  This is a matter having to do with the mechanical design of the rotors, crankshafts, peak power demand, and lubrication issues involved in the specific installation.  So be mindful that you may be able to combine the capacity controls with startup and shutdown needs. 

 

 HydrogenCompression.xlsx   154.53KB   99 downloads

 Reciprocating Compressor Capacity Controls.docx   36.05KB   115 downloads