If you want to quantitatively compare Brake Horsepower demands from a recip and from a centrifugal compressor, you have to reveal the gas or vapor in question, the inlet and outlet pressures as well as the inlet temperature. If the gas is a mixture, you have to furnish the thermodynamic properties and any condensables in the mixture.
If you are only after a qualitative comparison, then the FAQ mentioned by zdas04 is probably all you need.
You haven't mentioned the purpose of the need, so I assume all this is for an estimated Bhp, correct? The quality of the estimate is going to be directly proportional to the amount of information and basic data you furnish.
We are in the process of selecting gas boosting and conditioning stations for a gas cum naptha based power plant for a Gas Turbine Generator.
The Natural Gas specification is mentioned below:
Quantity - 66000 nm3/hr
Net CV - 9507 - 8913 kcal/m3
Specific Gravity - 0.6517 - 0.6982
Delivery Pressure - 2 to 5 kg/cm2
Temperature - 20 - 40 deg cent
Dust grade provided at suppliers end - loading of 0.1 mg/sm3 and max particle size of 50 microns
Normal gas pressure required at GT supplier terminal point - 29.5 bar
Gas Supply Temperature point at GT Supplier's terminal point - 11 deg cent more than the dew point with respect to gas supply pressure
Composition of NG
Methane CH4 - 86.84%
Ethane - 7.34%
Propane - 2.65%
We need to select on the basis of Project life cycle Cost Concept that is it would include
Design Alternatives
Capital Costs of Design Alternatives
Energy Cost Evaluation
Economic Life Determination
Maintenance and Replacement Considerations - Annual Recurring Maintenance and Non-Uniform Repair or Replacement
Comparing Alternatives with Different Economic Lives
Basically it would more focus on the operating expenses as energy consumption etc. Kindly advise as whether we should go in for Reciprocating or Centrifugal and other important/advantageous features of the advised compressors.
Easy. For each, calculate compression ratio, and BHP/throughput unit. For recips, use fuel of about 7200 BTU/HP/Hour. For centrifugals, use fuel of about 7800 BTU/HP/Hour. Done.
Hooken,
This is interesting. I was looking at the Ajax (integral recips) book today and saw that at 50% load the fuel-consumption number ranges from 15,000 to 22,000 BTU/hp-hr depending on frame. At 100% load it is "down" to 10,000 to 16,000 BTU/hp-hr.
For seperables, I have a hard time understanding why a Waukesha F-18GL would use 8% more fuel per BHP-hr (I suppose in addition to the gas-effeciency hit you're already taking?) driving a centrifugal than a recip. Any clarification?
Also, looking in both the Cat and Waukesha books, the fuel value varies pretty widely from frame to frame.
Reciprocating - Power Consumption at Part Loads are substantially Lower as compared to that at Full Load. Recip Compressor having Lower Power Consumptions at part Loads have overall less Energy Bills.
Centrifugal - Centrifugal Compressor has Marginally Lower Specific Power Consumption only at Full Load, Overall Energy Bills are Higher due to Higher Power Consumptions at all the Part Loads.
apalod,
Interesting, I guess I should just discard this wall of reference material I've collected over the years 'cause you say "it is true". I guess I'm just a fuddy duddy since I'll probably keep them. Can you point me to a reference that supports your contentions?
My experience tells me that at part load an engine has higher specific fuel requirements. The manufacturer's references in my office support that. This is true regardless of whether the engine is pushing a GenSet, a bulldozer, a recip, a screw, or a centrifigual compressor.
Different sorts of compressors require different amounts of hp to compress the same gas volume from the same suction conditions to the same discharge conditions. The fuel required to do that work is completely dependent upon the the amount of hp transmitted through the shaft, and has nothing to do with what kind of widget is at the other end of that shaft. Consequently, I would expect that for the same suction/discharge/volume a centrifigul would have a slightly lower specific-fuel requirement than a recip (because it is using more hp to compress the chunk of gas) while burning slightly more fuel (again, because it is using more hp).
imok2,
Did you really mean "centrifugals"? I've seen a lot of rotary screws used in air conditioning, but I've never seen anyone claim a centrifugal compressor was more effecient than any other technology.
I put in various max interstage allowable dP and temp values (to keep the number of stages under 20)and got about the same hp requirements. The Gas Hp required is 233 hp.
Then I went to Gardner Denver's RotoSize Program and let it pick the compressor for the same conditions and got 181 hp for a flooded screw.
Using an Ariel compressor wheel calculates to a recip that would require a 2 stage machine and 165 hp.
All of the values are just gas hp and disregard any interstage cooling hp load (for the centrifugal and recip) and pump load (for the screw).
