Electric or Steam driven Air Compressors? 1

[BioBob]

I have a design conditon where I have to decide whether to use electric or steam driven air compressors.

The requirement for electric is 300 250HP (200kW) motors that will move 1200 CFM at 90 PSIg. If I run them 24/7/365 at $0.08/kWh (an annual average), the cost is 200*24*365*0.08 = $140k/yr. 300 of them will cost $42 Million/year to operate. They also cost $100k each, but that's a one-time expense at $30 Million.

The steam turbine that generates the electric in the plant is available and I can draw off steam for the compressors but I'm at a loss to do a comparison since I have not seen any figures for steam driven air compressors to do a comparison.

My questions are...
1. Is there a preference bewteen electric or steam (if steam is available)?
2. How much steam am I going to need to drive steam driven air compressors to move this kind of air? Am I going to need more than what's available at a 500MW Power Plant?
3. How much are steam driven air compressors compared to their electric equivalent?

Thanks,
bob...
bob@hytekbio.com
HY-TEKBio
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[zdas04]

The choice between steam and electric usually comes down to either a loss analysis or an excess capacity analysis. The $0.08/kW number includes a significant allowance for generating losses, line losses, etc. A steam driven compressor will have significantly less total losses and will generally result in a lower cost for the same work if the existing boiler, pumps, etc. are adequate to the task.

Now, why are you looking at THREE HUNDRED 250 hp motors? I've got to assume it is a geographical distribution issue, but my god. There has to be a way to do what you are planning to do with 75 1,000 hp units or 15 5,000 hp units. If you are going to use steam as an alternative power source then the hp will have to be close together (or the heat loss will kill you) and you might be able to do it with 3-4 steam units. Something like half the cost of a compressor package is stuff like the skid, suction/discharge vessels, etc. So if you can combine 4 $100k skids into one $300k skid you are way ahead.

With $30 million in capital in hand there are a bunch of companies that would love to help you balance the trade offs and I would be really surprised if the recommended configuration was three hundred units.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[dcasto]

The steam turbines will require ove 2,000,000 lbs/hr of steam in non condensing turbines and you'll have 2,000,000,000 BTU/hr of condensable steam energy.

If you used condensing steam turbines, its only 1,000,000 pounds per hour and you'll need 150000 gpm of cooling water.

 

[BioBob]

David/dcaasto,

Thanks for the quick reply.

Our design requires 300 tanks of liquid all linked together in a series and we are passing 1200 cfm of gas through the series of tanks. I have to get the gas from teh top of the 1st tank and into the bottom of the next tank and overcome a 40 foot column of water plus a sparger back pressure that's why I need 90psig at the bottom of each tank. I must contain the gas through the entire tank system.

I can make a minor change in teh design and get away with oonly 60psig at the bottom of each tank but that's it.

The 250HP motor was based on a 120psig at the bottom.

We're sitting at a Power Plant so I have pressurized steam from the low pressure turbine output before it goes into the condensor, but I believe that's at 90psig and I'm not sure how much volume I have to push 300 pnuematic motors.

Quite a predicament.

bob...
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[zdas04]

You've explained the 90 psig, but not why you need 300 compressors. It sounds like you need to get pressurized air to 300 tanks, but doing that with a single compressor (or a few compressors) and a header still sounds possible.

David

 

[BioBob]

zdas04,

It's more than just getting compessed air to each of 300 tanks. We have a containment of air that starts at the first tank and is passed through each of the 300 tanks and is vented at the end. That gas has to be maintained separate.

How do I get that gas from the top of 1 tank to the bottom of the next with enough pressure to overcome the column of water pressure and the sparger back pressure without another compressor?

We were discussing this problem yesterday in some detail and the suggestion was made to use one compressor at teh head of teh series of tanks to generate a head of pressure and feed that to every pnuematic air compressor at the top of each tank. I'd have to find a source of pnuematic air compressors and determine how much pressure would be required to operate all of the compressors to find out how large the main air compressor would have to be. Using air driven air compressors would eliminate the electric cost to run each motor but I'd still have to pay for it at the head end if I use an electric air compressor there? I'm still trying to find out how much pressure and volume of air I have coming out of the final low pressure turbine to see it there's enough to do the job. My gut reaction is "not even close".

We're still trying to figure this one out. We have plenty of time... so far.

bob...
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[dcasto]

draw this out, my mind says you only need to lift fluid 40 feet one time.

 

[BioBob]

Drawing attached

 

[zdas04]

That is clearer. Thanks. Are you getting a process benefit from using the same chunk of air over and over again or is it just conservation of pressure? With 120 psig at the bottom of the first tank what pressure are you predicting at the top of the first tank?

David

 

[dcasto]

You will need a 20 psi differential pressure blower, it'll be under 30 HP

 

[BioBob]

zdas04 - We are not getting any benefit from using teh same air if you're talking about the air we're moving through the tanks. Let's say it's toxic and we have to contain it throughout the series of tanks until it's all consumed in teh process. That's not exactly accurate, but it's a good analogy.

dcasto - I believe you've hit the nail on the head! We really only need to over come the head of water pressure of about 20 psi between the top of the first tank and the bottom of the next tank. Who makes comperssors that will take me from 40psi to 60 psi at 1200cfm? I need to talk to them to find out what those compressors cost to purchase and cost to operate. That way, I only need to generate 60psi at 1200cfm with the "head" compressor going into the first tank in the series. That should drop my power requirements down considerably.

Who should I call???

bob...
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[zdas04]

You could also use an ejector, but I think the cost would end up being higher than the blower.

If the toxicity must be "consumed in the processes" then you are getting a benefit from the tanks being in series.

David

 

[dcasto]

I googled natural gas blowers and got this

http://www.spencerturbine.com/products/blowers/custom-blowers-gas-boosters/default.html

here is a site that gives some insite.

http://www.hydrocarbons-technology.com/contractors/compressors/gas_compressors/

This should get you going....