Do I lose energy through this expansion process?

[jcoots]

I have a case where I have several boilers at 430(psi) @ 550(deg F) feeding into a high pressure header. I also have a few boilers at 275(psi) feeding into a low pressure header that feeds the factory equipment. I am considering joining the two headers having an expansion valve between the two thus converting the 430(psi) to 275(psi). After looking at the situation I find that through the expansion I trade pressure and temperature for greater volume. My specific volume starts at 1.282 cu.ft/lbm and expands to 2.003 cu.ft/lbm, the temperature decreases from 550F to 524F. I also find that the enthalpy remains the unchanged at 1,274 Btu/lbm. Thus my conclusion is even though the steam expands cools off and looses pressure I have the same amount of energy per pound. I would like someone to verify this conceptually at least. Theoretically it seems right but practically I think something is amiss.

The reasoning behind this setup is: The factory is out of wack, they have several boilers of different pressures and ratings. We are trying to get the best use out of them as a whole. The factory equipment is designed for 275(psi) and they already have a high pressure boiler on a generator. We are trying to find the best way to utilize the high pressure boilers.

Thanks,

 

[TBP]

Check out back-pressure steam turbines. There could be a payback, depending upon hours of operation, and the lb/hr steam flow. The further apart you can get the inlet and outlet pressures, the better. The turbine could drive a generator, pump, compressor, etc.

 

[gkennedy34]

Enthalpy change across a valve is zero.

 

[vc66]

I believe that is part of the thermodynamic definition of an expansion (throttle) valve - enthalpy delta is zero. I could, of course, be mistaken.

V

Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."

- R. Buckminster Fuller

 

[rmw]

Expansion across a pressure reducing valve or orifice is essentially isenthalpic (heat loss through the valve body and noise generated is considered negligible or ignored.)

What type of "factory" is your factory? Depending on your process, TBP's suggestion is the best solution. You get the free work of the expansion of the steam and you get the steam exhausted at a pressure that can be utilized. Surely there is another pump or some other kind of rotating equipment that can be steam driven.

Can any of your current 275 psi users be converted to the 430 psi steam system?

rmw

 

[quark]

First, you should check whether your low pressure header can take care of extra steam capacity. Your high pressure steam is already superheated. The steam after reducing the pressure will also be in superheated condition.

You can either go for a back pressure turbine, if you steam flowrate is economical for that investment, or you can desuperheat the steam to get approximately 0.089lb/lb at saturated conditions.

Desuperheating the lowpressure steam (i.e after pressure reduction) gives slightly better advantage in your case as the sensible cooling is less.

 

[rmw]

Quark,

Your post assumes that the lower pressure steam system doesn't want any superheated steam. If this is the type of system that I have frequently seen in more than a few sugar factories, (a mix/mash of boiler and steam header pressures as older plants were added on to) there could well be turbine drivers on the low pressure system that would want superheated steam.

rmw

 

[omarbakr]

Well this is called Throttling, It's a process that has a constant enthalpy but its main idea is that it reduces preassure at contant enthalpy. it can be seen in auxillary lines that takes steam from high pressure from a certian boiler for a low pressure required steam. well to utilize the steam you have you can lower the heat rate input to the small boiler and make the steam of the high pressure boiler enter the steam drum of the smaller boiler where it will mix with water as superheated steam and eventually steam and water mixture will increase steam flow rate with less energy from the small boiler. or it's better if you have a rise header for the connection to go to it not the steam drum.

 

[quark]

rmw,

You are correct. I assumed saturated conditions as the temperature of low pressure steam was not mentioned.

I Wish a Happy, Prosperous and Peaceful New Year to you all.

 

[TXiceman]

Remember that you can neither creat or destroy energy. It is still there and you will have changed it's state.

Ken

Ken
KE5DFR

 

[jcoots]

Thanks for all the input. I've been out on vacation and holidays so thats why I haven't replied until now.

First, thanks for the confirmation, insight, and suggestions. To 'rmw' the answer is a Sugar Factory. Currently they are running all their boilers into one header at 300 psi. The problem with this is the low pressure boilers are designed for 275psi and the high pressure boilers are designed for 430psi.

It will take me a little while to digest this information, but will keep you informed.

Thanks again.

 

[rmw]

Jcoots,

I thought it sounded like a familiar scenario. I have seen this type of scenario all too often in the sugar world. I suggest that you take a hard look at some of your larger turbines, your mill turbines or your shredder turbines and see if any of them have a casing design that permits operation at the higher pressure and if so, convert them to the higher pressure steam. I also assume that most all of your turbines exhaust into your 15-20 psig header for the evaporators and pans.

If none of these turbines will withstand the higher pressure, it may behoove you to look at obtaining one that might.

You will have to look at your steam balance and see how much excess steam you have at the higher pressure and then find some horsepower user or users that can utilize or be converted to utilize this higher pressure steam.

You can drop it through a throttling valve but that is a lot of free work that is going to waste in an operation that has an awful lot of horsepower users, pumps, fans, etc.

Speaking of fans, what pressure are the turbine drivers for the fans for the HP boilers operating at? Chances are they were sized for the HP steam of the HP boilers and if they are on the LP system, that would be a travesty.

If you are going to spend money for a pressure reducing station (and you may need a desuperheating station too depending on where the pressure reduced steam is to be used) you might as well put that much money toward the purchase of a new turbine or turbines that can utilize the HP steam and exhaust into your exhaust header.

If you have excess generation capacity you might also look at taking some driven unit(s) (pump, fan, mill drive, etc) off of steam and putting it on electricity and using more of the HP steam through your generating turbine. But this is only viable if your generation turbine is an extracting turbine and the steam can end up in your exhaust header.

For me, (maybe you can tell) it would be a last resort to just drop the pressure through a throttling valve.

rmw