Superheated steam at low pressures 2

[acr2003]

This is the first time I am working with steam and I must build a line where I must generate steam at a temperature of 600C. My application requires low flow rates (4kg/h) and the dry steam must be introduced into my system at atmospheric pressure. Is it possible to generate superheated 600C steam at atmospheric pressure without major drops in the temperature? If so, how would be the pressure reduction along the line, I mean, what kind of valves should I use? Should pressure be reduced after the steam generator or after the superheater?
I appreciate the help

 

[linux]

hi acr2003
well i dont thimk its posible to have one without the other
its called steam enthalpy enthalpy = preasure and temperature combined however i will look into this for you

 

[acr2003]

Hi Linux,
Thanks for your reply. You mean enthalpy (temperature plus heat capacity)? I know that if we treat superheated steam as an ideal gas (what can be done at that temperature), playing with the PVT equation, keeping V constant, theoritically you could do it. But, in practice I am not sure. In the line I am trying to build, what I effectivelly want to know is if I could make pressure drops to reach the 15psi pressure, without droping T. If you could check it for me, it will be great help.
Thanks a lot
acr

 

[ChrisConley]

Maybe I'm not quite understanding your situation. But what will push the steam through the system? If you are introducing at atmospheric. It would make more sense to generate steam at pressure, and then regulate down as required. Then the steam could be transported without large heat losses. Having said that, what exactly is your application?

 

[davefitz]

For the very small flows you indicated, it sounds like you would probably use an electric boiler with an electric superheater.

For the final temp of 600 C, the best control would be to control the electric superheater heating element at 610-615C. The electic boiler and superheater would operate at about 1 bar gauge with a 0.8 bar pressure drop at the outlet pressure reducing valve.

The casing for the electric superheater could be ferritic SA335 P22 or P92 or a ferritic stainless steel ( can't use austenitic stainless if boiling water).

The relief valve on the unit would need to consider the max thermal duty the heater can generate, so it is prudent to deliberatley design the circuit breakers to limit this value to perhaps 125% expected max steam flow.

The temperature drop that occurs when steam is throttled from 2 bar a to 1 bar a is only 0.2 F ( 0.1 C), less than the accuracy of the thermocouples and control system.

The outlet valve would be a needle valve - for the flows you are considering, use a swagelock valve and tubing and fittings.

 

[acr2003]

DaveFitz,

Thanks for your precise reply and tips on the valves, piping and fittings, etc. It cleared me a lot and definetly will help mea lot in designing my system.
I am really intending to use an electric steam generator and superheater, due to my low flowrates. Just one point I didn't get: I will be placing the pressure reducing valve just after the superheater?
acr2003

 

[arup213]

hi acr,
i think it is possible to get 600c at atm. press.
The steam is produced at a higher pressure and the corresponding superheat temp.(<600c). Now if the pressure is reduced to atm. pressure by a press reducing valve( or if the steam is led in a vessel of reqd. volume such that the steam expands and attains atm. press as in the case of SEPARATOR) and considering isenthalpic process the temp. of the steam attains superheat with the degree of superheat much more than the initial degree of superheat.
Pl. check with H-S diagram and consider the enthalpy to be constant, it may be possible.
thanx

 

[davefitz]

You can place a pressure reducing valve at the outlet of the superheater or it can be placed at the outlet of the boiler/evaporator. You can also conduct the entire process at atmospheric pressure and have no valves. A lot depends on how you choose to control the process and what sort of dynamic response one needs.

If fast response to changes in flow demand is required, then it helps to have the unit under pressure and to use valves. There are several different configurations possible. Typically one would have city supply=>check valve or reverse flow preventor => feed control valve => once thru evaporator/boiler with outlet slightly (5-10% wet) less than saturated => steam/water separator, with liquid blowdown to drain => outlet orifice or fixed needle valve with a pressure relief or pressure control valve in parallel venting excess steam to atmosphere => electric superheater => line to process. This &quot;once thru&quot; avoids the need for a drum or pressure vessel and no need to recircuolate the water, but one can change to another configuration if wanted.

 

[acr2003]

Hi DaveFitz,

Your suggestions for the steam line were very welcome. I need to have pressurized steam leaving the boiler, or at least a diferential pressure at the boiler outlet, as I need a steam flow (for fluidizing a bed). I was checking some steam generators that could deliver pressure at around 50psig and then thinking of a pressure regulator after the boiler, to drop the pressure according to my needs, as I will be adjusting the steam flowrate ate diferent satges of my experiment.

I could then fixed the pressure before heating the steam (I believe that just heating the gas/steam won't increase pressure in the line) and then, after the heater, use a flowmeter to set the flowrate I will need.

Let me ask you another question. Do you know how electric steam boilers work? I am asking because I don't undesrtand why soem boilers require a water supply pressure a little higher than the operating pressure. I thought it was just the case of heating the water of an internal reservoir and then producing steam, but it doesn't seem so.

 

[KeItSiSt]

How are you going to get water into your boiler if the water inlet pressure is less than the boiler operating pressure?

There are two types of electric boilers; electrode and resistance heating. Both are basically a tank with elements immersed in the water. ON a steam boiler the top portion of the tank is used as steam disengaging area.

I've designed a few electric steam superheaters but none as high as 600C which will need to take careful consideration of your materials at that temperature. I have only designed units up to around 370F. You will need to know the empalthy required to reach your superheated state from your saturated state. The steam is then circulated or passed over the elements making sure there is no stagnation point or your hairpin will explode if the heat is not carried away. There is no water accumulation in the superheater and a flow switch is highly recommended as well as a steam separator. I have never designed one to operate at atmospheric...Some pressure is needed for proper flow which is critical when superheating steam. Personally, I am not real sure this is a project for a beginner and kind of scares me actually. Be careful. Superheated steam can be very dangerous especially at these high temperatures. Even small water droplets at these conditions can be devastating.

Oh, and welcome to the wonderful world of steam!

 

[KeItSiSt]

Did I say 370F? Sorry, meant to say 370C...about 700F has been my limit.