7 replies to this topic

[TxT]

Hi everyone.

I'm in my final year of my graduation, and in my project, I need steam as an utility, to use in the distillation columns reboilers (at 10 bar and 190ºC, and at 40 bar and 260 ºC).

I can assume that I just buy this steam, but i would like to produce it "myself". The problem is that i don't have any kind of experience doing this.

How can I design a boiler to produce the steam that I need?

Also, should I use two boilers for the two steam levels?

Sry to bother,

Thanks

[Profe]

Hi TxT

About your qurry, try the Spirax Sarco site and consult (review and study it)

http://www.spiraxsar...consumption.asp

And if you prefer, take a tour for the site and you get more knowledge about the steam subject.

I think that this will be useful.

Good luck

[TxT]

Hi Profe, thank you for the answer.

The link you gave me is very theoretical, and really hasn't helped me. Maybe i did not made myself clear: what i want to know is how to design a boiler to produce this steam, how can i do this?

Thanks

[ankur2061]

TxT,

Saying that you want to design a boiler is so very general. What do you want to do? Do you want to have a size for the boiler with dimensions? Do you want to decide whether it should be a fire-tube or water-tube boiler? Do you want to do heat and mass balance calculations to know how much fuel would be required to generate a given quantity of steam at given conditions of pressure and temperature is produced?

You need to give a lot more clarifications then just saying I want to design a boiler. This is a specialized area and there are experts who are involved in boiler design and who have spent many years in this field.

As a student I would expect you to only do an enthalpy and masss balance for a boiler. If such is the case then your stoichiometry textbooks should be provide you enough guidance for an enthalpy and mass balance.

Regards,
Ankur.

[TxT]

Hi ankur2061,

Doing the mass and enthalpy balances are quite easy, that is not the problem.

The main dificulty is to decide the type of the boiler (fire tube or so), and the main dimensions of it. Obviously, i'm not supposed to do anything that professional, i just want to know the dimensions of the boiler to know how much it costs.

Another question, maybe the most important: I have two types of steam - 10 bar and 40 bar. How may I produce it? I necessarily need two different boilers? Or its normal to use the same boiler, and somehow, creating diferent pressure steam?

Thanks in advance

[kkala]

You may learn a lot about utilities by elementary occupation on boiler design, despite the difficulties. I have seen only Babcok's book on boilers, rather a reference book. However it may be a good idea to consider the main boiler as a ready package, and specify its peripherals, e.g. daily fuel tank, fuel pumps, deaerator, boiler feed water pumps, cleaning gas facilities, stack, pressure reducing station ( from 40 barg 260 oC to 10 barg 190 oC). You can also include a demineralisation unit for the feeding water, but you can stop somewhere, you could consider that demi water comes ready from an existing unit. Below is some rough data to start, giving a general familiarisation of the task, although the data may need some revision later during the work. It is mainly based on a boiler of 40-60 ton/h steam.
1. Main boiler as a package, supplied and installed by Vendor.
Water tube steam Boiler, Pressurized, of the natural circulation radiant type, with steam and mud drums and bent water tubes, bottom supported, firing (specify fuel) and generating steam of 40 barg, 260 oC. It shall be equipped with one forced draft fan to introduce air into combustion chamber.
Note: Specify capacity of produced steam, cost data my Peters & Timmerhaus indicate capacities starting from 3 ton/h of steam. Specify fuel and fuel consumtion, assuming ~85% efficiency based on LHV for liquid fuel (assumed below).
2. Fuel daily tank. Useful for liquid fuel. Net capacity = specified hourly fuel consumption at max capacity x 24 hr, plus margin. Heat traced, if stores fuel oil.
3. Fuel pumps. Discharge pressure to burners depends on them, assume ~ 10 barg. One pump operating, another standby.
4. Deaerator. Drum's capacity for 20 min surge time (at max capacity). Deaerating tower on top of the drum by vendor. Introduced low pressure steam will keep water temperature at (say) 120 oC, pressure ~ 1 Barg. Deaerating steam (coming to air with displaced O2) negligible.
5. Boiler feed water pumps. One operating, another standby. Capacity ~1.3*boiler, discharge pressure higher than 40 barg to cover losses and the control valve at the discharge of these pumps, say 54 Barg. Install them below deaerator, NPSHa critical.
6. Cleaning gas facilities. Though depending on fuel and local regulations, Electrostatic precipitator would be needed for particulates (ash+smoke), if liquid fuel is burnt. Also selective catalytic reduction (SCR) would be necessary to lower NOx emissions, even if the burner system is characterized as low NOx. SO2 emission will be determined by the sulphur content in the fuel.
7. Stack : Exit gas temperature will be determined by the acidic dew point (due to SO2) of flue gases. Assume 15% excess air and calulate flue gas composition. Exit gas velocity 15 m/s. Over here stacks of 60 m height have been installed on boilers burning fuel oil (for dispersion of nonxious gases).
8. Pressure reducing station. You can assume a online control valve to reduce the pressure, while demi water is injected downstream to control temperature to 190 oC. Flow rates will be estimated by heat and material balance. Yes, you can take part of produced steam to pressure reduce and desuperheat it.
Note: Above info does not concern design, but preliminary plant design (to support capital estimate) as a student exercise. Any comment / correction would be welcomed for consideration.
Discuss the matter with your instructor, to know whether schedule of the work can accept above activities.

Edited by kkala, 06 December 2011 - 05:17 PM.

[Nikhiln]

Regarding two different pressure levels, the common way to do it is to design the boiler to produce the higher-level steam & then provide a letdown station (basically a CV & desuperheater) to obtain the lower-level steam.
Having said that, it would also depend on the required capacity. If your requirement is a high flow of lower-level steam & very less higher-level steam, it wouldnt make economic sense to design a single boiler for the highest level. Then multiple boilers would make sense.

[TxT]

Thank you for your answers. Nikhiln, I need 15 ton/hr of 10 bar steam and only 3 ton/hr of 40 bar steam, so I guess it's better having 2 differente boilers, right?