8 replies to this topic

[Propacket]

Hi Everyone,

Here is brief description of my question:

When sizing fired reboilers, we come across sizing of fire tube. We normally size fire tube based on the maximum heat flux. We find the surface area of fire tube based on the reboiler duty and heat flux:

Heat Transfer Surface Area = Process Heat Duty/Max. Allowable Heat Flux

After calculating the surface area, we select the fire tube dia and length that satisfies the calculated surface area. Only thing which limits the fire tube dia is heat density. We have to keep heat density below the maximum value to avoid instability of flame.

I am concerned about heating value of fuel because in all above calculations, heating value has not been used. If heating value is large, flow of fuel gas and flue gases will be small. Result is low velocity and low heat transfer rate. If heating value is small, flow of fuel gas and flue gases will be large which will increase the velocity and heat transfer rate.

I want to know the effect of change in heating value of fuel gas on performance of the reboiler. Please advise any other method which takes into account the heating value of gas.

Thanks

[Art Montemayor]

P.engr:

In dealing with the design of the combustion chamber in a direct-fired reboiler, I am afraid that you are making several erroneous assumptions - or assertions.

The size and configuration of the fire tube to the combustion chamber is strongly dependent on the type and size of combustion flame you proposed to create within the fired heater. THIS is what sets the type and size of burner - as well as the type of fuel gas that you propose to use. You should be dealing with a reliable burner manufacturer that can assist you in mechanically designing the combustion chamber.

Why are you varying (or considering to vary) the heating value of the fuel gas? I presume that you are dealing with only a fuel gas, and not with changing to a liquid fuel. You will find that the variation of heating values within the various hydrocarbon fuel gases is slight - or small - at best. But I have to assume that you are not proposing to changing to something of the order or similar to Hydrogen! Please be specific and do not generalize. If this is a specific and true industrial application, then come right out and state ALL the basic data - including the identity and composition of the proposed fuel gases. Your reluctance to do so only prolongs and complicates any valuable comments you can harvest from our Forums.

Await your reply.

[Propacket]

Art,

Thanks for the reply.

We have an installed 2 MMBtu/hr reboiler which takes fuel gas having 980 Btu/scf heating value.
Installed Fire tube has 16 in Diameter and 35 ft U-tube length. It has been sized based on 7000 Btu/hr.ft2 heat flux and 70% heater efficiency. Fuel gas flow rate is 0.049 MMSCFD.
Now this reboiler has to use a different stream as fuel gas which has a heating value of 600 Btu/scf.
The flow rate of new fuel gas is 0.08 MMSCFD. I just want to know what would be the effect on performance of the reboiler if we use this fuel gas with low heating value. Will the reboiler require any modification?

[Art Montemayor]

P.engr:

You are proposing reducing the fuel heating value (LHV, I presume) by 38.8%. That is a significant reduction. You have not furnished the requested compostion information. This is clearly not a “fuel gas” by any definition – probably a waste gas stream of some sort or other.

What you are proposing will add a considerable volumetric load on the direct-fired reboiler tube bundle and this will be relected in the pressure drop through the bundle as well as the back pressure placed on the burner. Whether or not this will work depends on your working together with the burner manufacturer – as I have previously stated and stressed. Depending on what your burner manufacturer tells you, I would doubt that you can apply the waste gas as a fuel. To achieve the 2 MM btuh reboiler load, you are going to have to generate a considerable amount of flue gas – much more than the tube bundle would normally be designed to handle. You may get by by applying an induced flue gas blower on the stack, but again this is an issue to be worked out with the burner manufacturer.

Please stick to the basic and important information. The fuel gas flow rate is not what is important in this case. It is the RESULTANT flue gas that is generated due to the poorer heating value. You must generate detailed and accurate flue gas calculations (stoichiometrics) in order to discuss this with your burner supplier and also to determine the expected tube bundle pressure drop.

You have to do the stoichiometric calculations and you have to determine and decide if your reboiler tube bundle will require any modification. By the way, what is a "70% heater efficiency" and what relation does this bear on the size and configuration of the reboiler tube bundle? Are you assuming that the same efficiency applies to BOTH of the different fuel gases? I don't think that is credible.

I hope this advice helps.

[SSWBoy]

I assume 70% efficiency refers to the fired duty of the burners, and the absorbed duty on the process side. Given that there is only radiant heating (no convection banks) flue gas temperature is higher and efficiency is lowered.

[Art Montemayor]

SSWboy:

Yes, that would be my assumption also. But that is all it is - an assumption based on pure speculation and subject to the OP's confirmation and delivery of all the basic facts. The Forum can't furnish specific answers based on generalities or speculation.

A direct-fired reboiler has a very short radiation area and the rest of the remaining heat transfer is effected by CONVECTION heat transfer - contrary to what you state. It is essentially the same type of process as that in a Scotch Marine boiler (fire tube boiler) where the tubes are not exposed to radiation and only employ convection heat transfer.

[Propacket]

Art,

Efficiency I have referred is the thermal efficiency which is defined as below:

Thermal Efficiency=Useful heat transferred/Heat Input

It depends upon percentage of excess air used and stack temperature. Greater the excess air used, smaller the thermal efficiency. Greater the stack temperature, smaller the thermal efficiency. Attached is a graph from GPSA Chapter-8 which is used for estimation of thermal efficiency.
If thermal efficiency is low, heat density (Process Heat Duty per unit Cross-Sectional Area of Fire Tube/Thermal Efficiency) of fire tube is high and high heat density makes the flame unstable. To reduce heat density, we have to consider larger dia fire tube.
From your detailed reply, I have come to conclusion that reboiler performance due to the change of heating value can only be predicted by working with the burner manufacturer. However, your question about the thermal efficiency has actually struck my mind. Because I believe that this is the parameter which is going to affect the reboiler performance.I completely agree with your understanding that reboiler efficiency cannot be same for two different heating values.

Attached Files

•  GPSA-Ch 8.pdf   143.2KB   198 downloads

[kkala]

Having found the thread interesting and educational, I would say the opinion that a gas of poor heating value does not always result in more flue gases compared to a gas of high heating value (for same rate of heat supplied).
In the considered case combustion heat rate is 49000scfd*980Btu/scf=80000scfd*600Btu/scf=48 MMBtu/d=2 MMBtu/h. Fuel gases according to attached fuel.xls have low heating values quite close to the mentioned fuel gases, yet the “poor” gas produces less flue gases compared to the “rich gas” under same combustion heat supplied (excess air = 10% for both gases).
Flue gas rate could increase in case of “poor gas” by increasing excess air, if this is needed (e.g. to keep tube heat flux within the limit – transfer 7000 Btu/h).
Consequently one could first look into resulting pressure drop along whole furnace, based on actual (calculated) flue gas composition in the case of two alternative fuel gases. Assuming that same pressure drops would roughly result from same volumetric flow rates of flue gases, a way out is to increase / decrease new fuel gas flow until more or less same flue gas rate results compared to previous fuel gas. Then check tube heat flux , burner capacities, combustion air supply fun, etc.
Well, easy said – hard done, furnace & burner suppliers can be of help.

Attached Files

•  combustion.xls   17.5KB   214 downloads

[djack77494]

P.Engr,
I think you've received some excellent advice. A fired heater is a complex piece of equipment. It is designed with consideration for a large variety of parameters that may significantly affect performance and operability. Some have been identified here, but only an engineer who has previously designed fired heaters would be aware of all of them. Unless you are in that catagory of experts, do not assume that you can make anything other than quite small changes and still achieve a predictable outcome.