4 replies to this topic

[Art Montemayor]

Binz:

Of course a kettle type reboiler can be used as a waste heat recovery unit. I've used them before as such. You can use just about every mechanical type of heat exchanger as a waste heat recovery unit.

However, a lot depends on what your waste heat stream is in order to qualify any selection of mechanical design as practical, operational, safe, and economical. It makes a lot of difference if your waste heat stream is a gas or a liquid stream; it makes a difference if your waste temperature is above 500 ^oF; it makes a lot of difference if your stream has solids, dust or is corrosive.

In short, I've answered your basic question, but because you are not specific in your query I can't be specific in my response. If you want a specific answer from an engineer, you must be specific and give all the required basic data and scope of work.

[Art Montemayor]

Binzee:

This is another response to your query, but based on your recent data submittal.

First of all, you are furnishing more specific information and data than your later query – but still not enough.

Like all Chemical Engineering design projects, your profs are looking hard at your response to this problem in order to judge whether you can toe the line with respect to the most valued and priceless commodity an engineer has – COMMON HORSE SENSE. You don’t state the quality of your gasifier gas and whether it’s got solids in it, if it is corrosive, or if it is at a high pressure. I’ll take the optimistic assumption that it is clean gas, is inert, and is essentially at 1 or 2 psig pressure. Before you do anything, use your common sense to address the following challenging questions:

1) Do you honestly believe you can find a gasket material for the U-tube bundle in the kettle reboiler that will withstand 3,900 oF ??? (I don’t think so …)

2) Do you think it would be economically practical to generate 1,500 psig steam in the shell side of the kettle?? (I don’t think you want to generate the steam in the U-tubes …)

3) Knowing beforehand that the kettle reboiler type of exchanger is the largest and bulkiest of any exchanger design, take into consideration that your gasifier gases have to tolerate a pressure drop through the tubes while under their total Acfm design capacity. If I don’t miss my guess, your gas capacity is rather large (another basic data you failed to state) and will require an atrocious and extremely large tube sheet (or various tube sheets in various kettles) that will have to be extremely thick in order to withstand the very high steam pressure. Do you think this is going in a practical direction??

I could go on with more questions or practical notations that will defeat the kettle design – and the shell and tube design as well!

Stop and think about what other options you might have and you’ll come to the realization that you’re much smarter and more practical if you generate such high pressure steam inside of a tube instead of a shell. Also, if you have a low pressure hot gas and can only tolerate a low pressure drop, the way to start thinking is to employ a design very much like a high-pressure steam boiler: use a low pressure, refractory-lined vessel with steam (water tubes) – like those in a boiler. In other words, you’ll have two horizontal drums – one below and another at the top. Both drums will be connected by specially designed, curved tubes that probably will be finned on the gas side. A water level is maintain in the upper drum and steam is generated within the tubes and separated at the top drum. The hot gases are directed around the multiple, curved tubes and exit out of the refractory chamber after transferring their heat. This mechanical design is capable of handling large gas volumes with low pressure drops, developing high steam pressures, and is self compensating in metal tube stresses through the use of curved water tubes.

This is what I would expect as an apparatus to handle hot gases while generating high pressure steam. A kettle type exchanger is NOT going to cut it from a practical (or economical) point of view.

I hope this response helps you orient your thinking towards resolving this heat transfer application in a way that your profs and you concur as being the most effective, practical, and economical.

Art Montemayor