practical limitations to using superheated steam in a process heating application 1

[Hotown]

I am working through a feasibility study regarding the use of superheated steam in a process heating application. We are required to use the superheated steam at 500 psia / 570 F. This steam would be supplied to heat exchangers for preheating various process streams with inlet temperatures ranging from 100-300 F.

I am familiar with feedwater heaters and their design, but I am struggling to determine what the practical limitations will be. My questions are:

1) What is a reasonably achievable TTD? I generally use 5F as a TTD for feedwater heaters, but I understand superheated steam can achieve a negative TTD.

2) What is a reasonable DCA? Again, I've used 10F for feedwater heaters but would obviously like to go much lower, assuming a feed stream at 125 F and heating steam at 500 psia.

3) Is it possible for the entirety of the heating to take place in a single heat exchanger? Or would it be necessary to use multiple heaters in series to accomplish such a range? This could be split into multiple streams if necessary, so I don't think total load plays a factor for the questions I'm trying to answer today. Assume it varies from 100 to 1000 MBtu/hr.

4) Are there any industry processes that typically operate under similar conditions? Maybe hydrocarbon processing?

Thanks ahead of time.

 

[marty007]

Hotown,

1) Yes, you can achieve a negative TTD with superheated steam. Essentially it's possible to have a heat transfer zone where you have convective heat transfer of water vapour transferring heat with the process fluid. What value of TTD is achievable? This depends on a bunch of factors, but most notably your relative flow rates of process fluid to superheated steam.

2) You can pinch the DCA as much as you choose, it'll just result in a bigger and bigger heat exchanger. What's reasonable depends a lot on your service and style of heat exchanger selected. It sounds like you want to have a subcooling liquid/liquid heat transfer region. In this case 10F is pretty tight as-is.

3) Again, anything could be done in a single unit. That said, if you want to pinch your DCA, it would likely make sense to at least have a separate liquid/liquid heat exchanger so you can optimize your fluid velocities and heat transfer rates.

4) I'm not too sure of major industries. I've personally designed a very small heat exchanger using superheated steam to heat CaCl2 for the Chlorine industry, but it was a very small unit.

Cheers,
Marty