# When is a heat exchanger worth it?

I am currently working on a huge industrial process for a student project and we need to transfer around 25MW of energy between dozens and dozens of streams, some larger and some smaller ones. Many even with gas/gas exchange, and those tend to get quite large fast. I was wondering if there was a general rule of thumb to determine if the investment for a heat exchanger is worth it in order to save on operating costs. Something like, "don't build liquid/liquid exchangers for less than 250kW and gas/gas for less than 500kW" something along those lines. Any ideas?

• No. There are too many factors to consider for a rule of thumb. I built a process with multistage heat exchangers at only 200W, but we had very limited input power so every watt counted. How would they affect maintenance? How would they affect the footprint? Is hot water a waster product without them? Would this be an environmental hazard to dispose of? Jul 2 '20 at 17:49
• What is the actual task you are doing? You can get an order of magnitude cost estimate from equipment fabricators if you can compile the different heat duties for the different stream combinations you are analyzing and have them quote you something. More generally, look at how much fuel you theoretically save by using the residual heat in some of these streams - that might allow you to narrow down candidates for further development. Jul 2 '20 at 22:00
• I did a lot more calculations than I had hoped, but I found that most heat exchangers for our project with less than 200kW are generally not worth it, except for special cases with extreme temperatures. Should be a good enough estimate for my project, but of course no general rule of thumb, indeed. Jul 9 '20 at 18:50

If this is a student design problem to see whether you understand the principles, make it as easy as you can, use steam and cooling water. Calculating all the pressure drops on the weird pipe systems and valves you need when using one stream to cool/heat the others will drive you insane. Not to mention what it can do to your choice of materials of construction an therefore building cost.

On the other hand, consider the place and resources available - e.g. if the area has low water resources you might find your design doesn't make it if your water use is too high. Or you may not have a large enough steady supply of coal/oil/gas/electricity to heat/cool all that water.

If you are designing something that will actually get built, take into acount the maintenance needs. Some designs look wonderfully efficient until someone has to hang upside-down or pull out half a mile of pipe to find a leak or change a valve. Don't get yourself lynched by the maintenance crew for making everyone's life difficult.

Cost is not usually the deciding factor.

Usually we need to keep the fluids separate for a reason:

one fluid needs treating the other doesn’t

Or the systems are at different pressures

Or the fluids need to be different due to later use.

• You're right, I misread. Keeping the fluids seperate is a given, though, since we are working on a chemical plant with vastly differing product streams. The question is, do I heat/cool the smaller streams by running piping all over the place or should I just use steam/ cooling water? I am not too sure if the 100, 200, 300kW streams matter when compared to the 5 MW ones Jul 2 '20 at 18:29
• Especially the gas/gas exchangers would get much much smaller and cheaper when I cool with cold water in a gas/fluid exchange Jul 2 '20 at 18:31