Yes it is a lot. But keep the other costs in mind. If you can save 1 m^3 of fuel salt that is worth $3 to $7M depending on the fissile concentration at startup - and has other fairly important side benefits too. Likewise, assuming we are using a Brayton cycle then reducing the Thot_in - Thot_out by 50C would be worth $17M/year or roughly $140M!!! As David mentions, for the first units we may well go with steam just to save on up front R&D expenses in which case we can't use the higher Thout_out anyway but in the future it will be worth a whole bunch of Hastalloy-N to reduce the Thot_in-Thot_out.mlippy38 wrote:Wow, $35M. That is even larger than I was expecting, and is particularly encouraging for my design.
Won't it be nearly zero in this case?- If you lengthen the heat exchanger infinitely, the two fluids will reach an equilibrium temperature (approach temperature equal to 0). Thot,in - Thot,out would be very large.
In the extreme, won't Thot,out = Tcold,in if the HX was essentially zero length?If you make the HX very short, it won't give the streams room to exchange heat. Thot,in - Thot,out would be minimal.
It will be interesting to see where you land. Do you have the hc,hh, and t/k numbers for the MSBR design? I recall reading where they knurled the insides of the tubes to increase the turbulence.This comes from the "A" term in q = U*A*Cmin, where A is the heat transfer area (perimeter*length).
- U comes from the convection terms of each fluid and the material by which they are separated. 1/U = [(1/hc)+(1/hh)+(t/k)] where hc is the convection HT coefficient of the cold fluid, hh is the convection HT coefficient of the hot fluid, t is the thickness of the boundary separating the flows, and k is the thermal conductivity of the boundary material. The convection terms are an interesting study themselves, as they reach peak values in the transition from laminar to turbulent flow. Due to the high pressure drops of the small channels already existing in compact heat exchangers, laminar flow is maintained to limit the issue from worsening. Turbulent flow always provides better heat transfer (higher hc and hh), but the price of dramatically higher deltaP is prohibitively high.
I'll tackle understanding Nusselt numbers another day - I just came off of a very long work shift.