It has been a little while since I studied my thermodymanics, so I need a little help here.  Reducing steam pressure through a PRV is an isenthalpic process (no enthalpy change), correct?  So If I have 100 psig (338 deg F) saturated steam and reduce it to 15 psi, what changes?  I assume that I will have superheated steam at 15 psi, right?  How can I determine what the temperature is?

Where I am going with this is that I currently have a steam to water heat exchanger being served by 15 psig steam.  However, this 15 psig steam is coming immediately out of a PRV that reduces it from 100 psig.  I want explore the possibility of just running a 15 psig main and not having to reduce the pressure, but a saturated 15 psig main is only about 250 deg F.  I assume that I have a higher temperature steam now, but how high?  Is that even inportant in the grand scheme of heat transfer, since I know that the latent heat is the driving force - at least I think it is.

Typically you can find a steam property calculator that you can find the enthalpy of the steam into the PRV - then you enter your new pressure with the enthalpy of the entering steam.... and voila... you should have your steam temperature on the downstream side of the PRV.

You are correct about the temperatures. At 100 psig saturated the enthalpy of the steam is 1189.6 btu/lb. The enthalpy will remain the same as it expands to 15 psig, giving a theoretical temperature of 300 deg F, compared with saturated 15 psig steam temperature of 250 deg F.

But this increase in temperature may actually reduce your heat transfer, rather than increase it. A portion of your exchanger area will be used to transfer the sensible heat from this superheated steam, bringing its temperature down from 300 F to 250 F without any condensation. The heat transfer coefficient for sensible heat transfer is usually much less than it is for latent heat transfer (condensation)/Forged Steel Valves. You may well find a 5 to 1 ratio of heat transfer coefficients.

So, depending on your temperature differences, you could end up with a situation where a significant portion of your area is consumed by sensible heat transfer, and you are actually getting only a small amount of heat transferred in that area.

My point was that your image of condensing superheated steam is flawed.  As long as the water at the steam inlet end of the exchanger is less than the saturation temperature of 15 psig steam, condensing will start immediately and there will not be a dry desuperheating section as you alluded.  For somewhat similar reasons, that