[Bill.D.Katt. (OP)]

We were assigned a group HW assignment that nobody (7 person class) can figure out. It starts pretty simple: Heat exchanger, saturated steam @ 200 psig, heating up process fluid from 80-200 F (heat capacity 7 J/g C), but here's the weird caveat: steam condensate returns to the steam boilers at 275 F. We've had problems where we have to calculate sensible heat loss from a pipe as a fluid returns, but could this sensible heat loss happen in the heat exchanger? As in, could the heat exchanger condense the steam and also remove sensible heat? None of us are sure if this happens in the real world.

[chris]

Can you please explain to me what "sensible heat" is?

[Bored chemist]

It's the counterpart to latent heat.

[chris]

I still don't understand. Please put me out of my misery...

[SeanB]

Latent heat, that which causes a phase change, but no temperature change ( at least on the macroscopic scale, like adding heat to water and it boiling at a fixed temperature.

Specific heat is a fixed amount of heat added to an object will always cause a specific temperature rise for the same mass. You heat up water from 10C to 20C needs the same energy as heating the same mass of water from 20C to 30C.

As to the OP, I am not sure, returned steam is 275F, so it will probably be condensed water at 275F, as the saturated temperature should be 388F there, so the steam will be almost entirely pressurised water, so you will have had a lot of latent heat released, plus nearly 100F of specific heat released in the exchanger to cool it down to the exit temperature.

Steam tables say that saturated steam at 200psig has to be at 388F to stay saturated.

http://www.turnkeyips.com/assets/steam_temperature_pressure_table.pdf

[chris]

Thanks. So what does @Bill.D.Katt. mean by "sensible heat"?

[SeanB]

Sorry, not specific heat but sensible heat, causes the temperature for any mass of fluid to change by a set amount so long as the fluid does not undergo a phase change.

Airconditioning use is you have hot humid air coming in at 30C, and you are pulling heat out of it, you can have no sensible heat taken out if the air comes out at 30C but condenses water at 30C, or you can have both if it comes out at 20C, where you have been removing specific heat energy as well as latent heat, condensing the water out along with actually cooling the air.

[Bill.D.Katt. (OP)]

Thanks very much! Sorry I wasn't specific about definitions here. But @SeanB is correct, sensible heat is energy from temperature change, while latent heat is energy from phase change. So what you're saying is that sensible heat can be lost in a steam condensing heat exchanger?

[SeanB]

Yes, as you will always have some degree of superheat in your steam, so there will be opportunity for temperature change in the heat exchanger in any case, and it is very rare to have only saturated steam, you will almost always have superheat if only to prevent slugging in the pipes joining the system. Most of the energy will come from the latent heat, but there will be some from the cooling of the incoming saturated steam down roughly 100F to make the exit water 275F, as it would be 388F hot water when it condenses, and now you would need to lose 100F roughly to cool it down to the exit temperature, so there will be some sensible heat loss, and a lot of it will be in the water cooling down at the outlet to heat up the incoming 80F fluid to some temperature well under 200F.

Sorry can't help you further much, been a long time since I ever really had any intro to thermodynamics, and most of that was not really steam related, more ideal gases only.