Some help

[mizzoueng]

I need to calculate the output temperature of a deaerator. I know the input mass flow rates and enthalpys, is there a way to determine the output Temperature from this?

 

[TouchDown]

From MERM, it looks like the function of the Deaerator is basically a high pressure Feedwater heater in mixing both steam and feedwater... The attempt is to bring the mix to a saturated temperature (at pressure) where the gas solubility is zero.

If you can bring water to saturation, then entrained gases want to come out.

Assuming it's working properly, I would think you could almost assume that the exit temp is NEAR saturation (or like) at pressure. Use steam tables to find sat. temp.

If it's more than a ballpark, and you want to confirm that it's working properly, then adiabatic energy balance says:

Mdotwater(enthalpywater) + Mdotsteam(enthalpysteam) = Mdotmix(enthalpymix)

Assuming that it's saturated water, and you know mass fractions, then solve for enthalpy of mix and see if it matches the saturation temp AT the pressure you're operating.

Just a guess. I don't work with these, just attempting to understand what MERM says. Good Luck.

 

[TouchDown]

Gimme some numbers and let's see if I come up with similar. I have a steam table here.

 

[mizzoueng]

I have 7 flows going into the deaerator. All are steam returns off turbines, except one is a fresh water intake (or make-up).

1. 3687 lb/hr @ 724F @ 1360 btu/lb

2. 33564 lb/hr @ 489.6F @ 1204.1 btu/lb

3. 50355 lb/hr @ 489.6F @ 1204.1 btu/lb

4. 537989 lb/hr @ 485.64F @471.1 btu/lb

5. 15071 lb/hr @ ___ @ 1197.8 btu/lb

6. 159566 lb/hr @ ___ @ 350.6 btu/lb

7. 400000 lb/hr @ 70F @ 28.08 btu/lb <----- water

I know the output is 1262589 lb/hr @ 348.24 btu/lb

And I assume MERM is "mechanical engineers reference manual", where can i get this holy grail? amazon?

 

[TouchDown]

Since you have so much information on the exit already, do you know the exit pressure? That pressure would allow us to identify the MIX of outgoing. If you assume it's saturated liquid - it would correlate with 375 deg. F exit - steam table.

PS - doing the Mdot x h calcs, your inputs add up to 4.45E8 Btu/hr, whereas exit equals 4.39E8 Btu/hr, which shows you that it's not truely adiabatic. It's approximately 98.8% energy efficient? Just looking at energy loss through heat loss.

The deaerators are supposed to be exit of saturated liquid. If that's not the case, you need to increase water flow, right?

Incoming steam looks to be superheated on the first three inputs - running at around 620-620psi. Item #4 is almost saturated and running at near 593 psi. The other 2 are not known because of unknown temps.

 

[mizzoueng]

I figured out some of it. Turns out I did not consider 4, 6, and 7 as saturated liquids but as saturated vapors. Once I figured that out and did a little research on the feedwater heaters they were going through I was able to back calculate the enthalpys and temperature and then verify them and the pressure with SteamTab.

I have much to learn. Feels like college did nothing and I am starting over.

BTW, where did you get your copy of MERM?

 

[TouchDown]

Doubtful that anyone that has passed the ME PE exam would ever really get rid of the book. Plus, you'll want a relatively new published revision.

"the other board" is the place to get it along with other study materials. If anyone has other options to recommend, then here's where to drop the hints.

I really didn't have any issues with "the other board", the MERM was great information (maybe not the best test preparation because of the problem formulation), but still an extremely important reference and knowledge building tool.

YOu can see a lot of links to "the other board" around.

I think it's ppi2pass.com

Later and good luck. One recommendation - when tackling problems, make sure to write out and validate assumptions. Sounds like you have a good tool to evaluate data - was my number (guess) even close?

 

[mizzoueng]

I calculated it at 434 Deg F. My boss is going to Double check my work for me so I guess we will see.

 

[TouchDown]

From steam tables, the 434 deg. F output result and 348.24 btu/lb enthalpy would mean that the water is liquid and not completely saturated.

My assumption was that you were at the point of saturation. <_< Probably a bad assumption.