Discharge temperature for N2 vaporizer 4

[VFreshEngineer]

Hi all,

I'm a fresh mechanical engineer with no others in my company, so I don't really have anyone to discuss stuff with. I don't have much experience with thermodynamics, so I would really appreciate all the help I can get. I have to provide the minimum discharge temperature when at the highest rate and lowest pressure going through vaporizer. The vaporized medium will be Nitrogen. The pump specifications are as follows:

Any ideas how to approach this?

 

[LittleInch]

So 25 psi to 10,000 in one go? multi cylinder?

That's a compression ration of about 300:1??

What's your start temperature?

But you're looking at 1000C+ I think - just look up temperature after compression. Remember to use absolute terms (psia or bara and degrees K)

Remember - More details = better answers
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[MortenA]

it dosnt make much sense, 689.48 bar (very precise and then forgets to mention if barg or bar a ). Then a vaporizer is mentioned? Vaporizing liquid N2? Usually this would be a atm - and all you then need is to know the boiling point of N2 at atm and wikipedia will tell you this: -196ºC. If you are vaporizing at 690 bar then you cant since its above the critical point although you may have a form of sublimation from solid to supercritical phase depending on your temperature...

--- Best regards, Morten Andersen

 

[VFreshEngineer]

Hi LittleInch and thank You for the answer.

As You assumed correctly it's multi cylinder, triplex pump.

Simplified schematic (no valves, TI, PI) for LN2-GN2 line looks like this:

Since I have discharge pressure, flow and cylinder volume I can probably calculate temperature somehow. However, as mentioned before I'm totally green in this field of study.

 

[Latexman]

What is the suction temperature and state of phase? We know the pressure is 1.72 bar (gauge?).

I'm trying to figure out, is it a pump or a compressor?

Good Luck,
Latexman

 

[LittleInch]

Ah.

It now seems your are pressuring liquid nitrogen (LIN) on your table and also LN 2 on your diagram.

Only AFTER the pump do you go through a VAP - vaporiser? - to create nitrogen gas at 10000 psi

So pumping liquid N2 I can't see much temperature increase myself. It will go up a bit but compression of a liquid is a lot harder than gas. Maybe 25 C?

Bung it into HYSYS and see what it says....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[VFreshEngineer]

I don't know suction temperature, but I guess it's below -196 degC as it's liquid nitrogen.

I'm trying to figure out, is it a pump or a compressor?

If You mean the one on the inlet side, it's a charge pump (centrifugal).

Part of datasheet for triplex pump:

Part of datasheet for centrifugal pump:

Part of datasheet for vaporizer:

 

[LittleInch]

OK.

I would forget any temp increase from the pump or maybe assume 25C.

Then you need to work out the mass flow rate. Then the amount of energy required to heat up your liquid n2 from-196C to say 0C.

Then the huge amount of energy to boil the N2.

Then figure out the energy coming from your water supply.

Then play around with the figures until one matches the other.

What temperature are you getting at the moment?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

-196 C! Yikes!

If the water flow stops suddenly, which is not uncommon, what's going to stop the vaporizer from freezing and busting almost instantaneously?

Good Luck,
Latexman

 

[Caloooomi]

This must be a nitrogen pumper. They have a standard rating of 180K SCFH / 10000 PSIg. (also 360K / 540K up to 15000 psig).

Tank -> pumps (booster / triplex) -> vaporiser.

Normally they are direct fired or direct steam vaporisers. I guess in this case it would be a compact heat exchanger, more along the lines of a water pot vaporiser (pancake coils). Pulling off engine jacket coolant to heat up a closed loop water circuit for producing nitrogen gas. If the water pump stops then yes, you can free up the coil. There is typically some residual heat before the ice growth becomes too large though, and you should trip before that happens... !

At this pressure you are not boiling -> there is no latent heat of vaporisation considered. It is a supercritical liquid -> supercritical gas.

The specs from the vaporiser there are, water side: 120 GPM with 45F dT.
This is around 800 kW.

The duty for 180K SCFH from full subcooled (-196 degc) up to +20 degC @ 700 bar is actually only 550 kW.

So more than enough energy for the process.

What will dictate the gas outlet temperature though would be the surface area of the coil.

Have you measured dT on the water side?

 

[VFreshEngineer]

Thanks for all the answers, I really appreciate this discussion and all of your tips.
I haven't been clear enough with explanations and I'm sorry for that.

Have you measured dT on the water side?

No, this is planning phase, so everything is just theoretical. I need to provide minimum discharge temperature when at the highest rate and lowest pressure going through vaporizer to a company that chooses it.

As Caloooomi have correctly assumed, this is a nitrogen pumper with heat exchanger on cooling loop. However, this is how it's been done before. Now, they want to electrify the whole unit, so el. engine instead of diesel, el. booster pumps, el. vaporizer etc. No hydraulics involved, just pneumatics.

 

[jari001]

In case you need it you can find thermophysical properties of nitrogen from NIST (

https://webbook.nist.gov/chemistry/fluid/

and

https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379&Mask=4)

and you can see a phase diagram for Nitrogen with Wolfram (

https://www.wolframalpha.com/input/?i=nitrogen+at+180K+and+10000+psia).

 

[MortenA]

But you still cant vaporize N2 at 690 barg since the critical point is at around 33 barg. At 680 barg and -196 C you would have solid N2 and it will sublimate into the supercritical state:

https://www.oreilly.com/library/vie...fluid/9781498724449/xhtml/29_Appendix04.xhtml

enthaphy diagram for N2.

Look at the top of the liquid condensation/vaporisation line

--- Best regards, Morten Andersen

 

[georgeverghese]

Suggest an exit temp of about 25degC from the vaporiser, so that letdown temp at low user pressures will be above 0degC - see T-S diagram for N2 in Perry Chem Engg Handbook.
All LN2 vaporisers I've seen use ambient air for heating LN2, but I suppose steam or water based units can be used also for larger heating duties if heating medium flow is always kept above a safe minimum and tubeside film temp on heating medium side is at least 5-10degC at the cold end for a cocurrent setup, when at low heating medium flow.

 

[Caloooomi]

Ask CS&P the question as based on the datasheets, they supplied it.
They have the details of the vaporiser so will be able to supply the discharge question based on 240k SCFH @ 8000 psi, based on the picture suplied.
Assume the water side is the lowest flowerate -> 120 GPM @ 185 degF.

You can do a mass / energy balance but you need the construction details of the heat exchanger in order to model an outlet temperature. It could be that there is no overdesign on surface area for the 180K case, so you would be looking at a significantly reduced outlet temp!

 

[VFreshEngineer]

Thank You all for the answers, I appreciate every single one of them.

Additional question on the sidelines out of curiosity. What actually determines the SCFH value in this case? Is it Triplex Pump in combination with Vaporizer? In other words - If I wanted to design a system with higher capacity than 180k SCFH, which components would be critical to get more than 180k SCFH?

 

[Latexman]

Both.

Good Luck,
Latexman

 

[jari001]

Pump to get the mass balance right, vaporizer to get the energy balance for the phase change right.

 

[VFreshEngineer]

Thanks Latexman and Jari!

Going further with this thought ... Caloooomi mentioned that

The duty for 180K SCFH from full subcooled (-196 degc) up to +20 degC @ 700 bar is actually only 550 kW.

. What would be the duty for 270K SCFH? How can I calculate this? Would I need an engine with better performance?

 

[LittleInch]

550 kW x (270/180)?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.