6 replies to this topic

[flash]

Hello,
Are there any proven and established techniques/processes involving liq NH3 at -30degC being sensibly heated to 0degC. The pressure is about 15barg which will keep NH3 in liquid state at the outlet temperature. I know there is one EG process but would appreciate if somebody can inform about the other options.

Cheers,

[Art Montemayor]

Flash:

If you venture to be specific and detailed in your problem/question composition and presentation, you will become aware that your liquid ammonia existing at -30 oC is probably SATURATED and at atmospheric pressure.

If that is the case (which I suspect), then there is no way you are going to add sensible heat. You have to have a SUBCOOLED liquid in order to be able to add sensible heat. Any heat added to a saturated liquid will boil it immediately.

Please explain exactly and accurately as to what you mean or what it is that you are trying to do. For example, are you trying to convert a saturated liquid into another state? If your liquid ammonia at -30 oC is actually a subcooled one, then tell us all at what pressure it exists. We need full, accurate thermodynamic phase and conditions descriptions.

Await your reply.

[flash]

Art:

Thanks very much for replying. However, I did mention the pressure condition in my original message. NH3 is at 15barg which will keep it subcooled even at 0degC. It wont start boling untill the temperature reaches 41degC. Do let me know if you need further info. I am looking for a suitable heat exchange device that can sensibly raise the liq NH3 temp from -30 to 0 degC.

[Art Montemayor]

Flash:

Thanks for clearing up what I failed to catch and understand. Now that I understand better, my opinion is that you should have no problems. You have a wide variety of heat transfer equipment that you can select to sensibly heat up your liquid ammonia. And you can get by with carbon steel. Depending on the size of the flow rate, you can use a TEMA shell & tube or another type of exchanger, such as a spiral or a coil.

Attached find a workbook that calculates the sensible heat for the subcooled ammonia in the range you want. All you need do is fill in the correct flow rate and you obtain the heat load on the exchanger.

I hope this helps.
 Heating_subcooled_Ammonia.xls   49KB   361 downloads

[flash]

Thanks for your effort and time replying my message.
Infact the problem is water cannot be used as heating media due to freezing issues in shell & tube. That means it has be to air heated or electrically heated or by some other means. Ethylene Glycol system might have been used but I am looking for alternative designs.

[Art Montemayor]

Flash:

This is the first mention you have made of using water as the heating medium. In fact, you have not previously mentioned that selection of the heating medium was your problem. Additionally, I have not even contemplated the subject of the heating medium in my spreadsheet or in my responses.

A heating medium is not a problem, if that is what is bothering you. Also, you can use an electric heater. However, I would almost always design for an intermediate heating medium, custom selected for the low temperature range.

[Qalander (Chem)]

Flash:

This is the first mention you have made of using water as the heating medium. In fact, you have not previously mentioned that selection of the heating medium was your problem. Additionally, I have not even contemplated the subject of the heating medium in my spreadsheet or in my responses.

A heating medium is not a problem, if that is what is bothering you. Also, you can use an electric heater. However, I would almost always design for an intermediate heating medium, custom selected for the low temperature range.

Dear

Although 'Fairly Good and Comprehensive response' given by our Art through above posts.

Just out of curiosity may I ask that If water is dis-regarded from being appropriate only on 'Freeze Point' characterstic.

Water's Freeze Point is surely manageable in the form of salt solution or 'Brine' of adequate concentration;

in case there are no other material,corrosion etc. related concerns!

Hope this gives another point to ponder!