1 reply to this topic

[nagi]

Hello there,

I am checkrating a process gas chiller using vaporizing ammonia as the chilling medium. The exchanger is a kettle reboiler (TEMA type - BKM) with complete vaporization of all the liquid ammonia entering. I have two questions:

1) Is it possible that all the liquid ammonia entering will be completely vaporized all the time? How can this be ensured? Isn't it anyway necessary to have a liquid outlet with a level control?

2) The exchanger design given by the licensor has ~250 tubes fitted in a 1500 mm shell ID. These tubes can be fitted in a 500 mm bundle diameter. Can you tell me any reasons why a high shell ID and a higher kettle ID (~1700 mm) are required for this application? are there any liquid entrainment concerns which have led to such a large dimensions?

Any inputs on the above are highly appreciated.

Regards,
Nagi

[Art Montemayor]

Nagi:

You are posting in the Industrial Professional Forum. As such, I have to presume you are a professional engineer and not a student. If I am correct, then you should first concentrate and make an effort to communicate as a professional engineer. You fail to tell us the usual and basic data required to understand your query. Please note and reply to the following questions:

1) If this is a gas chiller, what side of the kettle evaporator (not a reboiler) are you using for the ammonia refrigerant? I usually use the shell side because I employ the flooded concept of an ammonia evaporator. But you should tell us, since we know nothing about your process.

2) What are the process temperatures and pressures on both sides of the kettle evaporator?

The basic concept of a flooded evaporator is that you chill your product within the tube bundle by having the entire bundle submerged in a bath of the cold liquid refrigerant. The cooling of the product in the tubes is carried out by vaporization of the equivalent liquid refrigerant in accordance to the latent heat of vaporization of the refrigerant. It is that simple. You are vaporizing refrigerant (liquid ammonia, in your case) at a rate that the product in your tubes is transferring the latent heat. Therefore, your question: "Is it possible that all the liquid ammonia entering will be completely vaporized all the time? How can this be ensured?" doesn't make sense and shows you don't seem to understand the basics of what is happening in the kettle. There is no need "to have a liquid outlet with a level control". There simply can be no liquid exiting the kettle. Once the liquid refrigerant enters the kettle, the only way it can exit is only as a saturated vapor – never as a liquid. If it does, then the term "evaporator" does not apply!!! Does this not make sense?

The reason a higher kettle ID (~1700 mm) is required for this application is because of liquid entrainment concerns which have led to such a large dimensions. As I have stated previously, the basic design concept of a kettle evaporator is that NO LIQUID CAN EXIT the evaporator. The basic reason for this rule is that there is a compressor downstream that has to recompress the evaporated refrigerant vapor and if any liquid should get into it, it could be ruined or wrecked.