4 replies to this topic

[chocopie]

Hi! I'm a sugar tech engineering student

I'm calculating the length of the barometric column through the following equation:

H = Ho + h + s (source: Hugot, 1986, Cane sugar engineering, p.881)

where Ho = head of water corresponding to the maximum vacuum in the condenser

It is said that Ho can be taken as a lower limit for the temperature of the outgoing water a minimum of 30 degrees C. and the corresponding vacuum is 73cm


I'm kinda confused by this bit.
In my design,

The saturation temperature of vapour leaving from last vessel to the condenser is 56 deg C. The cooling water temperature is 30 deg C. and the approach temp is 5.6 deg C. Hence i'm projecting the outlet water temperature from the condenser to be 50.4 deg C.

How do i find the maximum vacuum in the condenser in my case?

I'd really appreciate some help, thanks

Edited by chocopie, 20 July 2010 - 04:15 AM.

[katmar]

Your posting is confusing because you have titled it "Tailpipe Sizing" but in the body of the post you ask "How do I find the maximum vacuum in the condenser in my case?".

I assume that what you want to know is what vacuum you can achieve with a water exit temperature of 50.4 deg C. From your steam tables you will see that this temperature corresponds to a water vapour pressure of 12.6 kPa absolute. You talk of a maximum vacuum of 73 cm, which I would take to be 73 cm of Mercury and to be a negative gauge pressure. (You need to be more specific with your units.) The vapour pressure of 12.6 kPa abs is equivalent to 9.45 cmHg absolute. To convert the absolute pressure to a vacuum you need to know your local atmospheric pressure. At sea level this would usually be taken as 76 cmHg. The maximum vacuum you could achieve whould therefore be 76 - 9.45 = 66.55 cmHg. Your 73 cmHg at 30 deg C corresponds closely to an atmospheric pressure of 76 cmHg, so I guess you are close to sea level.

[chocopie]

Thanks for your prompt reply, Katmar. I do get it now.. oh and i'm sorry for the confusing title.

I am actually confused about why the book says that Ho should be taken at a lower limit for the temperature of the outgoing water, a minimum of 30 degrees C when the actual outlet water temperature is 50.4 deg C.

Edited by chocopie, 22 July 2010 - 10:45 AM.

[katmar]

The length (height) of the tail pipe should be calculated for the lowest possible cooling water temperature because this is the temperature you will see if there are no vapours coming from your pan. At this low temperature you will be able to pull a stronger vacuum (lower absolute pressure) and the water in the tail pipe will be at its highest level. If the height is not correctly calculated you could conceivably get cooling water flowing down the vapour inlet to the pan.

[chocopie]

Thank you Katmar, things are much clearer in my head now. I've also been checking your posts concerning barometric leg calculations. Thanks for helping students like me I really appreciate it.