7 replies to this topic

[ashishmishra1234]

Dear Sir,

I want to design heater working on superheated steam of 160 deg C and 7 kg/cm^2 gauge pressure to heat water of 80 C to 115 C. Water is pumped to the heater to increase its pressure to 7 Kg/cm^2 .g. Please suggest me how to start. I am highly thankful to you for the same.</p>

Regards

Ashish
 steam -water.xls   45.5KB   35 downloads</p>

Edited by ashishmishra1234, 18 July 2012 - 12:18 PM.

[Art Montemayor]

ashishmishra1234:

Why do you use superheated steam to heat water when you could use less expensive and more available saturated steam to do the job better and in a smaller heat exchanger?

Steam heating in industry and in a heat exchanger is practically always done with saturated steam because it is very difficult to condense the superheated steam due to having to first remove the superheat. Superheated vapors and gases have a terrible film coefficient for heat transfer and a lot of exchanger surface has to be dedicated to that chore. When only superheated steam is available in industry, it is common practice to employ what is called a de-superheater first to remove the superheat.

Another way to heat water with superheat is to simply not use a heat exchanger at all. You can simply sparge the superheated steam into the water and heat the water that way.

I have made some comments on your workbook and am attaching it as Rev1.

I hope these comments help you.

Attached Files

•  steam-waterRev1.xls   52.5KB   37 downloads

[DB Shah]

At 7 kg/cm2g, sat temp will be approx 169°C. Hence at 160°C & 7 kg/cm2g pressure you will have a subcooled water below saturation temp.

I agree with Art, why need an exchanger when you can straight away sparge the hot fluid in to the cold one (both are water) unless one of them is contaminated.

[ashishmishra1234]

Sir,

My process requirement is 115C 7kg/cm^2 superheated water. I have availability of 160C - 180 C superheated steam at 7 kg/cm^2g.

Please suggest me how can I calculate steam required for this purpose.

I think first I will calculate the saturated steam requirirement and then decrease it by approx 10% for its degree of superheat. What will be the heat transfer coefficient for this superheater.

Please give me your valuable comments to calculate quantity of superheated steam required.

Thanks

Regards

Ashish

[srfish]

First calculate the heat exchanged from the water flow and its inlet and outlet temperatures. Then usng the enthalpies of the inlet and outlet steam/condensate and the heat exchanged, back out that flow.

[Art Montemayor]

Ashish:

My response to your further comments and questions is in red:

My process requirement is 115 ^oC 7kg/cm² superheated water. I have availability of 160 ^oC - 180 ^oC superheated steam at 7 kg/cm²g.
There is no such a thing as “superheated water in a pure water system. You cannot heat a liquid beyond its boiling point at the pressure it is at. The hottest a liquid can get is its saturated temperature. Beyond this temperature and at the same pressure, it will revert to the vapor phase. In the case of water, this means steam. Your product water at 115 ^oC and 7 kg/cm² is actually subcooled, not superheated. It is subcooled because it is at a lower temperature than its corresponding saturated temperature at that pressure.

Please suggest me how can I calculate steam required for this purpose.
As both DB Shah and I have suggested, there is another method used to obtain hot water by using steam – either saturated or superheated. But you have failed to tell us WHICH method you want to proceed with to produce your hot water.

I think first I will calculate the saturated steam requirement and then decrease it by approx 10% for its degree of superheat. What will be the heat transfer coefficient for this superheater.
This is not an acceptable calculation for the required steam flow rate. What is your engineering basis for doing it this way? All you have to do is make a heat and material balance around your heat exchange. This should be a very elementary and easy calculation.

Await your response.

[ashishmishra1234]

Sir

I will go for superheated steam with heat exchanger

Sir I have seen the paper of spirax sarco for superheated steam (page no 7) in attached file.


Regards

Ashish  block2module3.pdf   252.58KB   28 downloads

[DB Shah]

Dear Ashish,

It is not that you cannot size a heater using superheated steam, your degreee of superheat is less (~10°C) I donot find much problem with Ud, also you will be compensating with higher LMTD due to degree of superheat.

The point is why go for heater when you can sparge steam. Contact Forbes marshall, they may give you a better solution. (I assume both fluids are compatible from contamination point of view)

Divyang