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wuddog (Mechanical) (OP)
15 Sep 08 11:58
Hello,
I have what I think is a quick question.  I have a currently installed steam heat exchanger that I want to double in capacity.  Per the design data I am only using 1/3 of the exchanger's capacity (U=180 but I am only using U=53 BTU/(f^2*h*F)).  As I understand it, to squeeze the additional capacity, I need only to increase my steam flow rate through the exchanger.  I am currently supplying 300 lbm/h @ 60 psig and want to increase to 600 lbm/h @ 60 psig.  I have a 3/4" supply line and a 1/2" condensate line.  I though I would just need to increase my supply and return line sizes, but they appear to be sized correctly for the 600 lbm/h rate.  Am I missing something?  I don't want to mearly re-pipe to a bigger size without numbers to justify the work.  Any input would be appreciated.
TBP (Mechanical)
15 Sep 08 22:33
I know what lb/hr is, but what is lbm/hr?
metengr (Materials)
15 Sep 08 22:43
pound mass/hr
katmar (Chemical)
16 Sep 08 2:32
The length of the 3/4" line from the header to the exchanger is a critical piece of information in determining the viability of increasing the flow rate.  On the face of it 3/4" is too small for 600 lbm/hr, and unless the line is very short it is probably undersized even for 300 lbm/hr.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

Helpful Member!  hacksaw (Mechanical)
16 Sep 08 7:47
300 lb/hr results in a line loss of 18 psi/100 ft pressure drop or so, 600 lb/hr four times that, the steam velocity is also at 150 ft/s or so, so yo definitly have a bottle neck. You are already flow limited so a larger feed pipe is required.

Thew pressure drop is on an effective pipe length basis, not lineal footage so be careful.

The condensate removal design also needs to be checked.



 
wuddog (Mechanical) (OP)
16 Sep 08 18:02
katmar & hacksaw, thanks for your posts.  After reading your posts, I re-checked my numbers and realized I made a huge error with the specific volume (used vf instead of vg).  Now I realize I need 1.5" for 600 lbm/hr...if I want the flexibility to move up in flow I should probably go to 2".  Pipe run is about 15 feet, so it should not be too major of an upgrade.  Re-sizing condensate line to 1-1/4".
Sometimes it just takes another set of eyes to inspire an answer.
Thanks for the help.
chief (Marine/Ocean)
16 Sep 08 18:18

 The condensate line is always of greater size than the supply line.

Offshore Engineering&Design

wuddog (Mechanical) (OP)
16 Sep 08 18:27
Interesting, chief.  I've always found it to be the opposite.  I thought it was because steam occupies more volume than water, thus the line size for steam is greater than those for condensate.
Would love to hear other opinions.
chicopee (Mechanical)
16 Sep 08 19:35
Check out some steam books or the web for recommended steam flow velocitiea thru steam lines;  I am out of the country so I can not help you out with this info.  

Using equation:
       d(m)/dt= density (@ T&P) X Cross sectional area of   pipe X velocity of steam, you can calculate velocity of steam and compare value with recommended values
katmar (Chemical)
17 Sep 08 0:42
As a general rule the condensate line is smaller than the steam supply line, but it is always better to size them properly than to rely on rules of thumb.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

chicopee (Mechanical)
28 Sep 08 16:19
I am back home so here are recommended steam velocities:
            5,000 ft/min for heating service
           10,000 ft/min for high pressure saturated steam
           14,000 to 20,000 ft/min for high pressure super heated steam  

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