What is the effect on condensate loading on steam coil design?

[MSB119]

How do I determine the number of passes for internal coils? I have two internal steam coils, one at 650 ft2 and one at 715 ft2. The two combined will discharge about 30 gpm condensate. How do I determine the number of coil passes needed? The coils have a 7" pitch. How do I determine the area required for free drainage of the condensate?(the area not available for condensing due to condensate coverage?)

 

[speco]

Dear MSB119,

Since none of the real experts have weighed in on this one, I'll give it a try.

Tubeside steam condensers are generally presumed to operate about the same regardless of the pass configuration and velocities with respect to the heat transfer coefficient of the condensing steam. A typical number for the steamside heat transfer coefficient is around 1500 BTU/(hr-ft^2-degF), but lower for low pressure steam.

Most steam condenser applications operate with more-or-less saturated steam at the inlet and condensate out at essentially the same temperature. They are usually controlled by a simple trap, and operate with a downward flow pattern to keep from loading up with condensate.

The real question here is, what kind of velocities are reasonable. A rule of thumb to consider is an inlet velocity in the range of 100 feet/sec. Low pressure steam can run at a bit higher velocity. What you don't want is to have extremely an extremely high velocity anywhere in the system. This creates pressure drop which lowers the effective operating pressure at the coil.

Here's an example I once saw in a plant: They had a dedicated low pressure boiler with a 12" steam header, feeding a bank of immersion plate coils in a tank. The line feeding the bank of coils was about 4". There was a 2" control valve controlling the steam into the bank. The pressure drop through the small diameter piping and the control valve was so extreme that the effective temperature difference between the steam and the fluid in the tank was reduced by about 40%. Needless to say, the system was not performing very well.

Regards,

Speco (

http://www.stoneprocess.com)