Condenser sizing 2

[atlan99]

Hi all,
could somebody advise me with a design of air cooled condenser. In the calculation I have superheated vapor (one phase) and liquid+vapor (two phases) regions. In each the region I have different heat transfer coefficients. In the overall heat transfer coefficient, however, I have only one heat transfer coefficient for air side and one for the condensated vapor side. How can I implement both these regions the superheated vapor (one phase) and liquid+vapor (two phases) into the overall heat transfer coefficient?
Thanks
Atlan

 

[ione]

Well, you will have a portion of the HE where the superheated vapour flows and so it behaves as a dry gas (desuperheater with much lower heat transfer coefficient) and, when temperature has reached saturation,another portion of the HE with a condensing vapour. Each zone will have its own heat transfer coefficient.

 

[IRstuff]

They are essentially added together, are they not? The definition of the conductivity is W/K, where the temperature difference used might be something like an LMTD calculated value. You'd have to verify that the values scale correctly with area changes and temperature changes.

TTFN
faq731-376

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[srfish]

The operating pressure can have a big effect on evaluating this type of condenser. Above a certain operating pressure,for the desuperheating zone you use a vapor heat transfer coefficint all the way to the specified dewpoint temperature. This procedure can be done in all cases but it will be too conservative at low operating pressures. At low operating pressure the tube-side heat transfer coefficient will be relatively low. In this case the tube wall temperature needs to be calculated. If this temperature is lower that the dewpoint the tube wall will have a liquid condensate film and therfore a higher heat transfer coefficient than that for vapor. Some call this the wet-wall method of calculation

 

[davefitz]

I had thought that for power plant air cooled condensers, the incoming steam is always saturated or about 10% liquid by weight. The exception might be during use of the turbine bypass valve- US codes may require slightly superheated steam, but if the spray logic is modified to the "enthalpy control method" then that steam source can also be controlled to 10% liquid by weight.

The condensing steam heat transfer coefficient is relatively high, such that the main resistances to heat transfer are the fin efficiency and the air side heat transfer coefficient, and one could nearly ignore the resistance due to steam condensation, but with a resisstance due to a liquid film barrier added.

Other off-desing affects willlead to a higher pressure, such as air side fouling ,and steam side noncondensable gases.

"Nobody expects the Spanish Inquisition! "

 

[EdStainless]

Even if this not a steam condenser you prob want to de-superheat in an separate unit, a tank with direct contact spray would be good. You don't want to waste condenser on this portion of the cooling.

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Plymouth Tube

 

[StoneCold]

atlan99
Can you give us some insite as to your process conditions and flow rates. That will impact the appoach used. If the flowrates are relatively small you may use one heat exchanger, if the flow rates are large you may take some different appoaches discussed above. Please come back with some data so we can help you in a more specific way.

Regards
StoneCold