Shell/Tube flow rate?

[reggie]

Can anyone tell me if there is a rule of thumb calculation for water flow rates through shell and tube evaporators?
Obviously depending on the application the water pump will be be designed for an appropiate head but what about the minimum litres/sec flow rate in comparison to the tonnage rating of the heat exchanger??

[frank]

Hi Reggie

just use the proper formula Q = m c delta t

where

Q = refrigerating effect of the evaporator in kw

m = mass flow of water in kg/s (sorry, couldn't get the dot above the m)

c = specific enthalpy of the fluid in kj/kg (for water it's 4.19)

delta t = difference in entering and leaving water temperature (C)

so to find the flow rate of water through the exaporator you will need to know the refrigerating duty of the evaporator and then transpose the formula thus

m = Q/c x delta t.

if your evaporator is rated at, say, 80kw and your entering water temperature is, say,12C and the leaving temperature is, say, 6C then the water flow required would be

80/4.19 x (12 - 6) = 3.182kg/s or l/s

you could also use Q = m x delta h from the Mollier chart

hope this helps

Frank

[Nico]

Don't forget the limitations Reggie. Limitations for minimum flow when running at critical conditions like evaporation below freezing point and maximum flow causing turbulent flow and after a while causing leakage. Depends also whether the refrigerant is in the tubes or the water.
Condensers are sensitive for minimum flow as well due to scaling and biological growth.
Nico.

[Frosty]

Oh Frank

You're such a knowledgable bas*ard! Hee-Hee

Mr Frosty

PS - you wouldn't have happened to have been to college for approx 2.5 years would you? LOL

See you soon, Franky Boy

[frank]

Mr Frosty

How nice it is to see you back on the forum - what's up? is the fishing season over? - put the bike away for the winter have we?

You can't beat the basic's mate - they work every time! - why don't you copy my previous post and teach it to your students they might learn something for once

Give me a ring if you need me to explain it to you!

Frank