6 replies to this topic

[shin29]

Dear all

Greetings

I am talking about a Air heated heat exchanger , verticlly installed , with longitudenal fins, and heated by natural draft of air. 

the fluid inside is at subzero temperature ( -20 deg c ).

 

How should we design this exchanger specially frosting on the tubes  in terms of time and legnth of tubes.

 

thanks in sdavance for your kind responce

 

sahil

 

[Art Montemayor]

Are you dealing with a cryogenic, saturated liquid vapaorizer?  This is what is done every day in order to vaporize liquid nitrogen, oxygen, argon, hydrogen, etc.

 

Frosting externally on the tubes is something that is normal to this type of operation.  It can't be prevented when you are using atmospheric air to heat a sub-freezing  fluid.

[Bobby Strain]

You should purchase the exchanger from a reputable vendor who knows how to design it.

 

Bobby

[shin29]

Are you dealing with a cryogenic, saturated liquid vapaorizer?  This is what is done every day in order to vaporize liquid nitrogen, oxygen, argon, hydrogen, etc.

 

Frosting externally on the tubes is something that is normal to this type of operation.  It can't be prevented when you are using atmospheric air to heat a sub-freezing  fluid.

Sir,

it is a natural gas at subzero temperature after pressure let-down , and it is below its dew point .. The thing is if i keep it in line for a long time , how can i calculate the ice formation on tube and fins , in terms of time of operation and tube legnth coverd with ice.

[thorium90]

What is the design flow rate of the vaporizer? The manufacturer will typically have a maximum value for you. The number implies that going above that flowrate will typically have excessive ice formation and the warming will not be effective enough. Stay below that flow and you will likely get the outlet temperatures you want. They might also give maximum duration of use (not always, depends on design) and the ambient conditions

If large flows or surges are expected, some switchover mechanism might be installed to allow one vaporizer to de-ice

 

The calculation I believe would be more complicated than you would expect. Depending on ambient temperature and humidity, on the time, on the amount and direction of wind, airflow characteristics wrt to time as the flow changes when the ice grows, heat conductivity wrt to time as the ice grows and the thickness inreases, changing temperatures profiles in the gas in the tubes wrt to time and ice formation and melting of the ice in the sun (probably some amount of radiative heat transfer). Looks like the iterative solution of multi dimensional heat transfer PDEs and CFD simluations of the air flow around the exchanger.

Of course, for something so complicated, Im sure there are spreadsheets out there with some empirical equations to estimate it...

Edited by thorium90, 18 February 2013 - 11:14 PM.

[shin29]

What is the design flow rate of the vaporizer? The manufacturer will typically have a maximum value for you. The number implies that going above that flowrate will typically have excessive ice formation and the warming will not be effective enough. Stay below that flow and you will likely get the outlet temperatures you want. They might also give maximum duration of use (not always, depends on design) and the ambient conditions

If large flows or surges are expected, some switchover mechanism might be installed to allow one vaporizer to de-ice

 

The calculation I believe would be more complicated than you would expect. Depending on ambient temperature and humidity, on the time, on the amount and direction of wind, airflow characteristics wrt to time as the flow changes when the ice grows, heat conductivity wrt to time as the ice grows and the thickness inreases, changing temperatures profiles in the gas in the tubes wrt to time and ice formation and melting of the ice in the sun (probably some amount of radiative heat transfer). Looks like the iterative solution of multi dimensional heat transfer PDEs and CFD simluations of the air flow around the exchanger.

Of course, for something so complicated, Im sure there are spreadsheets out there with some empirical equations to estimate it...

 yes sir,

there is sure a design flow and switch over mechanism is reccomended by the manufacturer , what i want to realise is whether we can use both the exchanger witch means half of the design flow to each ... with out any switch over in between .

 

regards,

sahil

[thorium90]

Well if the valves and piping permit the possibility, then it can be done. But since you would not be having any spare, you would need more monitoring, like checking if the ice formation becomes excessive. I've seen such vaporizers topple on its side due to the weight of the ice formation