Freezing Steam Coil

[maddocks]

I need to install an air preheater for an air cooled exchanger such that the winter air here in Northern Alberta is preheated to avoid over cooling of the process fluid. If i install a steam bundle, the bundle at some point will simply freeze with -30F air. Is there a type of steam bundle that will not freeze and crack?

 

[TBP]

If there's a temperature control valve that controls the steam, the coil must be piped correctly to avoid freezing. They very much tend to freeze and split during spring and fall, when the air temps are just below the freezing point, rather than during very cold weather. (During cold weather, the steam valve is wide open, supplying lots of pressure to push condensate out of the coil.)

The condensate must be able to gravity drain to a vented receiver, and the coil must have vacuum breaker. Check out "Hook Ups" by Spirax Sarco for some excellent info on this.

 

I agree with "TBP" on looking up information in the "Hook Ups" book. It is a good resource. There is another approach to the coil freeze up problem. You need to avoid coil "stall," where the steam pressure in the coil cannot overcome the condensate return system back pressure under low pressure steam conditions; or worse yet, the coil goes into a vacuum and draws condensate back into the coil. Once the coil starts flooding, opportunity exists for freeze-up. The other solution is to use a "pump/trap" combination. You can either place a pressure powered pump in series with a steam trap (usually a large F&T trap) with the F&T downstream or you can get a product that has both features in one unit (a pump-trap). You will need help in properly "sizing" the units, thus contact your local steam specialty/trap supplier. You will also need a small section of reservoir pipe upstream of the unit to allow condensate backup while the pump-trap is operating. Even if the steam coil goes into a vacuum, the condensate can drain out of the coil into the reservoir and pump body and then the pressure powered pump can push the condensate out. No vacuum breaker required for this setup. As a final note, you may want to have vertical coils for better drainage too. Good luck with your application.

 

[TD2K]

Good recommendations.

We used a steam heater to preheat H2 and that hydrogen came off at about -75C in the cold train. So, it can be done but you need to ensure you remove the condensate as soon as it forms.

Get some user names and talk to them, steam coils as you've described are pretty commonly used and they do work well, if selected properly.

For a steam coil, that shouldn't be difficult. You are just going to turn the steam on full I assume and the coil needs to be sloped to the steam trap (or traps). If you need turndown capability (for when the air is 0F rather than -30F), put in parallel steam coils with their own traps rather than throttling the steam pressure to one big coil.

 

[kbander]

Hi maddocks,

I've seen many problems (I'm also in Alberta) of breaking steam coils (or hydrocarbon coils with water) due to freeze-up during cold weather. Waste steam condensers can be a particular problem because they are usually designed wrong for our climate.

The only design that I've seen that works well for waste steam condensers is to add ducting to recirculate warmer exaust air from the fin-fan back to the inlet plenum. on a forced draft fan, this would involve "boxing in" under the fan with louvers that would open based on the fan inlet temperature. Warm air from the outlet could be routed back under the fan or exhaust to atmos (again using automated louvres).

This may even work for you instead of using a steam coil.

I've often seen finfans in hydrocarbon service tarped to keep heat in. One site I worked at had a pretty good method to "roll" tarps in place. I'd rather do this than have a steam coil...


I hope this helps....cause I understand the mess and hassle broken steam coils can be when it's 30 below.


Regards,


Bob