Air assist for a steam pre-heat coil 1

[nerobo]

Yes, I'm thinking way out of the box here.

The air handlers serving our new lab supply 100% outside air and use steam preheat coils. A face and bypass design was somehow discarded during design and modulating steam valves were used instead. At lower temperatures (10-50degF), the coils are at atmospheric due to the opening of the vacuum breaker downstream of the steam valve. With so little head, condensate sits in the coil waiting to get out the tiny trap orifice. Meanwhile the steam/air mixture above the water cools and the freezestat trips.

My idea before resorting to more expensive options, is to tap into the pipe downstream of the steam valve and add a 5psi or so compressed air supply. The coil already gets induced air from the vacuum break, I'm just trying to give it some oomph to clear the condensate. Crazy??

 

[MechEngNCPE]

You would think that the steam valves would modulate OPEN at lower temps, providing steam to the coil. Why is the coil at atmospheric at lower temps? You may need to resize the steam control valve. It sounds like the valve is oversized, where it will modulate open, but then the supply air temp swings too high and it shuts off, causing the condensate to build up and quickly freeze. Maybe im way off here though, who knows...

 

[MechEngNCPE]

I don't think anyone is gonna say thats a good idea...

 

[ChrisConley]

Why is the vacuum breaker opening at lower temps? I agree that the valve sounds oversized.

Another solution would be a 'pump trap' that uses steam to pump the condensate. I'd go this route first, deliberately adding air to a steam system sounds like the wrong solution to me.

 

[TBP]

Installations like this will typically freeze & split the coil at air temperatures at or just below freezing. This is due to "stall".

The piping needs to be absolutely correct on installations like this. The condensate MUST - in short order - flow from the trap to a vented receiver. The trap itself MUST be sized for very little pressure drop across it.

Spirax Sarco's publication "Hook-Ups" explains "stall", and provides an excellent explaination and diagram of exactly how the system needs to be laid-out.

 

[cry22]

I say you need to break the eggs to make the omelette, without FBP (actually, even with FBP), no matter what you do, the coil CAN and most likely WILL freeze, and YOU will be blamed for it.

I'd go the REAL Safe route - I'd consider a small shell&tube heat exchanger on a wall nearby (I am sure you can figure out some space, even ceiling mounted if need be) and use a small pump with a glycol per-heat coil. You've got steam right there anyway.

Ultra Safe and fairly cheap. Somehow, we in the mechanical trade, we only get noticed when things go wrong, when things go right, everyone take it for granted.

Instead of you losing sleep each time the weather goes below freezing, why don't you go out and spend some of that Preventive maintenance money.

 

[TBP]

cry22 - I've installed a number of steam heated air intake coils, and if you follow the installation guidelines as shown in the Spirax Sarco literature - and no doubt other manufacturers have similiar stuff - then there will be no problems whatsoever.

Only air temps at or just below freezing are the problem. Once the air is several degrees below freezing, the steam control valve is open sufficiently to keep the vacuum breaker from operating.