air assist for steam pre-heat coil

nerobo · March 2011

Trying to think out of the box here.

The air handlers serving our new lab supply 100% outside air and use steam preheat oils. The proven face and bypass method was somehow discarded during design and modulating steam valves were

used instead (a proven problem for steam pre-heat applications).  At lower outside temperatures (10-50degF), the steam valves are allowing just enough steam to maintain a 55deg discharge temp. The coils (which would be in vacuum due to the rapidly condensing steam) are at

atmospheric due to the opening of the vacuum breaker downstream of the

modulating valve.  With so little head on the traps, condensate backs up in the coil waiting

to get out the tiny trap orifice.  Meanwhile the steam/air mixture above

the relatively warm 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 5psig (or so) compressed air supply. This air supply will only enter the coil as the coil pressure drops below 5psig. The coil

already gets induced air (at 0psig) from the <span style="color:#000000"><span style="text-decoration:underline;"><a href="http://www.eng-tips.com/viewthread.cfm?qid=294261&page=1#">vacuum[/u][/color] break, I'm just trying to give it some oomph to clear the condensate. Feasible? Crazy?</a></span></span>

Steamhead · March 2011

It might be easier

to lower the traps, so they'll have more head available.

Dave in QCA · March 2011

Pre-heat Coils and Proper Setup

I am quite familiar with a whole range of air handler units that operated in a hospital facility that I use to manage maintenance and facility planning. The 100% OA units always had some kind of steam preheat setup, and the setpoint was always 55F. The primary concern is that the bottom portions of the coil will be prone to freezing if controlled only by a modulating valve.

We had a couple from in the 1968 section of the building that had large blast heaters hanging inside a large (small room size) mixing box. The steam valves would modulate on these, the fans ran pretty much continuous in cold temps, and then, because of the ability of the Blast Heater to mix up the air in that chamber, both the blast heater and the air handler coils were protected from freezing. This was an odd setup, but it worked OK.

The rest of the air handling units (AHU) had a preheat coil with a face an bypass damper system. Generally, the secquence of operation was such that above 40F, both the steam valve and the face and bypass dampers modulated together to maintain 55F downstream of the preheat coil. Above 55F outdoor air, no heat was needed, and both the steam valve and the face and bypass dampers would be in the closed position. Below 40F, the steam valve would index to the open position and the control was achieved by the face and bypass dampers, maintaining a downstream temperature of 55F. This worked perfectly and protected the steam coil from freezing.

The setup that you describe in your system would work if it was not 100% outside air. But running on 100% OA, the coile is going to be very prone to freezing, the air will be very stratified, with hot temps at the top of the coil and cold (freezing) temps that the bottom of the coil, thus tripping out the freeze-stat. Good thing for the freezestat, or you WOULD be freezing and rupturing the steam coil.

Sounds like it is time for the purchase of some equipment. I will never run correctly as it is currently setup.

Dave in QCA · March 2011

Since this is new equpment.....

I am assuming that an engineer was involved in the design. Were there plans and specifications? Is it constructed exactly according the plans and specs.? If not, the mechanical contractor should have to make the corrections at his own expence, unless there was a change order issued accepting what was installed.

The first thing I would do is consult with the engineer (if there was one) and tell him what is going on.

If there was no engineer, and the system was designed by the mechanical contractor, ask the contractor how its supposed to work at 10F, and why doesn't it do that.

Once you start making modifications, you will have a hard time getting satisfaction from the parties that should be responsible for giving you a system that operates correctly.

nerobo · March 2011

air assist

The innocent have been punished and the guilty promoted.

The air assist idea...why wouldn't it work?

Dave in QCA · March 2011

Freezing coil

Even if you push the water out with an air assist, you are still going to have condensation in the form a moisture on the inside of your coil tubes. If the temperature is below freezing, that film of water will freeze. And as more condensation trickles down, it will freeze and build up until you have enough ice to rupture the coil. It doesn't have to be filled with water all at once to ruin the coil.

Another tactic that you may have already tried, during those really cold days, do you need cooling anywhere in the building? Probably not. If not, you will be able to increase the set point on the preheat coil without causing any problems in the building. Running the coil at a higher temp may help the problem somewhat.

Or, dig your feet in, propose the correct solution. A Face and Bypass Damper upstreem of the coil. The bottom line on a copper fin tube coil that is exposed to freezing temperature, the coil must be fully charged with steam, all the way to the trap. Any control of the air temp leaving the coil must be with a face and bypass damper. Anything else is a bailing wire solution and will ultimately fail, cause damage, and according to Murphy, it will happen at the worst possible time. I'm all for saving money, and when I was in charge of the hospital facility, we saved tons by keeping good old equipment in service and doing good routine maintenance. We had one airhandler that dated back to 1956 and we kept it going strong. I woudn't want to be the party responsible for a solution that ended up costing thousands of dollars in damages. You probably won't be as lucky as others and get a promotion out of it.

Brad White · March 2011

All good advice, I must say.

So far, everyone has added solid, good advice. Simple is best in any application, so dropping the trap elevation gets a solid first nod. Dual vacuum breakers, one at least after the control valve and one just upstream of the trap, will be of benefit, should one fail.

The air boost? I would stay away from that- you will not dry up the line enough to stop a freeze and if a sudden introduction, say a solenoid valve, you could cause hydraulic hammer and possibly rupture your vacuum breaker.

So, plenty of time for post-game quarterbacking, but the F&BP damper arrangement is hard to beat. Dave has your back there in all of the variations.

One technique we have also detailed for 100% OA units is a one-row steam coil, usually 4 to 6 fins per inch, as the first thing the air sees, after a roughing filter. This coil is "thin" enough so that it cannot heat the air above discharge setpoint when "running wild". Instead it is sized to take the air from outdoor design up to above 32F. There is only a 2-position non-modulating valve on this coil and in earlier days, manual valves only. This "stage-1 coil" would prevent freezing downstream and allow the "performance coil" to be sized with a much smaller control valve and capacity for accurate control by valve alone.

The stage-1 coil control valve would open first and a condensate line thermostat would enable the OA damper to open. A damper end switch would then enable the fan to run. Peace of mind.

Dave in QCA · March 2011

Setup sounds great.

Brad, I have not seen that setup used, but it makes perfect sence and I'm sure it would work great.

air assist for steam pre-heat coil

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