Brad White- Proper steam coil piping

Glen

lbs per hour capacity of each coil - subject to freezing??? piping arrangement? Your choice of trap is critical to avoid the freeze up ...

Ross_7

Brad White, Proper steam coil piping

Hi Brad,

Can you give me some examples for proper steam coil piping for modulated steam coils in an AHU, under 15 psig, which may be exposed to freezing temperatures? I think that good piping practices support Heatcraft's version, and an older book that I have " New Economy in steam heating" which includes using a vacuum breaker, and an additional air vent (thermostatic radiator trap)piped in for proper venting of the coil, with F&T traps serving as the main traps for the coil. TIA
Ross

Ross_7

losing ground

better re-post before I'm off the wall.

Ross_7

hello?

Brad? Are you there?

Brad White_203

Here

Sorry, Ross! I had to earn some money, but I am here now.

Rather than me second-guess specifics provided by manufacturers, here is what I normally specify/detail for such coils:

1) Valve-Only Control #1:

In order of flow: Gate valve, strainer, reducer/union/control valve/union/increaser, tee with vacuum breaker (with split tee in branch with a compound pressure gauge), flange or union to coil connection. On the outlet of the coil, dirt leg, tee, branch with gate valve, (bypass tee and globe valve optional), (second strainer optional), union/trap/union, check valve, gate valve, bypass re-entry if bypass is used.

I use F&T traps for variable flow applications. If the flow is more constant, inverted bucket traps.

On coils large enough, a compound gauge or T-P port can be helpful. Also, if load dictates, I use two valves in parallel, sized 1/3-2/3, to allow low level and full level modulation for better control.


Full valve modulation but vacuum breaker is the safety device. A freeze-stat to shut down fan, close the OA damper and open the RA damper (if any) is essential. Send out an alarm too.

Valve-Only Control #2:

Main Coil piped as above but upstream install a "first stage coil", usually a one-row, 6 FPI steam coil which has enough capacity to take the air above freezing but below target setpoint. The control valve modulates to maintain mild-weather setpoint, but when it is below 38 degrees, the control valve goes wide open and stays there.

This thin coil keeps the downstream side above freezing and in mild weather can make setpoint.

Valve Control with Face and Bypass:

This is my preferred method. Coil piped as above but preceded by a face and bypass damper. Valves modulate to maintain setpoint down to 38 degrees ODT. Below 38F ODT, the valves are fully open but the face and bypass damper takes over temperature control.

That bypass duct? Make sure you install a volume damper or perforated plate to equalize the pressure drop so that the airflow will not overshoot in bypass.

There are variations to the above so go by a manufacturer's guidelines first and always, as you do, use common sense.

Ross_7

lbs. per hour..

This was just a general question. Face & bypass method is typical of our installs. Freezing temperatures(outside design temps are -12) on occasion. Freezstats are always installed. Mixed air is set at 55F. I was mainly looking at faster venting for the coil,hence the extra air vent (radiator traps) and installed with a vacuum breaker on the outlet side of the coil prior to the trap. Existing coils were piped with WW-273 F&T traps with Powell 15 degree swing checks serving as vacuum breakers, and O2H WW radiator traps piped pallell to the F&T's serving as auxilliary air vents. Thanks for your time Brad!

Brad White_203

Vacuum Breakers

Hi Ross,

You are welcome.

Another thought on vacuum breakers, I usually put them at the top of the coil bank (top of the steam pipe after the control valve) before the coil inlet connections, to evacuate condensate all the way down.

I have seen vacuum breakers on the outlet side upstream of the trap, but in the case of a hang-up, the inlet to the vacuum breaker nipple could be underwater and defeat it.. Just a thought.