"Our problem is that five out of hundreds of univentilator coils freeze up on a regular basis. They are part of two pipe steam systems with pneumatic control valves and condensate pumps. The F&T traps have been replaced and the coils are grading down to the returns. The traps on some are two feet or so below the coil outlet. The boiler pressure on all buildings runs between 4 psi and 8 psi and the boiler shuts down at night until the coldest room in the building requires heat. If the outside temperature falls below -15 C the boiler stays on pressure. Do you have any ideas on how to solve this problem?"

I figured he was a Canadian (the Metric system gave him away), and if you're going to freeze steam coils, what better place to do it? I wrote back and told him that he should look and see if there's a vacuum breaker between the control valve and univentilator. Vacuum breakers do exactly what their name implies, and vacuum is what you get when steam condenses, unless air can get back in to break it. Unless it's a vacuum system, vacuum is not your friend.

What's going on in that coil is exactly what goes on when you hold your finger over the top of a straw that's filled with water. Think of your finger as the vacuum breaker. Lift your finger and gravity does the rest.

But when it comes to steam coils, you can't depend entirely on gravity to get rid of the condensate. The guy in the letter told us that his traps are two feet below his coils, right? Okay, think like condensate. You're building up inside the coil. The control valve is closed because the space is warm enough. The vacuum breaker just opened. You now have just two feet of vertical pipe available for you to stack and build static pressure. How strong are you feeling right about now? You're producing less than 1-psi pressure down there at the bottom of your stack, aren't you? There's a float & thermostatic steam trap down there and you're pressing against it with a measly 1-psi pressure.

Feeling wimpy? Me too.

And all of this assumes that the vacuum breaker is in place and working, of course. If it's not, there will be a vacuum tugging you back into the coil and there will be little or no pressure on the inlet side of that F&T trap. Which is why vacuum breakers are so important, if you'd like to keep the specter of frozen coils out of your life.

The challenge here (and I'll bet this is a big part of our Canadian friend's problem) is that a lot of people select these F&T traps for a differential pressure that's going to occur when the control valve is fully opened. They're figuring that steam at the maximum available pressure (4-8 psi in this case) is actually there. And a lot of people will assume that the pressure at the outlet side of the trap is atmospheric because the trap probably drains toward a vented condensate receiver.