Published: May 22, 2017 - by Dan Holohan

Categories: Steam

jenningsunittypecondensationpump

Many big buildings in our older cities use vacuum pumps to create a negative pressure in their condensate-return lines. This below-atmospheric pressure, which averages around 5.5 inches of mercury, lets the Empire State Building get away with running just 1.5-psig steam pressure on the coldest days of the year.

            Vacuum pumps can do these things for a big heating system:

 

  1. They can get the air out of the system very quickly.
  2. They can get the steam from the boiler to the furthest radiators even quicker.
  3. They solve a lot of the steam distribution problems and heat the building more evenly.
  4. They let the designer use smaller pipes for both the supply and return sides of the system, as well as smaller steam traps. They can do this because of the greater overall pressure differential the vacuum pump creates. This saves a lot of money on the installation.
  5. And, where needed, they can also lift condensate from a low point in the system.

 

            All of this means that once it’s a vacuum system, it will always be a vacuum system. You can’t easily turn it into something else. The pipes are too small to work well as a pressure system. They may even be in the wrong place if they’re coming back to the pump from below.

            When you first start it up, air fills the entire system. The vacuum pump and the burner come on together. The pump creates a vacuum inside the entire piping system. Air moves through the pump and leaves the system through a single main vent line.

            Before long, the boiler is making steam. By this time, the vacuum pump has much of the air out of the system, so the steam goes roaring up toward the radiators because high pressure always goes to low pressure. Everything is working on a push/pull principle here. The steam’s pressure pushes and overcomes piping-and-fitting frictional resistance, while the vacuum pump pulls the air through from the return side. Because of this, the engineer probably sized the pipes to be one size smaller than they would have to be in a standard steam system without a vacuum pump. Having pipes of this size isn’t a problem – as long as the vacuum pump is working.

            The steam arrives at the radiators. It closes the steam traps on temperature and gives up its latent heat to the radiator’s cold metal. With the steam traps closed, the vacuum pump continues to pull on the returns until it reaches an average vacuum of 5½ inches of mercury. Once the steam turns into condensate inside the radiators, the trap elements cool, open, and the condensate flows back to the boiler by gravity.

            But then one day, after years of faithful operation, the radiator trap elements begin to fail. This isn’t a dramatic event; they don’t all fail on the same day. They die one at a time over the course of a few years. No one notices this because the radiators continue to heat. The big difference, of course, is that since the traps aren’t closing, the steam is roaring into the return lines, creating water hammer, killing good traps, and working its way steadily back to the vacuum pump.

            It will take time, probably years, but the day the steam arrives back at the vacuum pump, it will destroy it. Vacuum pumps can’t pump steam. They'll just whine, scream, cavitate and die. The building superintendent will hear this and shut off the vacuum function.  Now the vacuum pump operates solely as a condensate pump, and this will affect distribution. The folks on the first floor are roasting and the guy on the top floor is freezing. He’s calling the Building Department. The superintendent responds by raising the boiler pressure, and that makes the folks on the first floor even more miserable. They open the windows.

            The superintendent tries to solve the problem by adjusting the central heat controller. He fiddles with the settings, making it run longer or shorter than it should. He even tries moving the temperature sensor around in the building piping. He tries this riser and that. He puts it on a wet return, and then on a steam main. Nothing seems to help so he tells the building owner there’s something wrong with the central controller. The owner gets on the phone with the control manufacturer and complains.

            Everybody’s mad at everybody else.

            Meanwhile, the steam is still working its way back to the vacuum pump, and since the super has the boiler pressure cranked way up, the condensate is hotter than ever. Steam shoots out of the pump’s receiver vent, the pump whines, screams and dies.

            The owner calls for service. A pump-repair guy shows up and rebuilds the pump. As soon as he walks out the door, the steam and red-hot condensate jump right back on the pump again.

            The owner makes another call. He’s mad at the pump-repair guy, so this time he calls a plumber. The plumber figures correctly that the bad traps in the building are killing the pump, but since the owner won’t go for new traps because of the expense, the plumber installs one very large steam trap at the inlet to the vacuum pump. He figures this will take care of the whole building.

            The pump comes on and pulls a terrific vacuum, but only as far as that big steam trap, which sits about two feet away from the pump. This doesn't help steam distribution, so the superintendent cranks the pressure up even higher. Now the big steam trap has about 10 pounds of steam pressure on its inlet side. When it opens, it vomits 240-degree condensate straight into the face of the newly rebuilt vacuum pump and causes it to whine, scream, cavitate and fail. Again.

            Last winter, a contactor called to tell me about a 17-story building that had a vacuum system. It also had some new motorized zone valves so they could heat certain parts of the buildings at certain times. Now when a motorized valve shuts, a vacuum will form around it as the steam condenses and shrinks back into water. Someone anticipated this and installed vacuum breakers on the outlet side of every motorized valve. Their thought was that this would allow air to enter the system so that the condensate could flow back to the boiler by gravity. It's the way we do things in systems that don't have vacuum pumps.

            But this building did have a vacuum pump, and when you add vacuum breakers to a system that has a vacuum maker, you're going to be spinning your wheels for a long time. It's like having a humidifier and a dehumidifier in the same room. Good luck with that.

            The contractor who called b was having a tough time trying to explain this to the building owner. They didn't want to have the vacuum breakers removed because they had paid to have them installed. The contractor needed me to tell him that he was right so he could tell the owners that I said so.

            I obliged him and haven't heard a word since. I like to think they listened, but who knows? People can be pretty strange when they pay for things that are wrong and don't work out as planned. It's easier to blame something else than it is to admit you made a mistake by hiring the knucklehead in the first place.

            A deep breath and some common sense can help.