Boiler Behavior Question

edited April 2018 in General

Hey gang, I was at a customer's place this week trying to help them understand how to operate their still.

Prior to my arrival they were running their boiler (and subsequently their kettle) with a thermostatic controller. So they would essentially set to a specific temp, and once temps are achieved the boiler turns off. Naturally this method sucks for distilling.

So we installed a globe valve and pressure gauge at the still, set the thermostat on the boiler at the highest temp (240°F I think) and manually adjusted as needed at the still. Seemed to work pretty well. They have a million BTU boiler so no issues with being under sized.

My question is that that gauge was telling us that the pressure seemed to vary on its own through out heat up and into the run. as we got more into the run the pressure seemed to more or less stabilize. I'm not really sure i understand this behavior compared to how absolutely precise work with electric heat input.

Can someone explain in plane language why the pressure seemed to bounce around during the run rather than stay relatively stable?

StillDragon North America - Your StillDragon® Distributor for North America


  • I'm sure @grim will get you figured out, but pressure during the run is always more stable than during heatup.

  • The situation inside the boiler is unstable untill all the liquid reaches the same temperature. This is also reflected by the heating up sounds of the kettle suddenly reducing at the time all the liquid is now boiling.

  • @punkin

    So hot.

    The kettle I mean.

  • edited April 2018

    Steam Pressure = Voltage

    Steam Flow Rate = Amperage

    Power Input = Steam Pressure * Flow Rate

    Since we can't afford mass flow devices to measure the pounds of steam delivered, it's not really easy to know directly the power input. We expect the pressure gauge to react like a watt meter, but that ain't happening. We only see one variable, and it isn't rock solid like electrical power.

    By turning the valve open, we increase the flow rate, but that doesn't necessarily correlate with steam pressure, especially if the kettle is cold. Steam will condense in the jacket as fast as you can feed it, you'll only build pressure if you can flow more steam than the jacket can condense.

    Likewise, a quick snap of the globe valve to a position that's more closed, and it's common to see the steam pressure crash, even go vacuum, and then come back up. Why? Because the steam in the jacket condenses faster than the new valve position lets in steam. Small downward changes in steam flow usually create curiously big swings in the pressure, something you don't see when you open the valve.

    Driving a steam still is like piloting a really big boat. You make small changes, and you wait. It's not like driving a Formula 1 car (Electric Direct Elements), that respond nearly instantaneously with laser precision. Steam is like a big ass cruise ship. Try to move fast and you end up overcorrecting.

    Common issue I see with steam boilers is the steam installers set them up for heating duty, meaning something like 3 psi on the cut-in, cut-out. Big swings of the boiler pressure. You'll commonly see them set the top end at like 11psi or so, so that 3psi differential - that's something like a 30-40% swing in steam pressure. My plumber nearly had a heart attack when I told him that I reduced the differential to 1 pound. Burner is going to fail, boiler is going to fail.

    Tight steam pressure on the input makes for a more consistent operation. On a big 1 million BTU boiler, it's likely that a 3psi differential might be 20 minutes between peak and low pressures. Couple that with overcorrecting on the globe valve, it's easy to be all over the place. A globe valve isn't a regulator, the higher the pressure on the input of the valve, the greater the steam flow through the valve. The best systems I've ever seen used 150psi high pressure boilers with 15psi regulators. The steam control was always rock solid.

    As boiler gets up to temp, the pressure gauge starts to be a more meaningful indicator of power input. Usually at that point you can easily build and hold pressure in the jacket, something more difficult to do during heat-up.

  • Ok awesome.

    So based on your answers, I would say that the system is relatively well behaved now. And also their newly minted operator has a much better handle on what to expect.

    I suspect they'll discuss further with their boiler service provider to try and get the unit as dialed in as comfortably possible.

    Thank you for the feedback.

    StillDragon North America - Your StillDragon® Distributor for North America

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