Greetings from Yakima, Wa

[SidWell]

Greetings,

I am very new to the world of steamboating. I have built an ultralight airplane and a car from the ground up, but, I have no experience with steam. I love doing new things that are a challange. I just purchased a very rough 26' whaleboat. I hope to power it with an engine in the 6 to 10 HP range. I have no equipment other than hand tools so I will need to purchase a ready built boiler and engine. I believe I can do the rest with the assistance from this forum.

After looking at several web sites for steam boat equipment, most of it is obvious except for one piece: vacuum pump. What purpose does a vacuum pump serve for a steam engine?

[boatbum]

The condensate pump helps to create a vacuum in the condenser to aid in drawing the steam from the engine into the condenser and to de-airate the feed water going from the condenser to the hotwell (you want as little oxygen as possible disolved in the water as it's hard on the boiler). The vacuum also makes the overall efficency of the engine slightly higher than non condensing engine set ups.

[SidWell]

So is the complete steam system closed from the atmosphere? Is the hot well under a vacuum as well?

[artemis]

Greetings,

...After looking at several web sites for steam boat equipment, most of it is obvious except for one piece: vacuum pump. What purpose does a vacuum pump serve for a steam engine?

Steam engines not only "leak" steam through/around all kinds of things like valve stems, piston and valve rod packing glands, threaded pipe joints but, when pressures "inside" drop below atmospheric (like a condensing engine where a partial vacuum is created in all the piping leading to the condenser) they will "leak" air into those places (science 101: nature abhors a vacuum). All the air entrained in the steam from the atmosphere and carried over in the boiler also contributes. When the exhaust steam is condensed to water the volume of the water is about 1600 times less than that of the steam. Any residual air expands to fill the space and represents a much greater volume than that of the water. In large vessels the volume of this "air" can cause serious problems, absorption of the "air" back into the "condensate" a distinct possibility. In large vessels this is handled by having a large "air pump" and a much smaller "condensate" or "water pump" remove same from the condenser. This is unnecessary (and impractical) in small engines and a single pump is fitted, but because it is doing double duty it is sized as though it is for "air" and the name has stuck.

So "air pump" as applied to our little boats (under 50' ?) simply means a suction pump that "sucks" everything - gas or liquid - from the condenser and pumps it into the hotwell. However, since this "everything" contains air, it is best discharged into the hotwell a couple of inches above the surface of the water so that the air is not re-absorbed back into the feedwater.

Post Scriptus:
Two items you should do immediately:
1. Join the Northwest Steam Society - http://www.northweststeamsociety.org
2. Buy the steamboater's bible: Steamboats and Modern Steam Launches - available from the publisher: Elliott Bay Steam launch - http://www.steamlaunch.com

[Maltelec]

A vacuum pump lowers the pressure on the engine exhaust which in turn lowers the steam temperature (a specific steam pressure has a specific temperature for dry steam).

Because the real energy in the steam is the heat contained within it, a larger pressure difference across the engine = a larger temperature difference = more energy out of the steam.

This extra energy you get from the vacuum is greater than the energy required to create the vacuum.

Same reason for superheated steam, more heat = more energy. For the same power out you use less steam, or for the same steam you get more power. Either way the efficiency goes up.

[SidWell]

Thank you all for your excellent answers. I think I have the idea now: The vacuum in the line between the engine and the condensor reduces the amount of free oxygen in the water. This reduces rust in the boiler. The vacuum also reduces the backpressure between the engine and condensor. This results in a horsepower gain that is greater than the energy required to operate the pump. Sign me up for a vacuum pump!

In case any of you are interested, here is a link to my past projects. I will soon be adding a new set of photos for my steam boat project.

http://s6.photobucket.com/home/SidEllegard/index

[87gn@tahoe]

Welcome to the forum Sid. Where do you hail from?

[fredrosse]

The vacuum pump (or air pump, oe wet air pump if it also pumps water) is locared downstream of the condenser, not between the engine and the condenser. It removes air that has leaked into the steam system, but only after the condenser, otherwise the vacuum pump would have to pump both the exhaust steam plus the air, and would therefore be very big.

The main purpose of the vacuum in the exhaust of the engine is to increase efficiency of the power plant. For our steamboats this can amount to several percent more output power. Many boats run with the steam exhaust direct to atmosphere, and have no condenser and no vacuum pump. My steam sidewheeler has a wet vacuum pump, but I have been running with exhaust directly overboard since I have not yet built the condenser. Usually lake or river water is OK to use directly in our boilers. Our steamboat group has about 10 boats, none of them typically run condensing, although a few of the boats do have the machinery to do this. The real compelling reason for a condenser is when running in salt water, as using salt water in a boiler is definately not a good idea, so one must condense the exhaust steam back into water, then pump this condensate back into the boiler.

Boiler corrosion is lower with deaerated water, and this can be achieved with or without a vacuum pump and condenser arrangement. You can condense the steam with or without a vacuum pump, but with the vacuum pump you can get higher power and higher efficiency.

[SidWell]

@FredRosse: Boy did I miss the boat on that one. Thanks for clearing that up for me. I plan steam in the resivors of the Colombia River, inland lakes of Washington and Coeur d'Alene, Idaho so I should be able to avoid the vacuum pump / condenser configuration for now. Maybe some day we will make it to Tahoe as well.

@Artemus: Thanks for the suggstions, I will get the book and join the club.

@Wes Harcourt: Yakima Washington and before that, Anaheim Calif.

[87gn@tahoe]

Anaheim explains the photos of flying over the desert and CA license plates on your car.

If you do make it down to Tahoe, give us a ring and we'll have a "mini meet"