What is a "steam dryer"?

[hartleymartin]

Being somewhat familiar with steam locomotives and the concept of superheating, I must ask what is a "steam dryer"?

I know that it isn't quite the same thing as a superheater, as you don't necessarily want superheated steam in a marine boiler application, particularly for things like small steam launches.

[Scotty]

In large watertube boilers it is an apparatus in the steam drum to remove the water from the saturated steam
before it goes to the superheater. Also called a demister.
On a high pressure boiler you really don't want any water (and if only drops) in the superheater.
Mostly a large packing of stainless steel wool, sometimes also a cyclone.

Not the best picture, but it shows the principle.
http://www.shg-schack.com/en/products-a ... team-drum/

With our small plants this is mostly not practicable.
There I would call a small superheater with no more than 20 °C superheat a steam dryer.

Scotty

[fredrosse]

In historic practice, horizontal boilers often had a steam offtake pipe that ran horizontal in the very top of the boiler, with many small holes where the steam would enter before leaving the pressure vessel. This pipe was called a "dry pipe". They are also found on some VFT boilers, same concept, except a circular bent "dry pipe" in the top of the boiler shell, running around just inside, and at the top of, the boiler shell. The intent of these pipes was to preclude water droplets being entrained into the main steam flow.

Modern industrial watertube boilers often have "dryers" mounted in the steam drum to preclude water carryover to the steam outlet pipe. These devices use centrifugal or chevron action to separate the water from the steam as the two phase steam/water mixture exits the steam generating tubes.

Steam dryers can also be external to the boiler proper, typically a small vertical pressure vessel, a separator tank with a tangential steam inlet near the middle, a steam offtake near the top, and a drain at the bottom. If the boiler steam is wet, or the boiler feeds a slug of water into the steam pipe, this vessel protects the engine from being damaged by the water induction. My steamer has such a separator fitted, made from a 12 inch long x 3 inch diameter piece of pipe. A steam trap automatically drains any water accumulation.

[hartleymartin]

I want to see if I understand this concept. Let's say that I have a vertical fire tube boiler.

1.) If I run a ring of copper pipe around the top of the cone (the equivalent of the firebox on a horizontal boiler) which isn't directly above any of the fire tubes, would this function as a dryer/small superheater?

2.) Rather than just a pipe coming straight out of the top of the vertical boiler, should there be an internal copper pipe with a series of holes drilled in the bottom to separate the water from the steam, so that any water will tend to fall back into the boiler rather than come out the main steam pipe?

3.) Whilst we're on the subject of boilers, assuming that it is a "wet-back" firebox, is it fine just to use plain hardwood for cladding, or should there be a layer of insulating fibreglass tape?

[Lopez Mike]

At the firing rates you will see on a small launch you will not need a drier. The bubbling and turbulance is moderate at best.

Take the steam out of the side as near to the top as convenient and don't run your water level above perhaps a couple of inches below it. And have your steam outlet at the front of rear of the boiler so power boat wakes won't slosh the water up near the outlet.

Keep it simple.

[steamdon-jr]

As far as cladding goes if you have a wet bottom VFT boiler you will be fine with no insulation under the cladding. the small amount of heat from the ash pan ring is no problem. however if you do not have a wet bottom VFT then it will be subject to the full heat of the fire.

[hartleymartin]

Thanks, that simplifies things greatly.

[Lopez Mike]

I have tried the insulation both ways and ended up with a 1 1/2 to 2 inch thick fiberglass blanket under the wood. First, my wood, mahogany, tended to char just a bit and the heat dried out the varnish badly and the wood began to split and deteriorate where it was near the steam pipe and such places.

Second, I noticed a very noticeable change in the behavior of the boiler. The boiler will hold pressure much longer when I'm at the dock with a spot fire. Since I burn wood I have no easy way to measure consumption under way though I feel confident that it is lower with the insulation. But when stopped, the reduced energy loss becomes very noticeable.

The drawback is the increased diameter of the boiler. Three to four inches more right where I have to squeeze past it to pay attention to the babes in the bow.