Steam engineering question

bob_50

Sarco puts out a little book called Hook Ups. I think you would find it worth the effort to get one.

Dan_22

Steam engineering question

Last week I installed a Market Forge boiler which supplies steam to two commercial kitchen components. One 60 gallon steam kettle with 119 lb/hr of steam at 15 psi. Also a two compartment food steamer which runs at 5 psi. The problem began when I set the boiler exactly as the manual stated with a 12psi cut in and a 14 psi cut out. It would overshoot and cut out at the 15 psi high saftey limit. I called the manufacturer and the tech rep told me the manual is wrong and to run the boiler at 5psi cut in and 7psi cut out. I question him about the decrease in capacity this would cause with the kettle and also how the same company can design a kettle which is engineered to run at 15 psi and not supply the boiler to run it. He told me the kettle output would not be affected because the 300mbh boiler will still give the same output to the kettle at 7 psi. This just does not ring true with me but I have nothing to dispute the fact with him. My reasoning is even if the boiler can supply the kettles required steam the velocity through the 3/4" supply would have to be around 15,000 ft/min to satify this btu requirement at 5psi. Is it even possible for 7psi steam to reach a velocity of 15,000 ft/min? And if so it still doesn't seem like a good idea. Does anyone know the math formulas I would need to prove that 5psi through a 3/4" pipe running 8feet to feed a kettle which requires 119 lb/hr of steam is not a good idea. Then maybe when I call back Market Forge I will have something to say. Thanks for any help anyone can give

Tony Conner_2

The...

... 3/4" line should be big enough (just). It's a short run. You could try 5 and 10 PSI if you need to. If you're dealing with short runs, and velocity noise isn't much of a concern, you can hit some pretty impressive velocities with steam.

What controls the steam to the kettle? There isn't some nasty quick closing valve like a solenoid in the steam line, is there? If there is, it'll snap shut when the temp is made, and the flywheel effect of heat input to the boiler won't be able to react before the boiler trips or the safety lifts.

Systems like this (or any other) are like links in a chain. Any one link affects the ones on either side of it, and it just dominos-out from there. From the sounds of it, I think you'll be able to tweak this set-up and make it work.

Dan_22

Thanks for the answer. There is no quick closing valve just a manually operated valve that controls steam to the kettle. Do you know the actual math formula for figuring the max velocity of steam. I mean can steam actually reach the speed of sound? It just doesnt seem possible. There has to be a limit.

Dave_12

there are two issues to look at

1. The temperature of 5 psi steam is lower than 15 psi steam. What does your process actually require.

2. Watson McDaniel says that steam lines should be sized for 4800 to 7200 fpm. I don't know how to calculate velocity without a lot of research, however go to www.spiraxsarco.com/esc. There are some excellent pipe sizing calculators available there. Make sure to reset units to ft/min, psi gauge, lb/hr etc. From the numbers I ran there 3/4" sched. 40 seems just shy for 15 psi steam and very short for 6-7 psi steam.

Good luck.

Tony Conner_2

Every Fluid...

... has a critical or sonic velocity, which is the speed of sound for that gas or liquid. Flow, velocity and pressure drop are all inter-related. Pressure drop drives everything with respect to flow. (No pressure drop, no flow.) The greater the pressure drop, the higher the flow, the more velocity. That is, until you hit the critical velocity for the particular fluid you're looking at. Once you reach that stage, increased pressure drop won't generate any more velocity. The fluid is moving as fast as it's going to, and that's it. For people working in the field, you'll hit pressure drop problems - things just won't work properly - long before you ever see critical velocity.

The valve ahead of the kettle - is it a ball valve? Production people have a habit of slamming them shut. It'll give you the same effect as a solenoid valve.

The one sure way to determine if the line is too small, is to install a pressure gauge just ahead of the kettle, and see how it compares to the pressure gauge on the boiler when things are operating.

Dan_22

The valve feeding the kettle is not a ball valve but it can be shut quickly by spinning the handle shut. The kettle is installed at a prison facility so the equipment is not treated very carefully.
I dropped the pressure to 12psi cut-out and 10 psi cut-in and the kitchen seems to be happy. It has not tripped out on high limit since I lowered the pressure.
I still have a little issue with Market Forge as to why they would design this system to require such high velocities to meet the required 119 lbs/hr load. Thanks again for the reply.

Tony Conner_2

Main...

... industrial supply runs may get taken up a pipe size or two to allow for future expansion, but the shorter shots maybe 50 to 100 feet long - to specific loads - I'll usually size to peak about 12,000 feet per minute. The velocity noise of steam whistling through the lines at operations like corrugated box plants is a non-issue compared to the roar of the other equipment.

Dan_22

The run is very short only 8 to 10 feet of flexible steam hose. The components are bolted together. I guess the engineering of these components bring the steam to its limit to reduce cost and space. It does cause an unstable waterline in the boiler though. From what I've read steam under 50 psi should not exceed 6000ft/min. Other than the unstable water line velocity noise is not really a problem in a commercial kitchen. It sounds as if you have played a part in designing steam systems or equipment. I did not know that the velocity of gases or liquids is governed by their speed of sound velocity. Is there any way to force them to exceed the speed of sound? Is there a chart that lists the peak velocities of gases and liqids?

Dan_22

Thanks for the reply. I don't think the temperture would be as much of an issue as the amount of steam you can deliver under various pressures at various velocities. I think there is a about a 30 degree f difference between 5psi steam and 10 psi steam. I am not sure how much this would effect the btu transfer rate.

Dan_22

I got the book online. It is exactly what I was looking for. Thank you very much for the info.

Tony Conner

The...

...lower the steam pressure, the more BTU/lb you get, in terms of latent heat. And latent heat is where the action is with steam. The downside of low pressure steam, is that each lb of steam gets BIGGER, and you can't fit as much through a given size of pipe, valve, or heat exchanger connection. Also, with higher pressure steam, the temperature is higher, so you get a better LMTD (log of the mean temperature differences), which is a big factor in sizing heat exchangers. The bigger the temp differences, the bigger the LMTD, the less surface area required to get the job done. There are are benefits and drawbacks to either way of looking at things.

Tony Conner

There's A Book...

...well, it's actually pretty thin, and soft covered, entitled "Crane Technical Paper No. 410". This is THE book on fluid flow in pipes and valves. In a lot of places, it gets pretty deep in math I never originally knew, or knew 25 years ago, and have long since forgotten. However, it's a great publication.

They also put out some software that's pretty neat. My version is old, and won't run on any kind of new machine, but it does everything I need it to, so I'm not planning to pop the $800 for the new version until absolutely necessary. When you get using it, you can see very quickly just how much of a difference the length of pipe makes. It's HUGE. If you've only got, like 8 or 10 feet of line, man, you can stuff a lot of steam down that short run.

In my copy of "Hook-Ups", there's a velocity chart on page 4. I carry a photocopy (it's old enough that it's not a scan - and it's looking a little rough) around tucked into my little pocket calander notebook. I've used that chart for over 20 years, and it's never let me down. When talking about steam flows, I size for the PEAK loads to hit the top of that chart. I don't try to have the regular load actually LIVE at 200 feet per second. You need to keep in mind, that for runs over maybe 100 or 150 feet, you'll need to back off a little, on this chart, and for short runs, you can "kick it up a notch".

The flip side to the short pipe runs, is, of course, LONG pipe runs. I used to work in district heating, and we had pipe that ran for a lot of city blocks. The Crane program was good there. If you tried to take any kind of velocity at all, the pressure drops would get you over the long distances. You'd start off at 125 PSIG in the plant, and at the end of the system, on a cold day, you'd be down to 75 PSIG if you were trying to work with a velocity of 100 feet per second. You'd have to look at having an end of system pressure of maybe 115 PSIG to have things still work properly, which might mean velocity of 50 or 60 feet per second.

Dan_22

I looked up "Crane Technical Paper No. 410 and I am going to see if I can get it at the library or buy it. All seem to agree this is the authority for fluid flow dynamics. I checked out page 4 in the hook up book. The chart does push the limits but it looks like it doesn't break the law of physics. I ran across one web page calculator that was giving me velocities exceeding 800mph. My interest in steam has recently peaked since I took over as head of Maintenance at a prison facility build in 1934. Dan Holohans book "The Lost Art of Steam Heating" has helped immensely. Once I start to learn about something I have a habit of always wanting to know more and go deeper than most books will. Thanks for pointing me in the right direction.

Tony Conner

Between The...

... Lost Art and Hook-ups, you've probably got most of the info you'll likely need. I've picked-up a ton of excellent old books off eBay. You can sometimes tell when Grampy has passed-on, or gone into a home - there'll be a row of listings of from the same seller - 20 or 30 books - on mining, or refineries, marine architecture, etc. And every once in a while - powerhouses and piping.