Hot water boiler - maximum temperature needed for special use heating - What Delta T can I achieve?

some_wood_guy

Hi all, this is an extension of my previous post about steam heating (I'll link it later) and after that discussion this has lead me to look back at hot water boilers for my situation.

First let me say thank you for any help you can throw my way - my hot water heating experience is in infancy and I will require time & experienced folks like yourselves to "show me the way". So, thank you for answering my beginner's question. My situation is still in the planning stages, so I'm all ears as far as if this is a good use of hot water boiler, if it's reasonably achievable, or any other input you might have.

I'd like to potentially use a hot water boiler in order to heat-treat firewood. I have an idea to use fanned heat exchangers in a 10ft x 10ft x 6ft room. The room is well insulated with polyiso foam insulation, including foil faced insulation on the ceiling. I would have the fanned heat exchangers on 2 walls facing one another, with metal IBC style totes of stacked, split firewood resting in between them. There would be anywhere from 2 or possibly 4 totes of firewood in the room at any given heating session, each tote containing between 800-1000lbs of firewood.

Needfully, the hot water heat running thru the heat exchangers will be enough to heat the cut & split firewood to a minimum internal temperature of 160F for a minimum time period of over an hour. I have sourced a used 96K BTU Burnham 204 LP hot water boiler as well as some ordinary copper tube / aluminum fin heat exchangers, as well as some high-heat automotive radiator fans (and will include some high temperature fan wiring). I will also source a high-temp room thermostat that will be used to turn on/off the boiler as needed to avoid any overheating in order to protect the insulation. The boiler would be located in an adjacent "room" that is separate from the heating chamber and the piping runs will be very short - probably 30ft overall not including the heat exchangers. Note that this room and the external "room" I have framed for the boiler will not be climate controlled, and will experience freezing temperatures. I assume, but need clarification if, the LP boiler can accommodate periods of freezing temperatures with the proper precautions. Would this include treating the water loops with antifreeze - and what else?

Realizing that an LP boiler in a non-climate controlled room may be less than ideal, I can of course explore a wood fired hot water boiler - I have plenty of firewood, after all :) But I was thinking that the LP boiler would be ideal especially with the ease of running it off a thermostat.

What I'm trying to work out is: can a hot water boiler get hot enough for effective heat transfer both in terms of efficiency and time? I see that the boiler I have has a maximum water temperature of 250F. While I don't expect to be able to run the water that hot, one of my questions is: How high can I crank the water temperature to be safe and within reason allow maximum Delta T thru the heat exchangers and into the wood? Can I expect temps of 200F or 210F perhaps? I don't think that an ordinary boiler water temp of 180F will provide enough Delta T to heat the wood in a reasonable time, nor am I sure that a small room temp of 160F can be achieved at all given a Delta T of only 20F. So in general: can I achieve a delta T of 40F or 50F in order to provide a much faster and certain 160F wood temperature?

Also of note, I don't think pex can handle this high of water temperature, so what other piping options would be appropriate? Lastly, I will of course seek professional help to install this system. I'm not trying to learn how to install a system like this over the internet, but I am trying to gauge if my heating idea is feasible and achievable.

Thank you very much for any input you have!

some_wood_guy

Useful inputs from the other post that are noteworthy here:

+ Yes, I will need high-temp (and appropriate sized) wiring for the fans and the thermostat wire, I have found some teflon insulated wiring for this use.

+ Yes, I will need appropriate insulation or heat barrier for the insulation in order to handle the high heat.

And also, I am not drying the firewood, per se, with this heat-treatment kiln. My only goal here is to heat-treat the firewood to the sufficient internal temperature satisfy Department of Agriculture regulations regarding eliminating pests/parasites.

Derheatmeister

This sounds like a lot of time and energy will be enbeded into this operation to perform this regulation by the USDA.

Is this firewood then also transported from the original location to another location?

some_wood_guy

https://forum.heatinghelp.com/discussion/comment/1799957#Comment_1799957

Yes, that's right.

Derheatmeister

Is it going to be worth it , Does this make economic sense ?

Maybe this whole operation will increase your Carbon footprint.

EdTheHeaterMan

One problem I see is the fan motor that is located inside the 180°+ room that will circulate the room air over the heat exchanger.  The standard HVAC fan motors are not designed to operate in that environment.  You may want to think about a belt driven fan in the kiln that can have the motor located outside the kiln. 

The standard heating boiler for residential and light commercial use, can operate with water temperature near 220°F with 15 PSI water pressure in the boiler.  I however would try to design the room using 210°F water or lower because you will be operating below the boiling point at sea level.  The boiling point of water at higher altitudes above sea level is somewhat lower.  

I only recommend this because, when you have a situation where you have water under pressure above the boiling point in a pressure vessel, if you ever have a failure that is not related to the relief valve, you could have a condition where the water pressure drops rapidly causing the high temperature water to flash into steam and that rapid expansion of water to steam can cause an explosion if enormous proportion.  This event is known as a BLEVE.  Boiling Liquid Expanding to Vapor Explosion.

That is where for example 220°F water under 15 PSIg pressure has a rapid drop in pressure to 0 PSIg.  Now that 220°F water will be above the boiling point at 0 PSIg so the water flashed to steam instantly.  The steam expands 1700 times its size of the water and the boiler and pipes and heat exchanger is not designed to hold that rapid expansion of pressure, so parts go flying in all directions.   

Under normal conditions the relief valve would hold the discharge to a slower pressure drop. This allows the smaller amounts of steam flash to increase pressure at a slower volume which will cause the small amount of flash steam to repressurize back into water and leave the pressure vessel intact.  I just think that operating the boiler at 200° or below is good design, to avoid the possibility of a BLEVE.  

As far as the differential is concerned, there are "off the shelf" controls that can be set at 200°F with adjustable differentials as low as 5°F so that will keep your heat exchange water at an average temperature of 190°F 

some_wood_guy

https://forum.heatinghelp.com/discussion/comment/1800138#Comment_1800138

I'm doing the math for my operation in order to know if it's worth it. But let me ask in particular: what part would not make economic sense? If I can find boiler, short piping runs, and fanned heat exchangers for a reasonable upfront cost, what other costs are you seeing that would make you question if this is worth it? I'm legit asking if I'm missing something.

Example of what I may be missing: Does the Burnham 204 have a built-in circulator pump, or am I supposed to source that separately?

At 80,000BTU boiler, it's about $3/hr for heat. As long as I have a decent delta T in order to heat the wood as quickly as possible, yes I do think it will be worth it.

some_wood_guy

https://forum.heatinghelp.com/discussion/comment/1800164#Comment_1800164

Thanks for the input on the fan motors. I'm hoping and assuming that the automotive-grade radiator fans I will be sourcing will be able to handle the heat.

Also, thanks for the input with the temp and flash steam risk. In this case, I would need to run antifreeze with the water, so the boiling point will be slightly higher at 223F. Thus, I'm hoping and expecting that I can run at 210F with a decent degree of error to prevent the risk of flash steaming in the event of pressure loss. Am I on the right track with that assumption?

EdTheHeaterMan

I think you are on the right track. What elevation is your location. Boiling point of a liquid is always measured at sea level. That is 0 PSI Gauge or about 14.7 PSI Absolute. If your ambient pressure based on elevation above sea level is lower that 14.7 PSI Absolute, then that will affect the BLEVE pressure. For example Denver, CO. is 5200 feet above sea level so the boiling point of water is about 203°F. So as long as your location is about 2000 feet above sea level or lower, you should be fine with 210° boiler 40% antifreeze mix temperature.

Steam heat is also at a lower temperature when you are above sea level, A steam boiler will make a 205° radiator in Denver, but it will make that same radiator 215° in New York City. There is a lot of things to consider when you look at using a boiler for your needs.

As far as using an automotive radiator fan to operate in that room, just remember to look at the motor specifications to make sure you are not constantly replacing motors because the "ambient room temperature" is 160° or higher.

I remember the great comedian Stephen Wright quote: "Room temperature is what ever the temperature of the room is."

leonz

The fellow that sells firewood to vermont castings has a steam boiler in his wood shed and he uses dry steam to heat the wood that he sells to them using radiators. A hot water boiler will not do what you need to do even for a small amount of firewood.