A Better Way to Determine Stored BTU's

[jebatty]

I have used sensors and thermometers to determine the amount of heat (btu's) stored and how much more I can burn before storage is full, as well as when I need to burn to recharge storage. While I have reduced this almost to an art (LOL) through weighed wood burns, which works very well, there is an easier way than using so multiple sensors, etc. -- the system pressure gauge.

The problem with temperature sensors or gauges on a storage tank are that it is difficult to know the status of the entire tank,while temperature at various levels is clear. Estimates can be made with experience. The simplicity of using the pressure gauge is that it reflects the average temperature of the entire tank. The closer system pressure approaches maximum, the higher the average storage temperature, which in many cases is the information most important for some applications, especially low temperature radiant systems.

A single system temperature gauge, measured in psi at maximum desired system pressure, 25 psi for example, and at minimum usable temperature pressure, 15 psi for example, will easily provide the info needed on the status of stored btu's and the amount of wood to burn to bring storage up to maximum.

Over my next few burn cycles I will relate psi to temperature on my burns. Currently I use mid-tank temperature as the gauge by which I do my burns, including the weighed wood burns. I think using system psi would be even easier and more accurate.

Thoughts?

 

[Pat53]

[quote author="jebatty" date="1321764860"]I have used sensors and thermometers to determine the amount of heat (btu's) stored and how much more I can burn before storage is full, as well as when I need to burn to recharge storage. While I have reduced this almost to an art (LOL) through weighed wood burns, which works very well, there is an easier way than using so multiple sensors, etc. -- the system pressure gauge.

The problem with temperature sensors or gauges on a storage tank are that it is difficult to know the status of the entire tank,while temperature at various levels is clear. Estimates can be made with experience. The simplicity of using the pressure gauge is that it reflects the average temperature of the entire tank. The closer system pressure approaches maximum, the higher the average storage temperature, which in many cases is the information most important for some applications, especially low temperature radiant systems.

A single system temperature gauge, measured in psi at maximum desired system pressure, 25 psi for example, and at minimum usable temperature pressure, 15 psi for example, will easily provide the info needed on the status of stored btu's and the amount of wood to burn to bring storage up to maximum.

Over my next few burn cycles I will relate psi to temperature on my burns. Currently I use mid-tank temperature as the gauge by which I do my burns, including the weighed wood burns. I think using system psi would be even easier and more accurate.

Thoughts?[/quot

Yup, thats what I do also. I can check the pressure gauge on the propane boiler in the basement without going into the garage. My system is pretty much the same as yours....15 PSI when the tank needs recharging, about 125F, and 25 PSI when its up to a 195F.

 

[ewdudley]

Over my next few burn cycles I will relate psi to temperature on my burns. Currently I use mid-tank temperature as the gauge by which I do my burns, including the weighed wood burns. I think using system psi would be even easier and more accurate.

Thoughts?

Very nice. But be sure to use a sealed-reference gauge, not an atmospheric-reference gauge, or you may be off by a over a psi every time a hurricane passes through Minnesota!

 

[Sawyer]

Running a non-pressurized system with a large volume, I find my water level gauge to be a better indication of temperature than the storage temperature or the supply temperature.

 

[jebatty]

Water level would be the equivalent to psi in a non-pressurized system. Maybe you have a simple scale next to your water level indicator showing, for example, "empty, 1/3, 2/3, full." I am going to put arrow markers like that on my psi gauge, maybe blue, green, orange, and red.

 

[Tennman]

Actually very clever. For our launcher at work we compute the energy stored in the compressed air at a known volume. From that energy it's easy to compute enegy required to accelerate a know mass from nothing to a desired velocity. You're basically doing the same thing since for all practical purposes the majority of the "stored pressure energy" is in the accumulator. I think the volume growth due to increase of pressure in the rest of the system would be very small. If you had a good handle on your accumulator's air volume before and after, you could very precisely compute energy to the accuracy of your pressure gage. Actually the pressure measurement error in inexpensive mechanical pressure gages would be your biggest measurement error. The trick it to get a pretty good handle on that air volume that your "compressor" is doing work on. Very cool thot here to get a handle on total stored system energy. Gotta run. BTW the equation to get from compressed air to energy is very simple.

 

[Sawyer]

Water level would be the equivalent to psi in a non-pressurized system. Maybe you have a simple scale next to your water level indicator showing, for example, "empty, 1/3, 2/3, full." I am going to put arrow markers like that on my psi gauge, maybe blue, green, orange, and red.

I marked supply temperature degrees on the water level indicator I made for the Garn out of ¾†PEX in the spare bung on the front. I did this so I could monitor the water level in the tank without climbing up to the manhole and got a bonus temperature gauge that does not seem to be effected by stratification. I got a surprise when I realized the cover had been leaking and my water level was low. Not so low as to trip the low water shut off but lower than I like to see. I would think it would be best to have the chemically treated water as high as possible to assure a good treatment for the steel.

I am surprised how much the level changes from 100* to 205*.

 

[in hot water]

What about a BTU meter on the boiler output? It would measure the gallons flowed and the temperature rise. That is how you pay for energy pulled from a district system of cities that sell steam to users.

Sun Reports.com uses a 3 sensor BTU calculation. Energy into the tank with flow sensor and ∆T and also tank temperature.

hr

 

[PassionForFire&Water]

I like the BTU-meters. Could not find any pricing on them.
One thing I know is that you will need to flow-meter in the supply or return line or a clamp-on.
These flow meters are expensive, $500 for in-line model to $1,000 fro clamp-on

 

[Tennman]

Actually very clever. At work we compute the energy stored in compressed air at a known volume/temperature. From that energy it's easy to compute pressure required to accelerate a know mass from nothing to a desired velocity. You're basically doing the same thing since for all practical purposes the majority of the "stored energy" is in the accumulator. I think the volume growth due to increase of pressure in the rest of the system would be negligible. If you had a good handle on your accumulator's air volume before and after, you could very precisely compute energy to the accuracy of your pressure gage. Actually the pressure measurement error in inexpensive mechanical pressure gages would be your biggest measurement error. The trick it to get a pretty good handle on that air volume that your "compressor" is doing work on. But that may be irrelevent. All you care about it getting a pressure measurement from YOUR system that represents YOUR desired stored energy.

Very cool thot here to get a handle on total stored system energy. Digital pressure measurement systems are inexpensive and would dramatically increase the precision of your measurements. Gotta run. BTW the equation to get from compressed air to energy is very simple. I'd be glad to help provide a simple spreadsheet to get a first cut on BTUs from pressure change.

 

[Tennman]

Oops. Got message yesterday my reply failed. Apparently not. Sory about double post. Anyway if I add storage next year I think I'll play with this idea.

 

[woodsmaster]

I allso use the pressure more than tank temp to determine how much heat is in storage at 10 psi I need to be building a fire at
20 psi the storage is full.

 

[woodsmaster]

Dont have any data to back it up but, it seems like from 20 - 15 psi goes faster than 15 to 10 psi.

 

[ewdudley]

Dont have any data to back it up but, it seems like from 20 - 15 psi goes faster than 15 to 10 psi.

Boyle and Mariotte might agree.