Time to heat storage

[Bob Rohr]

Nofo's description is very good. It is all about freshman thermodynamics.

Anytime a heat exchanger, coil or plate, is used to heat a storage tank, it is nearly impossible to achieve an output temperature from the heat exchanger which is equal to the boiler supply temperature. The heat exchanger output temperature will be less than the boiler supply temperature. This difference is called approach temperature. A somewhat standard design results in a "normal" approach temperature of 10F; that is, if boiler supply is 185F then output from the heat exchanger will be 175F. This also means that the maximum temperature that can be reached in a storage tank when using a heat exchanger will be about 10F less that the maximum boiler supply temperature. It is possible to design a heat exchanger to obtain a closer approach temperature than 10F, and of course the cost to do this also goes up. I am well aware of a system using a Garn WHS3200 that has an approach temperature of 5F while moving about 70 gpm through a plate hx.

A heat exchanger usually is required when using open storage, and one is not usually required when using pressurized storage. This difference means that it is possible to obtain maximum temperature in a storage tank about equal to the maximum boiler supply temperature. This can be important, especially in systems which require high temperature supply water, and it also is important because it can extend the time between boiler firings by having a greater quantity of hot water available from storage.

Through my several posts on weighed wood burns I have demonstrated with pressurized 1000 gallon storage that it is possible, even quite easy, to load my storage tank to 193F water, top to bottom, and 193F being about the maximum supply temperature of my Tarm before the controller shuts it down to prevent overheat, and moreover to accomplish this without any boiler idling.

 

[Bob Rohr]

The main difference with an un-pressurized tank with a copper coil is you only have one pumped flow, so the coil sits in a "pool" of warmed water. Only the thermal induced movement in the tank creates the driving ∆ T.

With a pressurized system you have only one HX and that is the boiler itself, the tank or load flow goes directly to the HX and it is a moving flow against the HX.

That the essence of a plate stye HX, you have two pumped flows and you have a much more efficient heat exchange. It is possible to size a plate style HX (close approach) to within a few degrees, A side to B side. It amazes me how much energy a 5X12" 30 plate HX will transfer. I generate my DHW instantly with a single pass through a 30 plate HX. 120F boiler side, 55F incoming water gives that huge ∆ T

Adding fins, or more surface area to the smooth copper coils would speed things up, but moving the flow around as well as through the coil would be a big plus. Many tank manufacturers now use the corrugated stainless coils, more surface area, plus the rough inside surface encourage additional HX via turbulance. Just like the turbulators you add in the boiler flue gas passages.

E dub is correct a BTU is BTU. If the boiler is firing then the energy is going somewhere. If the boiler goes into idle mode with a warm tank, then you lack heat exchange efficiency. If it has a mixing valve the operating temperature rating PLUS the differential needs to be combined. A 140 F element with a common 18 F differential = 158 F to the return before the bypass shuts down. Not all thermostatic valves have a 100% bypass shut down, so you are always slipping the clutch so to speak. I just dropped my thermostatic sensor cartridge to a 130 F. I may drop to a 115F.

Remember the dewpoint with wood varies greatly depending on the moisture content. It's not like gas, oil, or LP with a predictable dewpoint and 130- 140 F is realistic. With wood it is a balance more air to the combustion process lowers the dewpoint, but more air also carries more energy up the flue

Heat goes to cold always, but also the rate of hot to cold depends on the differential. The large cold start firings show quick temperature rise, as the load or tank warms the exchange slows. That is where a more efficient HX kicks in. Seems those last 5- 10 degrees are painfully slow to transfer, especially when you are watching and expecting.

http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf

 

[heaterman]

That chart ^ should be mandatory instruction for anyone contemplating a wood fired heater of any type. It graphically illustrates how high moisture content in the "fuel" screws up combustion and robs efficiency. Nice Hot Rod

What is being hashed around here can be broken down into the two aspects of efficiency in any boiler. First you have combustion efficiency which is fairly straight forward to figure out. The second factor is a little more difficult to pin down out in the field and that is heat transfer efficiency or how well the heat exchange surfaces in your boiler extract the heat from the fire side to the water side. One can have a good combustion process going on which burns very clean but if your transfer efficiency is poor your overall result can really stink. A good illustration is about any typical OWB. Once they get going they usually burn halfway decent if seasoned wood is used The tell that the overall efficiency stinks is in the 700*+ flue temps commonly observed. The ability to capture the heat from the combustion process in most of them is very poor.

For all normal gassers I use an average OUTPUT number of 60% of rating to get a handle on total heat generated over the course of a burn. Even a Garn with the firebox/heat exchanger completely submerged in the "storage" will have somewhat of a bell shaped efficiency curve over the course of a load.

 

[Bad Wolf]

With a 1200 gallon stoarge tank and no load from the house I get about 8 rise /hr. So 140 to 172 is about 4 hours. Thats at the top of the tank, there is usually a 20 differance between the top and bottom of the tank.
I think the math works 1200 gallons is 10,000 pounds. 8 x 10,000= 80,000 BTU. The TARM is rated at 102,500 BTU/hr x 75% efficiency (TARM claims over 80%) = 76,875 BTU/hr Thats close enough for me.

It must be a function of the pump speed but I can only get a 12rise when the TARM is firing.