[Stoves] 'BBM' and the accuracy of RTD's (Way off topic)

Mon Jun 8 11:57:52 CDT 2015

Dear Prof. Lloyd and Crispin, Stovers, 

One thing I have found is getting accurate temperature readings using thermocouples is not easy. You don’t know this until you compare the reading of a couple systems side by side or use a certified thermometer.  So I like the idea of temperature change. And I find very interesting its use to determine when maintenance is needed at the mine. Perhaps off topic to expand on this I would like to better understand the process. Fluid (water & chemicals?) pumped down into the mine under high pressure has a temperature reading and the low pressure on the outflow at atmosphere pressure has another? and the resistance due to the pump create heat and high values means maintenance required? something like that?

Heating water in a pot without stirring will create hot zone at the bottom and especially if there are obstruction in the pot like you have with the coils - is my guess. Super heated water at the bottom. If this is the case you will need a specified size coil for each specified and shape pot. I am thinking a better plan is to have the coil outside the pot submerged in cool recirculating water bath. The long end of the pipe going to the bottom of the cooking pot and the short end placed just below the surface. Water pumped through the coil from the cooling pot and returned will mix in the cooking pot and the system be suitable for any size pot. Calibrate two thermocouples against each other using one pot of hot water then place one at the inflow and the other at the out flow and record debts-T. 

Question:
Can you determine the heat getting to the pot by bring the pot of water to a boil them remove the heat source and measure the curve of it cooling?   So this is a Blank of heat loss from the pot? so the two values (blank and water temperature) is the heat from the stove that actually gets to the pot?  

Question:

Is the 4.186 j/g required to raise the temperature one deg. C the same going from 10c to 11c as it is from 80c to 81c? OR is it just an average of the total range and we need some complex math to determine the actual energy that went into a system going between two specified  delta-Ts?

Thanks

Frank

Frank Shields
franke at cruzio.com

> On Jun 8, 2015, at 1:25 AM, Philip Lloyd <plloyd at mweb.co.za> wrote:
> 
> “The delta T in the pipes is not large – maybe 3-7 degrees.” Importantly, delta T is something you can measure really accurately with thermocouples.  We used that to determine the efficiency of high-pressure (200 bar) pumps on deep-level mines.  The delta T accurate to  
> ±0.01 deg C between low and high pressure lines was an excellent measure of efficiency and told you precisely when maintenance was necessary.
>  
> Prof Philip Lloyd
> Energy Institute
> Cape Peninsula University of Technology
> PO Box 652, Cape Town 8000
> Tel:021 460 4216
> Fax:021 460 3828
> Cell: 083 441 5247
>  
>  
>  
> From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org <mailto:stoves-bounces at lists.bioenergylists.org>] On Behalf Of Crispin Pemberton-Pigott
> Sent: 08 June 2015 12:59
> To: 'Discussion of biomass cooking stoves'
> Subject: Re: [Stoves] 'BBM' and the accuracy of RTD's (Way off topic)
>  
> Dear Frank
>  
> This will give you the idea. The water is circulated through a heat exchanger, not into and out of the pot. That way I can leave the pot to have its local temperature variations and collect heat from a broad area – more representative of the bulk temperature than any single point.
>  
> First attempt (pipes a little short)
> <image001.jpg>
>  
> Pot on a traditional stove.
>  
> <image002.jpg>
>  
>  
> >Seems to have several advantages. One being continue mixing of the pot water  as the water is returned, no need to decide on a simmer temperature and a boiling temperature (based on elevation) 
>  
> The point is to measure the heat gained by the pot, not to cook anything. A lab test is a simulation. This is a reasonable method of determining the heat gained for as long as you want to run the fire. What we have largely been doing in the past is trying to work out after the fact what the heat flow was. When evaporation Is involved it can get messy, plus no one has been considering the heat lost from the pot. 
>  
> >Using K thermocouples? I would think J would be more to measuring the range around boiling. 
>  
> K is what most people have and they are flexible: can use them in the fire and stack and so on. They are OK for most things.
>  
>> >>the flow through the heat exchanger is 35 cc per second. There is five litres of water in the pot. 
>>  
> >The flow is constant? 
>  
> Surprisingly constant. It is governed by the pressure in the line (or regulator) and temperature. The delta T in the pipes is not large – maybe 3-7 degrees. Obviously it can be tuned. The SEET Lab in Ulaanbaatar has a flow meter on it logged by the computer. It can detect to a precision of a few Watts.
>  
> >And the temperature is kept the same in the pot? 
>  
> The water mass and pot temp can vary as you like. I prefer to keep the water temp below 70 so there is no evaporation at all. The Indian test 13152 uses 95 degrees and doesn’t consider evaporation which is a source of experimental error. We set the flow to match the heat gain rate such that it doesn’t rise above 70. Remember that if you consider the losses from the pot to radiation and convection, there is an influence. There is a difference between the heat gained by the pot and the Net heat gained. 
>  
> >Its the same 5 liters of water just being circulated?  
>  
> No the water is wasted. The inlet is from the wall.
>  
> >The only thing left is the change in the energy required to keep the cooling water / heating water in the heat exchanger to maintain the temperature of choice. 
>  
> Yes. 
>  
> What it does very well (because the pot water temperature change is integrated with the coolant and Delta T) is we can find out the heat gain rate at the full power range of the stove. 
>  
> Because the CSI Technical Test operates the stove according to local practise, and that power level is adjusted from time to time depending on the meal choices, we are able to determine the total heat gained for any selected test cycle. That is the ultimate test of the stove: what is the thermal and emissions performance when ‘run that way’?
>  
> While the flow rate can be monitored continuously, in fact it doesn’t vary much and there are still things one can do: have the water exit into a large bucket that sits on a scale, log the mass change and watch the flow rate at different temperatures.  That provides a correction factor (if needed).
>  
> An ultrasonic flow meter is really, and surprisingly, accurate. Because the temperature range is low – 15-30 degrees – the influence of thermal expansions of water is tiny compared with other variations. The water will expand but so does the pipe… J
>  
> The spreadsheet used is the same as that use for heating water in a radiator supply (like the Chinese and Mongolian low pressure boilers). In the short term we will make it able to accommodate 2 pots at the same time to test two-pot stoves. 
>  
> I was very surprised at the low Delta T on the low pressure boilers. The inlet and outlet are 4-7 degrees apart if there is a small pump. We also found in January that the pumps are not constant flow which means a flow meter is absolutely required. For the pot heat exchanger connected to the tap there is very little variation.
>  
> I will discuss this setup with Julien in July so he can try it independently using his temperature logger. What do you say Julien?
>  
> Regards
> Crispin
>  
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