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

Sun Jun 7 17:59:16 CDT 2015

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)

Pot on a traditional stove.

>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… :) 

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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