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<DIV><FONT face=Arial>Dear Crispin</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>The question of "What is an acceptable Turn-Down Ratio?"
(TDR) is not a simple one.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial>Fundamentally, the Cook has to control the heat to the pot
for two reasons:</FONT></DIV>
<DIV><FONT face=Arial>1: To cook it properly</FONT></DIV>
<DIV><FONT face=Arial>2: To cook it efficiently</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial>When heating a "watery food" such as a soup, there would
be little concern for burning the food, in that the large amount of water
present, and its fluidity, would make burning or scorching very unlikely,
and a small TDR would work for the Cook. At the other extreme,
a "non-fluid" or "pasty" food requiring a long cooking time, like beans or
a stew, should have a large TDR to avoid burning or scorching of the
food. </FONT> <FONT face=Arial>If the stove does not have an
adequately high TDR, the Cook may be able to compensate by:</FONT></DIV>
<DIV><FONT face=Arial>1: Adding extra water and allow it to boil
off</FONT></DIV>
<DIV><FONT face=Arial>2: Move the pot to a cool part of the stove
top.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>In a test, like WB 4.2.1, the test is run with no cover on
the pot. There can be a very significant heat loss from the pot, even though its
temperature is below boiling, as a consequence of evaporative cooling. Thus, a
TDR of perhaps only 2 may suffice with a non-covered pot, while a TDR of 6 may
be required if the pot is covered. While an open top pot man enable a low TDR of
say 2 or 3 might hold the pot contents within the simmering range, the foods
being cooked may not be able to accept a water addition part way through the
cooking process. The required TDR will depend on whether the cooking pot is
covered or not.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial>From a fuel efficiency standpoint, the cooking is best
done with a covered pot:</FONT></DIV>
<DIV><FONT face=Arial>a: heat the pot to boiling temperature on "High
Heat"</FONT></DIV>
<DIV><FONT face=Arial>and then</FONT></DIV>
<DIV><FONT face=Arial>b:turn down the stove to the point where there is just an
occasional wisp of steam escaping from under the
cover. </FONT> </DIV>
<DIV><FONT face=Arial>This is the way that most "pot cooking" is done in the
real world. One notable exception would be when the Cook purposely wants to boil
away excess water, to thicken the food.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>The nature of the pot, and ambient conditions have a big
effect on the required turn-down ratio. Also, the maximum heat input rate to the
pot, ie "pot power", will have an effect on the required TDR. </FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial>For example, if a covered pot of water was heated to
boiling, and if its heat loss rate was 500 watts at boiling temperature, then a
stove with a maximum "pot power input" of 1 kw would be perfect with a TDR
of 2. On the other hand, if the same pot was set on a stove with a maximum pot
power of 3 kw, then the TDR would have to be 6.<FONT
face="Times New Roman"> </FONT><FONT face=Arial>However, the first pot
would be very slow to heat up to boiling temperature</FONT></FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>In summary:</FONT></DIV>
<DIV><FONT face=Arial>1: For most efficient cooking, and to reflect most
common cooking practises, the pot should be covered.</FONT></DIV>
<DIV><FONT face=Arial>2: Heating pots with no cover will require more cooking
fuel, but by addition of water, burning or scorching of foods can be
prevented, with a lower TDR. This assumes that adding water during cooking is
acceptable to the Cook.</FONT></DIV>
<DIV><FONT face=Arial>3: .A stove heating a covered pot will require a larger
TDR than a stove heating an open top pot with no cover.</FONT></DIV>
<DIV><FONT face=Arial>4: A stove with "high pot power input" will require a
larger TDR than a pot with a lower "pot power input."</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>The actual TDR required can be measured with present day
instrumentation in common use. Once the maximum "high pot power input" rate is
measured, the fire can be turned down, or allowed to die down slowly, and the
point where water temperature starts to fall can also be measured. Knowing the
rate of fuel burning at that time enables calculation of the required TDR. It
would also allow calculation of fuel efficiency at the lower required
TDR.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Best wishes,</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Kevin</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV>----- Original Message ----- </DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=crispinpigott@gmail.com href="mailto:crispinpigott@gmail.com">Crispin
Pemberton-Pigott</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=stoves@lists.bioenergylists.org
href="mailto:stoves@lists.bioenergylists.org">'Discussion of biomass cooking
stoves'</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, May 02, 2013 11:12
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [Stoves] Stove Definition -
controllability</DIV>
<DIV><BR></DIV>
<DIV class=WordSection1>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">Dear
Friends<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">I
would like to remind everyone that it is pretty important to a cook to be able
to control the fire in some manner. There are lots of precedents so I won’t
repeat them. I would like to have a minimum control exerted over the cooking
power in order to qualify as a ‘cooking stove’. There are many appliances
which are used for heating water, showers (like the Geyser 2000 etc) or drying
fish and so one and on. But in order to ‘cook’ the fire has to be
controllable.<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">For
an electric or gas stove this is fairly easy. In order to start things off,
what does everyone think about a turn down ration of 4:1 where the turn down
is ‘willful’ meaning it is controlled by the cook be either removing fuel,
controlling airflow or by some other means. The reason is that stoves are
appearing which definitely burn fuel and provide heat but are not very
controllable (or not at all controllable). While one car argue that by
brilliantly fuelling the stove in just the right manner a fire and its burn
can be exactly matched to a cooking need – agreed this is possible – but is it
‘cooking’?<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">When
sitting in the field with cooks it becomes obvious that most cooking involves
controlling the power at some point. How much control should be applicable to
a stove in order to qualify as a ‘cooking stove’?<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">If
I ask for a water heating stove, it would not have to have any controllability
at all – it just needs to heat the water within a certain time after which it
can go out – no one will mind. But if we want to present a ‘solution’ (a
cooking alternative to an open fire or sheltered fire) it will have to be
manageable ‘to a certain extent’.<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">Thus
if someone says, “Here is my new cooking stove,” I can say, “Prove it can
cook.”<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">If
I ask for a maximum power of X and ask for a demonstration that it can be
controlled to X/4 is that reasonable as a minimum standard of
proof?<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US">Thanks<BR>Crispin<o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN
style="FONT-FAMILY: 'Calibri','sans-serif'; COLOR: #1f497d; FONT-SIZE: 11pt; mso-fareast-language: EN-US"><o:p> </o:p></SPAN></P></DIV>
<P>
<HR>
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