How to measure bake chamber parameters

by Stephen St. Clair-Thompson
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A heat flux data logger sits between rows of cookies to measure the heat conditions within the oven during baking.
 

 

It is important to think of the oven as a device for producing heat flux and not in terms of temperature. An instrument that helps with understanding flux is the oven data logger. These small devices have one or more sensing heads and a recorder in a well-insulated box. It is placed among the dough pieces at the infeed of the oven and retrieved from the baked products after the oven. Both temperature loggers and heat flux loggers are commercially available, and some loggers can distinguish between convective heat flux and radiant heat flux.

 

A glance at the data logs from a 3-zone convection oven shows why it is important to think in terms of heat flux and not temperature. In this case, the convection dampers on zone No. 2 were closed, producing a markedly different heat flux than during the other two zones, although the temperatures were generally the same. What would have happened if the process had to be moved to another oven and only the temperatures had been replicated? Clearly, to obtain consistent products, we need to provide consistent heat fluxes.

 

Providing consistent temperatures is only one, and an insufficient, way of achieving uniform results. We know this fact in our daily lives. Those of us lucky enough to go skiing will know it better than most. Consider a skier standing at the top of a mountain on a still bright day. He may be pleasantly warm because he gets a warming dose of sunlight and is not subject to any convective effects. But he sets off downhill, skiing fast through a belt of trees at the same air temperature. He will rapidly feel cold because the sun’s radiation is denied him, and his velocity relative to the air provides him with a substantial (negative) heat flux. Thus, the heat flux in a convective oven is a function of both the air temperature and the velocity of the air as it passes over the dough pieces.

 

The following table describes the terms used to measure heat transfer and heat flux. (The unit “kW per hr,” frequently seen in magazine articles, is meaningless. The correct unit is “kW” alone or “kWhr” as noted by the table.)

 

Heat Measures

Imperial

Metric

Heat (energy)

British thermal unit (BTU)

kilowatt hour (kWhr)

Heat transfer (power)

BTU per hr

kilowatt (kW)*

Heat flux (rate of transfer of heat per unit area)

BTU per hr per sq ft

kW per sq m

 

Between the mixer and the oven stand a variety of machines and equipment systems that accommodate the processing stages of intermediate proofing, final proofing and retarding. Each applies time, temperature and humidity to bring out the desired characteristics required for a high-quality finished product.

 

More on this topic can be found in “Baking Science & Technology, 4th ed., Vol. II,” Page 495, by E.J. Pyler and L.A. Gorton. Details are in our store.

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