Mixing to a constant final temperature is necessary for uniform production of doughs. Considered on the basis of weight alone, flour is the biggest single contributor to batch temperature. If flour comes into the mixer cool enough, then the temperatures of ingredients other than water almost do not matter.
As configured today, flour cooling takes place by injection of the cold gas or dry ice snow at various process sites. It can be injected into the pneumatic pipe, or enclosed auger conveyor, that transfers flour from silo to receiving hopper or batching vessel. It can be introduced at a batching vessel such as a pneumatic blender. And it can be injected at the mixer. Safe venting of the spent gas must be part of the final system design.
Ice, chilled water, or injection of carbon dioxide (CO2) in the mixer bowl accomplishes somewhat inefficient and imprecise compensation for winter-to-summer differences in ingredient temperatures. Conditioning of flour temperature through injection of CO2 during pneumatic conveying is a substantial improvement over current temperature reduction methods.
Reduction of flour temperature occurs through the injection of liquid CO2 directly into the flour conveying system at a point or points before the mixer. When the liquid CO2 passes through an injection nozzle into the flour line, it instantly flashes into fine particles of CO2 “snow.” The fine particles mix with the fluidized flour as they sublime into CO2 gas. At the operating temperature of -78°C (-109°F), the CO2 causes an immediate temperature reduction of the flour and any air conveyed with the flour.
Most vendors differ only in the method of CO2 injection. Some provide a series of nozzles, located at multiple sites through control valves in the horizontal flour lines, while others handle coolant injection with multiple nozzles at a single location on the vertical flour line immediately ahead of the flour use bin. Chief consideration must be given to the length of time (a function of the length of the pipe in horizontal lines) and fluidization of the flour within the blend of cryogenic gas and conveying air. The vertical system is reported to be more efficient in fluidization to ensure uniform mixing of the CO2 and flour.
The vertically installed flour temperature reduction system consists of five parts: (a) CO2 piping, (b) electronic control panel, (c) mechanical control panel, (d) CO2 injector and (e) flour temperature thermocouple. At the system’s control panel, the operator sets the desired temperature for flour as it enters mixing. The inline cooling system automatically activates when the flour valve at the silo is opened. Thermocouples located in the flour line detect the temperature of the flour and send a signal to the temperature control system. Based on the difference between the flour temperature and the set point, the control system determines the flow rate of CO2 through the injector valves needed to reach the set point. Liquid CO2 flows through the nozzles mounted around the circumference of the injector until the temperature of the flour reaches the set point. This action results in achievement of the desired flour temperature at the weigh hopper or mixer.
A similar system adds a phase separator to ensure that the cryogen (liquid nitrogen) is always present at the injector, and there is not loss of cooling when the nitrogen sublimates, turns to gas, in the manifold. This system controls flour temperature to ±0.5°C (±1 F°). Liquid nitrogen’s boiling point temperature, -195.8°C (-320.4°F), is much lower than that of carbon dioxide, -78.6°C (-109.5°F). The length of this vertical injection system varies from 16 to 50 in., depending on the diameter of the flour line.
Another design injects the cold CO2 into flour scale hoppers above mixers, which means it works with both pressure and vacuum conveying systems. A thermocouple measures the temperature of incoming flour and dry ingredients, reporting this data to the control system, which calculates the amount of cryogen required and opens the valve sending the cold gas into the scale hopper. The hopper’s fluidizing bed ensures distribution of CO2 throughout the dry ingredients. All these activities take place during the normal time delay between delivery of dry ingredients to the scale hopper and the end of mixing for the previous batch.
Mechanical conveyors can also be fitted with flour cooling systems. Configured as heat exchangers, the cooler circulates cold water or glycol in a double-walled jacket around the stainless-steel tubes carrying the flour.
More on this topic can be found in “Baking Science & Technology, 4th ed., Vol. II,” Page 382, by E.J. Pyler and L.A. Gorton. Details are in our store.