Gas Pressure: Pressure is defined as a force per unit area, that is a force divided by the area over which the force is distributed. It is difficult to measure force exerted by gas molecules.

Liquid Pressure: The pressure of liquid depends only on the height of the liquid column and the density of the liquid. Consider a liquid with density d, contained in a cylinder with cross-sectional area A, filled to a height h;

Barometric Pressure: The device used to measure the pressure exerted by the atmosphere is called a barometer.The height of mercury in a barometer, called the barometric pressure, varies with atmospheric conditions and with altitude.

The Standard Atmospheric Pressure (atm) is defined as the pressure exerted by a mercury column of exactly 760 mm in height, when density of Hg = 13.5951 g/cm3 at 0⁰C and the g = 9.80665 m/s2. Then, 1.00 atm = 760 mm Hg = 760 torr = 101.325 kPa = 1.01325 bar = 1013.25 mbar

Example: What is the height of a column of water that exerts the same pressure as a column of mercury 76.0 cm (760 mm) high ?

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In the experiment of spray drier we have discussed the general principle of drying process. It depends on mainly the relative humidity change of a product. While the processing the humidity of sample is decreasing. In the machine there are some important parameters like inlet and outlet temperatures, flow rate of liquid and the air. In order to determine the flow rate we have looked up the velocity of air with kumpas. Also cross sectional area is needed for the determination of flow rate. It was measured by anamometer. We have measured the wet bulb and dry bulb temperatures with psyhrometer.

In our experiment we have used onion liquid to make it powder.However,we have come up some diffuculties.In spray drier,nozzle is the dispersion device but onion liquid was so dense that nozzle couldn’t disperse liquid and it was stopped by dense liquid particules.If we had used atomizer instead of nozzle,we would have been able to disperse liquid through spray drier reqularly without having any problem.

Also when we got onion powder,we measured moisture content of powder by using moisture analyzer and it was nearly 65%.When we compared moisture contents of powder and liquid we saw that moisture contents were nearly the same value.This was unusual because moisture content of powder was expected to be lower than that of liquid.This might result from wet powder,waiting of powder in laboratory environment for a while and powder might absorb some moisture from environment.These could be reasons why moisture contents were nearly the same.Neverthless,moisture content of powder should be around 5-10 % in certain circumstances.

Consequently, spray driers can be used for many food products and by using spray drier,we produce powders and we can use these powders for preparation of foods easily without consuming time and taste will be nearly the same with original products.
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