Where does the pressure difference in coffee roasting come from? Why use "differential pressure gauge"?

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What is a differential manometer?

The pressure difference between the two ends is called a differential pressure gauge. A differential pressure meter is installed in the exhaust duct to measure the pressure difference between the inside and outside of the bean dryer boiler in Pa.

It can be simply divided into mechanical pressure difference meter and digital pressure difference meter:

Mechanical differential manometer (pictured above), which uses the pointer corresponding scale to judge the immediate pressure difference.

Digital pressure difference meter, that is, the use of electronic digital display of real-time pressure difference, in decimal point as a unit, can more accurately display the pressure difference, but also use a computer to record the change of the pressure difference.

How does the pressure difference occur?

Pressure difference: a pressure difference measuring point is installed in the exhaust air pipe to measure the pressure difference between the exhaust pipe (or inside the baking cylinder) of the roaster and the outside world through the pressure difference measurement, in Pa. In the case of a passively ventilated drum roaster, the stronger the exhaust, the greater the pressure difference (that is, the smaller the pressure in the drum).

Draught

The hot air type bean dryer produces hot air from the combustion chamber and blows into the boiler, resulting in a positive pressure difference, while the direct and semi-direct rolling bucket bean dryers draw air from the boiler with an exhaust motor to produce negative pressure, so the cold air outside the fuselage is introduced into the combustion chamber for heating.

Baked bean reaction

In the process of baking, free water slowly evaporates to produce water vapor, and gas is slowly produced inside. After dehydration period, Mena reaction period and development period, various chemical reactions make water vapor and carbon dioxide in the cell be discharged in a short time, which will not only increase the humidity and air pressure, but also cause the pressure difference between the boiler and the outside.

Why observe the differential pressure?

Relationship between pressure difference and air volume

Bernoulli's law:

V = fluid velocity

G = gravitational acceleration,

H = the height of the fluid

P = the pressure strength of the fluid

ρ = fluid mass density

Constant= constant

It can be known from Bernoulli's law that when the atmospheric pressure is fixed, the variables are (1) and (3), so v ^ 2 (fluid velocity) is proportional to p (fluid pressure). That is to say, the pressure difference is proportional to the square of the wind velocity (exhaust air volume).

For example, when the throttle is 50% and 100% open, the measured pressure difference is about 10pa and 40pa, then a very accurate corresponding relationship can be obtained, so that we can use the pressure difference to accurately calculate the immediate airflow when baking beans.

Monitor air volume

* traditional throttle

* relationship between traditional throttle angle and air volume

If the bean dryer itself does not have a precise air volume control mode, it is usually based on the throttle scale, but the traditional throttle scale does not show a linear relationship with the air volume, which can easily lead to inaccurate air volume adjustment results.

Bean bakers of different structures produce positive or negative pressure in the exhaust duct when they are running. Because of Bernoulli's law, we know that the magnitude of air flow can be observed through differential pressure readings, and the value can reflect the force of motor ventilation. Therefore, it can be regarded as a throttle scale, allowing the baker to control the current exhaust air value at any time, in order to stabilize the air volume of pumping and supplying air.

Stable baking

In the process of using the silver collection bucket and exhaust fan, oil and dust are gradually attached, and the exhaust air volume will be reduced because of blockage, resulting in exhaust burden, leading to the gradual weakening of exhaust effect, such exhaust decline phenomenon, will then have a great impact on the baking effect. If it is a traditional throttle scale, you can only test the cooked beans repeatedly before you have a chance to find out, which may waste a lot of raw beans.

Of course, it is recommended that the bean baking machine should be fixed clean in order to get to the root of the problem. The advantage of the differential pressure meter is that when the dust affects the exhaust air, the relationship between the pressure difference and the air volume is abnormal, or when the condition affects the ventilation, we can immediately judge whether the exhaust air path is blocked from the value of the pressure difference meter, so that the air volume can be adjusted and the pressure difference can be maintained at the normal value, at least the pot can be baked normally. Clean the exhaust pipe at the end to eliminate blockage.

Influence of pressure difference change

Is the pressure difference between inside and outside of the boiler enough to affect the baking effect?

1 standard atmospheric pressure is equal to 101325Pa, assuming that the air door of a bean baking machine is fully opened after an explosion, it will produce a pressure difference of about 114Pa (usually this pressure difference is not small), while the real pressure change inside and outside the boiler is calculated to be 0.112% (114Pa/ (101325Pa-114Pa) = 0.001126), that is, only about 1/1000 difference, so when someone vividly describes that "loss of pressure in the baking room may cause impermeable beans" Such a subtle effect should be difficult to observe and detect!

The influence of mountain and typhoon days

The gasification and boiling of water vapor is also very important when roasting coffee beans. For example, although the water boils on a high mountain, the water temperature does not reach 100 ℃, which leads to the situation that instant noodles are not cooked and meat is not cooked well. This is mainly because the pressure on the mountain is much lower than that on the flat ground, the atmospheric pressure decreases with the increase of height, and the boiling point of water decreases with the increase of height.

The alpine atmospheric pressure of 3000 meters is only about 70000Pa (about 7 ℃ 10 of the ground atmospheric pressure). When the atmospheric pressure decreases by 100Pa, the boiling point temperature of water will drop by about 0.03 ℃, so on a mountain of 100 meters, the boiling point temperature will drop from 100 to 91 ℃.

In typhoon days, the pressure has the opportunity to drop from 101325Pa to 94000Pa, about 7% pressure change (101325Pa-94000Pa) / 101325Pa=0.0723), and the boiling point temperature also drops from 100 degrees to 97.8 ℃ (101325-94000) / 100), so the impact on baked beans should not be underestimated.

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