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Technical Interview Questions - 4 which contains questions about instrument air compressor, dew point,  color codes of pipes in industries,  orifice meter, venturi meter, batch process in chemical industry, continuous process in industries, osmosis process

41. What is dew point and why it is important in instrument air?
42. What are the color codes for cylinders of Oxygen, Carbon dioxide, and Chlorine?
43. Please give a comparison between orifice meter and venturi meter
44. Definitions: Partial pressure, Vapour pressure, Diffusivity
45. Difference 


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41. What is dew point and why it is important in instrument air?
  • The Dew Point is the temperature at which water vapor starts to condense out of the air (the temperature at which air becomes completely saturated). Above this temperature the moisture will stay in the air.
  • Electronic instruments uses instrument air received from instrument compressors must be free of any moisture. Small amount of moisture may condensate & it may harmful to instruments So the instrument air should be free of moisture. This is ensured by keeping the dew point below  400 C or below. 

42. What are the color codes for cylinders of Oxygen, Carbon dioxide, and Chlorine?

Oxygen – White
Carbon dioxide – Grey
Chlorine – Yellow

43. Please give a comparison between orifice meter and venturi meter

Orifice meter

orifice meter
Venturi meter

venturi meter

  1. The orifice plate can easily be changed to accommodate widely different flow rates, whereas the throat diameter of a venturi is fixed, so that its range of flow rates is circumscribed by the practical limits of Dp.
  2. The orifice meter has a large permanent loss of pressure because of the presence of eddies on the downstream side of the orifice plate; the shape of the venturi meter prevents the formation of these eddies and greatly reduces the permanent loss.
  3. The orifice is cheap and easy to install. The venturi meter is expensive, as it must be carefully proportioned and fabricated. Homemade orifice is often entirely satisfactory, whereas a venturi meter is practically always purchased from an instrument dealer.
  4. On the other hand, the head lost in the orifice for the same conditions as in the venturi is many times greater. The power lost is proportionally greater, and, when an orifice is inserted in a line carrying fluid continuously over long periods of time, the cost of the power may be out of all proportion to the saving in first cost. Orifices are therefore best used for testing purposes or other cases where the power lost is not a factor, as in steam lines.
  5. However, in spite of considerations of power loss, orifices are widely used, partly because of their greater flexibility, because installing a new orifice plate with a different opening is a simpler ma즈er. The venturi meter cannot be so altered. Venturi meters are used only for permanent installations.
  6. It should be noted that for a given pipe diameter and a given diameter of orifice opening or venturi throat, the reading of the venturimeter for a given velocity is to the reading of the orifice as (0.61/0.98)2, or 1:2.58.(i.e. orifice meter will show higher manometer reading for a given velocity than venturi meter).
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44. Definitions: Partial pressure, Vapour pressure, Diffusivity

Partial pressure
The pressure of component gas that is present in mixture of gases.
The total pressure of mixture of gases = partial pressure of the component gas which are present in mixture of gas.


Vapour pressure
The pressure exhibited by vapour on liquid or solid surface is known as vapour pressure.


Diffusivity
The ratio of the flux to corresponding concentration gradient. Unit is m2/sec.


45. Difference : H

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