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Millwright Third Year 2018 > Gas and Air Dryers > Flashcards

Gas and Air Dryers Flashcards

1
Q

What is the term used to describe the amount of water vapour

A

Grains. 7000 grains equals one pound of water

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2
Q

What is relative humidity?

A

It is a comparison of the amount of water a sample of air holds, to the maximum it could hold of saturated.

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3
Q

What is dew point?

A

The temperature at which the moisture comes out of the gas

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4
Q

What does pressure do to the amount of moisture air can hold?

A

As pressure increases, the amount of moisture it can hold decreases

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5
Q

What is another name for a large amount of liquid entering the compressor inlet?

A

Slug

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6
Q

What are some problems with moisture in a gas stream

A

It plugs instrumentation components,
mechanical damage due to liquids not being compressible.
If air is used outside it could freeze, paint applications require a dry are stream for adhesion

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7
Q

What are the four categories of dryers?

A

Adsorption
Absorption
Refrigeration
Mechanical

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8
Q

Describe adsorption

A

It is the adhesion of molecules in a thin layer on a surface. It uses the attraction of certain compounds to water

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9
Q

When is the adsorption drying agent no longer active?

A

When the vapour pressure of the film on the drying agent equals the vapour pressure of the air stream

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10
Q

What must be done when adsorption dryer no longer can remove moisture from the air stream

A

It must go through a cycle of regeneration (reactivation). This removes the film of moisture

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11
Q

Define regeneration

A

The process of removing the accumulation of moisture from the dryer bed

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12
Q

Approx how much dried air is used to regenerate the dryer bed?

A

About 10-15%

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13
Q

What is a benefit of adsorption dryers?

A

They can achieve lower dew points than any other type of dryer

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14
Q

What are some common desiccants used in an adsorption dryer?

A 
Activated alumina 
Granulated silica gel 
Bead type 
modified silica gel 
Molecular sieve
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15
Q

Describe activated alumina

A

It is a porous aluminum oxide, and is inert. Suitable for use in variety of gas streams, and s spherical shape is common

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16
Q

Describe granular silica gel

A

Irregular shape, large surface area to collect moisture. Commonly used in packaging of medicines, electronics, cameras

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17
Q

Describe bead type silica gel

A

Similar to granular, but spherical shape. It is very porous

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18
Q

Describe molecular sieve

A

Acts as a filter by restricting the size of particles able to pass through the bed. Used in applications such as: Hydrocarbons, nitrogen, hydrogen, H2S, CO2

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19
Q

Where are coalescing filters used?

A

Downstream of the dryer to ensure dust from desiccant that breaks down over time is picked up. It is also used as a pre filter to ensure desiccant isn’t loaded with oil or other contaminants

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20
Q

How is the flow for regen, and why?

A

The flow used is reversed through the bed for regen. They do this so the flow of air goes from least saturated. To most saturated , and it reduces compaction of the bed

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21
Q

What are the methods of regeneration?

A

Heater less, heater dryers, and removing contaminants

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22
Q

What is the relativity of temperature and water vapour

A

As air warms up, the amount of water it can hold, in the form of vapour, increases

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23
Q

What happens with heat when moisture is removed?

A

Heat is given off by the water and absorbed by the desiccant

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24
Q

What is a negative aspect of heatless regeneration?

A

Because 10-15% is going to regen, you have to make 10-15% more air than your demand

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25
Q

Describe a heater dryer?

A

The regen air is warmed, from an external source, and passes through to remove moisture

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26
Q

What is a hydrometer ?

A

It measures the amount of moisture in the gas stream. Sometimes called a wet bulb

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27
Q

What is an absorption?

A

It uses a chemical bond between the drying agent and the gas stream being dried. The moisture is absorbed by the material and forms a new solution

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28
Q

Are all absorption methods able to be regenerated?

A

no

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29
Q

What are the two types of absorption dryers?

A

Chemical dryer (deliquescent) Liquid dehydration (glycol)

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30
Q

What is another name for a chemical dryer

A

Solid dehydration

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31
Q

Why are chemical dryers not able to be regenerated

A

Because the deliquescent material is consumed by the drying process. It forms a solution called brine

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32
Q

What is the most common deliquescent?

A

Calcium chloride

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33
Q

Which way is the flow in a chemical dryer?

A

Upward

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34
Q

What is a typical dew point depression(lowering of dew point) in a chemical dryer?

A

Approximately 10 degrees C

35
Q

What is a problem with chemical dryers used in lid temperature applications?

A

The brine in the bottom of the tank could freeze

36
Q

Which drying method gives the best dew point depression?

A

Adsorption (desiccant)

37
Q

What is a danger with chemical dryer?

A

The carryover of bribe or salt dust is extremely corrosive

38
Q

Why is a chemical dryer impractical for systems requiring a constant supply of dry gas?

A

Because it must be shut down in order to replace deliquescent and the gas would then be bypassed around the compressor

39
Q

What is liquid dehydration?

A

It is a abortion method of drying using a drying agent such as glycol

40
Q

How does liquid dehydration work?

A

The gas is brought into contact with the drying agent which is sprayed and atomized through nozzles. The air is scrubbed with the glycol which absorbs moisture from the air

41
Q

How does the wet glycol have the moisture removed from it?

A

It is passed through a still, where it is heated to between 190-205 C where the moisture is evaporated

42
Q

How pure is the glycol returning to the system after the still?

A

98% pure glyco

43
Q

What affects the efficiency of liquid dehydration?

A

Glycol temperature Concentration of glycol Amount of interaction between the gas stream and the glycol.

44
Q

What is the major consideration with most dryers?

A

The flow or speed through the dryer. Sufficient time is required to fully remove the moisture

45
Q

Describe a refrigeration dryer

A

There is no adsorption or absorption materials. It lowers the temperature of the gas stream causing the moisture to fall out

46
Q

What is s condenser and an evaporator?

A

Condenser turns the gas back into a liquid, and the evaporator turn the refrigerant into a gas

47
Q

What is a benefit of a refrigeration dryer

A

It is the least expensive both in operation and maintenance perspectives

48
Q

What is s heat exchanger?

A

It takes the heat from one place, and moves it to another

49
Q

What is latent heat?

A

Latent heat cannot be measured by thermometer. It is the heat required to maintain a phase change (water to steam)

50
Q

What is sensible heat?

A

Heat that can be measured. EG. Water raising from room temperature to boil can be measured 30C-100C when it starts to boil, it no longer gets hotter, but heat is being put into it. That is latent heat

51
Q

How much energy is required to raise temperature of water?

A

It takes one BTU to raise 1 lb of water 1F It takes one calorie to raise 1 gram of water by 1C

52
Q

Describe the air circuit through a refrigeration dryer

A

It enters hot air, and is slightly cooled by the cold air leaving. It then moves through the refrigerant heat exchanger and cooled down further. Here the condensate is seperated and drained by the water separator. The cold air then leaves and is slightly heated by the warm wet air entering

53
Q

Describe the refrigerant circuit through the refrigeration dryer

A

The refrigerant gas is compressed. It then goes to the condenser where it is cooled by large fan and becomes a liquid. It then moves to evaporator, it takes a great deal of heat to become s gas, it gets the heat from incoming gas stream

54
Q

How low can the dew point be lowered in a refrigeration dryer?

A

To 2-3C. Lower temperatures would result in a freeze up of the system, thus they cannot be used in s system that experience temperatures below 2-3C

55
Q

Having a high relative humidity hold more or less additional moisture?

A

Less

56
Q

The temperature to which a gas must be reduced before the water vapour it carries begins to condense refers to what?

A

Dew point

57
Q

Of the following, which one would NOT be used to remove moisture? Air receiver Intercooler Aftercooler Cyclone separator

A

Intercooler

58
Q

When a sample of air that is partially saturated cools, what happens to the relative humidity?

A

Rises relative to the temperature drop

59
Q

In a deliquescent dryer, which way is the airflow?

A

Upward through the bed

60
Q

Which type of dryer constantly regenerates itself while in operation?

A

Rotary desiccant dryer

61
Q

What is the least expensive method of dew point depression?

A

Refrigeration drying

62
Q

How much can liquid dehydration lower the dewpoint?

A

40-60°C from the temperature of gas entering the dryer

63
Q

The air leaving a refrigeration dryer is often reheated for what purpose?

A

To expand it so less air is needed

64
Q

Explain a mechanical dryer

A

They only use mechanical or physical means to cause the moisture to fall out of the gas stream

65
Q

List some mechanical dryers

A

Liquid separators , centrifugal separators, demister pads, over compression, coalescing filters

66
Q

What is a liquid separator ?

A

It is used to take the load off a dryer, used when there is s large volume of moisture in the gas stream. It is a reduction of velocity in a large vessel that allows liquids and other contaminants to drop to the bottom of the vessel

67
Q

What is the least complicated of all dryers?

A

Liquid separator.

68
Q

What are baffles used for in a liquid separator ?

A

To create a more torturous path for the gas. They aid in the diffusion and velocity reduction of the gas

69
Q

What is a centrifugal separator?

A

Sometimes called cyclonic separator. It uses centrifugal force to throw out the heavier moisture particles. These are very common after an after cooler

70
Q

What is a demister pad?

A

A fine wire mesh screen or pad. These provide a surface area for the mist to adhere to until the droplets become big enough to fall out. They are commonly used in natural gas processs and glycol coolers

71
Q

What is over compression?

A

This is the only mechanical cooling method that lowers the dew point. It increases the pressure of the gas making it unable to hold on to moisture. This is a very costly way to remove moisture because of the higher pressure needed

72
Q

What is a coalescing filter

A

A very effective method of oil and water removal. It can remove particles down to .01 of a micron (100 millionth of a meter)

73
Q

What is a normal drop over a system pressure?

A

5-10 PSI is considered normal.

74
Q

What should be suspected as a problem if the pressure differential across a system is unusually low?

A

A switching valve may be stuck open, or a filter may have a hole in it or maybe bypassing it

75
Q

What is a typical vapor compression cycle

A

Evaporation, compression, condensing, expansion

76
Q

Which is generally the most trouble free dryer

A

Refrigeration dryer

77
Q

What is the correct volume of refrigerant sometimes called?

A

The charge

78
Q

What should prefilters and after filters have installed To assist in filter condition assessment?

A

Differential pressure measurement

79
Q

How much can adsorption lower the dewpoint

A

It can lower the dewpoint to -70°C

80
Q

Describe molecular sieve desiccant

A

It can actually act as a filter in that it restricts the size of particles that are able to pass through the bed it can remove hazardous gases and sweeten gasses

81
Q

What Do all dryers require the use of in internal or Extertal heat source?

A

They require a high temperature lubricant in the switching valves

82
Q

What is another name for a rotary dryer

A

MD or rotary dryer

83
Q

What is the dewpoint depression in the chemical dryer

A

Approximately 10°C from the temperature of the incoming gas stream

Millwright Third Year 2018 (15 decks)

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