Gas and Air Dryers Flashcards
What is the term used to describe the amount of water vapour
Grains. 7000 grains equals one pound of water
What is relative humidity?
It is a comparison of the amount of water a sample of air holds, to the maximum it could hold of saturated.
What is dew point?
The temperature at which the moisture comes out of the gas
What does pressure do to the amount of moisture air can hold?
As pressure increases, the amount of moisture it can hold decreases
What is another name for a large amount of liquid entering the compressor inlet?
Slug
What are some problems with moisture in a gas stream
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
What are the four categories of dryers?
Adsorption
Absorption
Refrigeration
Mechanical
Describe adsorption
It is the adhesion of molecules in a thin layer on a surface. It uses the attraction of certain compounds to water
When is the adsorption drying agent no longer active?
When the vapour pressure of the film on the drying agent equals the vapour pressure of the air stream
What must be done when adsorption dryer no longer can remove moisture from the air stream
It must go through a cycle of regeneration (reactivation). This removes the film of moisture
Define regeneration
The process of removing the accumulation of moisture from the dryer bed
Approx how much dried air is used to regenerate the dryer bed?
About 10-15%
What is a benefit of adsorption dryers?
They can achieve lower dew points than any other type of dryer
What are some common desiccants used in an adsorption dryer?
Activated alumina Granulated silica gel Bead type modified silica gel Molecular sieve
Describe activated alumina
It is a porous aluminum oxide, and is inert. Suitable for use in variety of gas streams, and s spherical shape is common
Describe granular silica gel
Irregular shape, large surface area to collect moisture. Commonly used in packaging of medicines, electronics, cameras
Describe bead type silica gel
Similar to granular, but spherical shape. It is very porous
Describe molecular sieve
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
Where are coalescing filters used?
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
How is the flow for regen, and why?
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
What are the methods of regeneration?
Heater less, heater dryers, and removing contaminants
What is the relativity of temperature and water vapour
As air warms up, the amount of water it can hold, in the form of vapour, increases
What happens with heat when moisture is removed?
Heat is given off by the water and absorbed by the desiccant
What is a negative aspect of heatless regeneration?
Because 10-15% is going to regen, you have to make 10-15% more air than your demand
Describe a heater dryer?
The regen air is warmed, from an external source, and passes through to remove moisture
What is a hydrometer ?
It measures the amount of moisture in the gas stream. Sometimes called a wet bulb
What is an absorption?
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
Are all absorption methods able to be regenerated?
no
What are the two types of absorption dryers?
Chemical dryer (deliquescent) Liquid dehydration (glycol)
What is another name for a chemical dryer
Solid dehydration
Why are chemical dryers not able to be regenerated
Because the deliquescent material is consumed by the drying process. It forms a solution called brine
What is the most common deliquescent?
Calcium chloride
Which way is the flow in a chemical dryer?
Upward
What is a typical dew point depression(lowering of dew point) in a chemical dryer?
Approximately 10 degrees C
What is a problem with chemical dryers used in lid temperature applications?
The brine in the bottom of the tank could freeze
Which drying method gives the best dew point depression?
Adsorption (desiccant)
What is a danger with chemical dryer?
The carryover of bribe or salt dust is extremely corrosive
Why is a chemical dryer impractical for systems requiring a constant supply of dry gas?
Because it must be shut down in order to replace deliquescent and the gas would then be bypassed around the compressor
What is liquid dehydration?
It is a abortion method of drying using a drying agent such as glycol
How does liquid dehydration work?
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
How does the wet glycol have the moisture removed from it?
It is passed through a still, where it is heated to between 190-205 C where the moisture is evaporated
How pure is the glycol returning to the system after the still?
98% pure glyco
What affects the efficiency of liquid dehydration?
Glycol temperature Concentration of glycol Amount of interaction between the gas stream and the glycol.
What is the major consideration with most dryers?
The flow or speed through the dryer. Sufficient time is required to fully remove the moisture
Describe a refrigeration dryer
There is no adsorption or absorption materials. It lowers the temperature of the gas stream causing the moisture to fall out
What is s condenser and an evaporator?
Condenser turns the gas back into a liquid, and the evaporator turn the refrigerant into a gas
What is a benefit of a refrigeration dryer
It is the least expensive both in operation and maintenance perspectives
What is s heat exchanger?
It takes the heat from one place, and moves it to another
What is latent heat?
Latent heat cannot be measured by thermometer. It is the heat required to maintain a phase change (water to steam)
What is sensible heat?
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
How much energy is required to raise temperature of water?
It takes one BTU to raise 1 lb of water 1F It takes one calorie to raise 1 gram of water by 1C
Describe the air circuit through a refrigeration dryer
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
Describe the refrigerant circuit through the refrigeration dryer
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
How low can the dew point be lowered in a refrigeration dryer?
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
Having a high relative humidity hold more or less additional moisture?
Less
The temperature to which a gas must be reduced before the water vapour it carries begins to condense refers to what?
Dew point
Of the following, which one would NOT be used to remove moisture? Air receiver Intercooler Aftercooler Cyclone separator
Intercooler
When a sample of air that is partially saturated cools, what happens to the relative humidity?
Rises relative to the temperature drop
In a deliquescent dryer, which way is the airflow?
Upward through the bed
Which type of dryer constantly regenerates itself while in operation?
Rotary desiccant dryer
What is the least expensive method of dew point depression?
Refrigeration drying
How much can liquid dehydration lower the dewpoint?
40-60°C from the temperature of gas entering the dryer
The air leaving a refrigeration dryer is often reheated for what purpose?
To expand it so less air is needed
Explain a mechanical dryer
They only use mechanical or physical means to cause the moisture to fall out of the gas stream
List some mechanical dryers
Liquid separators , centrifugal separators, demister pads, over compression, coalescing filters
What is a liquid separator ?
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
What is the least complicated of all dryers?
Liquid separator.
What are baffles used for in a liquid separator ?
To create a more torturous path for the gas. They aid in the diffusion and velocity reduction of the gas
What is a centrifugal separator?
Sometimes called cyclonic separator. It uses centrifugal force to throw out the heavier moisture particles. These are very common after an after cooler
What is a demister pad?
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
What is over compression?
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
What is a coalescing filter
A very effective method of oil and water removal. It can remove particles down to .01 of a micron (100 millionth of a meter)
What is a normal drop over a system pressure?
5-10 PSI is considered normal.
What should be suspected as a problem if the pressure differential across a system is unusually low?
A switching valve may be stuck open, or a filter may have a hole in it or maybe bypassing it
What is a typical vapor compression cycle
Evaporation, compression, condensing, expansion
Which is generally the most trouble free dryer
Refrigeration dryer
What is the correct volume of refrigerant sometimes called?
The charge
What should prefilters and after filters have installed To assist in filter condition assessment?
Differential pressure measurement
How much can adsorption lower the dewpoint
It can lower the dewpoint to -70°C
Describe molecular sieve desiccant
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
What Do all dryers require the use of in internal or Extertal heat source?
They require a high temperature lubricant in the switching valves
What is another name for a rotary dryer
MD or rotary dryer
What is the dewpoint depression in the chemical dryer
Approximately 10°C from the temperature of the incoming gas stream
